Lucy completes its first Earth gravity assist after a year in space

by Lee Kanayama

A year after its launch, NASA’s Jupiter Trojan asteroid explorer Lucy has completed its first of multiple Earth gravity assists on its journey to explore nine asteroids. Lucy is a first-of-its-kind mission to visit several Trojan asteroids in Jupiter’s L4 and L5 Lagrange points. The mission’s closest approach to Earth occurred on Oct. 16 at 7:04 AM EDT (11:04 UTC).

Lucy Instruments and Flyby

Since its launch on Oct. 16, 2021, Lucy has been operating in cruise mode in a heliocentric orbit around the Sun. The cruise mode allows Lucy substantial autonomy as it coasts between significant events. The three primary science instruments are inactive during cruise mode; however, in November 2021, they were turned on for system checkouts. Mission controllers then turned them back off after the checkouts.

To ensure a close flyby of Earth, Lucy completed multiple correction maneuvers and minor burns to place the spacecraft on a trajectory as close to Earth as possible. In preparation for the flyby, Lucy completed a correction maneuver on June 21.

Since June, Lucy has been in limited communications caused by the thermal conditions of its position relative to Earth and the Sun. Because of this, its high-gain antenna is out of commission, and the craft currently talks to Earth via its low-gain antenna. With the flyby complete, Lucy will soon return to using its high-gain antenna.

A month before the flyby, Lucy was ~14.5 million kilometers from Earth. On Sept. 26, Lucy was 10.4 million kilometers away and activated its Long Range Reconnaissance Imager (L’LORRI) to begin observations of the Didymos/Dimophos binary asteroid system. Lucy started watching the system to capture imagery of NASA’s Double Asteroid Redirection Test (DART) impact of Dimophos—which successfully changed the orbital period of the small asteroid by 32 minutes.

DART featured a camera system named Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO). Both DRACO and L’LORRI were initially developed from the LORRI camera onboard New Horizons. Lucy successfully observed DART’s impact and will transmit its imagery in the coming days.

Lucy features three instruments to conduct observations throughout the mission.

L’LORRI is a high-spatial-resolution monochrome visible imager and will provide highly detailed images of the Trojan asteroid surfaces. The John Hopkins University Applied Physics Laboratory provided the imager for Lucy.

Similarly, the spacecraft’s Thermal Emission Spectrometer (L’TES) will detect radiation emitting from the Trojan asteroids. It was developed from OTES (OSIRIS-REx Thermal Emission Spectrometer), which was used on OSIRIS-REx, and the EMIRS (Emirates Mars Infrared Spectrometer), which flew on the Al-Amal Mars mission. Arizona State University built the spectrometer for Lucy.

The third and final instrument is L’Ralph, composed of two sub-instruments: the Multispectral Visible Imaging Camera (MVIC)—a visible-light imager—and the Linear Etalon Imaging Spectral Array (LEISA)—an infrared spectrometer. L’Ralph is based on the Ralph instrument flown onboard New Horizons and will be used to measure silicates, ice, and organics on the surface of the Trojan asteroids. The instrument was built by the Goddard Space Flight Center.

The high-gain antenna and radio telecommunications hardware will measure doppler shifts, helping to determine the mass of the Trojans.

Lockheed Martin built the spacecraft, incorporating the lessons learned from New Horizons and OSIRIS-REx—the latter of which the company also made.

Lucy before it was encapsulated in Atlas V’s 4.2-meter fairing. (Credit: NASA)

After traveling nearly one billion kilometers in space, Lucy began its approach to Earth. The spacecraft activated its instruments and readied its systems for the flyby. Approaching from the direction of the Sun, Lucy took images of the Earth and the Moon to help calibrate its instruments.

This calibration will ensure the spacecraft can get as precise science as possible during its flybys. During its Earth flyby, Lucy attempted to take photos of Ethiopia, where the Lucy fossil—the spacecraft’s namesake—was discovered.

