One in a Half-Million, Lucy Sees Double

On November 1st 2023, NASA’s Lucy spacecraft flew by its first asteroid, just over ten months after the previously unplanned encounter was declared. The approval of this early flyby not only brought the list of asteroid targets to a total of ten, but also allowed Lucy to begin planetary science operations two years ahead of schedule.

The small asteroid Dinkinesh was not targeted for any unique or stand out properties that make it a tantalising scientific target. The diminutive asteroid was simply in the right place at the right time for a close-up that would not consume too much of Lucy’s fuel. The primary purpose of the encounter was rehearsal, a test run of Lucy’s Terminal Tracking System and suite of scientific instruments. Lucy will be conducting six more fast flybys of asteroid systems over the course of its primary mission, so the chance to rehearse a flyby operation ahead of Lucy’s main targets is incredibly valuable. Any trouble or adjustments identified during the flyby of Dinkinesh become lessons that can be applied at Donaldjohanson, Lucy’s next target. However, Dinkinesh has now proved itself a far more interesting scientific target than originally anticipated.

Closest approach to Dinkinesh occurred at 12:54 PM EDT, and two hours later NASA confirmed in a blogpost that Lucy was once again communicating with Earth via the Deep Space Network. The health and situation of the Lucy spacecraft following the flyby was assessed and confirmed nominal, following this Lucy was instructed to begin downlinking flyby data. Around a day later, at 2:20 EST the first imagery from the encounter was revealed; and came with a major surprise.

Dinkinesh, the lucky asteroid chosen out of a sample size of half a million, was not just Lucy’s first asteroid close up – Dinkinesh was actually Lucy’s first two. As newly released imagery reveals; Dinkinesh is actually a binary pair – a system of asteroids that are orbiting each other. During the approach phase, mission team members began to suspect that Dinkinesh may be a double asteroid, which Lucy’s imagery went on to confirm. The surprise adds an 11th asteroid to Lucy’s list of targets, with Dinkinesh’s newly discovered smaller companion yet to be named. Dinkinesh and its moon show dramatic surfaces displaying large boulders, craters, high ridge-lines, and high cliffs in imagery delivered thus far.

The discovery marks only the second time an asteroid moon has been discovered by a visiting spacecraft, after Galileo discovered Ida’s moon Dactyl during its encounter in 1993. Furthermore this makes Dinkinesh the third binary asteroid to be visited in history, following DART’s mission to the Didymos system. Dinkinesh has also overtaken Eurybates as the first double asteroid to be visited by the Lucy Mission out of four. The double asteroid Eurybates-Queta and Polymele and its unnamed moon will both be visited in 2027, and Patrocles-Menoetius visited in 2033.

The imagery also represents a successful practice run for Lucy’s Terminal Tracking System, and what we have received so far. Receiving all of Lucy’s data from the flyby will take around a week, due to the 30-minute light delay between the spacecraft and Earth.

UPDATE: Less than a week after its encounter with asteroid Dinkinesh, newly returned imagery from the Lucy spacecraft has revealed that Dinkinesh’s previously unknown satellite was itself hiding a companion. The new image is taken from a different angle and reveals that Dinkinesh’s satellite is actually a contact binary, two asteroids stuck together. This is the second definitive contact binary observed in nature after Arrokoth (New Horizon’s target) and the first ever contact binary satellite discovered. Read more here.