Asteroid impact!
Kamal Lodaya, Bengaluru
No, an asteroid is not hitting us. We are hitting an asteroid, Dimorphos, about 27 crore km away.
At the end of September, NASA's DART spacecraft will reach the small asteroid Didymos, less than 1 km in diameter. Dimorphos is an even smaller moon (around 150 metres in diameter) which goes around Didymos in 12 hours. Didymos goes around the Sun in a little over 2 years. It can sometimes get close to Earth.
Around the middle of September, DART will jettison (eject) an Italian nano-satellite, a spacecraft called Licia which is little more than a camera. Such small satellites are called cubesats, a class of spacecraft which are really small and used for research purposes. Licia will follow DART a little slower and observe the impact as it reaches the "scene of the crime" about 3 minutes later. Licia will take some pictures of Dimorphos after the impact and of Didymos, and go into orbit around the Sun. It is not expected to last more than 6 months, and will eventually spiral in and fall into the Sun.
DART also has a camera, but the 500 kg spacecraft will be destroyed as it hits Dimorphos at a speed of 6 km per second, in a direction opposite to its revolution around Didymos. Another European spacecraft, Hera, will reach Didymos on 28 December 2026 and take more pictures of Dimorphos to see what are the effects after 4 years.
Why hit an asteroid?
In 2005, an American spacecraft called Deep Impact already hit a comet, Tempel 1, and took some pictures. Why do it again?
Because Dimorphos orbits Didymos, its orbit can be calculated very very precisely using the laws of Johannes Kepler and Isaac Newton. It goes around Didymos in 11 hours 56 minutes. What will the impact do to little Dimorphos? It is calculated that DART's impact will speed up Dimorphos's orbit to 11 hours 46 minutes. This difference can be observed and calculated from Earth.
So what DART's impact does is give us a chance to calculate and see exactly what an impact does to a tiny asteroid. So when we are suddenly faced with the prospect of an asteroid headed Earth's way, we can consider impacting it and changing its orbit (so that it no longer hits Earth) as a potential escape strategy.