At its closest approach, Lucy came just 351 kilometers from the surface of Earth, which is closer than most orbiting spacecraft—including the ISS. As it was so close to Earth, this made the spacecraft experience slight atmospheric drag that the teams had to compensate for.

“In the original plan, Lucy was actually going to pass about 30 miles (48 km) closer to the Earth,” said Rich Burns, Lucy’s project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “However, when it became clear that we might have to execute this flyby with one of the solar arrays unlatched, we chose to use a bit of our fuel reserves so that the spacecraft passes the Earth at a slightly higher altitude, reducing the disturbance from the atmospheric drag on the spacecraft’s solar arrays.”

The first location to spot Lucy was in Western Australia at 10:55 UTC. The spacecraft quickly passed overhead and disappeared at 11:02 UTC. At 11:04 UTC, Lucy was at its closest and started retreating from Earth. 22 minutes after the pass, observers in the western US got a view of the spacecraft.

Twelve hours before the gravity assist, Lucy could conduct a minor correction burn in case of a risk of collision with orbital debris. As Lucy approached Earth, there was a risk of collision with large amounts of debris and objects in orbit around the planet. This correction maneuver would have altered the close approach by a few seconds. While only a few seconds difference, it would have been enough to avoid a collision. To prepare for the possible burn, NASA scanned Lucy’s path ten days before the gravity assist to check for any potential impacts requiring the burn.

Once retreating from Earth, Lucy gathered a few more calibration images before leaving Earth’s sphere of influence.

As Earth and Jupiter are the closest the two have been in decades, Lucy is as close to Jupiter now as it will ever get. As Jupiter’s orbit is so large, the distance from the planet to its Trojan asteroids is so significant that Lucy—at the Trojans—will be further from Jupiter than during the gravity assist.

The purpose of a gravity assist is to use a planetary body’s gravitational force to alter a spacecraft’s trajectory. When performing a gravity assist, the spacecraft enters a planetary body’s sphere of influence to change its velocity using the planet’s gravity. Lucy used Earth’s gravity to gain speed, boosting the highest point—or aphelion—of its orbit around the Sun.

The gravity assist with Earth changed its aphelion to pass the orbit of Mars. With this, the orbital period for Lucy increased to two years. After completing a single orbit, Lucy will conduct a second Earth gravity assist to provide the energy needed to have its aphelion reach the orbit of Jupiter.

Lucy Update

Since launch, Lucy has operated flawlessly while in heliocentric orbit—despite the issue with one of its two solar arrays.

With a lanyard initially failing to spool and latch the solar array, NASA spooled the lanyard over multiple days to increase the stability and tension of the array—along with its power generation. The solar array is now deployed to around 353-357 of its planned 360 degrees, and the teams are confident in the array’s ability to complete Lucy’s mission in its current state. With the gravity assist complete, NASA and the teams will determine if they will continue with further spooling operations in the future.

The Lucy teams have also continued to observe the Trojan asteroids to understand the targets fully.

In August 2022, the Lucy team announced the discovery of a moon around one of its targets, Polymele. Lucy’s team discovered the moon on March 27. As part of a routine observation, Polymele passed in front of a star, blocking—or occulting—the star’s light. With multiple astronomers situated across the path of the occultation, this allowed the teams to measure the size, location, and shape of Polymele with impressive precision as the star’s light was obstructed or dimmed.

A graphic showing the distance and artist impression of Polymele’s Moon. (Credit: NASA’s Goddard Space Flight Center)

Teams were surprised, however, when multiple observers noticed the star blink twice as the asteroid passed by. The unexpected observations detected an object 200 km away from Polymele. Using the same occultation data, the teams found the moon to be 5 km in diameter, but 27 km long along its widest axis. Teams will not give a name to the moon until its orbit is better determined by either Lucy or a ground-based occultation observation.

