Home | Discovery and Naming | Physical Characteristics | Future Approaches
Asteroid 99942 Apophis, formerly known by its provisional designation 2004 MN4, stands as one of the most closely watched and scientifically significant near-Earth asteroids ever discovered. This potentially hazardous object measures approximately 450 meters by 170 meters in size, making it a substantial space rock that has captured the attention of astronomers, scientists, and the public alike since its discovery on June 19, 2004, at the Kitt Peak National Observatory by astronomers Roy A. Tucker, David J. Tholen, and Fabrizio Bernardi.
The asteroid's name derives from Apophis, the Greek name for Apep, an ancient Egyptian deity representing chaos and destruction. In Egyptian mythology, Apep was depicted as an evil serpent dwelling in eternal darkness, constantly attempting to swallow the sun god Ra during his nightly journey through the underworld. This ominous naming proved fitting given the initial concerns about the asteroid's potential threat to Earth, though the name also serves a scientific purpose: as Apophis transitions from an Aten-type to an Apollo-type asteroid following its 2029 encounter with Earth, the name cleverly bridges both naming conventions, being the Greek name of an Egyptian deity.
The discovery of Apophis marked a significant moment in planetary defense history. When astronomers first identified the asteroid at a very low solar elongation of 56 degrees and at a considerable range of 1.1 astronomical units, they could not have anticipated the international attention it would soon command. What made this discovery particularly notable was the rapid refinement of its orbital calculations and the subsequent realization of its potential threat to Earth.
In December 2004, initial observations indicated a probability of 2.7 percent that Apophis would impact Earth on Friday, April 13, 2029. This assessment caused unprecedented concern in the astronomical community and led to Apophis being rated at Level 4 on the Torino impact hazard scale on December 27, 2004. This represents the highest rating ever assigned to any celestial object on this scale, which ranges from 0 to 10. The asteroid also achieved a rating of 1.10 on the Palermo scale, indicating an impact hazard over twelve times the background level of similar-sized objects.
The brief period of heightened concern was eventually resolved through additional observations. Precovery images from March 15, 2004, identified on December 27, 2004, allowed astronomers to refine their orbital calculations. Combined with radar astrometry conducted in January 2005, these observations ultimately eliminated the possibility of an impact in 2029. However, concerns shifted to the possibility of Apophis passing through a gravitational keyhole during its 2029 approach, which could have set up a future impact scenario.
Apophis is a fascinating object from a physical standpoint. Based on radar imaging from the Goldstone and Arecibo observatories conducted between 2012 and 2013, scientists have determined that the asteroid is an elongated, bilobed object measuring approximately 450 meters by 170 meters. This unusual shape suggests it may be a contact binary, formed when two separate objects gently collided and stuck together in space. The asteroid's surface is relatively bright, with an albedo of approximately 0.35, indicating it reflects about 35 percent of the sunlight that strikes it.
The asteroid's composition appears to match that of LL chondrites, a common type of stony meteorite. Its spectral classification is Sq-type, indicating a somewhat weathered surface that has been exposed to the harsh environment of space for an extended period. The estimated mass of Apophis is approximately 61 million kilograms, though this estimate carries more uncertainty than the diameter measurements and could vary by a factor of three.
One of the most intriguing aspects of Apophis is its rotation state. Unlike most asteroids that spin around a relatively fixed axis, Apophis is a tumbler. Its rotation axis moves within the asteroid's reference frame with a complex pattern spanning approximately 263 hours. The asteroid appears to flip with an average period of about 30.56 hours, while the principal axis precesses around the angular momentum vector with a period of approximately 27.38 hours. This tumbling motion makes studying the asteroid particularly challenging but also provides valuable insights into its internal structure and formation history.
Apophis follows an elliptical orbit that brings it from just outside Venus's orbit at perihelion (0.746 AU from the Sun) to just outside Earth's orbit at aphelion (1.099 AU). The asteroid completes one orbit around the Sun every 323.6 days, or approximately 0.886 years. Its orbit has a relatively low inclination of 3.3 degrees to the ecliptic plane, meaning it stays relatively close to the plane in which Earth and the other planets orbit.
Before the 2029 encounter, Apophis approaches Earth approximately once every 7.75 years on average, though these encounters are not evenly spaced in time. The orbital geometry is such that close approaches tend to occur between December and April, when Apophis is in the outer portions of its orbit. Notable historical approaches include passages on December 16, 1889, December 18, 1939, December 21, 2004, and January 9, 2013.
The most anticipated event in Apophis's known history will occur on April 13, 2029, at 21:46 Universal Time. On this date, the asteroid will pass Earth at a distance of approximately 31,600 kilometers above the surface. To put this in perspective, this is about five times Earth's radius, one-tenth the distance to the Moon, and closer than the ring of geostationary satellites that orbit Earth at approximately 35,786 kilometers altitude. This will be the closest approach of an asteroid of this size in recorded history.
