The Hera probe, sent by the European Space Agency, is approaching the two asteroids known as Didymos and Dimorphos. Until now, its camera saw them as a single spot. In an image released on October 27, it finally manages to distinguish the two bodies. They are located about 197 million kilometers from Earth.
Artist's view of the European Hera probe passing close to Didymos, with the small asteroid Dimorphos visible on the right.
Credit: ESA-Science Office
This change is primarily due to the distance. When the image was taken, Hera was still 1,280 km from the pair. The smaller asteroid represented only about one pixel in the image, but its glow was already separated from that of the larger one.
The two rocks are very unequal. Didymos measures about 780 m in diameter, compared with 151 m for Dimorphos. The latter orbits its larger neighbor, at an average distance of about 1.15 km between the two bodies. For Hera, distinguishing them separately marks a new stage in its approach.
But this rendezvous is far from a simple visit. In September 2022, NASA deliberately crashed its DART probe into Dimorphos. The impact altered its motion around Didymos: it now takes about 33 fewer minutes to complete one orbit.
This artist's impression shows Hera approaching Dimorphos.
Credit: ESA-Science Office
If an asteroid were ever heading toward our planet, a small change to its trajectory, made early enough, could allow it to avoid us. The DART test showed that a controlled impact could indeed alter an asteroid's motion without destroying it.
Yet scientists still do not know precisely what the collision did to Dimorphos. Did it carve out a crater or fundamentally change its shape? What is its exact mass, and how are its rocks assembled? Hera must collect the observations needed to answer these questions.
Before that, the probe must approach carefully. Two engine burns have already reduced its speed relative to the asteroids to about 72 km/h. It can therefore draw closer gradually and obtain sharper images. The small luminous spot will then take on the appearance of a genuine rocky world.
The European Space Agency plans to unveil the first detailed images of Dimorphos on November 25. Hera will then closely examine its surface and structure. It is also carrying two small satellites that are to conduct their own observations and then attempt to land on the asteroid.