Lucy’s return to Earth

Today, NASA’s Lucy spacecraft will return to Earth. But it will not stay here, instead it will only do a flyby to pick up more speed to get to its destination. So, what is its destination, and what is its mission? Let’s take a closer look at NASA’s Lucy mission.

Launch and mission overview

The Lucy spacecraft launched on 16 October 2021 09:34 UTC on a ULA Atlas V from Florida. So, today is also its first launch anniversary. The planned mission duration is 12 years, which is quite a long mission duration. But this time is required because it’ll take Lucy this long to get to all of its destinations. Speaking of which, this will be a record-breaking number of asteroids. It will fly by one asteroid in the solar system’s main asteroid belt, as well as seven Trojan asteroids. No space mission so fare has flown to as many destinations in independent orbits around the sun. Take a look at the following diagram, to see how it’ll get there.

Lucy will be the first space mission to study the Trojan asteroids. Which leads us to another question.

What are the Trojan asteroids?

As the name “Trojan asteroids” might already suggest, they are a group of asteroids. To be more specific, they are actually two groups of asteroids in the orbit of Jupiter. One group is always ahead of Jupiter in its path, the other always behind. They are doing so because they are orbiting the Sun-Jupiter L₄ and L₅ Lagrange points. If you want to know more about Lagrange points, take a look at my previous post, where I go into more detail. For now, it’s probably enough to say that the asteroids stay at these points and travel along with Jupiter around the Sun.

So, why are they called Trojan asteroids? Does it have anything to do with the ancient city of Troy? It actually does. Astronomers agreed to use names from Greek mythology to name individual asteroids in these groups. More specifically, they choose the names of the heroes from the Trojan War. They are using names of Greek heroes to name asteroids in the leading group. The asteroids in the trailing group are named after Trojan heroes. However, there’s no rule without an exception. In this case, there is one asteroid in each group, which would belong in the other group. Both were already named, when astronomer Johann Palisa of Vienna suggested the naming rule.

The Trojan asteroids are interesting for scientists because they probably are ancient. Originating from the early days of our solar system, when the outer planets formed. They might be over 4 billion years old. Therefore, they might be time capsules, which will give us inside into the history of our solar system. By flying a spacecraft out there and examine them up close, we can learn a lot about the evolution of our solar system. Lucy will fly by seven different of these asteroids and carry out remote sensing to learn more about each of them. To get there, Lucy has to come back to Earth first.

Lucy’s return to Earth

So, what’s happening today? Well, Lucy is coming back to Earth and doing a flyby. It is doing this to get a gravity assist from Earth. Mission-planners decided to use multiple gravity assists to get Lucy to the Trojan asteroids in order to safe fuel. Which also saves mass and reduces the costs. The closest approach of Lucy will happen on 16 October 2022 11:04 UTC.

To get the most out of the gravity assist, Lucy’s flyby will take it as close as 350 km above the surface of Earth. This is closer than the orbit of the ISS. Which is why the mission-planners had to consider the effects of the atmosphere on the spacecraft. Originally, they planned for an even closer approach to Earth. A problem with one of Lucy’s solar array, however, prevented this plan.

After the launch, both solar arrays unfolded as planned, but only one did so completely and latched. The other one unfolded but didn’t latch. Doing remote trouble shouting, engineers managed to unfold it enough, that it is no issue for the mission. However, they didn’t manage to latch it. As they are not sure, how the solar array will interact with the atmosphere, they decided to reduce the risk and raised the flyby. Thereby reducing the effects of the atmosphere on Lucy.

What’s happening next?

The flyby will give Lucy additional energy for a two-year trip through outer space. Then it will return to Earth for another flyby on 12 December 2024. This one will send Lucy out to the main belt asteroid (52246) Donaldjohansen (20 April 2025) and the first group of the Trojans. It’ll encounter its first Trojan asteroid, (3548) Eurybates on 12 August 2027. After returning one last time to Earth on 25 December 2030, Lucy will travel to its final destination in the trailing cloud of asteroids, where it’ll arrive on 3 March 2033.

Credits for image at the top: Southwest Research Institute