One and a half year Before NASA’s Double Asteroid Deflection Test (DART) changed the course of a space rock, astronomers observed a fear-inducing object near Earth.
It was the asteroid Apophis. When it was discovered just 18 years ago, NASA feared that this 1,100-foot-wide rock could hit our planet in the near future, with possibly dire consequences. AND
Fortunately, when Apophis flew away from Earth in March 2021, new measurements revealed that it won’t be an issue for at least a century. And when the time comes, a mission like DART could mitigate the threat.
But to determine how the attack by the small asteroid Dimorphos on September 26 may also have changed the trip around the Sun that its larger partner Didymos makes, a mission called ACROSS will need to know Didymos’ precise location.
Recently, astronomers took an important first step towards that goal: they finally caught Didymos blocking the light from distant stars.
ACROSS, or Asteroid Collaborative Research via Occultation Systematic Survey, successfully identified Didymos passing in front of background starlight for just 0.13 seconds on Saturday (October 15), reports the European Space Agency (ESA). But it wasn’t easy – ESA officials say the ACROSS collective’s efforts were “enormous and unsuccessful for several weeks”.
ACROSS built on the amazing work of another ESA mission called Gaia, which is currently surveying a billion stars in the Milky Way. The 8-year mission’s cutting-edge work has already manifested itself in three data releases, the most recent in June this year. Astronomers were able to track Didymos in the Gaia data, helping them track its position over time.
Stellar occultation – that is, when an object passes in front of a distant star from our point of view – is a useful phenomenon that in the past has helped in the search for objects beyond the orbit of Neptune. Closer to home, it helped reveal asteroids that travel around the Sun between the orbits of Mars and Jupiter. But near-Earth asteroids, or NEAs, are much harder to find.
“NEAs move fast and are small,” says Paolo Tanga, ACROSS project leader and planetary scientist at the Côte d’Azur Observatory in France, in the ESA statement. “Thus producing shorter events and much narrower shadows cast on the ground.”
But like Apophis, some NEAs can be threatening. Now that Didymos has been detected via stellar occultation, however, astronomers can assess the progress of potentially lifesaving kinetic deflection tests such as DART.
In October 2024, ESA will also launch a mission to closely check the consequences of the DART impact. This spacecraft, called HERA, is currently expected to arrive at Didymos and Dimorphos in December 2026.
