Lucy Has its First Asteroid Target in the Crosshairs

NASA’s Lucy spacecraft launched almost one year ago, in October of 2021. Its journey is an ambitious one, and long. It’ll visit eight different asteroids in its planned 12-year mission. Two of them are main belt asteroids, and the other six are Jupiter Trojans, which share the gas giant’s orbit around the Sun.

Lucy’s first, and smallest, target asteroid is now in the spacecraft’s sights.

The first asteroid on Lucy’s bucket list is the small 152830 Dinkinesh, whose name is Ethiopian for the Lucy fossil that NASA named the mission after. The Lucy fossil is significant because it’s so complete, and it tells us so much about human evolution. NASA is hoping that their spacecraft can do the same for our understanding of Solar System evolution by looking at asteroids, which are like relics from the Solar System’s early days.

Lucy is getting its first look at Dinkinesh, a small, 900-meter (3,000 ft.) stony asteroid in the main belt. Its closest approach will be on November 1st, when it will come within 425 km (265 miles) of the asteroid.

Lucy’s first pictures of Dinkinesh show little more than a tiny speck of light moving through space. The images were taken from 23 million km (14 million miles) away. But it’s a start, and as we know from other missions like New Horizons, it’s fun to follow along and anticipate the actual rendezvous less than two months from now. What will Dinkinesh look like?

To Lucy, Dinkinesh will be little more than a speck of light leading up to the encounter. After all, the asteroid is a tiny speck of rock in space. Surface detail won’t be visible until the day of the closest approach.

Lucy’s encounter with Dinkinesh has a purpose beyond studying the asteroid. Dinkinesh was added to Lucy’s itinerary after the spacecraft was launched. The encounter gives mission operators an opportunity to test some of the spacecraft’s functions, especially its tracking systems, which are designed to keep the asteroid in view of Lucy’s instruments. All of Lucy’s visits will be flybys, and the spacecraft is moving fast, about 4.5 km/s or 10,000 mph. So it’s critical that its instruments are on target. The visit with Dinkinesh is a chance to get things right for the rest of the mission.

This image spans about 120,000 km or about 75,000 miles, and in it, one star outshines the others. It’s HD 34258, a star in the constellation Auriga that’s invisible to the naked eye. But even though the star is so dim, Dinkinesh is far dimmer. It’s about 150,000 times fainter than the star.

Lucy’s orbital path through the inner Solar System and main asteroid belt is convoluted. After leaving Dinkinesh behind, it’ll visit Earth. But that won’t be until another year has passed, on December 13th, 2024.

After it leaves Earth behind, it’ll spend another four months before it reaches its next target, the asteroid 52246 Donald Johanson, named after the discoverer of the Lucy fossil. It’s a C-type asteroid in the inner main belt. It’ll keep visiting other asteroids until 2033 when it visits the last one on its list, the binary Jupiter Trojan asteroid named 617 Patroclus–Menoetius.

Why visit all of these asteroids?

There are two groups of Trojan asteroids and they’re particularly interesting. They both have the same orbit as Jupiter around the Sun, but one group is in front, and one is behind. Everything in the Solar System is in motion, and Lucy’s path takes it looping around and back and forth as it meets up with its different targets.

The Trojans are a diverse population of small objects that are remnants of the early Solar System. They’re now on stable orbits, and there are almost as many of them as there are asteroids in the main belt. Our models of planetary formation strongly suggest that the Trojans are leftovers from the same primordial material that formed the outer planets. They hold clues to the Solar System’s formation over four billion years ago.

We don’t know everything about the Trojans, but we know they’re diverse. We know that there are different types, including S-types and C-types. We know that they’re different sizes.

But Lucy and its instruments will tell us a lot we don’t know.

The spacecraft will examine each of its target asteroid’s surface geology. It’ll learn about layering, composition, crater count and distribution, and the different ages of its surface units.

It’ll show us each asteroid’s colour and surface composition. It’ll measure the properties of its regolith and the distribution of minerals, ices, and organic species. The spacecraft will tell us about the density and composition of the asteroids and will investigate their interiors by examining crater impacts, fractures, exposed bedrock, and ejecta.

Some of the Trojans may have rings, or even satellites like the final target does, the binary asteroid 617 Patroclus–Menoetius. If they do, Lucy will spot them, too.

To do its work, Lucy carries the types of instruments that most space probes carry: imaging cameras, infrared spectroscopic mappers, thermal infrared spectrometers, and other instruments.

Lucy has a planned 12-year mission length. But it likely won’t end there. After it visits 617 Patroclus–Menoetius it’ll likely be extended. At that point, Lucy’s convoluted orbit with its gravity assists will be behind it, and it’ll be in a stable orbit between the L4 and L5 Trojan asteroid clouds.

The mission extension will be influenced by what the spacecraft learns about its initial targets. One of the fun things about space missions is that there are always surprises, things that scientists just can’t foresee.

Maybe some of Lucy’s findings will surprise us, and maybe the mission extension will be in search of an answer to a question we haven’t even asked ourselves yet.