As the dinosaurs could attest, it’s worth keeping an eye on any big space rocks that might be buzzing around Earth. A fleet of observatories are doing just that, and astronomers have now announced that we’ve just ticked over the milestone of 30,000 near-Earth asteroids (NEAs) discovered.
Asteroid impacts can be dangerous events, devastating areas the size of a city, country, or if we’re really unlucky, the entire planet. Having some forewarning of an impending doomsday rock could give us time to intervene – especially since NASA’s DART mission has recently demonstrated that we can in fact alter an asteroid’s orbit.
So, since the turn of the century astronomers have ramped up efforts to catalog objects whose orbits bring them close to Earth. These near-Earth asteroids are officially defined as those that come within about 45 million km (28 million miles) of our planet’s orbital path around the Sun. That’s about 117 times the distance to the Moon.
A range of observatories are keeping watch, and discoveries are gathered and tracked by ESA’s Near-Earth Object Coordination Centre (NEOCC) and NASA’s Center for Near-Earth Object Studies (CNEOS). And in early October, this list finally clocked the 30,000 object mark.
Among the 400 newcomers that tipped the tally over the line, 50 were estimated to measure more than 140 m (460 ft) wide. A rock of that size could wipe out a city with a direct hit, or cause a decent amount of destruction to a region, so those are the ones that astronomers want to watch. Just one of these new NEAs measured more than 1 km (0.6 miles) wide, which would devastate a much greater area on impact.
Thankfully, the list of potentially hazardous asteroids grew more slowly. Only eight of those new NEAs make the cut for this list, which is defined as being over 140 m wide and coming within 19.5 lunar distances of Earth. The 1-km-wide rock was one of them.
Most of the time, further observations rule out any chance of impact in the coming decades. However, there are currently 1,426 asteroids with a “non-zero” chance of impact, which are being carefully studied as part of the NEOCC’s Risk List.
Currently sitting right on top is a huge rock known as 1979XB, which earns its Most Wanted status in a few ways. For one, it’s estimated to have a diameter of about 700 m (2,300 ft), which would devastate a small country if hit. Worse still, it hasn’t been seen since 1979 so astronomers can’t pin down where it is now with any real certainty – but there’s a small chance it may strike Earth in 2056.
In the more immediate future, there’s a 0.05% chance that 1979XB makes a close (but safe) pass by Earth in December 2024, in which case astronomers would be able to lock onto it again and calculate its orbit more precisely and probably rule out future impacts.
As devastating as an asteroid impact could be, it should be reassuring to know that there are so many eyes on the sky, and they’re getting better and better at finding these rocks.
“The good news is that more than half of today’s known near-Earth asteroids were discovered in the last six years, showing just how much our asteroid eyesight is improving,” said Richard Moissl, ESA’s Head of Planetary Defence.