Roughly 66 million years ago the Chicxulub asteroid with an estimated diameter of 10 km struck Earth at the Yucatan peninsula in Mexico. It caused an extinction of 75% of all plant and animal species, including the dinosaurs.

In 1908 the Tunguska meteorite exploded above a remote region in Russia, flattening about 2000 km² of forest. Ongoing discussion if it was a comet or an asteroid. Estimated size 30-80 m.

On 15 February 2013 an asteroid, size approximately 20 m, exploded at an altitude of ~ 30 km above the Chelyabinsk district in Russia. The shockwave caused substantial damage, many people were injured by broken glass.

And last few weeks there has been “alarming” news in the media about several “space rocks” threatening to collide with Earth and cause havoc. Foremost in this was the British tabloid Daily Express. Here are a few of its headlines (click on the image to see the corresponding article)

7 September:

Asteroid shock: NASA warns of ‘100 percent’ chance of asteroid impact

27 September:

Asteroid alert: NASA tracks three space rocks heading past Earth at once – Will they hit?

2 October:

Asteroid warning: NASA panic as four killer space rocks avoid horror impact with Earth

NASA panics, warning of a ‘100 percent’ chance of asteroid impact? Hm, time for a post about asteroids and their danger for Earth 😉

Asteroids are “small” rocky objects, billions of them, orbiting the Sun, most of them in the (main) Asteroid Belt, between the orbits of Mars and Jupiter. Small is relative, more than 150 million are larger than 100 meter and the largest asteroid, Ceres, has a diameter of 945 km! Notice the units, used in the picture below. One AU (Astronomical Unit) equals 150 million km, the average distance between Earth and Sun. This unit is often used for distances in the Solar System. For example, Mars orbits at a distance of ~ 1.5 AU around the Sun and the main Asteroid Belt is located between around 2.2 and 3.2 AU.

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There are also asteroids outside the main asteroid belt, orbiting closer to the Sun. When their closest distance to the Sun (perihelion) is less than 1.3 AU , they are labelled Near Earth Asteroids (NEA’s). They are interesting for several reasons. One of them is the possibility of mining them in the future. Another is the possibility that a NEA could come so close to Earth that it might collide!

Starting in the 1990’s numerous surveys have been set up to discover and monitor NEA’s. During the last decade the Catalina Sky Survey and the Pan-STARRS surveys have discovered more than thousand NEA’s yearly and every day new ones are discovered. The basic technique is to compare pictures of the sky, taken on different dates and looking for “light points” that have moved, using automated software.

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In the figure below the cumulative total is shown (October 2019), split according to the estimated size of the asteroid. Total: 21059 , Larger than 140 m: 8817, Larger than 1 km: 900 (10 October 2019).

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Fortunately most of these NEA’s will never come so close to Earth that there is a risk of collision. A subcategory has been defined of Potentially Hazardous Asteroids (PHA), asteroids that come closer to Earth than 0.05 AU and are larger than 140 m. At the moment the number of PHA’s is about 2000. They are of course monitored more closely.

In the left graph the total number of PHA’s is given (from 1999 until September 2019. Each year ~ 100 new PHA”s are discovered. The right graph shows the number of PHA’s larger than 1 km. The last decade only a few more have been found.

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Here is a graph showing the orbits of the ~ 1400 PHA’s known in 2013.

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After this explanation about NEA’s and PHA’s, you might be a bit scared that the UK tabloids were right in their warnings about imminent asteroid collisions resulting in disasters.

Let’s have a look at Sentry, operated by CNEOS, the Center for Near Earth Object Studies (objects because both asteroids and comets are studied). It is a highly automated impact prediction system, that continually monitors the most current asteroid catalog for possibilities of future impact with Earth over the next 100+ years. At the moment of writing this post, it monitors 945 objects. The probability of impact and the impact energy result in a number on the Torino Scale, comparable with the Richter index for earthquakes.

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Here is the reassuring result of Sentry: All the 945 objects have a Torino index 0 ! They form no risk for Earth in the next century.

