Protecting the Earth from Cosmic Clashes

Q: What are asteroids and why should we be worried about them?

Oli: An asteroid is “just” a rock, or a pile of rocks, that orbits the Sun. Some asteroids are dead comets — those that have lost their ices so the comet is covered by a rocky crust. My work at ESO focuses on minor bodies such as asteroids, comets and trans-Neptunian objects — including those with the potential to smash into Earth. We call these Near-Earth Objects, or NEOs. Currently, we know of about 17 000 asteroids and 100 comets that are classified as NEOs.

Meteor Crater on the Colorado Plateau in Arizona. This crater is 1.2 kilometres in diameter and was created by a 46-metre asteroid 50 000 years ago.

Credit: NASA Earth Observatory

A key number to remember with NEOs is 10: if an asteroid 10 metres across hit Earth, it would release about the same amount of energy as the Hiroshima bomb. As its effect would be localised to within a few square kilometres around the impact site, it’s unlikely to do a large amount of damage. Remember that the surface area of the Earth is huge and a lot of it is taken up by the ocean, so it would be incredibly unlikely — and extremely unlucky — for an asteroid 10 metres across to severely damage a populated area. But the energy that an object releases is proportional to the cube of its size — so in comparison, a 100-metre asteroid (with the same composition and speed as the 10-metre asteroid) would release 1000 Hiroshimas. An asteroid with a diameter of one kilometre would do much greater damage, and an asteroid of 10 kilometres would be like the one that killed off the dinosaurs. It would sterilise an entire continent and cause major global damage.

On average, one of these huge 10-km asteroids strikes Earth every 50 million years, and the last one was 65 million years ago — meaning we are now overdue. Of course, I should mention that I’m not too worried about an asteroid wiping out all of humanity. We know about most asteroids of this size in the Solar System – we’ve studied their orbits, their characteristics, and we can predict their chance of impact. But as the asteroids get smaller, the less we know of them. We estimate that about 70–80% of asteroids from 500 metres to 1 kilometre in diameter are known, but only about 10% of asteroids 100 metres in diameter are known. The International Asteroid Warning Network (IAWN) is working to improve these numbers.

Q: So what is the IAWN?

Andy: The International Asteroid Warning Network aims to detect, track, and physically characterise Near-Earth Objects to determine which are potentially dangerous to Earth. The network is made up of scientific institutions, observatories, and a variety of interested groups — all of which can make observations of asteroids and NEOs. Participation in the network is voluntary and partners are funded with their own resources. They also agree to a policy of free and open exchange of all data submitted to the network.

The banner of the International Asteroid Warning Network.

Credit: Dan Durda

Q: How did the IAWN form?

Andy: The network has its roots in the United Nations Committee on the Peaceful Uses of Outer Space (UN COPUOS), which was established in 1959 shortly after the launch of Sputnik. NEO detection happened for years by observatories around the world, including ESO, but in 2002 the UN committee decided there should be a single team to oversee the detection, risk analysis and communication of NEOs and their dangers. By 2008 two vital organisations had been set up: the International Asteroid Warning Network (IAWN) and the Space Mission Planning Advisory Group (SMPAG). The establishment of these groups marked a tangible and essential step in protecting Earth from potential asteroid impacts, and the IAWN, in particular, was crucial in collecting and sharing information about potential space hazards. Then in 2013, the Science and Technology subcommittee (STSC) gave IAWN the official role of NEO detection. By cosmic coincidence the Chelyabinsk meteorite struck the atmosphere above Russia on 15 February 2013 during the STSC meeting, giving immediate impetus to this work!

A photo from the first COPUOUS meeting.

Credit: UN Photo

Q: What is ESO’s role in the IAWN?

