Landing on Mars

Landing a spacecraft on the planet Mars is not a piece of cake!

After several failed attempts the first successful landing took place in 1976, when two(!) spacecrafts, the Viking 1 and 2, landed safely on the surface of the Red Planet. And a Red Planet it was. Here are the first (color) pictures taken, left by the Viking 1, right by the Viking 2

The next successful landing was more than 20 years later, the Mars Pathfinder in 1997. The lander contained a small separate vehicle, a Mars rover, that could independently explore the surface. Here you see the Sojourner, after it had just rolled down from the Pathfinder. It is a tiny vehicle of 63 x 48 x 28 cm and with a mass of about 12 kg.

The next mission was the Mars Exploration Rover in 2004. Two separate missions actually, landing two rovers on Mars, the Spirit and the Opportunity. Both missions were very successful, the two rovers were planned to operate for 90 Sol’s (a Sol is a Martian solar day), but Spirit remained active until 2010 and Opportunity until June this year. Actually they are still trying to contact Opportunity, hoping it survived the huge dust storm that raged on Mars this year. Check this update for the latest info.

Here is an artist impression of the Opportunity rover. Compared with the Sojourner this is a big boy..:-) , 2.3 x 1.6 x 1.5 m, mass 180 kg. Until the loss of signal on Sol 5111 (June 10, 2018) it had traveled 45.16 km.

Four years later, in 2008, the Phoenix landed on Mars, for the first time a landing in the polar region. It confirmed the existence of water ice on Mars. Here is an artist impression of the Phoenix landing. Mass about 350 kg

In 2012 the Mars Science Laboratory (MSL) mission landed a rover on Mars, the Curiosity, which is still active at the moment. Dimensions 2.9 x 2.7 x 2.2 m. mass 900 kg.

This photo shows the difference in size. In the foreground the Sojourner, left the Opportunity and right the Curiosity.

The last successful landing took place two weeks ago, 26 November 2018, when the InSight lander touched the surface of Mars. Diameter of the lander 1.5 m (without its solar panels), mass 360 kg.

Main mission is to get more information about the interior of the planet. A seismometer will record “marsquakes” and a “drill”  designed to burrow as deep as 5 m, will measure the heat flow from the interior. Here is an artist impression of the lander with the solar panels deployed. Foreground left the seismometer, right the drill.

Here is a map of Mars with the location of the eight successful landings.

More than half of all missions failed, for example the Beagle 2 in 2003 and the Schiaparelli in 2016.  For a full report , see the Wikipedia article Mars Landing.

Why is landing on Mars so difficult and risky?

Let us look in more detail at what is called the Entry, Descent and Landing (EDL) phase of a  Mars mission. This phase starts when the spacecraft enters the atmosphere of Mars and ends about 6-7 minutes later, when it lands on the surface.

For the MSL (Curiosity) mission in 2012, NASA created a fascinating YouTube video: 7 Minutes of Terror, in which scientists and engineers explain how many things can go wrong in this phase, while they can only watch helplessly. Watch the video, it takes only 5 minutes and gives a better explanation than I can provide here…:-)

But let me try. I will concentrate on the Curiosity lander because until now it is by far the most complicated mission of all.

The atmosphere of Mars is very thin, but the spacecraft enters with a high velocity of about 20.000 km/h and would be destroyed if it was not protected by a heat shield. Here is an artist impression of the so-called aeroshell in which the Curiosity (and all other landers) is safely stowed away. It consists of a backshell and a heat shield.

Here is the aeroshell in the assembly hall. The diameter is 4.5 m. You can see the Curiosity inside the backshell. On top of the backshell is the cruise stage which controls the spacecraft during the cruise from Earth to Mars.

An exploded view of the aeroshell. From left to right the cruise stage, the backshell, the descent stage, the Curiosity rover and the heat shield.

All Mars landing missions have three parts, two of which are basically the same: 1. slowing down by atmospheric friction and  2. further speed reduction by a parachute. You need one more step, because the Martian atmosphere is so thin that a parachute can not reduce the speed to (almost) zero at ground level. One way or another, you need (retro) rocket power for the last part

When the spacecraft is a lander, the “easiest” way is to provide it with retro-rockets. After the lander detaches from the backshell, it will unfold its legs and use its rockets to land. See the picture above of the Phoenix. The InSight used the same solution. Here is a picture of the InSight landing.

Rovers have to move around, so it doesn’t make sense to burden them  with the extra weight of these rockets. That’s why for the Pathfinder, Spirit and Opportunity another solution was developed. Put the retro-rockets in the backshell, lower the rover on a tether connected to the backshell,  protect it with airbags(!), use the rockets until almost zero speed, then drop the rover. Here is a collage of what it would look like for a Martian observer. Left, the airbags are already inflated, the rover is still hanging under the backshell, which is retro-firing. Middle, the rover has touched the surface but is still bouncing many times, before it comes to rest (right). Then the airbags will deflate and the rover is ready for action. Here is an animation of the landing of the Spirit rover.

The Curiosity is too heavy and voluminous for this airbag technique, so a spectacular new (and expensive) solution was developed. A sky crane!

Here is  a schematic view of the EDL process for the Curiosity. The first phase, atmospheric braking, looks normal, but there is a difference. Before the aeroshell enters the atmosphere at an altitude of 125 km, with a velocity of 20.000 km/h,  some mass is ejected one sided (“Cruise Balance Device Separation”). The resulting “unbalance” has as effect that the aeroshell will not move ballistically (like a projectile) but can be “steered” a bit through the atmosphere. The Martian atmosphere has turbulence,  storms, pressure differences etc, affecting the trajectory of the aeroshell, resulting in a considerable uncertainty in the final landing location.  The “hypersonic aero-maneuvering” reduces this uncertainty, important for Curiosity which had to land close to the rim of  the Gale crater.

At an altitude of 10 km from the ground, when the velocity is about 1500 km/h, a huge parachute (diameter 17 m!) is deployed, slowing down the aeroshell further. The heat shield is ejected 20 seconds later. From that moment,  using radar, the exact altitude can be measured, and the precise time when the descent stage & rover have to detach from the backshell. The descent stage starts firing the retro-rockets, first to move horizontally away from the backshell and the parachute. Meanwhile the rover is lowered 7.5 m  on cables, it deploys its wheels, while still connected through an “umbilical cord with the descent stage. Here is an artist impression.

                                                                                                                                             As soon as the rover touches the ground, the connecting cables are cut and the descent stage will fly up and away, to crash a few hundred meters from the rover

Curiosity has landed!                                                                                                                                                                                                                                                       All this  (and numerous details I have skipped) must happen  in less than 7 minutes. Seven minutes of terror, because everything is automatic. If something goes wrong, nobody can do anything. Besides, the radio signals back to Earth take about 14 minutes, so, when Mission Control gets the message that the spacecraft has entered the atmosphere, it has actually already landed (or crashed….) Here is an animation of the Curiosity landing. Spectacular.