A few comments

1. Keep in mind that the Torino Scale is defined only for the next 100 years. There exists another scale, the Palermo scale, that is more sophisticated, with both negative and positive values. The result is the same: none of the objects have positive values.
2. When a new NEA has been discovered, its orbit is not yet well defined. It happens quite regularly that temporarily such an object has a positive Torino/Palermo index. Subsequent observations reduce the index .
3. Sentry monitors also NEA’s that are smaller than 140 m and therefore officially no PHA’s.
4. Asteroids, smaller than ~ 20 meter, will disintegrate in the atmosphere, leaving a streak of light, a fireball.

Earth is continuously bombarded from outer space by rocky objects. Every year about 40.000 tonnes. Occasionally a small asteroid, more often remnants of a comet or an asteroid. They are called meteorites and will “burn” in the atmosphere. Here is a map of these fireballs, recorded between 1988 and present. Size and color of the circles indicate the energy of the impact. The large orange circle is the Chelyabinsk asteroid, mentioned in the introduction of this post.

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So, what about the scaremongering articles in the media? Here are again the last two captions. In total seven space rocks, horror impacts, NASA panic.

NASA tracks three space rocks heading past Earth at once – Will they hit?
NASA panic as four killer space rocks avoid horror impact with Earth

Here are the 7 scoundrels: 2019 SH3, 2019 SN3, 2019 SP. 2019 SE8,
2019 SM8 , 2018 FK5 and 2019 SD8.

The last column gives the closest distance, expressed in the Lunar Distance (
384402 km) . The size is approximate (see appendix).

Name	Size	Close approach	Distance	in LD
2019 SH3	~ 27m	2019-Sep-30 01:37	1202.000 km	3.1
2019 SN3	~ 16m	2019-Sep-30 11:45	845.000 km	2.2
2019 SP	~ 44m	2019-Sep-30 17:41	2540.000 km	6.6
2019 SE8	~ 5m	2019-Oct-01 13:56	159.000 km	0.41
2019 SM8	~ 15m	2019-Oct-01 15:12	1085.000 km	2.8
2019 FK5	~ 6m	2019-Oct-01 22:56	5094.000 km	13.3
2019 SD8	~ 12m	2019-Oct-02 02:29	532.000 km	1.4

In an appendix of this post I will explain how you can extract these data from the invaluable CNEOS website. In case another alarmist article will published, you can check yourself if you have to get worried 🙂

All seven asteroids are NEA’s, but none of them are Potentially Hazardous Asteroids and NASA will not have panicked at all. Actually these events are common, the past year around 2400 NEA’s have passed Earth, 73 of them even closer than the Moon (like 2019 SE8 in the table above). About 370 of them were PHA’s, but none of them came closer than 7.4 LD’s

Let’s have a look now at the first article

Asteroid shock: NASA warns of ‘100 percent’ chance of asteroid impact

A ridiculous title but the content is much better. Although there are no PHA’s that will hit Earth in the next 100+ years, on a larger time scale it may happen, so humanity should be prepared for such a situation.

Here is a informative infographic created by ESA, the European equivalent of NASA. Notice in the bottom row, how many asteroids in the medium range (100-300 m) still have to be discovered: more than 80 %!

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When a big PHA asteroid is discovered on collision course with Earth, there is basically only one realistic option to avoid a disaster: to deflect it. When you are able to do that (many) decades before its impact, a relatively small change in its course might be sufficient. Many ways to deflect an asteroid are described in this Wikipedia article: Asteroid impact avoidance . And bi-annually a Planetary Defence Conference is organized, the last one was held in May 2019, here is the report .

One section of this conference was dedicated to asteroid Apophis, of special interest to me, because I have published three blog posts about it in 2010-2012 😉 . For a while after its discovery in 2004, there was concern that this 370 m big rock might hit Earth in 2036, after a close encounter in 2029.
I wrote Will the Earth be hit by Apophis in 2036? followed by a (technical) post Again Apophis . In 2012 the winner of the yearly competition for students and young professionals Move an Asteroid had a winner who proposed to deflect Apophis by paintballing 🙂 My report Paintballing Apophis! explains how he wanted to do it.

Apophis is not a PHA anymore, but will still pass Earth on 13 April 2029 at the VERY short distance of 31.000 km (less than 0.1 Lunar Distance!), so at the conference there were numerous suggestions how to make use of this opportunity.

