Just-discovered ‘dinky’ asteroid moon is actually something far stranger

Dinkinesh, the asteroid that was the first target of NASA’s Lucy mission to Jupiter, is still throwing up surprises.

The first surprise came when the Lucy spacecraft zipped past the kilometre-wide asteroid and found an orbiting tiny natural satellite.

The tiny moon, now called Selam, was thought to be about 220m wide. Lucy’s tracking camera captured the first pictures of the satellite as the spacecraft cruised by at 16,000km/h.

But then the second surprise – Lucy analysts have discovered Selam isn’t a single moon.

Instead, the object is a ‘contact binary’ or 2 moons that have fused. How this fusion and the creation of Selam occurred remains a mystery, however in their research published today in the journal Nature, they speculate a collision with its larger pair could be the cause.

“There’s a lot more complexity in these small bodies than we originally thought,” says University of Maryland astrogeologist Jessica Sunshine.

“With the additional observations taken by the spacecraft, we were able to better analyse features such as Dinkinesh’s rotation speed and Selam’s orbit pattern.”

Credit: NASA/SwRI/Johns Hopkins APL/NOIRLab, Supplied

Selam – the child of a breakup

Dinkinesh has a strange trough along its surface that appears to point to a violent fracture in its past.

Sunshine and her colleagues think the fracture is the result of a spinning incident, where Dinkinesh’s fast rotation caused about a quarter of its structure to break off and reform as Selam.

The scarring on the surface of the larger asteroid would likely result from any leftover fragments bombarding its rocky skin.

As a binary asteroid system, Dinkinesh and Selam bear some resemblance to Dimorphos and Didymos, which were the subject of 2022’s DART mission.

“I’m personally very excited to compare the Didymos binary system with this one,” says Sunshine.

“Especially as they appear to share many similarities such as size, general shape and possibly composition despite being in totally different parts of the solar system.

“The Didymos binary system is located in a near-Earth environment while the Dinkinesh system is located much farther away from Earth in the main asteroid belt. They have very different features but we think they may have undergone similar processes to become what we know of them today.”

Scheduled to run for 12 years, Lucy has the longest pre-launch mission duration of any NASA expedition to date (the Voyagers were only meant to last 5 years).

The spacecraft is currently beyond the orbit of Mars preparing for a final Earth flyby on 13 December.

It will use that gravity assist to fling itself towards Jupiter, where it will find its way to the ‘Trojans’ – clusters of asteroids that orbit the Sun on the gas giant’s path. 

Its first targets – scheduled for survey from 2027-2028 – will be the ‘Greek camp’ asteroids Eurybates, Polymele, Leucus and Orus, before revisiting Earth in 2030 for another gravity-assisted slingshot towards the ‘Trojan camp’ where it will scout the asteroid Patroclus-Menoetius.

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