Can we spot a killer asteroid before it hits Earth?

And then. There it was, a tiny chunk of black, dusty stone that came from outer space. About a month earlier, a team of astronomers had predicted in what part of the world meteorites from a particular asteroid would fall. That asteroid, named 2018 LA, exploded in the night-time sky in Botswana several hours after it was seen hurtling towards our plant.

Finding the fragment proved the astronomers right – and it was only the second time in history that fragments of a meteorite had been found from an asteroid previously observed in space.

One of the teams that contributed to this fantastic find is hoping that their system of telescopes can one day warn people on Earth in advance of a bigger, potentially fatal, impact. But how will they do it?

While still in space, 2018 LA was first spotted by the Catalina Sky Survey, a Nasa-funded project. But it was then also observed by the Asteroid Terrestrial-Impact Last Alert System (Atlas) at the University of Hawaii. Atlas is a system of telescopes that has been designed with one ultimate goal in mind: save earthlings from giant space rocks.

It was set up by John Tonry, an astronomer. He was inspired to start the project years ago after constantly hearing that the chances of a dangerous asteroid hitting Earth were very low – once every millennium or so.

“It really bugged me that that number was always handed out without any uncertainty and the actual, most recent event of that type was only 100 years ago,” he explains.

Tonry is referring to the 1908 Tunguska event in Siberia, in which an asteroid exploded in the atmosphere, creating a 50-100m wide fireball and flattening about 80 million trees. Reportedly, one person died in the incident. Had the impact site been a more populated area, the effects would have been horrific.

Atlas consists of two telescopes in Hawaii, though Tonry is currently working with colleagues on setting up a third in South Africa, to observe the sky in the southern hemisphere. A fourth telescope has also been funded. Once the full system gets going, Tonry hopes it will offer us a good chance of spotting a major potential impact and, if needs be, it could provide enough notice to evacuate the predicted impact site.

Thanks to this summer’s fragment discovery in Botswana, we know Atlas can get it right. And pinpointing the landing place of 2018 LA was all the more impressive since it was such a small asteroid, less than 2m across.

“It’s a very good initiative, the Atlas system” says Clemens Rumpf, a visiting researcher at the University of Southampton who specialises in the study of asteroids.

“Even today we miss a lot of asteroids that are potentially dangerous.”

The space around Earth is full of rocks. Atlas’s job is to pick out the ones that might threaten us. Tonry explains that in a single night of observations, he and his team might detect about a million objects. A lot of those will be stars or exploding stars (supernovae), or known asteroids on safe trajectories. There might be just ten or twenty that are new to us, and not necessarily any that are dangerous.

If something headed for Earth does crop up, Atlas can post an update to its web pages. Astronomers at institutions like Nasa or the International Astronomical Union’s Minor Planet Center have automated scripts that scour such web-pages for new information. This is how they know as soon as a possible discovery has been made. Astronomers can then begin plotting trajectories and predicting impact sites.

Rumpf points out that some larger asteroids are in a regular orbit around the sun, usually an oval-shaped path that may or may not intersect with Earth at some point in the future. Possible future impacts of such asteroids are easier to predict – but not all space rocks play nice.

“[Some] are on irregular orbits because they are not bound by the sun,” explains Rumpf. “They kind of come out of nowhere.”

Systems like Atlas might be particularly useful, in theory, at spotting those just in time.

By studying how light reflects off different kinds of rocks, such as those with higher metal content and so on, scientists can then make predictions about asteroids flying towards Earth based on the light they themselves reflect, says Alessondra Springmann, a planetary radar astronomer at the University of Arizona.

“If the asteroid comes close enough you can use radar – if you have shape, volume and composition then you can perhaps work out the density,” she adds.

That can help predict what kind of explosion a large asteroid might make as it enters Earth’s atmosphere. The more solid and the heavier it is, the worse the news could be for mankind.

What if we find an asteroid so big that it could threaten thousands or millions of lives? Or throw up so much debris into our atmosphere that the climate would be damaged for a very long time?

Ideally, we’d spot such a monster long before it arrived, which might give us time to defend ourselves. Springmann notes that there are a couple of ideas for how to do this – including launching a “gravitational tractor”. This would involve blasting a spacecraft towards the asteroid and positioning it close enough that the, albeit tiny, gravitational pull of the craft could steer the asteroid off-course.

“If you don’t have a lot of warning, the last ditch effort is to send a kinetic impactor,” she says, before clarifying: “a bomb”.

If we do ever get the chance to shatter a life-threatening space boulder, it’ll be thanks to observations made by scientists here on Earth that tell to some degree of confidence us how big it is, how fast it’s moving – and how likely it is to kill us all.

Featured image by BBC Studios | The Planets