Here is a fascinating animation of Apophis, passing Earth on 13-4-2029 (click on the image). All the blue dots are man-made objects, orbiting Earth! The red dot orbiting Earth is the International Space Station.

As in earlier conferences, part of the program is a tabletop exercise about a hypothetical asteroid threat. The participants have to discuss how to respond, which action to take, etc. Very realistic, it reads like a thriller. Scroll down to page 31 of the (pdf) report. Here are the “press releases” given daily to the participants.

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In the exercise, the participants decided to deflect the asteroid by using Kinetic Impactors, heavy spacecraft that crashes with high speed into the asteroid. It is the most common way to deflect an asteroid.

THEORETICALLY! Because this technique has not yet been tested in a real situation. Quite amazing, and a source of concern for many scientists.

Finally there is now one space mission in progress to test this kinetic impactor technique and I will end this blog with a description of the Asteroid Impact and Deflection Assessment (AIDA) mission.

The mission is a cooperation between NASA and ESA and the original plan consisted of two spacecraft, a large NASA impactor called Double Asteroid Redirection Test (DART) and an ESA spacecraft , the Asteroid Impact Mission (AIM), that would watch DART crashing into the asteroid and observe the immediate effects of the impact.

Target of the mission is the asteroid Didymos, a PHA with a diameter of ~ 800 m, discovered in 1996 as 1996 GT . Don’t be surprised, but it has actually a “moon”, nicknamed Didymoon, diameter ~ 170 m, orbiting Didymos in about 12 hours.

This was the original plan: December 2020 AIM was to be launched, to arrive at Didymos in May 2022. It would go in orbit around the asteroid and study Didymos and Didymoon.

Dart would be launched in July 2021, arrive at Didymos October 2022 and crash into Didymoon, while AIM was watching! After the crash AIM would measure the change in Didymoon’s orbit, to see if this Kinetic Impactor technique is an effective way to deflect dangerous asteroids in the future.

Here is an artist impression of the mission.

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And here is a very informative video, prepared by ESA in 2016 about AIM

But in December 2016, AIM was cancelled by ESA, after Germany withdrew the 60 million Euro funding for the project, to use the money instead for the ExoMars project. The Washington Post commented : Europe will send a rover to Mars but won’t protect Earth from an asteroid and a planetary scientist said “A cool project has been killed because of a lack of vision – even short term – and courage, and this is really sad“

NASA decided to continue with the DART mission and measure the effects of the impact on Didymoon using earth-based telescopes instead. And ESA is planning to launch a much simpler spacecraft, named HERA, in 2023, after the crash of DART! After arrival at Didymos it would study the effects on Didymoon. But the decision to actually fly the mission still has to be taken, in November this year.

As an European I feel rather ashamed that Europe has acted this way.

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Appendix

In this appendix I will explain how you can find reliable information about any asteroid, when you know its name.

First the naming convention for “minor planets” . The year of discovery is followed by two letters and (sometimes) a number. The first letter gives the half-month in which the object was first observed. The second letter stands for a number 1 until 25. (the I is left out to avoid confusion with the J) and counts the objects, discovered in that half-month. When the system was introduced, probably it was thought that there would not be more than 25 objects discovered in a half-month. But nowadays hundreds of objects are discovered every month! The number tells how many times you have to repeat the alphabet! Here is the coding table.

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An example. Recently a NEO has been discovered: 2019 SP3. In the table we see that the S stands for 16-30 September and the P for 15. So this asteroid is the 3 x 25 + 15 = 90th object, discovered in the second half of September 2019.

To find the properties of this asteroid we go to the JPL Small-Body Database Browser. Enter the name 2019 SP3 (case sensitive) in the Search box.

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Lots of information, mostly about the orbit (left table) and the discovery history (upper right table). Important for us are two numbers in the other tables, the MOID = 0.00252421 AU and the absolute magnitude H =26.98.

The minimal orbit intersection distance (MOID) tells us how close the orbits of 2019 SP3 and Earth can get. 0.00252421 AU = 378000 km. Less than the distance between Earth and Moon!

The absolute magnitude H indicates how bright the asteroid is. It gives us an indication about the size of the asteroid. A large asteroid will reflect more sunlight and therefore appear brighter. But this reflection also depends on the structure of the asteroid, is it coal black or more shining. This reflection property is given by the albedo , which can have a value between 0 (no reflected light) and 1 (perfect reflection).

