The near-Earth asteroid Apophis (artist’s impression) will fly within 40,000 kilometers of the planet by 2029 – much closer than this month.Credit: Detlev from Ravensway / SPL
A 340-meter-wide space rock called Apophis whizzed safely past Earth on March 6. The next time it returns, in 2029, it won’t be so boring: Apophis will come within 25,000 miles of the planet and skim just above the area where some high-flying satellites orbit the Earth. It will be the first time that astronomers can see such a large asteroid passing so close to us.
Last week’s fly-by gave scientists a chance to test the global planetary defense system, with astronomers quickly estimating the likelihood of an asteroid hitting Earth as they follow its path across the night sky. “It’s a fire drill with a real asteroid,” said Vishnu Reddy, a planetary scientist at the University of Arizona in Tucson who coordinated the observation campaign.
The Apophis fly-by shows how much astronomers have learned about near-terrestrial asteroids – and how much they have yet to learn. Since 1998, when NASA began the largest search for asteroids near Earth, scientists have discovered more than 25,000 of them. And 2020 was a record year for discoveries. Despite the COVID-19 pandemic interrupting much of the research, astronomers cataloged 2,958 previously unknown near-Earth asteroids during the year (see “Space Rocks”).
Source: NASA Center for Near Earth Object Studies
A large number came from the Catalina Sky Survey, which uses three telescopes in Arizona to hunt threatening space rocks. The operations were briefly shut down last spring due to the pandemic, and a forest fire in June caused a longer shutdown, yet the Catalina investigation still uncovered 1,548 near-Earth objects. These include a rare ‘minimoon’ called 2020 CD3, a small asteroid less than 3 meters in diameter that was temporarily captured by Earth’s gravity. The minimoon broke off from Earth’s gravitational pull last April.
Another batch of discoveries last year – 1152 – came from the Pan-STARRS survey telescopes in Hawaii. The finds include an object named 2020 SO, which turned out not to be an asteroid, but a leftover rocket supercharger that has been orbiting in space since it helped launch a NASA mission to the moon in 1966.
Close conversations
Some of the asteroids discovered last year came close to Earth – at least 107 of them passed the planet at a closer distance than that of the moon. Last year’s close shaves included the small asteroid 2020 QG, which skimmed just 2,950 kilometers above the Indian Ocean in August. That made it the most famous approach – a record broken just three months later by another small object, 2020 VT4. It passed less than 400 kilometers from the planet and was not seen until 15 hours after it flew by. If it had hit, it would likely have disintegrated in Earth’s atmosphere.
All these discoveries make astronomers more aware of the nature of the billiard ball in the solar system, where many asteroids roam in space near Earth. The recent effort to observe Apophis shows how astronomers around the world can work together to assess the threat from asteroids, Reddy says. “It’s been a huge international effort,” he says, “and a lot of fun.” By the time Apophis returns, eight years from now, scientists will have an even more detailed count of threatening space rocks.