Compared with the Curiosity mission, the landing of InSight was a lot simpler, basically the same as the Phoenix in 2008. The mission of InSight is to study the interior of Mars, the precise landing location is less important, as long as the surface is flat. Therefore no “guided entry” through the atmosphere was needed. The InSight is also much lighter (360 kg)  than the Curiosity (900 kg), so it was decided to provide the lander itself with rockets.

Although the EDL phase for the InSight was 6 minutes,  the catchy description “7 Minutes of Terror” was again used in the media  for this mission…:-) The BBC:    Nasa’s Mars InSight mission heads for ‘7 minutes of terror’

Of course it is still a major technological achievement! NASA published a very good explanation of InSight’s EDL phase:  InSight landing on Mars .

Here are three pictures taken by the InSight. The lander has two cameras, the Instrument Context Camera is a fisheye camera, mounted underneath the lander deck. In the first picture (left), taken a few minutes after landing, the lens is still protected by a transparent cover, because of the dust whirled up by the rockets. In the right picture the cover has been removed. and as you can see, still a lot of dust has managed to crawl under the cover and stick to the lens. Unfortunate, although the images will still be usable.

The second camera is mounted on a robotic arm, Here is a superb picture taken by this camera. The scientist are very happy with the sandy, rock-free location. The reddish box is the seismometer which later will be deployed after the best location has been determined.

The latest news about the InSight mission can be found here

 

Hayabusa2

In 2014 I have published several posts about Rosetta, the spacecraft that has explored the comet 67P. Click here for my reports. I am very interested in these Close Encounters between spacecraft and celestial bodies. Here is a new one, the Japanese Hayabusa mission. Actually there are two Hayabusa spacecrafts, the first one was launched in 2003, the second one in 2014.

Their mission was basically the same: Fly to an asteroid,  land on it, collect some asteroid material, then fly back to Earth to deliver the collected material.  An ambitious project!

Hayabusa was the first spacecraft ever that has landed on an asteroid and returned to earth with some asteroid material. Not as much as was hoped for, because the mission encountered quite a few technical problems. Therefore a second, improved Hayabusa2 spacecraft, was designed and launched on 3 December 2014.

Here is an artist impression of the Hayabusa2. The boxlike spacecraft has dimensions of 1 x 1.6 x 1.25 m and a mass of 609 kg

Destination? A tiny asteroid, 1999 JU3.. In an earlier post I have explained the complicated naming of the minor planets. The J stands for the first half of May, U stands for 20 and the subscript 3 means 3×25. So this asteroid was the 95th minor planet, discovered in the first half of May 1999. This provisional name is replaced by a number and sometimes a name, in this case 162173 Ryugu. It is the 162173th minor planet and the name has been suggested by JAXA, the Japanese counterpart of NASA.

Here is the route followed by Hayabusa2 to Ryugu.The Earth orbit in blue, Ryugu in green and the Hayabusa2 in purple.

It reached the asteroid, 3.5 year after the launch,  on 27 June 2018, . One day earlier it took this picture of Ryugu, from a distance of about 40 km

Properties of Ryugu:  not really spherical, diamond-shaped, a diameter of ~ 900 m and a rotation period  of 7.6 hours. The gravitation at its surface is about 80.000 times smaller than on Earth!

Until December 2019 Hayabusa2 will stay near Ryugu, at a distance of 20 km (HP, home position), where the gravitational attraction of the asteroid is almost nothing.  From there it will investigate the properties of the asteroid and several times it will descend for a short time to the asteroid.

On 20 September the spacecraft started a slow, controlled descent and one day later, 55 m above the surface of Ryugu, it dropped two Minerva rovers. While they were falling down to the surface, Hayabusa2 ascended to HP. Both rovers are working correctly, a huge relief for the scientist,  They are really tiny, diameter 18cm, height 7 cm, mass 1.1 kg Here is an artist impression.

Both rovers have multiple cameras and temperature sensors on board They can move around by “hopping” and do this autonomously! Wheels like for example the Mars rover has, would not work in this low gravity world.  One hop can take 15 minutes and move the rover horizontally ~15 m.

This is a picture of the Ryugu surface, taken by  one of the rovers.The scientists are surprised that the surface is so rough.

On 3 October, the spacecraft descended again to Ryugu, to drop the MASCOT lander, developed by the German and French space agencies. A bit larger and heavier, size of a shoebox, mass ~10 kg. Contains cameras and various scientific instruments. No solar cells like the two rovers, but battery operated, able to provide power during 16 hours. Also able to hop, like the rovers.

This is an artist impression of the MASCOT, leaving its container in Hayabusa2.

During the descent, Hayabusa2 was able to follow the lander. The yellow line is the actual path, the blue line is the projection on Ryugu’s surface. The times are given as “hhmmss”. After it hits the surface, it bounced several times. During the last part (straight blue line),no pictures were taken. The location 02:14:04 is the final landing place. The separate location 00:55:09 +1 is taken one day later and proves that MASCOT has managed to hop. The shadow is from Hayabusa2

Here are two images taken by MASCOT itself. Left while descending to the surface and yes, that is its own shadow top right. Right after landing, again showing a very rough surface.

After the successful landing, MASCOT started to use its scientific instruments (spectrometer, magnetometer and radiometer) and sent the data back to Hayabusa2 within the limited timespan of 16 hours. Actually the batteries lasted one hour more, a bonus. It hopped two times.

Until now the mission has been very successful: two rovers and one lander have touched an asteroid for the first time in history!

What will be next? The main mission is to collect material from Ryugu and bring it back to Earth. How to do that? Here is a schematic view of Hayabusa2. Notice the .Sampler Horn at the bottom

This is the procedure: the Hayabusa2 will descend very slowly to the asteroid until the horn touches the surface. Then a small (5g) bullet will be fired inside the horn, hit the surface at high speed and surface particles will fly up and be collected at the top of the horn. Hopefully at least 0.1 gram, maximum 10 gram. This will be done at two different locations.

The third and last one is quite spectacular, an attempt to collect material below the surface. Here is another view of the bottom of the spacecraft. Next to the horn you see the Small Carry-on Impactor.

It is an explosive device, meant to create a crater in Ryugu, so that Hayabusa2 can collect the debris. Here is how it works . The explosive will deform the copper shield (2.5 kg) into a projectile, that will hit the surface at a speed of 2 km/s, creating a crater with a diameter of several meters

By the way,  this is where the idea may have come from…:-)

The explosion must of course not damage Hayabusa2 itself! The  scientists have found this spectacular solution:

  1. Hayabusa2 approaches the surface of Ryugu.
  2. It releases the bomb and also a camera.
  3. Then it moves up and sideward to hide itself behind the asteroid!
  4. The bomb explodes and creates a crater.
  5. The camera takes images and sends them to Hayabusa2.
  6. Hayabusa2 appears again and descends above the crater.
  7. The horn will collect debris of the explosion

Here is an artist impression, where Hayabusa2 is descending above the newly formed crater.

