Lucy space probe discovers double asteroid moon

Luy and Dinkinesh

When the Lucy space probe flew past the asteroid Dinkinesh, its satellite appeared as a compact, rounded moon in the first image (A). However, looking back (B) it turned out to be a contact double system. © NASA/Goddard/SwRI, Johns Hopkins APL, APL/NOIRLab

NASA's Lucy space probe is actually on its way to Jupiter's Trojan asteroids. But during an initial test of its systems in the asteroid belt, it discovered a double surprise during its flyby of the asteroid Dinkinesh: The first images taken during the approach to the 700-meter-large asteroid initially revealed that this asteroid has a previously unrecognized companion - one around 220 meters in size satellites. Now new images that the probe created after its closest approach to Dinkinesh show that this asteroid satellite actually consists of two touching parts. Such a contact double system has never before been observed in an asteroid companion, NASA reports.

NASA's Lucy spacecraft launched in October 2021 on a mission to find out more about a special type of asteroid: the Trojans of Jupiter. These are tens of thousands of asteroids up to 250 kilometers in size that accompany the gas giant 60 degrees in front of and behind it on its orbit around the sun. Because the attractive forces of the Sun and Jupiter are balanced at these two Lagrange points, objects can orbit there in stable orbits for a long time. Planetary researchers suspect that many Jupiter Trojans were captured from further out areas of the solar system during the inner migration of young Jupiter. These asteroids could therefore reveal whether this migration of the planet took place. But before the Lucy space probe arrives at Jupiter's Trojans in 2027, it must pass through the asteroid belt that lies between Mars and Jupiter. It offers the probe the best opportunity to carry out a few flybys of “normal” main belt asteroids and collect comparative data.

An asteroid satellite turns out to be a duo

On the night of November 2, 2023, Lucy completed her first close passage in the asteroid belt: She flew past the approximately 700 meter large asteroid Dinkinesh at a distance of just 430 kilometers. This passage served as the first test of the probe's tracking system, which keeps its science instruments and cameras aligned with the target asteroid during the rapid flyby. As the probe's data showed, this dress rehearsal was a success: "They prove that the tracking system is working exactly as it should," says navigation engineer Tom Kennedy from Lockheed Martin. But the first images of this flyby also revealed something completely unexpected: behind the larger asteroid, a smaller, around 220 meter tall companion peeked out. Accordingly, Dinkinesh has a satellite. “The fact that there are actually two asteroids makes this even more exciting,” says Keith Noll of NASA’s Goddard Space Flight Center. “In some respects, these two asteroids are similar to the asteroid pair Didymos and Dimorphos visited by the DART mission, but there are some very interesting differences that we will now examine in more detail.”

Since then, the Lucy space probe has sent further images of its first asteroid passage “home” - and these brought another surprise: the approximately 220 meter small asteroid moon is not a single, compact object, as it seemed in the first images. Instead, it is a double moon made up of two touching, rounded chunks. In such a case, planetary scientists speak of a contact binary system. “We never expected anything so bizarre,” says Lucy project member John Spencer of the Southwest Research Institute. “We have only seen a few such contact binary systems up close - and never one that orbits another asteroid.” The scientific director of the Lucy mission, Hal Levison from the Southwest Research Institute, adds: “What amazes me most is that that the two components of the satellite have the same sizes.”

This is how the Lucy mission continues

The Lucy space probe has not yet sent all the data and recordings from its flyby of Dinkinesh back to Earth. In the next few days, even more information about the asteroid's double moon could arrive. “It's wonderful when nature surprises us with a new mystery,” says NASA's Tom Statler. “Science makes us ask questions that we didn’t even know existed before.” After this first, fruitful asteroid passage, the Lucy space probe is now heading back towards the inner solar system. It will gain momentum again on its elliptical orbit around the sun in December 2024 with a flyby of Earth before setting course for the asteroid belt again. In 2025 there will be an encounter with the main belt asteroid (52246) Donaldjohanson before Lucy flies further towards Jupiter.

In 2027, the space probe will first reach the first group of Jupiter Trojans, which are moving ahead of the gas planet in its orbit. The target objects there are the asteroid Eurybates with its moon Queta, as well as the asteroids Polymele, Leucus and Orus. After Lucy has completed this “visit list” by November 2028, she will be directed back into the inner solar system to Earth - a first in the history of space travel. But only by making this big loop and flying past the Earth again can the space probe change its orbit so that it can next head for the rear cloud of the Jupiter Trojans. Lucy will reach these objects orbiting the Lagrange point L5 in March 2033. There are further asteroids on their investigation program.

Source: NASA, Southwest Research Institute

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