99942 Apophis

99942 Apophis is an S-type (silicaceous¹) asteroid with an approximate diameter of 1,210 feet. This is an asteroid that primarily consists of silicate minerals and nickel-iron. It has a relatively low albedo² of 0.23, indicating a somewhat reflective surface.

The asteroid’s mass is estimated to be about 2.7 × 10¹⁰ kilograms, and it has a rotation period of about 30.6 hours. Apophis was discovered on June 19, 2004, by astronomers Roy A. Tucker, David J. Tholen, and Fabrizio Bernardi at the Kitt Peak National Observatory.

Initially designated as 2004 MN4, it was later named after the ancient Egyptian demon Apophis, an enemy of the sun god Ra, reflecting its perceived threat to Earth. Apophis follows an eccentric orbit around the Sun with a semi-major axis of 0.922 astronomical units (AU) and an orbital period of about 323 days. Its orbit crosses the Earth’s orbit, categorizing it as a Near-Earth Object (NEO) and specifically an Aten asteroid³. One of the most notable aspects of Apophis is its extremely close approach to Earth on April 13, 2029.

During this encounter, it will pass within 31,300 kilometers (19,400 miles) of Earth, closer than the orbits of geostationary satellites. This event will be visible to the naked eye in some regions and will allow for extensive observation and study.

Initially, Apophis was thought to have a significant chance of impacting Earth in 2036, with early estimates suggesting a probability of up to 2.7%. However, further observations and orbit refinements have since ruled out any impact in 2036. The closest approach in that year will occur on April 13, but it will be at a safe distance.

Another close approach is expected in 2051, though it will not be as close as the 2029 encounter. Current predictions indicate that Apophis will pass at a distance of several million kilometers, posing no immediate threat to Earth. Further into the future, Apophis will make additional close approaches in 2066 and 2068. The risk of impact during these years remains extremely low,

with continuous observations ensuring accurate predictions. Since its discovery, Apophis’s orbit has been closely monitored, with significant refinements made through various observational campaigns.

These refinements have been essential in reducing uncertainties and ruling out potential impact scenarios. Observations in 2005 and 2011, utilizing radar and optical telescopes, significantly improved our understanding of Apophis’s orbit. These campaigns provided precise measurements of its position and velocity, contributing to more accurate predictions.

In 2013, further refinements were made using data from the European Space Agency’s (ESA) Herschel Space Observatory. This data helped to improve estimates of Apophis’s size, shape, and thermal properties. Additional observations in 2015 continued to refine the asteroid’s orbit, further reducing uncertainties and improving impact risk assessments.

During the 2020–21 observational window, Apophis was studied extensively using radar and optical methods. These observations provided critical data to refine its trajectory and physical characteristics ahead of the 2029 close approach. When Apophis was first discovered, the probability of impact in 2029 was estimated at 2.7%, causing widespread concern. However, as more data was collected, the impact probability decreased, and by 2009, it was effectively ruled out. Continuous observations have since confirmed that there is no significant risk of impact for the foreseeable future.

Should Apophis impact Earth, the effects would be catastrophic. An impact would release energy equivalent to over 1,200 megatons of TNT, causing widespread devastation, potentially triggering tsunamis if it landed in an ocean, and leading to significant loss of life and environmental damage. The risk corridor for a potential 2036 impact was a narrow band across Earth’s surface, known as the keyhole.