How to avoid an asteroid apocalypse

Shooting stars, or meteors – pieces of rock that come from outer space and burn in the Earth’s atmosphere when their trajectories cross that of our planet – provide a unique spectacle. When we are lucky to spot one, we make a wish. But when it comes to bigger pieces of rock, kilometres in diameter, the only wish we can currently make is for it not to hit us.

The problem is that statistically the day must come when a trajectory of one of those oversized shooting stars will cross that of our planet. It happened in the past. So, we’d better be prepared.

“We have evidence of asteroid impacts on Earth in the form of craters,” says Ian Carnelli, manager of the Hera asteroid-deflection mission at the European Space Agency. “We can also use proxies like other planets and the Moon to estimate how frequently we can expect to be impacted by an asteroid.”

Data on near Earth objects, small bodies including asteroids and comets that orbit the Sun at distances relatively similar to that of Earth, are globally catalogued by the Minor Planet Centre of the International Astronomical Union.

Currently, there are more than 20,000 such known bodies, but the catalogue is nowhere near complete. The good news is that astronomers have a better idea of trajectories of the big asteroids, 1km in diameter and larger. Impact by such an asteroid would have similar effects on the planet to those of the Chicxulub asteroid, which is believed to have hit Earth some 66 million years ago, causing the extinction of dinosaurs.

“Based on the effort of ESA, Nasa and other agencies to detect asteroids, we know today that about 90 per cent of the very large asteroids that are bigger than 1km in diameter are not expected to collide with Earth in the coming centuries,” says Carnelli. “But even asteroids that are tens or hundreds of metres in diameter would cause devastation on the scale of a country or a continent, and about those asteroids we know much less.”

For example, an asteroid only 50m in diameter would destroy the entire London metropolitan area, causing devastation equivalent to that of the explosion of the Hiroshima atomic bomb.

Carnelli estimates there might be millions of such smaller asteroids orbiting in the solar system compared to the few hundreds of thousands of those larger than 1km. The problem is that the smaller the asteroid, the less light it reflects, and therefore the lower the likelihood of it being spotted. “This is where our efforts are focused – on the detection of asteroids of this size – but also on the mitigation, so that we can avoid, as human beings, such a natural disaster,” says Carnelli.

In 2021, Nasa plans to launch its Double Asteroid Redirection Test (DART) mission, the first ever attempt to change a trajectory of an asteroid. Its target is part of a so-called binary asteroid system called Didymos (meaning ‘twin’), which was discovered in 1996 and classified as potentially dangerous to Earth. The system, consisting of the main Didymos asteroid and a smaller moon, Dimorphos, orbiting around it, frequently passes within a few million kilometres from the Earth. For example, in November 2023, it is expected to fly past Earth at the distance of 5.9 million kilometres. For comparison, the distance between the Moon and Earth is 384,400km.

DART, a very simple spacecraft by design equipped only with a navigation camera, will hit Dimorphos at a speed of 6.6km/s. The impact is expected to slow down Dimorphos by only about half a millimetre per second. Such a tiny change in speed, however, is enough to alter an asteroid’s trajectory so that it wouldn’t hit Earth if it were on a collision course with it.

“This is the technique that the international community considers the most promising and mature technologically,” says Carnelli. “But we have never done it. It works well in computer simulations, but the point is to test it in orbit to really make sure that we get the same results as we get from our computer models and therefore gain a certain level of confidence. If one day we need to use it, we would know how it works.”

DART is Nasa’s part of the Asteroid Impact and Deflection Assessment (AIDA) mission, a partnership with ESA, which approved its contribution at the agency’s ministerial council in November 2019.

In fact, the target asteroid owes its name to this joint mission. To date it has often been called either Didymos  B or Didymoon, but in June 2020 the International Astronomical Union accepted the name Dimorphos (‘having two forms’). This was suggested by planetary scientist Kleomenis Tsiganis, a member of both the DART and Hera teams, because the body “will be known to us in two very different forms, the one seen by DART before the impact, and the other seen by Hera a few years later.”

‘This is where our efforts are focused – on the detection of asteroids larger than 1km in diameter– but also on the mitigation, so that we can avoid, as human beings, such a natural disaster.’

The spacecraft that ESA will build will arrive at Dimorphos almost four years after DART hits it in 2022. However, Carnelli insists that such a delay will have no effect on the scientific benefits the international community expects to gain from Hera.

“While the objective of DART is to go as fast as possible against Dimorphos, Hera needs to arrive as slowly as possible to rendezvous with the asteroid, stay in orbit around it for a number of months and inspect the damages and effects caused by the impact of DART,” Carnelli says.

Since there is no weather in space, the crater will be as fresh as on the day after the impact, he adds. In fact, even if Hera was able to reach the asteroid at the same time as DART, she wouldn’t be able to observe the impact directly since that would put her at risk of being hit by the material ejected during the impact.