The first touchdown of the spacecraft itself was planned for end October, but it has been postponed until January 2019.

Why?

Because the surface of Ryugu is much rougher than expected!

The horn of Hayabusa2 extends about 1 m, therefore the touchdown area should not have rocks higher than 50 cm. The touchdown area must also have a diameter of at least 100 m because of navigational accuracy.

Such a location could not be found on Ryugu!. Below is the one finally chosen (red circle) free of rocks, but ONLY 20 m in diameter!

There is also some good news. The earlier launching of the rovers and the MASCOT showed that navigation (with laser range finders) could be done more accurately (within 10 m), at least until the altitude of 50 m above the surface.

In the next weeks, two rehearsals will be performed, going lower, to find out whether tis accuracy can be maintained until touchdown.

The first real touchdown is now planned for January next year. There is enough time because Hayabusa2 will stay at Ryugu until December 2019.

All this is happening at about 300 million km away from Earth. Amazing. Keep in mind that communication between Earth and the spacecraft takes about 15 minutes, one way!

The German Space Agency has published a very instructive YouTube video, illustrating what I have tried to explain in this blog. Not only about the MASCOT lander as the title suggests.  Worth viewing more than one time!

If more news becomes available I will write an update

 

New Horizons

The first time I wrote about the New Horizons spacecraft was in a February 2015 post: Close Encounters. Launched in 2006, its primary destination was Pluto.  During the long voyage it had gone into hibernation (to save energy) and now it had woken up successfully to prepare for the flyby of Pluto in July.

To give you an impression of the size of the spacecraft, this picture is taken in 2005 during preparation for the launch.

Note the black “tube” to the left, it is the RTG, the power source for the spacecraft.

Solar panels can not be used because of the large distance to the Sun, instead radioactive plutonium is used.

The heat of the radioactive decay is  converted into electricity by thermocouples.

My second post was titled Close encounter with Pluto and published July 2015, a few days after the successful flyby. Here is a picture of Pluto, in high resolution, taken by New Horizons. Although the flyby took only minutes, the transmission of all photos taken, took more than a year, because of the slow bandwidth. Analysis is still going on.

In that post I wrote that New Horizons would try to visit another member of the Kuiper Belt before it left the Solar System. Soon after the Pluto flyby, in August 2015,  it was decided that (486958) 2014 MU69 would be the next destination.

What a name ..:-). Let me explain. The Kuiper Belt is located outside Neptune and contains trillions of objects, remnants of the early solar system. Pluto, once seen as the ninth planet, is now seen as a Kuiper Belt object. The Minor Planet Center keeps track of all the observed Kuiper Belt objects and the present count is 779736 !

The target of New Horizons is minor planet no 486958, discovered by the Hubble Space Telescope in 2014.

In this image (taken by the Hubble telescope)  you see the object (surrounded by a green circle) at 10 minute intervals

The code  MU69  tells in a complicated way that the object was the 1745th Kuiper Belt object, discovered in the second half of June! Curious about the code?  Have a look at the Wikipedia item about Minor Planet naming.

 

After a public voting campaign, NASA announced a few months ago that 2014 MU69 would get the nickname Ultima Thule. In classical and medieval literature Ultima Thule got the meaning of any distant place located beyond the “borders of the known world”

First estimate of Ultima Thule’s size, based on distance and brightness, was about 30 km. After it was chosen as the next target of New Horizons, of course many more observations have been made. How to get more information about an object of ~ 30 km, at a distance of more than 6 billion km?

Well, it can happen that Ultima Thule passes in front of a background star! In that case it will block for a short while the light of this star. This is called an occultation. Last year Ultima Thule occulted three different stars in June and July. Such an occultation can only be seen from specific locations on Earth (similar to a solar eclipse).  Here are the three predicted occultation paths.

On 3 June 2017, the NASA scientists tried to observe the “shadow” of Ultima Thule from Argentina and South Africa, but detected nothing. It turned out later that the predicted occultation path was not accurate enough, so the telescopes had been placed in the wrong location..

The second occultation took place over the ocean, therefore the airborne telescope SOFIA was used, flying along the predicted occultation path.

Main purpose was to check for hazardous material around Ultima Thule, which could endanger the fly-bye of New Horizons.

First they thought that they had missed the shadow, but later analysis showed that there had been a short dip from the central shadow

The third attempt was very successful. 25 telescopes were placed along the occultation path in South Argentina and five of them observed the dip.

Here is an example. It is an animated gif, time between the frames is 0.2 seconds.

Watch the star in the centre and notice how it “disappears” for a short while!

Careful analysis of the “dip” gives a lot more information. Ultima Thule might be actually a contact binary, with a very elongated shape.

More information about this amazing scientific exploration can be found in this  NY Times article.

Here is an artist impression of Ultima Thule. The Sun is not more than a very bright star, you can see how New Horizons is approaching… 🙂   To the  left you see a “moonlet” orbiting Ultima Thule, for a while the scientists thought there could be one, but it is now disputed.

On New Year’s Day 2019 at 05:33 UTC, if everything goes well, New Horizons will pass Ultima Thule within about 3500 km.

New Horizons has woken up from its hibernation last month and is healthy. The coming months preparations will be made for the encounter.

It will be exciting to see how Ultima Thule looks in the real. But it will take time to transmit pictures back to Earth.  It takes almost six hours for data to bridge the distance between New Horizons and Earth!

An update will follow later.

‘Oumuamua

The Haleakala volcano is located on the island of Maui, the second-largest island of Hawaii. On the top of this extinct volcano, at an altitude of 3055 m, an astronomical observatory has been built. One of the telescopes in this observatory is the Pan-STARRS telescope, with  a 1.8 m diameter mirror and equipped with the largest digital camera ever built, recording almost 1.4 billion pixels per image

The function of this telescope is to scan the sky, looking for moving astronomical objects, like comets and asteroids. The idea is simple, you take two pictures of a part of the sky, at different times and compare them.

In 1930 the planet Pluto has been discovered this way. Here are the original photographs, taken 6 days apart, the arrows point to Pluto. In those days the astronomer used a gadget, called a blink comparator, which rapidly switched from viewing one photograph to viewing the other. The moving object would stand out by “blinking”.

Nowadays computers can do this much better than humans, and Pan-STARRS is connected to a sophisticated computer system, that not only analyses the images, but also communicates with other observatories all over the world, when moving objects are found.

The interest in these objects is not only scientific. Pan-STARRS is taking part in the NEO Search Program. NEO stands for Near Earth Object, and NEO‘s are objects, mostly asteroids, that could collide with Earth in the future. Readers who have been following my blog from the start, may remember my posts about the asteroid Apophis, published in 2010(!). In 2013 Pan-STARRS had already discovered 10.000 NEO’s

On 19 October 2017 a new moving object was discovered by Pan-STARRS.  At first it was assumed to be a comet, and named C/2017 U1   but as it had no characteristic comet tail, it was reclassified one week later as an asteroid : A/2017 U1


A short intermezzo about the classification of comets. The first letter describes the kind of object, P for a periodic comet, C for a comet with unknown period and A for an object that was first classified as a comet but is actually an asteroid. There are a few more categories. The letter is followed by the year of discovery and by a letter that indicates the “half-month” of discovery. A for the first half of January, B for the second half, C for the first half of February, etc. The letter I is not used, so the U means that the object was discovered in the second half of October. It is here followed by the number 1, because it was the first discovery in that half-month.


