Hera's First Look at a Deflected Asteroid
AFor most of history, people could do nothing to stop a large rock from space from hitting the Earth. Astronomers could track asteroids and calculate their paths, but if one had been found on a collision course, there would have been no tested way of pushing it aside. That situation began to change in 2022, when the American space agency NASA carried out the first real experiment in what is now called planetary defence. Today, a European spacecraft named Hera is arriving at the scene of that experiment to find out what it actually achieved.
BThe target of the original test was a small asteroid called Dimorphos. It measures roughly 151 metres across, which makes it about the same size as the Great Pyramid of Giza. Dimorphos does not travel through space alone; instead, it circles a much larger asteroid, Didymos, which is around 780 metres wide. The two bodies form what astronomers call a binary system, with the smaller one orbiting its partner at an average distance of just 1.15 kilometres. By space standards its movement is extremely slow, at only about 18 centimetres per second.
COn 26 September 2022, NASA's DART spacecraft was deliberately flown into Dimorphos at a speed of about 6.1 kilometres per second. The collision threw a vast cloud of rock and dust into space, and some of the debris travelled as far as 10,000 kilometres. More importantly, it changed the way Dimorphos moves around Didymos. After the impact, the small asteroid took about 33 minutes less to complete one orbit than it had done before. This was the first clear proof that a spacecraft could alter the motion of an asteroid on purpose. The technique, known as kinetic impact, is simple in principle: a fast-moving object strikes the asteroid and gives it a push.
DYet the experiment also produced a puzzle. Before the impact, scientists had used computer simulations to predict how much the orbit would change, and the real effect turned out to be far larger than these models had suggested. Several basic questions also remain unanswered. Researchers still do not know whether DART left a crater on the surface or whether it changed the shape of the asteroid significantly. They are also uncertain about the asteroid's mass, what it is made of and how it is structured inside. Until these questions are answered, kinetic impact cannot be regarded as a fully reliable method of protecting the planet.
EThis is where Hera comes in. Built by the European Space Agency (ESA), the spacecraft has been sent to carry out what its team describes as a "crash site investigation". On the evening of 18 October, as it approached its destination, Hera used its black-and-white Asteroid Framing Camera to photograph the two asteroids. At that moment it was about 1,280 kilometres from them, while the asteroids themselves were roughly 197 million kilometres from the Earth. For the first time, the camera showed Didymos and Dimorphos as two separate objects instead of a single point of light. The smaller asteroid took up only about one pixel, but the mission team was pleased that it was clearly separated from its larger companion.
FGetting this close required careful navigation. Earlier in October, Hera fired its thrusters twice, with a week between the two manoeuvres, reducing its speed relative to the asteroids to just 20 metres per second. Over the coming months, it will study Dimorphos with a range of scientific instruments. It will also release two small satellites, known as CubeSats, each roughly the size of a shoebox, which will make closer observations of the asteroids. Eventually, the mission is expected to include attempts to land on the surface. ESA has announced that the first detailed images of Dimorphos will be made public on 25 November.
GThe results will matter well beyond this particular pair of asteroids. By measuring exactly what DART did, scientists hope to improve their predictions of how future attempts to deflect an asteroid would turn out. If a dangerous asteroid is ever found heading towards the Earth, such predictions could be extremely valuable. In the meantime, members of the public can follow the mission as it unfolds through an interactive online platform called the Hera Space Companion.