The problem is that we have to guess what the albedo of our asteroid is. In general they are quite dark, with albedo between 0.3 and 0.05. Often a value of 0.15 is used.

Here is part of the conversion table :

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Using the value of H =26.298, we find that the size of 2019 SP3 lies between 14 and 34 meter, with a probable value of 19 m.

Conclusion: with an estimated size of 19 m and a minimal distance to Earth of 378000 km, 2019 SP3 is NOT a PHA.

This is what the Daily Express reported:

Potentially hazardous’ space rock to fly closer to Earth than Moon

Don’t be put off by the title of this blog, I’ll try to keep it simple 🙂

In Greek/Roman times it was thought that Earth was the center of the Universe. The Sun , the planets, the stars were all rotating around Earth.

Now we know that Earth itself is rotating, in 24 hours (although we keep saying that the sun rises in the East and sets in the West).

We also know that Earth is orbiting around the Sun in 365 days. As seen from Earth, the Sun seems to be moving across the sky. This apparent path of the Sun throughout the year is called the ecliptic.

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If Earth would be rotating like in the picture below, then during the whole year, the Sun would be above the Equator and there would be no seasons. . Day and night would be both 12 hour, everywhere on Earth, throughout the year.

But everybody knows that this is not the case. In the Northern hemisphere. daylight will be longer than night in summer and shorter in winter, whereas in the Southern hemisphere it is just the other way around. The reason is that the axis of rotation of the Earth is slightly tilted, about 23.5° .

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Notice that during Earth’s orbit, the direction of the rotation axis remains the same (pointing to the North Star).

In the picture below we can see what happens in a bit more detail. Let’s start with the 1st Day of Spring (20/21 March), when the tilt is not directed towards the Sun and the Sun is directly over the equator. Day and night are equally long all over Earth, it is called the March Equinox. The same happens on the 1st Day of Autumn (22/23 September), the September Equinox.

From March until June, the Sun moves North, days become longer and nights shorter until the 1st Day of Summer, 21/22 June, the Summer Solstice, after which the Sun moves back to the Equator. From September to December, the Sun moves South, days become shorter and nights longer, until the 1st Day of Winter, 21/22 December, the Winter Solstice, after which the Sun moves back again to the Equator. All this has been described from the perspective of the Northern Hemisphere. For the Southern Hemisphere everything is opposite.

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So the axial tilt of Earth is responsible for the seasons and there are scientists who believe that this tilt has been critical for life: Axis Tilt is Critical for Life , but this is quite controversial.

Until now I have been rather vague about the apparent position of the Sun. “Above the Equator”, “moving North”, “moving South”. Actually, we can be much more precise nowadays, using a coordinate system similar to what we use on Earth, with latitude and longitude. But how did the Greek and Romans do that?

To describe the location of the Sun against the backdrop of stars, the Greeks used the concept of constellations, patterns of stars that were given a name, often based on their myths and religion. A constellation most of you will know is Orion, the mythological Greek hunter. The left photo was taken by the Hubble telescope and shows the characteristic shape of Orion. Right an antique star chart, published in 1824. To guide the eye, I have connected the main stars with blue lines

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During a year, the Sun crosses twelve of these constellations , as indicated in the image below. These twelve constellations are called the Zodiac. The names of the constellations may be familiar to you, if you know your (Western) horoscope 😉 .

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At the moment that this post has been published , the Sun has left the Leo constellation and entered Virgo.

Nowadays many more constellations have been defined (88!), covering the whole sky. Below you see a map of the celestial globe. The Celestial globe is similar to the Earth globe, but everything is projected to the “sky”.

Notice that the horizontal scale (Right Ascension) is not in degrees, like longitude on the Earth globe, but in hours and going from right to left. Same as on the Earth globe the choice of the origin (zero) is arbitrary. On the Earth globe it is the Greenwich meridian, on the Celestial Globe it is the March Equinox. The traditional name for this origin is the First Point of Aries.

The name was coined by the Greek astronomer Hipparchus, who lived from c 190 BC until c 120 BC. At that time the Sun had just entered the Aries constellation during the Spring Equinox. The March Equinox is indicated by the Aries symbol ♈︎ .