Further observation of this asteroid, by many other observatories, showed that it came from OUTSIDE our solar system!

A few years ago I have published a post about NEO’s,  Visitors from Outer Space, but Outer Space stands there for the Kuiper Belt and the Oort cloud, both still belonging to our solar system.

So this was an EXTRAORDINARY event, the first observation of an object that came from another star! On 6 November 2017, less than three weeks after its discovery, it was reclassified again, as 1I/2017 U1 , where a new category was introduced, I, standing for Interstellar object. The number 1 in front of the I was added, because it was the first occurrence of this new category. And it was given a proper name: ʻOumuamua, which in the Hawaiian language means “a messenger from afar arriving first“. The symbol ʻ in front of the name is not a typo, but a ʻokina, a glottal stop.

How do the astronomers know that it comes from outside our solar system? By its speed and its orbit! Here is an animated gif of ʻOumuamua’s orbit.

The orbit is hyperbolic ! From far away it approached the solar system with a velocity of ~26 km/s. Attracted by the Sun, its velocity increased to ~88 km/s at perihelion, on 9 September 2017. When it was discovered by Pan-STARRS, 40 days later, it was already on its way out.

Here is another sketch of ʻOumuamua’s orbit, with dates. Try to view it “3-dimensional”, the orbital plane of ʻOumuamua is tilted with respect to the ecliptic (the orbital plane of the planets).

Is there anything more that we know about this visitor from outer space? For example from which star it started and how long it is underway?  It arrived roughly from the direction of the star Vega, 25 lightyear away. In that case, with its velocity it would have taken ~ 600.000 year to reach our solar system. But Vega was not in the same location, that long ago. We just don’t know, ʻOumuamua could have left its star system billions of years ago.

Surprisingly we know a bit more about its shape, its size and its color! The color is dark reddish and the size is roughly 200 x 30 m.  A kind of gigantic cigar…:-) Here is an artist impression

You may wonder how anything can be said about the shape, as the image of ʻOumuamua is only a single pixel in even the strongest telescopes! The answer is that the asteroid is “tumbling” with a period of ~ 8 hours. Therefore the amount of light it reflects varies.

Here are the measurements done by several telescopes, with a theoretical fit, assuming a 1:10 ratio between length and width  Click to enlarge.

Of course there are people who are wondering if it could be a spaceship…:-). In that case it would probably be out of control or abandoned, because of the tumbling. Anyway, both SETI and Breakthrough have been listening for any signals coming from ʻOumuamua. Without results.

For this blog I have used the very informative Wikipedia article about  ʻOumuamua and other Internet sources.

Physics Nobel Prize 2017

Last month  the Nobel Prize for Physics has been awarded to three American physicists for their ” decisive contributions to the LIGO detector and the observation of gravitational waves”

Here they are, from left to right Rainer Weiss (85), Barry Barish (81), Kip Thorne (77) .

All are retired professors and quite old, not unusual for Nobel Prize winners…:-; More unusual is that this Nobel Prize has been awarded for the observation of gravitational waves in September 2015, only two years ago! The time between a discovery and the Nobel Prize is often 10-20 years and tends to increase

In this case the physics community was pretty sure that the Nobel Prize would go to  LIGO, the Laser Interferometer Gravitational-Wave Observatory, where the gravitational waves were observed. Problem is that a Nobel Prize (with the exception of the Peace Prize) can not be awarded to an organisation but only to a maximum of three individuals (and never posthumously). And the article in Physical Review Letters, where the discovery was published in February 2016, has more than 1000(!) authors. Here is the beginning of the author list

In this blog I will explain why these three people were selected. But first I must tell a bit more about gravitational waves, and why physicists are so excited that they have been observed.

In 1687 Newton publishes his  masterwork “Principia” in which he presents the three laws of motion  and the universal law of gravitation.

Motion takes place in 3-dimensional space as a function of time. Both space and time are absolute concepts, independent of each other.

Newtonian mechanics works extremely well, but there is one disturbing fact, the speed of light c in vacuum turns out to be always the same, no matter how fast the light source is moving itself. Einstein  “solved” the problem in 1905 by accepting the constancy of c as a fact, which resulted in  his Theory of Special Relativity (TSR)

But it came at a price! Space and time are no longer absolute and independent in this theory, together the three dimensions of space and the single dimension of time form a 4-dimensional continuum, called spacetime .

Gravitation doesn’t play a role in the TSR, but in 1916  Einstein publishes his Theory of General  Relativity (TGR). In this theory gravitation is described as a curvature of  spacetime. A massive object like the Sun curves the spacetime in its surroundings and a planet like Earth just “follows” this curvature.

A consequence of this theory is that even light would follow this curved spacetime and will be deflected when it passes close to the Sun. This prediction was successfully confirmed only a few years later. During a solar eclipse the stars near the Sun became visible and their position was shifted in complete agreement with the TGR. It was front page news and made Einstein world famous.

 

Another prediction of the TGR was that (accelerated) motion of massive objects could produce waves and ripples in this fabric of spacetime. Mind you, in spacetime itself ! However, these waves and ripples were estimated to be very small, maybe only measurable if  those objects were extremely massive.

For example, two black holes or neutron stars, orbiting each other.

Here is an artist impression of the gravitational waves caused by two orbiting black holes. I have hesitated to include this image, because I find it very confusing, suggesting that the cells of the spacetime fabric are moving up and down, whereas the cells themselves are changing shape, stretching and contracting. But the image comes from LIGO, so who am I…:-)?

After this long(?) introduction it is time to go back to LIGO and the three Nobel Prize winners.

LIGO has a long and complicated history, starting in the 1960!  Here are some important dates. The names of the three Nobel Prize winners in blue.
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In 1968, almost 50 (!) years ago, Kip Thorne (Caltech) did calculations about the gravitational waves of black holes. Which, by the way, had not yet been discovered at that time, but their existence followed from the TGR! He came to the conclusion that detection should be possible. Also in the 1960s, Rainer Weis (MIT) proposed to use interferometry to detect the incredibly small variations in the fabric of spacetime. See below for more about interferometry.

In 1980, under pressure of the American National Science Foundation (NSF) , MIT and Caltech joined forces in the LIGO project. But progress was slow and funding not easy.

In 1994Barry Barish (Caltech) was appointed director of the project. He was a good organiser, proposed to build the LIGO detector in two phases. This proposal was approved by NSF and got a budget of  USD 395 million,  the largest project in NSF history!

In 2002, the first phase of LIGO became operational, but no gravitational waves were detected.