The 2029 encounter will be visible from Europe, Africa, and western Asia. During the approach, Apophis will brighten to magnitude 3.1, making it visible to the naked eye from rural and darker suburban areas, and easily visible with binoculars from most locations. Over the course of approximately one day, the asteroid will move rapidly across the sky, traveling from the constellation Centaurus through Perseus and into Pisces, covering an arc of 205 degrees.
As Apophis approaches Earth, it will be traveling at 6.0 kilometers per second relative to our planet. Earth's gravity will accelerate the asteroid to 7.4 kilometers per second at the point of closest approach, then slow it back to 6.0 kilometers per second as it departs. This gravitational interaction will dramatically alter Apophis's orbit, transforming it from an Aten-class asteroid with a semi-major axis of 0.92 AU to an Apollo-class asteroid with a semi-major axis of 1.1 AU. The perihelion will increase from 0.746 AU to 0.895 AU, while the aphelion will grow from 1.10 AU to 1.31 AU. The orbital period will change from approximately 324 days to about 423 days.
The close approach will subject Apophis to significant tidal forces from Earth's gravity. While the asteroid will not approach within the Roche limit where it would be torn apart, these tidal forces may alter its rotation axis and could cause partial resurfacing of the asteroid. This resurfacing might change its spectral classification from a weathered Sq-type to an unweathered Q-type, providing scientists with a unique opportunity to study fresh material from the asteroid's interior.
Following the 2029 encounter, Apophis will continue to make periodic close approaches to Earth, though none as dramatic as the 2029 event. In 2036, the asteroid will pass Earth twice, in March and December, at distances of approximately one-third the Earth-Sun distance. Around April 19-20, 2051, Apophis will pass within about 6 million kilometers of Earth, marking the first time since 2029 that it will come within 10 million kilometers of our planet.
Earlier predictions suggested possible impact scenarios for 2036 and 2068, but these have been definitively ruled out through improved observations and orbital calculations. Radar observations conducted at Goldstone in March 2021 helped refine the orbit to unprecedented accuracy, reducing the uncertainty in the 2029 approach distance from hundreds of kilometers to just a few kilometers. Based on these refined calculations, the Jet Propulsion Laboratory announced in March 2021 that Apophis poses no risk of impacting Earth for at least the next 100 years.
However, predictions beyond 2116 become increasingly uncertain due to the chaotic nature of orbital dynamics and the cumulative effects of small perturbations, including the Yarkovsky effect. This subtle phenomenon occurs when an asteroid absorbs sunlight and re-radiates it as heat, creating a small but persistent thrust that can gradually alter its orbit over time. By April 2116, Apophis is expected to pass about 2.6 million kilometers from Earth, but the uncertainty range extends from as close as 150,000 kilometers to as far as 15 million kilometers.
Apophis represents a unique opportunity for scientific study and planetary defense planning. The 2029 close approach will allow astronomers to observe an asteroid of this size at unprecedented proximity, providing detailed information about its composition, structure, and physical properties. Ground-based telescopes equipped with adaptive optics will be able to resolve surface features, while radar observations will provide three-dimensional models of its shape and rotation state.
The asteroid's tumbling rotation state makes it particularly interesting for understanding how asteroids evolve over time. The complex spin dynamics suggest that Apophis has experienced significant gravitational interactions or collisions in its history. Studying how the 2029 Earth encounter affects this rotation state will provide insights into how tidal forces reshape asteroids and potentially trigger surface changes or even structural failures in larger bodies.
From a planetary defense perspective, Apophis serves as an important test case for our ability to track and predict the orbits of potentially hazardous asteroids. The progression from initial impact concerns to definitive exclusion of near-term threats demonstrates both the capabilities and limitations of current observation and prediction methods. The asteroid has prompted discussions about the need for transponders or other tracking devices on potentially hazardous objects, as well as techniques for deflecting asteroids should a genuine threat be identified.
Asteroid 99942 Apophis stands as one of the most thoroughly studied and carefully monitored objects in the solar system. While early concerns about potential Earth impacts generated significant public attention and scientific scrutiny, improved observations have eliminated any impact risk for at least the next century. The upcoming 2029 close approach promises to be a spectacular astronomical event, visible to millions of people and providing scientists with an unparalleled opportunity to study a near-Earth asteroid in detail. As we continue to refine our understanding of Apophis and other potentially hazardous objects, this asteroid serves as a reminder of both the dynamic nature of our solar system and the importance of continued vigilance in monitoring objects that share Earth's cosmic neighborhood.