Apophis

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Lore & Background

The name Apophis is the Greek name of Apep, an enemy of the Ancient Egyptian sun-god Ra. Apep was an evil serpent that dwelled in the eternal darkness of the Duat and tried to swallow and kill Ra during his nightly passage. Apep was held at bay by Set and Horus with Ra's entourage, and Ra beheaded Apep to death forever once in the form of the great feline in the shade of Heliopolis sun temple's persea tree. Tholen and Tucker, two of the co-discoverers, are reportedly fans of the television series Stargate SG-1, which features a persistent villain named Apophis. However, Tholen denied reports that the asteroid was named after the TV character, stating the connection is coincidental. In the fictional world of the show, the alien's backstory was that he had lived on Earth during ancient times and had posed as a god, giving rise to the myth of the Egyptian god of the same name. The mythological creature Apophis is pronounced with the accent on the first syllable; in contrast, the asteroid's name is generally accented on the second syllable, as the name was pronounced in the TV series.

In Their Own Story

On 13 April 2029, at 21:46 UT, Apophis will pass Earth at a distance of about 31,600 kilometres above the surface. Using the June 2024 orbit solution which includes the Yarkovsky effect, the 3-sigma uncertainty region in the 2029 approach distance is about ±3.3 km. The distance is five times the radius of the Earth, one tenth the distance to the Moon, and closer than the ring of geostationary satellites currently orbiting the Earth. It will be the closest asteroid of its size in recorded history. On that date, it will become as bright as magnitude 3.1, visible to the naked eye from rural as well as darker suburban areas. The close approach will be visible from Europe, Africa, and western Asia. Over the course of about a day, Apophis will move northwest from Centaurus to Perseus and then southwest to Pisces, an arc of 205°. Approaching Earth its speed relative to Earth will be 6.0 km/s; Earth's gravity will accelerate it to 7.4 km/s at the time of closest approach, and then slow it back down to 6 as it departs. During the approach, Earth will perturb Apophis from an Aten-class orbit with a semi-major axis of 0.92 AU to an Apollo-class orbit with a semi-major axis of 1.1 AU.

Reader's Guide

Apophis is the target of a number of planetary defense missions to study the asteroid during its 2029 close approach, including the OSIRIS-REx and Ramses spacecraft. Based on Goldstone and Arecibo radar images taken in 2012–2013, Brozović et al. estimated that Apophis is an elongated object 450 × 170 metres in size, and that it is bilobed (possibly a contact binary) with a relatively bright surface albedo of 0.35±0.10. The axis of its angular momentum points 59° south of the ecliptic, meaning Apophis is a retrograde rotator. Apophis is a tumbler, not rotating around a fixed axis; the axis of rotation moves with a period of around 263 hours. The angle between the long axis of Apophis and the angular momentum vector swings between around 78° and 102° (90°±12°). The principal axis of highest moment and the rotation axis both move around the constant axis of angular momentum much faster, with a time-averaged period of 27.38 hours (precession). The result is that Apophis appears to be 'flipping', making a revolution on average every 30.56 hours.

Did You Know?

• During a brief period in December 2004, Apophis set the record for highest rating ever on the Torino scale by reaching Level 4 on 27 December 2004.
• It took until 2021 to rule out the possibility that Apophis might hit Earth at some later date in the next century.
• Apophis will evolve from an Aten- into an Apollo-type asteroid as a result of the 2029 encounter.
• In 2008, Denis Moskowitz proposed a symbol for Apophis based on ancient Egyptian depictions of Apep.
• Apophis's surface composition probably matches that of LL chondrites.