In 2004, funding and groundwork started for the second phase, “Enhanced LIGO”, four times more sensitive than the first phase.

In September 2015, after a 5 year overhaul of USD 200 million was completed, Enhanced Ligo started operating.

Within days, on 14 September at 9:50:45 UTC,  Enhanced LIGO detected gravitational waves for the first time in history.
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So, what is an interferometer? Here is a sketch of the LIGO interferometer

And who could better explain how it works than Rainer Weis himself?

What may not be fully clear from the video is the huge scale of this LIGO project.

Two “identical” interferometers have been built in the US, about 3000 km apart

Here is an aerial view of  the Hanford interferometer, each of the arms is 4 km long!

Both interferometers can be seen easily on Google Earth. Left Hanford, right Livingston.

As Weis explained, gravitational waves cause small differences in the length of the arms. Very, very small. In the order of  10-19 m, that is about 1/10.000 part of the size of a proton. Read that again and again, I still find it difficult to believe..:-)

The sensitivity must be about 1/10.000 part of the size of a proton.

New technology had to be developed to reach this sensitivity. Ultra-high vacuum, very precise mirrors, extremely stable lasers. Noise reduction (thru seismic vibrations, a passing truck, etc) is the main problem. That is also the main reason that two interferometers were built. Accidental noise should be different in both detectors, but a gravitational wave should reach both (with a slight time difference, because of the distance between the two detectors).

Critical is the suspension of the mirrors. They must be absolutely stable. Here two images of the damping and suspension systems. Click here for details

What kind of signal do we actually expect? Let’s concentrate on orbiting back holes (it is called a binary), like Thorne did. As shown in the earlier image, they produce gravitational waves, but those are way too small to be detected. However, the binary will loose energy sending out these waves, as a result over time the two black holes will get closer and closer. Until they come so close that they will merge into one larger black hole, a cataclysmic process that may take less than a second! It is during this phase that the gravitational waves are much stronger and might be detectable.

Here is a computer simulation of the merger of two black holes. The simualtion has been SLOWED down about 100 times, in reality the merger occurs in a split second. The “moving” background stars are a result of the extreme distortion of spacetime.

Click here to see the gravitational waves, sent out during the merger.

You will notice that before merging the two black holes spin faster and faster, distorting the fabric of spacetime more and more. It is a bit similar to a bird chirp: increasing frequency and loudness.  After they merge into one, no more gravitational waves.

So, what happened on 14 September 2015? The two interferometers were to start the first research run on 18 September and were already in fully operational “engineering mode”, when at 9:50:45 UTC both detected  the typical “chirp” signal.  For testing purposes sometimes “fake” signals were injected, to test the alertness of the system and the scientists. It took a few hours before it became clear that this was a real signal and not a test!

Here is the “Nobel Prize winning” signal. The red graph is from Hanford, the blue one from Livingston (the Hanford signal is also shown, inverted and shifted in time)  Notice the time scale, the whole merger takes place in a few tenths of a second!

The lower two graphs show a fit to the data, using Numerical Relativity. It is surprising how much information can be extracted from these two graphs. Here is a (partial) result

Two black holes, with a mass of 35 and 30 M☉. (solar mass) , at a distance of about 1.4 billion lightyear away from Earth, merged into a single black hole of 62 M☉. .

The mass difference of 3 M , was radiated during the merger as gravitational waves. That is an awful lot of energy!  The estimated peak emission rate was greater than the  combined power of  all light radiated by all the stars in the observable universe! If you don’t believe me, click here.

This first event has been named GW150914. GW stands for Gravitational Wave and is followed by the detection date 14-9-2015. In the past two years more gravitational waves have been detected, here is a list

If you look at the location, you see that in the first five events the location of the binary is not well-defined. The reason is that you need more detectors to determine the location accurately, two is not enough.

The sixth event, GW170814 was not only detected by LIGO, but also by Virgo , the European counterpart of LIGO. This interferometer is located near Pisa in Italy. Same setup as LIGO, slightly smaller arms (3 km)

Virgo was also designed in two phases. The first phase did not detect gravitational waves. In 2106 Advanced Virgo became operational and is now cooperating with LIGO.  Another interferometer will be built in India: INDIGO .

The last event, detected until now, GW170817 (about three months ago), is an interesting one, because it is not a merger of black holes! For the first time a merger of two orbiting neutron stars has been observed. The masses of the two stars are comparable with the Sun and the binary is closer to Earth, although still a respectable 130 million lightyear!  It is not sure if the merger resulted in a neutron star or a black hole. But anyway, a merger of neutron stars should result in visible light coming from the debris after the merger.

Because of the detection with three interferometers, it was possible to narrow the region of space where the gravitational waves came from.  The location predicted by LIGO/Virgo was still large, about 150 times the area of a full moon. Within hours after detection, alerts were sent to astronomers all over the world and a massive search started.

A few hours later the Swope telescope in Chili reported they had found  the source in galaxy NGC 4993  , 140 million lightyear away. This was soon confirmed by other observatories.

Here is an image of this elliptical galaxy. The inset shows the light coming from the merger, getting weaker and weaker, as expected.

More interesting discoveries can be expected in the future, this is just the beginning.

When you want to learn more about this fascinating new field of astronomy, you should read the book Ripples in Spacetime, written by Govert Schilling

 

Total Solar Eclipse 2017

Are you using Whatsapp and did you recently receive this picture? Getting excited already, that in a few weeks time you will observe a unique event?

Sorry to disappoint you, but this is complete nonsense. Yes, on 21 August 2017 there will be a total solar eclipse, and to watch it is an experience of a lifetime. But solar eclipses are common, almost every year there will be a solar eclipse visible, somewhere on Earth..:-)

Here is a list of solar eclipses between 2011 and 2020. In the third column the type of eclipse is given. Twenty four eclipses in total, five of them total. The last column gives the geographic region where the eclipse will be visible.

I do not want this blog to be very technical, but some explanation may be useful..:-)

A solar eclipse occurs when the moon passes between the sun and the earth.

The moon orbits the earth in about 29 days, so you would expect a solar eclipse roughly every month. But the orbital plane of the moon is tilted 5 degrees, therefore the shadow of the moon will not touch the earth every month. Also, because of the (big) size of the sun, the shadow of the moon (the white lines) just reaches a small part of the earth. The pink lines mark the region where the moon blocks the sun only partially.

Another effect is that the orbit of the moon is slightly elliptical, so the distance of the moon to the earth is not always the same. If the moon passes between the sun and earth while it is farther away from the earth, it can not block the sun completely, resulting in an “annular” eclipse.

Let us look in a bit more detail at the 21 August eclipse. The blue band is where you can see the total eclipse. Weather permitting of course…:-) The light blue lines parallel to the blue band indicate the regions where you have a 75%, 50% and 25% partial eclipse.

