The asteroid
340 metres of stone on a 324-day orbit
Apophis is not a comet, not a meteor, and not remarkable in itself. Millions of rocks of this size orbit the Sun. What makes this one worth a website is arithmetic: its path and Earth's arrive at the same place at the same moment in April 2029, closer than any object of its size in recorded history.
The numbers
| Diameter | 340 m | about the height of the Eiffel Tower |
|---|---|---|
| Rotation period | 30.56 h | it tumbles rather than spinning cleanly |
| Albedo | 0.35 | reflects about a third of the light that hits it |
| Spectral type | Sq | stony, silicate-rich |
| Orbit class | Aten | an Aten: it spends most of its time inside Earth's orbit |
| Orbital period | 324 | days to circle the Sun |
| Absolute magnitude | 19.09 | a measure of intrinsic brightness |
| Discovered | June 19, 2004 | at Kitt Peak, Arizona |
How it was found
Three astronomers at Kitt Peak National Observatory — Roy Tucker, David Tholen and Fabrizio Bernardi — picked it up on 19 June 2004. They tracked it for two nights and lost it. It was recovered six months later from Siding Spring in Australia, and only then did the orbit calculations reveal what it was going to do. Archive images from March 2004 were later found to contain it, which is why the observation arc starts before the discovery date.
The name
Apophis is the serpent of chaos from Egyptian myth, the enemy of the sun god Ra, who tries each night to swallow the Sun. The discoverers chose it deliberately. Two of them were fans of the television series Stargate SG-1, in which Apophis is a villain. The name was approved once the object had a permanent number, 99942.
What it is made of
Radar and spectroscopy put Apophis in the Sq class: a stony, silicate-rich body, similar in composition to the most common meteorites that reach the ground. Its albedo of 0.35 is high for an asteroid, meaning it is a relatively bright, fresh-surfaced rock rather than a dark carbonaceous one. It is elongated, roughly peanut-shaped, and its rotation is not a simple spin — it tumbles on more than one axis, with a period near 30.6 hours.
Where it goes
Apophis belongs to the Aten group: its orbit is smaller than Earth's, so it spends most of each 324-day year inside our orbit and only crosses outward periodically. The orbit is tilted just 3.3° from the plane of the planets, which is part of why encounters with Earth are so frequent and, in 2029, so close.
What happens after 2029
The 2029 pass will physically change Apophis. Earth's gravity will stretch and reshape its orbit, lengthening its year and moving it from the Aten group into the Apollo group. Tides may trigger landslides on its surface, and its tumbling may be altered. Two spacecraft are being sent to watch: NASA's OSIRIS-APEX, already in flight after its Bennu sample return, and the European Space Agency's Ramses, intended to arrive before the encounter and observe the changes as they happen.