Discovery and Naming
Apophis was discovered on June 19, 2004 by astronomers Roy Tucker, David Tholen, and Fabrizio Bernardi. Upon its discovery, it was given the provisional designation 2004 MN4. Though the provisional naming convention is a bit complicated to explain, this means it was the 113th asteroid discovered between June 1st and June 15th of the year 2004.
On June 24, 2005, the asteroid's orbit was sufficiently well-calculated to make it eligible for a permanent number: 99942. This in turn made it eligible for a name. On July 19, 2005, it was given the name Apophis, which is the Greek name for the Egyptian god Apep. In ancient Egyptian mythology, Apep was a destroyer; an evil god of darkness. Apophis was also the name of an alien in the science fiction series "Stargate SG-1" who was intent on destroying Earth.
The probability of Apophis ever hitting the Earth is now considered remote, but for the first nine years after its discovery, it was considered a potential near-term threat. Initial observations determined that Apophis would make a close approach to the Earth in April 2029. In December 2004, NASA estimated the chances of a collision of being 1 in 233 - the highest probability of any asteroid impact that had ever been calculated. Over the next few days, NASA continued to raise the impact probability. On December 27, the probability of impact was calculated as 1 in 37, which is more likely than drawing three of a kind in a hand of poker!
Subsequent observations in December 2004 and January 2005 showed that Apophis will definitely not collide with the Earth in 2029. However, on April 13, 2029, Apophis will pass by Earth within about 20,000 miles. This pass will be close enough to alter the asteroid's orbit. The exact course of the new orbit will depend on exactly how close Apophis passes by us. The next opportunity for an impact would be on April 13, 2036. On April 16, 2008, the probability of an impact on April 13, 2036 was calculated at 1 in 45,000. On October 7, 2009, the impact probability was reduced to 1 in 233,000. On January 10, 2013, it was reduced to 1 in 11 million.
While the Earth appears to be safe from Apophis in the near term, there is still a 1 in 256,000 probability of impact on April 12, 2068.
In a collision with Earth, Apophis' impact energy would be approximately 750 megatons. This is roughly equivalent to 47,000 times the energy released by the atomic bomb dropped on Hiroshima in 1945. The meteorite impact that occurred in 1908 in Tunguska, Siberia had an impact energy of 10 to 15 megatons. An impact of 750 megatons, therefore, would be very devastating at a local level, but it is not nearly enough to completely destroy the earth or be considered an extinction threat.
Apophis revolves around the sun once every 323.6 days. If its orbit were circular, and thus it always traveled at a constant speed, Apophis would pass by Earth approximately every 7 years and 9 months. Like most asteroids, however, its orbit is elliptical, meaning that its distance from the sun varies. The closer Apophis is to the sun, the faster it moves, and the further away it is, the slower it moves. Therefore, while its approaches to the Earth are generally spaced about 7 years and 9 months apart, each approach can vary by several months, and it is even possible for a single pass to take as much as four months to complete, with two close approaches in that pass.
Apophis' orbit also takes it close to Venus, although it does not ever approach Venus closely enough to collide with the planet or change the asteroid's orbit.
Apophis currently belongs to the Aten class of asteroids. These are the asteroids whose orbits are wholly or predominantly within Earth's. After its close encounter with Earth on April 13, 2029, Apophis' orbit will widen, causing it to generally remain outside of Earth's orbit. It will, however, still be a near-Earth asteroid and will cross Earth's orbit twice with each revolution. Asteroids with these orbital characteristics belong to the Apollo class.
Apophis's orbit is inclined (or tilted) at 3.331 degrees relative to Earth. This is a relatively small inclination, especially for such a small object. For example, the largest asteroid, Ceres, is inclined at 10.6 degrees, and Venus, which is almost as large as the Earth, is inclined at 3.4 degrees.
Apophis is only about 330 meters (1,083 feet) in diameter. Its mass is estimated at 40,000 metric tons.