Is there anything special about this eclipse? Yes..:-)  It will only be visible from the Unites States of America and no other country!  AMERICA FIRST…:-)  Probably Trump will  twitter one of these days that it is one of the successes of his administration…;-)

Of course there is a lot of interest in the USA for this Great American Eclipse . Here are a few advertisements, taken from the Internet.

But also for the USA it is not a unique event. The last total eclipse, visible in mainland USA, was on 26 February 1979 and the next one will be on 8 April 2024.

Total eclipses are spectacular. It gets dark, and the solar corona becomes visible. A reason for many people to travel to a region where the total eclipse can be watched.

Actually I was one of them, 8 years ago!

Friends  told us about a total eclipse, visible in China on 22 July 2009.  Here it is

We decided to visit China, Hangzhou region, around that time, hoping for clear skies. The full report you can read here: China July 2009 part I: Hangzhou.

Many people, locals and tourists,  full of expectation.

Actually it was rather cloudy. Here you see pictures during the start of the eclipse. We kept our fingers crossed.

And we were lucky. The clouds were breaking, it was not completely clear, but enough to see the “diamond ring” and the corona

It was an unforgettable experience. As it will be for the Americans (and the many foreign tourists) on 21 August.

When you compare the two eclipse maps, you see in the lower right corner the duration of the totality.  The China one 6m 39s, the American one “only” 2m 40s. CHINA FIRST!

The maps come from a very informative website about solar (and lunar) eclipses, EclipseWise.com

Jupiter and Juno

This post is about the planet Jupiter and the spacecraft Juno, launched in August 2011 and orbiting Jupiter since July 2016. The image shows both the planet and the spacecraft.

But we will start with some Roman (Greek) mythology. Jupiter (Zeus) was the king of the gods and Juno (Hera) his wife. Jupiter was an promiscuous god with numerous extramarital affairs and Juno was a jealous spouse, always keeping a eye on her adulterous husband.  Here are a few of his affairs

  • He lusted for Io, and transformed the girl into a cow, to hide her from  his wife. In vain, Juno asked him to give her the cow as a present.
  • He abducted Europa, disguised as a bull. King Minos of Crete was one of their children
  • He fell in love with the nymph Callisto and took the shape of virgin goddess(!) Artemis to seduce her.
  • He was so enchanted of Ganymede, that, in the shape of a raven, he took the beautiful boy(!) to Mount Olympos.

You will understand that as schoolboys we were always happy when our Latin and Greek  teachers told us about these myths…:-)

Back to astronomy. Jupiter is the largest planet in our solar system. The planet is so big that all the other planets would fit in it. It is the second-brightest planet (after Venus) in the night sky.

In 1610, Galileo discovered that Jupiter has four moons. In the image you can see their size, compared to Jupiter. They look small beside the planet, but they are actually big. The largest one, Ganymede, is bigger than the planet Mercury!

The four moons were named after the four lovers of Jupiter named above! Below you see a (resized) image of each moon and a painting with Jupiter in action.

Since Galileo observed the four moons, many more (smaller ones) have been discovered. At the moment 67  moons have been observed, of which 53 have been named, often after Jupiter’s girlfriends and boyfriends…:-) Here is the complete list of Jupter’s moons

It may now be clear why the spacecraft has been named Juno  🙂  After the launch of the spacecraft, NASA published a mission statement in which they explained the name of the spacecraft:

The god Jupiter drew a veil of clouds around himself to hide his mischief, and his wife, the goddess Juno, was able to peer through the clouds and reveal Jupiter’s true nature.”

Actually the mission of Juno is to explore Jupiter and not his moons…:-) Much is still unknown about this gas giant. Does it have a solid core? Does its atmosphere contain water? An important part of the mission will be the study of Jupiter’s gravitational and magnetic fields.

So, let us follow Juno on her exploration of Jupiter. It took her five years to reach Jupiter. Why so long? Here is the reason:

To give the spacecraft enough speed at launch to reach Jupiter would cost too much energy. Therefore it is first launched in an (elliptical) orbit around the sun.

The Deep Space Maneuvers one year later will bring it back very close to Earth, which will give it a gravitational slingshot. See my Rosetta blog for an explanation.

As a result the orbit becomes a hyperbole, at the right moment crossing the orbit of Jupiter, where it will be captured by the planet.

Here is a fascinating animation of the whole process.

Jupiter has to be approached carefully because of its intense radiation belts. The magnetic field of a planet traps charged particles like electrons and protons in a doughnut-shaped region around the planet. Earth has these radiation belts, they are called the Van Allen Belts. For Jupiter they are many thousand times stronger and can seriously damage the spacecraft.

To protect the instruments of Juno, the most sensitive ones have been placed in a titanium container with 1 cm thick walls and a weight of 18 kg.

Here is an image of the spacecraft during assembly. The Radiation Vault is the brown box on top of the spacecraft.

Note the size of the human!

To minimise the radiation risk, Juno has to be captured carefully in a polar orbit. Here is a YouTube animation:

The capture orbit is very elliptical with a period of ~ 54 days. The original plan was to reduce the period to 14 days, after two capture orbits (1 and 2). The first reduced orbit (3) would be a clean-up orbit, followed by 32 “science” orbits (4-36), each of them slightly shifted, so the whole surface of Jupiter would be covered.The image gives an impression of these science orbits. Mind you, during each 14 days only a few hours before and after perijove (the point of shortest distance to Jupiter) can be used for science!

However, during the second orbit, a few days before the planned Orbit Reduction Maneuver on 19 October 2016, a problem was found with some helium valves needed to operate the main engine, and a few hours before perijove, the spacecraft went into “safe mode”, because the onboard computer encountered unexpected conditions. The next two orbits were used for testing and diagnostics.

Finally, on 17 February 2017, mission control decided it was too risky to perform the Orbit Reduction Maneuver. So the  spacecraft will remain in  its 54 day orbit. Totally 12 science orbits will be performed until July 2018. The next perijove (orbit 7) will occur on 11 July.

It must have been quite a disappointment for the scientists, instead of new data every two weeks, they now have to wait almost eight weeks.

Are there results already? The instruments that are measuring the magnetic field of Jupiter and the composition of the Jovian atmosphere are collecting data, it seems the magnetic field is more lumpy than expected.

The most spectacular results come from the on-board camera Junocam. Here is an image of Jupiter’s south pole, not observable from Earth. Amazingly complex and turbulent.

And last week NASA published another picture, taken 19 May, just after Juno passed perijove 7. Keep in mind that these images are color enhanced! Part of the south pole region is visible. The white spots are part of the “String of Pearls”, massive counterclockwise rotating storms.

The next orbit will pass over the famous Great Red Spot, a storm on Jupiter that has lasted already for several hundred years and is so big that Earth would fit inside it. Will be interesting to see images.

At the end of the Juno mission,  the spacecraft will be directed into the Jovian atmosphere, where it will be completely destroyed. This will be done to avoid any chance that material of Juno might “contaminate” one of  Jupiter’s moons. If ever life forms are found on these moons, there must not be any doubt about its origin.

