NASA has launched a mission to deliberately crash a spacecraft into an asteroid in order to change its orbit. The aim is to test a planetary defence system, one that could prevent an asteroid from colliding with Earth in future. The mission, the first of its kind, should provide the first real data about what it would take to deflect an asteroid away from Earth. Carried aboard a SpaceX Falcon 9 rocket, the Dart (Double Asteroid Redirection Test) robot spacecraft lifted off at 6.21 a.m. UTC (Co-ordinated Universal Time) on 24th November 2021 from the Vandenberg US Space Force Base in California. The probe, about the size of a small car, was released from the rocket booster minutes after launch. It is now on a 10-month journey to its target, a tiny asteroid named Dimorphos lying around 11 kilometres (6.8 million miles) from Earth. The impact is scheduled to take place some time between 26th September and 1st October 2022.

24th November 2021

Asteroid collision

Earth is constantly being bombarded by small pieces of rock—fragments of larger asteroids or comets called meteoroids. These usually burn up in the atmosphere; we see them as brief streaks of light in the night sky called meteors, or "shooting stars". Once in a while, however, a much larger lump of rock crashes to the ground. Known as a meteorite, the larger it is, the more damage it causes. About 66 million years ago, a very large meteorite—often described as an asteroid—collided with the Earth. Vast amounts of dust and debris were ejected into the upper atmosphere, obscuring the Sun and causing food chains to collapse. This brought the Age of Dinosaurs to an abrupt end. It is possible that something similar could call time on our own age, too—unless we can find a way to deflect it.

Dimorphos is a 160-metre (520-foot) “moonlet” asteroid that closely orbits a larger 780-metre (2560-foot) asteroid called Didymos, just as a moon orbits a planet. (Dimorphos is a Greek word meaning "having two forms", while Didymos means "twin" in Greek.) Dimorphos is one of 27,500 known near-Earth asteroids. These do not orbit within the main Asteroid Belt but circle the Sun closer to Earth. Around 1800 of these are called Aten asteroids. Their orbits actually cross Earth's orbital path around the Sun and therefore are of particular concern. The Dart mission will help scientists perfect the technology needed to counter the threat they pose to our planet.

Mission schedule

About 10 days before impact, a briefcase-sized mini-spacecraft called the Light Italian CubeSat for Imaging of Asteroids (LICIACube) will separate from the main Dart spacecraft. Along with observations from ground-based telescopes and radar—plus an onboard camera that will capture the final moments before collision—LICIACube's images of the impact will be relayed back to Earth. The data should enable scientists to calculate the degree to which the impact has altered Dimorphos’s orbit.

The expectation is that the impact will move the asteroid only by a fraction of a single percentage point and alter its orbital period around the larger asteroid only by a few minutes. Nevertheless, over a span of millions of kilometres, the changes should easily be enough to eliminate the risk of an asteroid that was previously on collision course with Earth actually hitting our planet.