Will an asteroid hit Earth?

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.

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.

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).

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.

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

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.

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.

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).

NameSizeClose approachDistancein LD
2019 SH3 ~ 27m 2019-Sep-30 01:371202.000 km3.1
2019 SN3 ~ 16m 2019-Sep-30 11:45845.000 km 2.2
2019 SP ~ 44m 2019-Sep-30 17:41 2540.000 km6.6
2019 SE8 ~ 5m 2019-Oct-01 13:56159.000 km 0.41
2019 SM8 ~ 15m 2019-Oct-01 15:12 1085.000 km2.8
2019 FK5 ~ 6m 2019-Oct-01 22:565094.000 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 %!

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.

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.

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.

x x x x x x x x x x x

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.

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.

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 :

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

4 thoughts on “Will an asteroid hit Earth?

  1. Very comprehensive blog on the asteroid. I learn a lot from your article. It must have taken a lot of your time and effort. TQ.

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