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Lucy Zipped Past Earth to get a Gravitational Assist Towards Jupiter’s Trojans

Gravitational assists are marvels of orbital mechanics. Usually, they are done for a combination of fuel (i.e., cost) savings and speed, as it is sometimes faster to take multiple trips around planets in the inner solar system to reach locations in the outer solar system more quickly. Lucy recently made such a maneuver on its way out to the Trojan asteroids along Jupiter’s orbit, and its close pass was both a marvel of precision and speed.

The flyby, which went off without a hitch on October 16th, happened about one year into a twelve-year mission time for the Trojan asteroid probe. Almost half of that mission time will be spent in transit before reaching its destination in 2027. And that’s with the significant speed increase afforded by maneuvers such as the latest gravitational assist.

Already traveling at 8 km a second, Lucy isn’t exactly going slowly. But even that speed isn’t enough to get to Jupiter on time, so it needed a bit of a boost from its home planet. After this successful slingshot, it is now traveling at close to 15 kilometers per second.


A visualization of Lucy’s orbital trajectory.
Credit – NASA Scientific Visualization Studio

But to get there, the probe has to swing close enough to Earth that it was actually closer to the ground than the ISS typically is. So close, in fact, that if you happened to be in Western Australia on October 16th, you could have potentially seen Lucy with the naked eye for a few minutes around 7 PM.

Luckily, those observers didn’t see a fireball, as Lucy managed to avoid the numerous obstacles in its path. That required a bit of help from the US’s newly minted Space Force, which monitors crowded orbital paths, such as the some that Lucy had to maneuver through. Constant communication between the teams was vital as the probe made it through the densely packed orbital plane.

If there had been a potential collision, the team running Lucy had one of two options: either kick the thrusters on so the probe would pass by the potential collision either two or four seconds before or sooner. That might not sound like a large window, but, for perspective, a four-second speed-up would mean that Lucy was 48 kilometers away from the potential collision when it would have happened.


UT video describing the Lucy mission.

All this high-speed math and orbital mechanics are literally rocket science, and some of the best brains in the business have determined that this path was the best for Lucy to take, even it if was slightly dangerous. It turns out their bet paid off, and the probe is well on its way to the Trojan asteroids following Jupiter, where it will arrive in 2027.

However, there will be one more such maneuver – in 2024, Lucy will get another boost from its home planet on its way out to the outer solar system. It might not be as spectacular or visible as this one, but it serves the same purpose – to go where no probe has gone before.

Learn More:
NASA – NASA’s Lucy to Fly Past Thousands of Objects for Earth Gravity Assist
UT – Lucy’s Solar Array is Fixed! (Mostly)
UT – Lucy is off to Visit Jupiter’s Trojan Asteroids
UT – Another of the Lucy Mission’s Asteroids has a Moon

Lead Image:
Artist’s concept of the Lucy missions visiting an asteroid. Credit – NASA

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