See NASA’s Lucy Mission Fly Past Earth Tonight

An exciting mission is about to re-visit our fair planet this week on Thursday night/Friday morning, as NASA’s Lucy spacecraft passes Earth in a bid to pick up speed. The gravitational assist will have Lucy fly by around 11:15 p.m. EST on December 12th (4:15 Universal Time on Friday, December 13th). The closest approach will occur 354 kilometers (220 miles) above the Pacific Ocean just west off the coast of California.

Launched in October 2021, Lucy is headed to explore the two camps of asteroids that orbit stably on either side of Jupiter: the Greek camp located at Jupiter’s L₄ Lagrange point (ahead of the planet on its orbit) and the opposing Trojan camp at the L₅ point behind Jupiter. The mission will visit a total of 11 asteroids (three main-belt asteroids and eight Trojans).

To get there takes some complicated orbital gymnastics, and this flyby past Earth is the second of three. The flyby will increase the spacecraft’s velocity relative to the Sun by 7.3 kilometers per second (16,000 mph). After crossing the Moon’s orbit 34 hours prior to the flyby, the spacecraft will cross the skies above North America in just seven minutes, moving at 14.8 km/s relative to Earth.

Threading Earth’s Shadow

Unfortunately, the spacecraft will be a bashful one on this pass, as it hits Earth’s shadow just one minute prior to its closest approach above the surface. That will make it difficult to see. But things improve a bit on shadow egress, which occurs 20 minutes later at 4:34 UT; the southeastern U.S., western Europe, western Africa, and eastern South America could have a good view of Lucy as a +7th-magnitude binocular object. By that time, it will be more than 6,920 kilometers away. Although this puts the spacecraft beyond low-Earth orbit, it will still be interior to geosynchronous orbit.

While observers in Hawai’i could see Lucy zip through the twilight sky just before shadow ingress, southern Atlantic area observers are favored at egress due to the expected angle of the spacecraft’s enormous solar panels, which will still be reflecting sunlight. Observers in North America and Europe will be seeing the panels edge-on or from the back.

While Tony Dunn (San Francisco State University) doesn’t expect to see the flyby well from his location, he adds, “I’ll have to be happy simply knowing that it is only a few hundred kilometers above my head (!)”

NASA’s Lucy mission will visit Earth this weekend for a gravity assist that will send it to Jupiter’s Trojan asteroids. At its closest, it will pass about 250 miles above California, similar to the altitude of the ISS. But Lucy will be in Earth’s shadow then.

[image or embed]

— Tony Dunn (@tony873004.bsky.social) December 9, 2024 at 4:09 PM

How to Find Lucy

You can use NASA JPL’s Horizons ephemeris generator to create a list of minute-by-minute right ascension and declination search coordinates for your site. For example, from my own location in Miami, Florida, I expect a view of the receding spacecraft in Cancer, the Crab — just south of Mars. I would treat the search similar to watching for a satellite that’s below naked eye brightness: find a bright fixed star in the field using binoculars and keep watch at the appointed time to see Lucy slide by.

During the previous flyby, in 2022, controllers used the opportunity to test and calibrate instruments onboard Lucy, a campaign that went off without a hitch. This time, there won’t be a repeat. “We are not planning to do any observations during the flyby,” says principal investigator Harold Levison (Southwest Research Institute)

Astronomer Gianluca Masi tracked Lucy on its first Earth flyby, and will again host a live broadcast via the Virtual Telescope Project starting at 11:30 p.m. EST December 12th / 4:30 UT December 13th. You can also follow NASA’s social media hashtag #SpotTheSpacecraft this week.

What’s next for Lucy

The mission already made its first flyby past asteroid 152830 Dinkinesh on November 1, 2023 revealing that the asteroid possesses a tiny moonlet, later named Selam. Selam turned out to be a small contact binary pair, the first such pair seen orbiting an asteroid.

Now, this pass will send Lucy from a two-year orbit around the Sun to a six-year orbit, which takes it to its next target: the main-belt asteroid 52246 Donaldjohanson. Lucy will pass 922 kilometers from the asteroid on April 20, 2025. Then, on August 12, 2027, Lucy will reach its first “Greek Camp” asteroid, 3548 Eurybates.

Lucy will actually revisit Earth a third and final time on December 26, 2030, which will send it to the Trojan camp on the other side of Jupiter in its orbit. Lucy’s eighth and last target of its mission is the 617 Patroclus-Menoetius pair in 2033.