Oli: To search for asteroids you need a survey-type instrument such as Pan-STARRS, which continuously scans the whole sky with the aim of detecting moving objects. ESO’s telescopes are very powerful, but have a narrow field of view and so are used to observe specific objects; in other words, they are not suitable for discovering NEOs. So we work as part of a team. Other huge surveys detect asteroids, some of which are considered potentially hazardous — and some of these are threatening enough get on ESO’s “to-do list”. Our role is to target the high-risk asteroids that no other observatory can observe. If an object is small or far away, only big telescopes like ESO's Very Large Telescope get called on to hone in and measure it.

In collaboration with ESA, we’ve run an ongoing project on the VLT since 2015. The project is awarded 24 hours of observing time per year, and while this time is modest, it’s enough to follow-up all the potentially dangerous NEOs that cannot be observed by smaller telescopes. 58 high risk or difficult NEOs have been observed by the VLT so far, 24 of which were removed from the risk list—the others are still on the list, despite the VLT observations.

This panoramic photograph captures the Very Large Telescope (VLT) against a beautiful twilight on Cerro Paranal. The NEO observations are performed with Unit Telescope 1, the first large telescope on the left.

Credit: ESO/B. Tafreshi (twanight.org)

Andy: It’s really important to note that as an intergovernmental organisation, ESO has a great responsibility to the public who ultimately pay for what we do. The Director General decides on the 24 hours set aside per year for asteroid observations using the VLT.

Q: How do we calculate the risk and the probability of an asteroid hitting Earth?

Andy: Short answer: it’s complicated. The risk is a combination of the likelihood that an asteroid will strike, how soon it will strike, and the effects it would have on Earth. Astronomers use the Palermo Technical Impact Hazard Scale, which combines these values and also compares it to the ‘background’ level of risk. We have to consider many variables.

Firstly, the orbit of the asteroid must be determined, along with the chance its path could intersect with Earth’s and when this would happen. Next, the size of the asteroid is vital, as it provides the main indicator of its danger — a large asteroid would slam into Earth’s surface intact, while a smaller one would burn up harmlessly in our atmosphere. The danger also depends on composition; some asteroids are basically huge chunks of iron ore, which can hold together as they pass through the atmosphere, while others are loosely-bound dust, ice and rocks, which burn up more easily. Then we must consider the angle of incidence — whether the asteroid travels straight down or at an angle, passing through much more of the atmosphere. In this case, the asteroid experiences more friction and is more likely to reduce in size (and danger) or be vaporised altogether. For most NEOs, these parameters are unknown, so we have to work with average, typical values.

The Chelyabinsk meteoroid fell to Earth on February 15, streaking across the sky above the city of Chelyabinsk, Russia, at 9:20 am local time.

Credit: Marat Ahmetvaleev

Q: What happens if we find an asteroid at high risk of hitting Earth?

Andy: Certain criteria have been set up that trigger an impact response. If the probability of impact is greater than 1% for objects over 10 metres, IAWN must alert the Space Mission Planning Advisory Group (SMPAG), of which ESO became an observer on 11 October 2017. SMPAG then have the harder job of coming up with an international action plan and deciding on the criteria for action. If the probability of impact within 20 years is greater than 10% for objects over 20 metres, SMPAG must alert authorities and the United Nations to begin terrestrial planning, which includes determining a “risk corridor” on the earth’s surface. If the probability of impact within 50 years is greater than 1% for an object of over 50 metres, SMPAG must begin mission planning. Much of the current work of SMPAG involves analysing the various mission options.

Oli: There are many possible hazard mitigation methods that are being considered, all of which sound very dramatic and sci-fi. It might seem like the best option to avoid a large predicted impact is to destroy the asteroid — but this isn’t such a good idea. We don’t want to break up the asteroid because it would dramatically increase the number of impacts and the likelihood they’d hit human populations! Not to mention the difficulty of tracking all the fragments.

A close-up image of asteroid (25143) Itokawa taken by the Japanese spacecraft Hayabusa during its close approach in 2005.

