On 12 October 2017, a 20-metre asteroid passed just 50,000 kilometres (31,000 miles) from Earth. For weeks, dozens of astronomers from labs around the world mobilised, measuring everything they could about the asteroid in preparation for an impact.
This asteroid had been discovered five years before, and the astronomers knew that it wasn’t actually a threat to Earth. But they used the flyby as an important exercise to test astronomers’ ability to quickly coordinate a worldwide observation campaign. Scientists and legislators have grown increasingly concerned about the threat of near-Earth objects, thanks to high-profile meteorite impacts and the realisation that many countries are unprepared for a sudden asteroid threat.
“In the [US] Department of Defense, they do so-called ‘war games,’” Vishnu Reddy, associate professor at the University of Arizona’s Lunar and Planetary Laboratory who devised the test, told Gizmodo. “So why don’t we play pretend to test out the entire system, too?”
The Pan-STARRS1 survey—a series of telescopes and instruments that automatically survey the sky for moving objects—discovered the small asteroid, called 2012 TC4, on 4 October 2012 at a distance 15 times Earth’s radius. Earth’s gravity changed its trajectory such that it could have passed by us in 2017 at anywhere between two Earth radii and 45 Earth radii. Modelling demonstrated that it wouldn’t hit Earth, and its small size—less than 20 metres in diameter—made it not much of a threat (it’s smaller than the meteorite that caused a fireball over the Russian city of Chelyabinsk in 2013). Still, the asteroid’s close distance made it the perfect subject of Reddy’s “war-game,” in which astronomers pretended that it really would hit Earth.
The first step was to assume that after the initial detection, scientists didn’t know whether the asteroid would impact the Earth. They searched with the Very Large Telescope in Chile in the summer of 2017 and found 2012 TC4, and when it was discovered, it was the faintest near-Earth object ever detected, according to the paper published in the journal Icarus. “If an impact during the 2017 close approach had been possible based on the 2012 astrometric data, these recovery observations would have been sufficient to confirm or rule out the impact,” according to the paper.
Then, Pan-STARRS1 automatically detected the asteroid on 25 September 2017; had it not already been detected in 2012, this would have been the first time anyone had seen the asteroid, marking the start of the simulation. Followup observations allowed astronomers to determine its rotational period, its maximum size, and to which class of asteroids it belonged. Had the asteroid been a real threat, these details would allow researchers to model where on Earth the rock might hit and how much damage it would cause. They also performed a continually updating risk assessment based on the rock’s size and composition.
The exercise was largely a success, with a few exceptions. “There were definite successes but there were some things that went spectacularly wrong,” Alessondra “Sondy” Springmann, researcher in the doctoral program at the Lunar & Planetary Laboratory at the University of Arizona, told Gizmodo.
Springmann noted two main issues: a mountain-wide power outage from a fallen tree prevented the NASA Infrared Telescope Facility (NASA IRTF) on Hawaii’s Mauna Kea from observing the asteroid, and damage from Hurricane Maria prevented the Arecibo telescope in Puerto Rico from monitoring it. Backup sites were able to monitor the asteroid in their stead.
These exercises are important. The US Congress mandated in 2005 that NASA should try to track 90 per cent of near-Earth objects larger than 140 metres, a size at which an impact could be catastrophic to a country or the entire world. We’re only a third of the way there, reports Quartz, and independent analyses have demonstrated that some asteroid-detecting surveys suffer from systematic errors.
Scientists will continue performing tests like these, while others are working on ways to deflect with the asteroids if they actually do pose a threat to the Earth. Like painting them or nuking them or slamming something into them, maybe.
Featured image: Illustration: NASA/Don Davis