Apophis will pass inside the ring of geostationary satellites in 2029, and that is rare for an asteroid of its size. But smaller asteroids pass that close, and closer, much more often than most people realize. Many of them are found only after they have gone by. This article walks through some of the best-documented close approaches and what they teach us.
Astronomers measure distance in several ways. A flyby "within the Moon's distance" means less than about 384,000 km from Earth's center. Geostationary satellites orbit about 35,786 km above the equator, and the International Space Station is only around 400 km up. Apophis will pass about 32,000 km above the surface, so it will cross the geostationary belt's altitude. For comparison, see Apophis 2029.
On February 15, 2013, asteroid 2012 DA14 passed about 27,700 km above Earth's surface, inside the geostationary belt. It was roughly 30 meters across and had been discovered the previous year, so the flyby was predicted and carefully observed. By coincidence, the Chelyabinsk meteor exploded over Russia the same day. The two objects were unrelated, as they came from different directions, but the coincidence made 2013 a turning point in public awareness. Read about that event in Tunguska and Chelyabinsk.
On February 4, 2011, asteroid 2011 CQ1, only about a meter across, passed roughly 5,500 km above the surface. At the time it was the closest non-impacting asteroid flyby on record. It was found only hours before closest approach.
On August 16, 2020, a car-sized asteroid called 2020 QG passed about 2,950 km above the southern Indian Ocean. It was discovered by the Zwicky Transient Facility in California roughly six hours after its closest approach. Its path was bent noticeably by Earth's gravity. At the time it set a new record for the closest known non-impacting approach.
On November 13, 2020, asteroid 2020 VT4, a few meters across, passed roughly 400 km above the Pacific Ocean, lower than some satellites, and set a new record for the closest known approach by an asteroid that did not hit. It, too, was found only after it had passed.
On January 26 (U.S. time), 2023, asteroid 2023 BU passed about 3,600 km above the southern tip of South America. It was a few meters wide and had been discovered only days before by an amateur astronomer in Crimea, Gennady Borisov. NASA's tracking system calculated its path and showed that it would not hit.
On October 6, 2008, astronomers found a small asteroid, 2008 TC3, only hours before it entered the atmosphere. It was the first asteroid ever detected before it hit Earth. It exploded over the Nubian Desert in Sudan on October 7, and about 600 fragments, called Almahata Sitta, were later recovered. Since then, a handful of other small asteroids, such as 2014 AA, 2018 LA, 2019 MO, 2022 EB5, 2022 WJ1, 2023 CX1 and 2024 BX1, have been found hours before impact. None of these caused damage, because objects of that size break up high in the atmosphere.
Surveys are improving. NASA's tracking and impact-risk programs described in How NASA Tracks Asteroids help compute whether a newly found object will hit within hours.
All of the objects above are tiny compared with Apophis, which is about 340 meters wide and so thousands of times more massive than a 5-meter rock. A flyby of such a large object inside the geostationary belt is a far rarer event, expected only about once every few thousand years on average. It has also been tracked for more than twenty years, so it carries no surprise: we know its path to high precision. Learn more in Is Apophis Dangerous?
Facts in this article are drawn from public scientific sources. For primary information, see: