The asteroid 99942 Apophis, often referred to as the “God of Chaos” asteroid, is a Near-Earth Object (NEO) that captured global attention shortly after its discovery. Initial calculations of its orbit suggested a non-zero probability of impact, which caused significant public and scientific alarm. Apophis became a symbol for the threat posed by large asteroids. This article details its physical dimensions, its highly anticipated 2029 flyby, and the definitive assessment of its risk to Earth.
Pinpointing the Size and Mass
The question of how big the God of Chaos asteroid is has been refined through years of sophisticated radar observations. Current measurements place the mean diameter of 99942 Apophis at approximately 370 meters. Radar imaging suggests it is an elongated, peanut-shaped, or bilobed object, with dimensions roughly 450 meters by 170 meters along its axes.
The physical composition of Apophis classifies it as an S-type asteroid, meaning it is primarily stony and composed of silicate materials mixed with nickel and iron. Based on its size and estimated density, the asteroid’s mass is calculated to be in the range of \(6.1 \times 10^{10}\) kilograms. This substantial mass is a major factor in assessing its potential impact energy. Its slow rotation period of about 30.6 hours will be studied during its upcoming close approach.
The 2029 Close Approach
The specific event that brought Apophis into the spotlight is its upcoming close encounter with Earth on Friday, April 13, 2029. On this date, the asteroid will pass within approximately 31,000 to 32,000 kilometers of Earth’s surface. This distance is remarkable because it is closer than the ring of geostationary communications satellites that orbit our planet.
This close flyby will be the nearest an asteroid of this size has ever come to Earth since astronomers began tracking them. The asteroid will be visible to the naked eye from parts of Europe, Africa, and Asia, appearing as a fast-moving point of light. The primary scientific significance of the 2029 event is the dramatic alteration it will cause to the asteroid’s orbit due to Earth’s gravitational influence.
The gravitational tug will change Apophis’s classification from an Aten-class asteroid to an Apollo-class asteroid, lengthening its orbital period around the Sun. This massive tidal force will also stretch and squeeze the asteroid’s body, potentially causing quakes and landslides on its surface.
Current Impact Probability Assessment
The initial concern following Apophis’s discovery in 2004 was due to early calculations that gave it a 2.7% chance of impacting Earth in 2029. This probability caused Apophis to achieve the highest rating ever recorded on the Torino Impact Hazard Scale, reaching Level 4 for a brief period. The Torino Scale communicates the threat posed by NEOs, combining impact probability and kinetic energy.
Subsequent radar observations and refined orbital calculations quickly reduced this risk. The possibility of an impact in 2029 was completely eliminated within days of the initial high rating. The risk of a later impact in 2036, which depended on the asteroid passing through a small “gravitational keyhole” in 2029, was also ruled out.
Highly precise radar tracking, including observations made by NASA’s Goldstone Deep Space Communications Complex in March 2021, allowed scientists to definitively calculate the asteroid’s future path. The Jet Propulsion Laboratory announced that Apophis has absolutely no chance of impacting Earth for at least the next 100 years. The asteroid was removed from NASA’s Sentry Risk Table and currently holds a Torino Scale rating of 0, indicating a negligible threat.
Predicted Regional Impact Consequences
While the risk of impact has been eliminated for the foreseeable future, understanding the potential effects of an object the size of Apophis is important for planetary defense planning. If a 370-meter-wide S-type asteroid were to strike Earth, the energy released would be equivalent to 2,400 megatons of TNT. This is significantly more powerful than any nuclear weapon ever tested.
Such an impact would not be an extinction-level event, but it would cause catastrophic devastation on a regional scale. A land strike would create a crater several kilometers across and level an area the size of a small country due to the resulting shockwave and intense thermal radiation. If Apophis were to strike the ocean, the main threat would be colossal tsunamis that could inundate coastlines hundreds of miles away from the impact site.