To end this post in a lighthearted way, the Juno has three passengers on board! Figurines, specially crafted by Lego in the shape of Jupiter (with a lightning bolt), Juno (with a magnifying glass) and Galileo (with a telescope and Jupiter in his hand)

Preparing this post, I have made extensive use of a very informative web page: Juno Mission and Trajectory Design . Very detailed and sometimes quite technical, but worth reading.

 

 

Neighbour, here we come!

In a recent blog, Our nearest neighbour? , I reported about the discovery of the planet Proxima b, orbiting around a star, “only” 4.22 lightyear away from Earth. In several media it was suggested that within a few decades a spaceship could be launched to reach this planet. A spaceship is science-fiction, but there exists an ambitious plan to send a swarm of space-chips to Proxima b within a few decades. I promised to write a separate blog about this Breakthrough Starshot  Here it is.

In 1865 the French novelist Jules Verne wrote De la Terre à la Lune (From the Earth to the Moon), in which he describes how three adventurers travel to the moon in a projectile, shot from the earth by a large cannon. I have read it spellbound when I was a teenager. You can read it online here , it is fascinating (and hilarious too).

The illustrations are beautiful. Here  are some. From left to right the three adventurers climbing into the projectile, the comfortable interior and the firing of the canon.
verne1 verne2 verne3

Why this introduction? We know now that this method is not used in our space age. We don’t shoot our spacecraft to the moon or other planets, we use rocket propulsion.  The  Voyager 1 (825 kg) was launched by a Titan-Centaur rocket (600.000 kg). The images show the launch, the Voyager spacecraft and a structure diagram of the rocket. The Centaur is mounted on top of the Titan. A huge amount of fuel is needed to launch a “tiny” payload!

Voyager & Titan

voyager_001

After completing its mission, the Voyager is now leaving our Solar System with a speed of more than 60.000 km/h That is fast but it would still take about 75.000 year to reach Proxima b, if it was going in that direction (which is not the case).

So we can forget about  space travel to the stars, using rocket propulsion, at least in the foreseeable future. Is there another option, more in the style of Jules Verne?

Actually there may be one…:-)

One year ago Travis Brashears, a graduate student at the University of Santa Barbara in California, and his supervisor, Philip Lubin, professor of astrophysics and cosmology at the same university, published a paper Directed Energy Interstellar Propulsion of WaferSats in which powerful lasers “shoot” miniature (~ 1 gram only!) electronic chips away from earth in the direction of a nearby star, with a speed approaching the speed of light! Here are the (main) writers , Brashears left and Lubin right.

brashears-lubin

Does this sound as science fiction? For me it does. But apparently not for these guys.

Sure, light exerts pressure, there are several projects going on, using sunlight propelling a solar sail , a bit similar to the sail of a sailing boat being blown by the wind. One successful project is IKAROS, a solar sail of 196 m (!) , launched in 2010 by Japan. Here an artist impression of the sail, with Venus, its destination. The sail is so big. because the thrust of the sunlight is only small.

Ikaros

Next year March the LightSail 2 will be launched. To the left the actual spacecraft, a so-called cubesat. To the right an artist impression of the LightSail in space, with a deployed sail. Notice how small the cubesat is compared to the sail!

lightsail-table-002

lightsail1_space

These projects are using sunlight. The project of Brashears and Lubin is futuristiic.

  1. A ground-based laser will be used as a “shotgun”  Estimated power needed 100 GW. That is a lot! The Three Gorges Dam in China, the largest power plant in the world, generates 22.5 GW.
  2. The spacecraft will be a chip with a mass of about 1 gram, with a light sail of ~  1 m2   . The plan is to prepare about 1000 of these miniature “spacechips” and launch them simultaneously in a mothership, orbiting the earth. From there the starchips will be shot, one after another on a daily basis, during 3 year.
  3. The laser will give a spacechip in about 10 minutes a speed of 20% of the speed of light. That is fast , 60.000 km/s
  4. The spacechips will reach Proxima b in about 20 year. Hopefully at least a few of them will have survived the journey.
  5. They will send back pictures to earth.
  6. Estimated cost of the project US$ 5-10 billion.
  7. Proposed launch date about 20-30 years from now.

Here is an artist impression of the launch. Mind you, the spacechip is the tiny dot in the center of the light sail!

lightsail-starchip

Futuristic indeed. The time span of 20-30 year is because much of the technology still has to be developed. Designing a spacecraft on a centimeter-size, gram-scale chip, developing a light sail with a thickness of 1 micron or less, building a 100 GW laser and many more challenges.

Here another artist impression. The plan is to build a so-called “phased”  array of smaller lasers, with a combined power of 100 GW. If you use 100 kW lasers ( at the moment the maximum power available), you need a staggering 1 million of them.

Laser array

I am skeptic, as usual…:-) But not everybody is. Yuri Milner, for example is optimistic.  This Russian/American tech entrepreneur and multi-billionaire,  started as a physicist and is very interested in the big question “Are we alone in the universe“. In July 2015 he announced, together with the British physicist Stephen Hawking, the Breakthrough Initiatives , a program to search for extraterrestrial intelligence. At that time the program consisted of two parts.

  1. Breakthrough Listen.  Basically a large-scale version of the SETI project. Funded by Milner with US$100 million.
  2. Breakthrough Message. A prize pool of 1 US$ 1 million for the best (digital) messages that could be sent out into deep space. No concrete plan to actually send these messages, because for example Hawking thinks it might not be advisable to do that. See my blog Anybody Out There?

In April 2016, part 3,  Breakthrough Starshot, was announced by Milner and Hawking. Milner and Mark Zuckerberg (FaceBook) will contribute another US$ 100 million to explore the technological feasibility of the program outlined above.

Milner, Hawking and Dyson

From left to right Yuri Milner (holding a protoype of a spacechip in his hand), Stephen Hawking and “eminence grise” Freeman Dyson, a physicist and cosmologist, now 93 year old. If you are interested in really futuristic ideas, have a look at his Dyson Sphere 🙂

Below is an animation of the process. A few comments may be useful.

  1. There are 135 lasers in the array. You need at least 1 million.
  2. The spacechips are launched simultaneously in a container, but released and shot one after another.
  3. When they reach Proxima b after ~ 20 years, they will pass the planet at full speed (60.000 km/s). So fast that the camera on board can only take a few pictures. Also data will be collected about magnetic fields etc.
  4. These data will be sent back to earth, using miniature lasers on the spacechip, focused with the help of the light-sail.
  5.  About 4.22 year later, the ground-based laser array will receive these data. Hopefully…:-)

I have been working about two weeks on this blog, reading and collecting as much information as I could find. To be honest, I became more and more skeptic.

A few days ago Scientific American has published a very informative article about the Starshot Program: Inside the Breakthrough Starshot Mission to Alpha Centauri. Many scientists were asked for their opinion about the project. There is respect for the technological challenge, but scepsis about the scientific value.

