What is Apophis and will it hit Earth?
Apophis is an asteroid about 340 metres across that will pass very close to Earth on 13 April 2029. Scientists have ruled out an impact for at least the next century.
Asteroid 99942 Apophis was discovered in 2004. Early calculations suggested a small chance that it could hit Earth in 2029, briefly making it the most worrying asteroid known. Further observations ruled out a 2029 impact, and by 2021 radar measurements had ruled out any impact for at least the next 100 years.
The 2029 flyby. On 13 April 2029, Apophis will pass about 32,000 kilometres above Earth's surface, closer than geostationary satellites. It will be visible to the naked eye from parts of Europe, Africa and western Asia as a moving point of light. An asteroid this large passing so close is thought to happen only about once every few thousand years.
What it is like. Apophis is about 340 metres across, elongated, and probably a stony S-type asteroid, related to LL chondrites. Its name comes from an Egyptian serpent god of chaos.
Science opportunity. Earth's gravity will change Apophis's orbit and may alter its spin, perhaps even causing small landslides on its surface. NASA's OSIRIS-APEX spacecraft, formerly OSIRIS-REx, is planned to arrive shortly after the flyby to study it. ESA has developed plans for a mission called Ramses to accompany it, and other missions have been proposed.
Why it matters. Apophis shows how initial uncertainty in an asteroid's orbit can create concern, and how further observations refine predictions. It is a valuable natural experiment for understanding how planetary encounters affect asteroids.