The ecliptic is shown, with the constellations (in white) which the Sun is passing throughout a year. Following the ecliptic from right to left, you will notice that it starts at 0h in Pisces, the Sun moves North until 6h (Gemini) then back to the equator at 12h (Virgo), going down South until 18h (Sagittarius) and back to Pisces for the next cycle.

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Do you notice the contradiction? The First Point of Aries is now located in Pisces! How can that be? Time to talk about the Precession of Earth!

Above I wrote that the rotation axis of Earth, during its orbit around the Sun, is always pointing in the same direction (to the North Star). That is correct at the moment, but not forever, because there is a third movement of Earth.

Probably many of you have been playing with spinning tops. When a top (or a gyroscope) is spinning, it will often “wobble” under the influence of the force of gravity. The same happens with the Earth under the influence of the gravitational force of Sun and Moon. This wobble is called Precession.

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Here is a short video about the precession of a gyroscope

The Precession of Earth is a very slow process. One round takes about 25800 year. Here is an animation of the process.

The red line connects the two equinoxes, the March Equinox is marked with the Aries symbol. Notice how the March Equinox passes all signs of the zodiac during one precession. 360° in ~ 25800 year, means a shift of 1° every 72 year. Since Hipparchus’ time, the First Point of Aries has shifted ~30° , it is no longer in Aries, but in Pisces and will cross over to Aquarius
around 2600 AD.

The second part of the video shows how the North pole points to different stars during a precession. In the picture you see that “today” it is pointing at Polaris (the North Star) , but in 3000 BC it pointed at the star Thuban, and in 14000 AD it will point close to the bright star Vega.

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To summarise, here is a picture, with the three movements of Earth, the daily rotation (24 hours), the yearly orbit around the Sun (~365 days) and the precession of the rotation axis (~25800 years).

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Notice that the rotation of Earth and its orbit around the Sun are counterclockwise , while the precession is clockwise! So the yearly motion of the Sun through the Zodiac goes from Pisces to Aries to Taurus etc. But the Spring Equinox has, since Hipparchus , shifted back from Aries to Pisces and will shift to Aquarius around 2600 AD.

Let me end this post with some explanation about why I decided to write it.

Everything written above belongs to the realm of astronomy. Unfortunately there exists also astrology, a pseudoscience. Basically astrology states that the position of the heavenly bodies affects our lives. For example, an important factor is the location of the Sun on the day that you were born. That becomes the sign of your horoscope. Here is an example:

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I was born on 17 April, therefore I am a Ram. But that is nonsense, maybe true in Hipparcos’ time. Because of the precession I was actually born when the Sun was in Pisces (12 March – 19 April)

Of course astrologers are aware of the precession, see for example
Sidereal and Tropical Astrology. for various “solutions”.

Directly related to precession is the concept of Astrological Ages. As explained above, during one precession of 25800 year, the March Equinox traverses all 12 signs of the Zodiac, staying in one sign on average 2150 year. Such a period is called an Astrological age. At the moment we live in the Age of Pisces.

Astrologers claim that each age is characterised by certain properties. For example the Age of Pisces is the age of religion, the Age of Aries was the age of war and the coming Age of Aquarius will be the age of freedom.

You will not be surprised that in my opinion these Astrological Ages are even more nonsense than horoscopes.

A few months ago a friend told me about Matias de Stefano, who calls himself an Indigo Child and has memories from an earlier life in Atlantis. He is 29 year old and has many followers. I listened to his video lecture Total Recall. Here is a transcription. One quote to give you an impression of the lecture:

We are Beings (indigo/crystal) that come from the 6th to 13th dimension, to try to promote the 4th and 5th dimension inside the 3rd.

But the following statement is so ridiculous that I decided to write this post.

Earth spins around the sun in a process which lasts about 365 days, but at the same time, our Sun spins around another sun which is a lot bigger, called Syria, about every 26,000 years. As a year on Earth, the Sun’s year has its seasons, equinoxes, solstices and ages, too.

Apparently he has not the faintest idea what the Precession of the Equinox means.

I will end this post with a famous song from the 1967 musical Hair. ” This is the dawning of the age of Aquarius “

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 1994, Barry 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