Supermoon, 14 November 2016

Just a short post about the Supermoon of 14 November, widely publicised by the media the last few weeks as a not to be missed, once in a lifetime event. For example on Facebook

It’s a hype.

Supermoons are not rare, they occur regularly, on average every 14 months. The last one was 28 September 2015, the next one will be 4 December 2017.

Full moons have different sizes because the orbit of the moon is slightly elliptical. The image shows the moon orbit, exaggerated. The average distance to Earth is 385.000 km, but the moon can come as close as 356.500 km (perigee) and as far as 406.700 km (apogee). The moon orbit also rotates itself with a period of 8.85 year

lunar-phases-elliptical-orbit

As a result of these two effects, a full moon can sometimes occur when the moon is in or near its perigee. An observer on Earth will then see this full moon brighter and larger, than when it occurs in its apogee. Dividing the apogee distance by the perigee distance, we find 406.700 / 356.500 = ~ 1.14, so the moon will look ~14 % brighter and ~ 30 % larger. This effect is easily observable, as you can see in the image below.  By the way, the name Supermoon has been introduced by astrologers, the correct name is Perigee Full Moon.

mini-supermoons-of-2015

So, why this sudden interest in this particular Perigee Full Moon of 14 November?

The values given for apogee and perigee are actually averages. Because of the influence of sun and planets they vary slightly in time. Here are the perigee distances during the Supermoons of 2015 and 2016 :

  • 356.876 km in 2015
  • 356.511 km in 2016

The perigee distance on 14 November is a little bit smaller! To be precise , 365 km smaller, ~0.1%. So the Supermoon of 14 November will be 0.1% brighter and 0.2% larger. Observable for the unaided eye? Not at all, believe me…:-)!

Why the hype?  When you look at the Perigee Full Moons in the past and future, you  have to go back to 1948 to find an even smaller full moon perigee: 356.462 km (49 km smaller).  And from now on you have to wait until 2034 to find a smaller one: 356.447 km (64 km smaller). These Supermoons will be ~0.02 % brighter.

That’s why it is said: the brightest Supermoon in 86 year…:-). Technically correct, but….   a hype.

My suggestion, try to observe the moon tomorrow, when it is rising, just after sunset. The moon looks always larger when close to the horizon! This is an optical illusion, the Moon Illusion. Combined with the Perigee Full Moon it will be beautiful

And when you are not free tomorrow, it is not that critical. One or two days later you can still admire the Supermoon.

Our nearest neighbour?

As you may know from my  blog, I think we may be alone in the Universe. But of course I would be more than happy if (intelligent) life would be found outside our own planet. My PC is taking part in the SETI project, see my blog  Anybody out there? Last week there was excitement about a strong signal from a sunlike star, but: No alien signal, says SETI astronomer.

Numerous extrasolar planets have been found by now, as of 1 September 2016 the count was 3518. A few dozen of them might be able to support life (rocky, similar size to Earth, orbiting in the habitable zone of their star).

So, why did this Letter to Nature (one of the leading science magazines) :  A terrestrial planet candidate in a temperate orbit around Proxima Centauri cause so much commotion that it became front page news in the media?

The answer is simple: Proxima Centauri is not just one of the hundreds of billion stars in our galaxy. It is the star closest to our Sun, at a distance of 4.22 lightyear “only“,

Let’s have a closer look at this nearest neighbour of the Sun.  Where can we find it in the night sky? And can we see it  with unaided eyes or binoculars?

Here is the night sky (in Malaysia) in March, south-eastern direction. You will notice three constellations, dominated by Centaurus. The name comes from Greek mythology, where a Centaur is a half-horse half-man creatureSky-march

centaurus_contellation

Here is how the Greek saw a Centaur in the stars.

You may find it difficult to see a centaur, but the two bright stars in his left leg are conspicuous. Rigel Kent, better known as α Centauri, is the third-brightest star in the sky, after Sirius and  Canopus. 

Hadar (β Centauri) is also a bright star.

α Centauri is actually a star system, consisting of three stars. Two of them, α Centauri A and B are so close that they can not be separated by the unaided eye. Here is an image taken by the Hubble telescope.

Best image of Alpha Centauri A and B

 

α Centauri A (to the left) is slightly larger than the Sun, while B is a bit smaller. They orbit around each other with a period of 80 years.

 

 

The third component, α Centauri C is a red dwarf, much smaller and cooler (more reddish)  than the Sun. Very far away  (about 0.21 ly) from the other two. If it is bound by gravitation to A and B (not 100% sure), the estimated orbiting period is ~ 500.000 year. Here are A and B (seen as one star here) and C (in the center of the red circle). The other stars are Milky Way stars, much farther away.

The α Centauri system is closer to the Sun than any other star, about 4.35 ly away, and of the three components, α Centauri C is a bit closer (4.22 ly) and therefore it has been named Proxima Centauri.

Because of the close distance, the system has been studied intensively. A planet might be orbiting α Centauri B, but even if found to be true, it will not be habitable.

 

Now a planet has been found, orbiting the red dwarf in the α Centauri system. It has been called Proxima b. Very close to the star, orbiting it in about 11 days only. Compare this with Mercury’s period of 88 days. But because the star is less bright than the Sun, the planet is still in the habitable zone. Here is an artist impression how the planet could look like. α Centauri A and B are also shown, as bright stars.

Artist's_impression_of_the_planet_orbiting_Proxima_Centauri-002

Our closest neighbour! But a distance of 4.22 light-year means that Proxima b is still 40 trillion km away from Earth. At this moment spacecraft New Horizon, after taking spectacular pictures of dwarf planet Pluto, is leaving our solar system with a respectable speed of ~ 60.000 km/h. That is fast, but it would take ~ 80.000 year to reach Proxima b.

Here is what the Mail Online reported on 24/8. “The second Earth that we could visit in our lifetime”  and  “just four light years away

Actually there is an audacious plan to send a probe to Proxima b. Not a spaceship but a space-chip! Not one probe, but a swarm of them. Interested?  The project is called .Breakthrough Starshot and it deserves a separate blog post.

Here only a few comments on the idea of a “second Earth”.

  • As the planet orbits very closely to its Sun, it will probably be tidally locked, like Mercury. In that case the sun side will be scorching hot, the other side dark and freezing cold. Only the twilight zone might be able to support life
  • Proxima Centauri is a flare star, with occasional eruptions of radiation, comparable but much stronger than the solar flares. Not very suitable for the development of life.
  • Will there be water on Proxima b?  Earth got its water during the Late Heavy Bombardment. when numerous comets and asteroids, disturbed in their orbit by the giant planets, collided wit Earth.

In this very readable Scientific American blog more skeptical arguments are given.

Here are a few other habitable planets. Proxima b is not yet in this list, it belongs to the bottom row, Proxima Centauri is a so-called M star

Kepler-452b (top row) is sometimes nicknamed Earth’s Cousin..:-) But the distance to Earth is a whopping 1400 light-year!  It would take New Horizon about 25 million year to go there.

More about the Breakthrough Starshot project in a later blog post