NASA’s Lucy spacecraft will perform a close flyby of Earth on December 12, utilizing a gravity assist to adjust its trajectory towards the Jupiter Trojan asteroids.
This maneuver, the second such assist in its 12-year mission, follows an initial gravity boost in 2022. The spacecraft will pass only 220 miles above Earth, positioning it for subsequent encounters with main belt and Trojan asteroids. Observers may glimpse Lucy using telescopes in specific locations, though instrument calibrations will be inactive during this pass.
Lucy’s Close Encounter with Earth
On December 12 at 11:15 p.m. EST, NASA’s Lucy spacecraft will pass just 220 miles (360 kilometers) above Earth’s surface, skimming the planet’s atmosphere. This close flyby, known as a “gravity assist,” will adjust Lucy’s trajectory, sending it through the main asteroid belt and toward the Jupiter Trojan asteroids — a group of small celestial bodies that share Jupiter’s orbit around the Sun but have never been explored before.
This will be Lucy’s second Earth gravity assist, occurring three years into its planned 12-year mission. The first gravity assist took place on October 16, 2022, boosting the spacecraft from a one-year solar orbit into its current two-year orbit, which extends into the inner edge of the main asteroid belt. That maneuver set up Lucy’s first asteroid encounter with the small asteroid Dinkinesh and its companion, Selam.
The upcoming gravity assist will propel Lucy into a six-year orbit that will take it deeper into the main asteroid belt, where it will pass the asteroid Donaldjohanson. From there, Lucy will head toward the Jupiter Trojan asteroids, beginning its first encounter with the leading group of these ancient bodies in 2027.
Observational Challenges and Safety Measures
During the gravity assist, the Lucy spacecraft, from Earth’s perspective, will approach from the direction of the Sun. This means that observers on Earth will not be able to see Lucy approaching, as it will be lost in the Sun’s glare.
Lucy’s trajectory will bring the spacecraft very close to the Earth, even lower in altitude than the International Space Station (ISS). To ensure the safety of the spacecraft as it passes through this region full of Earth-orbiting satellites and debris, NASA has procedures to anticipate and avoid potential collisions.
If needed, the spacecraft will execute a small trajectory correction maneuver 12 hours before closest approach to alter the time of closest approach by 1 or 2 seconds — enough to avoid a potential collision.
Shortly after sunset, keen observers in the Hawaiian Islands may be able to catch a glimpse of Lucy as the spacecraft approaches Earth before it passes into Earth’s shadow at 6:14 p.m. HST. Lucy will speed over the continental U.S. in darkness, traveling over 33,000 miles per hour (14.8 kilometers per second), and emerge from Earth’s shadow 20 minutes later at 11:34 p.m. EST.
At that time, Lucy may be visible to observers with a telescope in the western regions of Africa and the eastern regions of South America as sunlight reflects off the spacecraft’s large solar panels (observers in the eastern United States will be looking at the much dimmer “back” side of the solar panels, making Lucy harder to see; go here for more information about observing Lucy during this gravity assist).
Lucy will then rapidly recede from Earth and return to interplanetary space, the gravity assist having increased the spacecraft’s speed with respect to the Sun by over 16,000 miles per hour (7.31 kilometers per second).
Upcoming Encounters and Instrument Calibration
During the first Earth gravity assist in 2022, the spacecraft imaged the Earth and the Moon as part of an instrument calibration. As no further calibrations are needed at this time, the instruments will be off during this encounter. Lucy’s next asteroid encounter will be with the small main belt asteroid Donaldjohanson on April 20, 2025.
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