The double-moon asteroid described for the first time

A team of researchers used NASA's Lucy space probe to discover a split moon orbiting an asteroid for the first time. “It's the first time we've seen a system like this,” said planetary scientist and co-author Stefano Mottola of the German Aerospace Center (DLR) in Berlin. The Moon consists of two parts that remain together due to the force of their gravity and orbit around the asteroid together.

“Gravity is very weak, but it's enough to hold the two pieces together,” Mottola said. Such an interplanetary double moon has not yet been discovered.

The researchers discovered the system in the first images shortly after the “Lucy” probe passed near the asteroid Dinkenish on November 1, 2023, and after evaluating all the data, they have now presented it in the specialized journal “Nature.” Therefore, the two touching satellites have diameters of 210 and 230 metres. It orbits Dinkenish in about 52.5 hours at a distance of 3.1 km.

When the images were evaluated, a moon appeared for the first time behind the large asteroid, which is 720 meters long. Looking at more images, researchers noticed that Dinkenish's moon consists of two parts. They refer to the object as a binary connection. It is not yet clear exactly how the double moon formed. However, researchers assume that the material needed for this comes from the asteroid.

Dinkenish flies around the Sun in the main asteroid belt between Mars and Jupiter. While flying, Lucy was about 470 million kilometers from Earth. This is equivalent to three times the distance from the Sun to Earth.

Like Dinkenesh (the superstitious one), the double moon was given an Amharic name: Salam (peace). Amharic is the official language in Ethiopia, the country where the three-million-year-old Lucy skeleton was found and after which the probe is named.

NASA's Lucy probe was launched in October 2021 from Cape Canaveral Spaceport in the US state of Florida. It's actually traveling to asteroids in Jupiter's orbit, and is scheduled to fly by seven of Jupiter's so-called Trojans from 2027. Like the mission's namesake — the fossilized humanoid Lucy, whose skeleton provides unique insights into the evolution Humanity – Lucy aims to revolutionize knowledge about the origin of the planets and the formation of the solar system.