The data that Hera will collect will enable the scientists to validate their computer algorithms and model with greater accuracy future deflection manoeuvres involving other asteroids. “The main contribution of Hera is that it will allow us to extrapolate the results of DART from this specific asteroid and this specific experiment on any other body,” Carnelli explains.

“Without Hera, we wouldn’t have enough data to know that what worked on Dimorphos would work on a different type of asteroid, but once we have the additional information from Hera, such as the shape of the crater, the structure, material and physical properties of the asteroid, we can put that into our computer models to recreate the same effect and therefore to calibrate our computer models.

“Then we can change, for example, the properties of the asteroid and we can tell what the effect on a different asteroid would be and then we can easily design a mission to a different asteroid and play with the parameters, knowing that the physics and everything behind that is correct.”

Hera will be the first spacecraft ever to inspect a binary asteroid, a somewhat perplexing group of asteroids that are, for some reason, more common in the near-Earth population than in the main asteroid belt between Mars and Jupiter.

“We have known now for almost 20 years that a large number of small near-Earth asteroids are binaries,” says Alan Fitzsimmons, professor of astronomy at Queen’s University Belfast, who coordinates British participation in the Hera project. “About 15 per cent of near-Earth asteroids are binaries. We believe that it’s the action of sunlight that spins up these asteroids to fast rates, which eventually cause them to rupture. We hope that with Hera we will be able to test some of those theories.”

Both Didymos and Dimorphos fall into the dangerous category of asteroids that would cause country-wide or continent-wide damage if they collided with Earth. While the primary Didymos asteroid is approximately 780m in diameter, the secondary asteroid Dimorphos is about 160m across.

Apart from its size, the astronomers also chose Dimorphos for the experiment because of the speed at which it orbits its larger sibling.

“Dimorphos is orbiting Didymos at the speed of only 5cm per second,” says Carnelli. “If you are trying to change the velocity of a body by half a millimetre per second, you get the best results if you measure it against a body that is moving slowly. If you tried to measure such a minuscule velocity change in a typical asteroid that is travelling at 30km per second, that would be very difficult.”

Moreover, since Hera will be absent at the moment of DART’s strike, astronomers want to be able to observe the event from Earth. The Didymos duo is expected to pass relatively close to our planet at the time of the experiment in 2022.

“We can measure the velocity at which the Moon orbits Didymos when we look at Didymos with our telescopes,” Carnelli explains. “The asteroid reflects a certain amount of light but every time its moon passes in front of our telescope, the light dims and we can see a pulsation. By measuring these pulsations before and after the DART impact, we will be able to measure the amount of deflection achieved by DART.”

To be able to deflect an asteroid, or even warn a population about an imminent danger, we first need to be able to spot the danger. In February 2013, a small asteroid only 18m in diameter damaged over 7,000 buildings and led to 1,500 injuries in the town of Chelyabinsk in Russia, near the border with Kazakhstan.

The space rock, which disintegrated in Earth’s atmosphere, came virtually out of the blue. The problem is that today, seven years later, even a bigger asteroid could still hit Earth unexpectedly.

“Chelyabinsk was quite small and was coming from the direction of the Sun so no one saw it coming,” admits Fitzsimmons. “If it was coming from the opposite direction, we would have been able to give a day or two warning. With bigger objects, even if their final approach is from the Sun, we might be able to detect them earlier in their orbit when they are in the night sky. But with small objects such as Chelyabinsk it’s difficult.”

Once the astronomers map the asteroid’s orbit, they can calculate its trajectory for the next 200 years and learn not only if it will hit Earth but also where exactly.

If an asteroid 50m across or larger was on a trajectory to hit a densely populated area, a deflection mission would be considered, according to Fitzsimmons. But he admits that today’s telescopes are still not very good at detecting asteroids below 100m in diameter.

“We can still be hit today or tomorrow by something the size of Chelyabinsk without any warning,” he says. “Over the next decade, however, we expect better technology and better telescopes to come online and to make a tremendous leap in our ability to discover and track these objects.”

ESA is currently installing the first of its next-generation asteroid-hunting telescopes on the Italian island of Sicily. The telescope, called Flyeye, consists of 16 cameras that provide it with a very wide field of view, allowing it to monitor a large portion of the sky. ESA plans to install further three such telescopes on other continents to achieve worldwide coverage.

According to Fitzsimmons, the Vera Rubin Observatory, currently under construction in Chile, will provide a further leap in observation capabilities and help the astronomers fill up the catalogue of potentially dangerous near-Earth objects.

“As we continue doing these surveys, we know that at some point we are going to find an asteroid that is going to hit us in the future,” Fitzsimmons says. “It’s a natural process; we know that it’s been happening in the past and it will continue to happen in the future. The only thing we don’t know is when. In fact, we can find one tomorrow on course to hit us in five years and then DART and Hera might no longer be just experiments.”