Credit: JAXA

A much better option is to deflect the asteroid. One idea is to spray paint one side of the asteroid white, making it more reflective — so when photons from the Sun bounce off, their momentum will transfer to the asteroid, pushing it off course just enough to miss Earth. This technique is based on the phenomenon of the Yarkovsky effect. Another idea is to send up a small rocket to push the object gently off course over a long period — say, 10 years. Basically, if we know about a potential impact long enough in advance, we can do something about it. We already have the technology today.

Andy: Like Oli said, the extent of our preparedness will largely depend on the amount of time we have — obviously a 20-year warning will be different to a 2-day warning! Lots of people are thinking about mitigating the hazard of asteroids — NASA has the Asteroid Impact and Deflection Assessment (AIDA) Mission, and ESA used to have Asteroid Impact Mission (AIM) although at present its funding is unclear.

Q: Does the recent discovery of the interstellar asteroid `Oumuamua affect our understanding of NEOs?

Oli: Our team at Pan-STARRS first spotted the `Oumuamua asteroid. This was an interstellar object — the first ever discovered — that briefly became a Near-Earth Object, except it was travelling much faster, meaning it would have been extremely damaging if it struck the Earth.

We think that over its lifetime, our Sun has ejected tens of trillions of objects into interstellar space, so it’s reasonable to assume that other stars, including our neighbours, have done the same. This means there are a huge number of interstellar objects travelling through space. But when you compare this with the sheer scale of the Universe, the likelihood of even coming across one is exceedingly slim — and the chances of an interstellar asteroid striking the Earth are negligible. Furthermore, there would be not much we could do to mitigate such an impact because we’d have just a few weeks’ notice. It is better to focus our efforts on the much higher risk from our own Solar System’s NEOs.

This artist’s impression shows the first interstellar asteroid: `Oumuamua. This unique object was followed closely by telescopes around the world including the VLT, which characterised its shape and colour.

Credit: ESO/M. Kornmesser

Andy: The recent discovery of `Oumuamua was also very interesting because it was extremely well-publicised. Events like this are definitely useful for alerting the public to this issue, which is really necessary, as this is one natural disaster we could avoid by doing something about in advance.

Q: How well do you think the public understands the risk of asteroids and NEOs?

Andy: That’s a tough question! Obviously, films have had a huge role in creating a cultural awareness about these scenarios. I remember “Deep Impact” and “Armageddon”, both from 1998. I wonder what happened around that time to warrant two films in the same year about asteroids!

Oli: It’s unfortunate to note that almost all the science and technical aspects in these two movies were wrong.

Andy: Yes — these kinds of movies can, of course, be quite silly and over the top. The assessment of risk and probability is complex, and not necessarily easy to quickly explain, especially in a movie.

But movies aren’t the only way the public can be made aware of asteroids. Asteroid Day aims to raise global awareness about asteroids. It’s held on the anniversary of Earth’s most harmful known asteroid-related event in recent history, the Tunguska event in Siberia on 30 June 1908. ESO recently became a partner of Asteroid Day, which is also officially recognised by the UN.

Artist’s impression of the planetary system around HD 69830 II, which likely includes an asteroid belt.

Credit: ESO

Of course, there are many ways of communicating asteroids to the public. The absolute key is to raise awareness without alarming anyone. It’s vital to highlight that the known risk from NEOs is really incredibly low, and also that it’s one of the few natural disasters about which we would have plenty of advance warning. Unlike an earthquake, hurricane or volcano, we also have the technology to attempt to stop an impact from happening!

Q: What does the future hold for asteroid detection and hazard mitigation?

Andy: The next meeting of the International Asteroid Warning Network will be on the 30 January 2018, in Vienna. On the agenda is a review of the guidelines for public communication of the threat—the Torino Scale, which is like the Richter earthquake scale but for asteroid impacts.

Oli: The next generation of survey telescopes is currently being built. In particular, the Large Synoptic Survey Telescope, an 8.4-metre telescope under construction in Chile, is expected to start observing in 2023. It will scan the sky for dangerous asteroids with a sensitivity about 100 times higher than current surveys, and it should discover up to 90% of the potentially hazardous asteroids larger than 150 metres within 15 years of operation.