This is not the first time the Lucy teams have discovered a moon around one of the asteroid targets. The Lucy teams previously used the Hubble Space Telescope to observe the targets for an unexpected binary pair. An undetected secondary asteroid could confuse the targeting system—or even risk losing the spacecraft. While looking at images from 2018, the teams discovered a moon around Eurybates. With other observations in early 2020, they confirmed the moon’s existence and named it Queta.

Queta is estimated to be less than 1 km in diameter. The discovery of the two moons increased the mission’s target count from seven to nine.

Lucy’s Future—A Flyby Frenzy

Lucy’s second gravity assist with Earth will occur on Dec. 13, 2024. The second gravity assist will begin the spacecraft’s multi-year journey to fly past multiple asteroids. Just five months after the second gravity assist, Lucy will fly by asteroid 52246 Donaldjohanson, located in the main asteroid belt between Jupiter and Mars. The probe will use Donaldjohanson as a rehearsal before reaching the Trojan asteroids.

While only a trial, this flyby will provide the teams with valuable data on C-type asteroids. Measuring only 4 km in diameter, it is one of the smallest objects Lucy will visit. The object is identified to be a fragment of a massive collision from 130 million years ago, which produced the Eriogone family of asteroids.

After flying past Donaldjohanson, Lucy will continue to the L4 Trojan swarm “in front” of Jupiter. Known as the “Greek camp,” the L4 Trojan asteroids congregate around the “leading” Lagrange point, creating a swarm of asteroids.

Lucy’s first flyby of the Trojan asteroids will take place on Aug. 12, 2027, visiting 3548 Eurybates. Eurybates is another C-type asteroid, like Donaldjohanson, but somehow ended up much further from the Sun than its sibling. Eurybates is 64 km in diameter and originated from the same collision as Donaldjohanson. Lucy will visit the asteroid and its moon, Queta, to understand more about how C-type asteroids form, why Eurybates ended up in the Trojan swarm, and more.

A month later, on Sept. 15, Lucy will fly by 15094 Polymele. As a P-type asteroid, this will be the first time a spacecraft flies past a member of the dark, reddish class of asteroid that could be rich in organics. Scientists believe Polymele formed from a collisional fragment of a larger asteroid; Polymele will be compared to other P-type asteroids to determine why it is smaller than others of the same type. Polymele also is accompanied by a moon; Lucy’s flyby will allow closer-up studies of the body to determine its orbit around Polymele.

After a short seven-month coast, Lucy will reach 11351 Leucus, a D-type asteroid. Unlike some other asteroids, Leucus rotates, having a 446-hour day. This rotation gives the surface of Leucus different heating over time compared to other asteroids. Studying Leucus will help scientists understand the composition and formation of D-class asteroids. During observations of the asteroid, its brightness varies from Earth’s perspective, suggesting it has an elongated shape. Leucus is approximately 40 km in diameter.

The final object Lucy will explore in the Greek L4 swarm is the D-type asteroid 21900 Orus. Orus is a larger asteroid, at 51 km in diameter, and can be compared to the smaller Lueucus. While the two are both D-types, Orus has a dark, red surface—much like Eurybates. Lucy will fly by Orus on Nov. 11, 2028.

After completing its flyby of the Greek L4 swarm, Lucy will then begin a return to the inner solar system for its third and final Earth gravity assist. On Dec. 26, 2030, the spacecraft will complete its gravity assist of Earth to aim for the L5 set of Trojan asteroids, also known as the “Trojan camp.”

As the last two targets for Lucy, 617 Patroclus and Menoetius are binary P-type asteroids in the L5 swarm. With diameters of 113 and 104 km, respectively, the two are in a relativity high-inclination heliocentric orbit at 22 degrees. Scientists believe the two may be primordial asteroids originating from the early solar system. Lucy will fly past the pair on March 3, 2033, completing its 12-year primary mission.

During each flyby, the spacecraft will pass 400 to 965 km away from each asteroid, allowing for detailed surface observations. Studying these asteroids will allow scientists to better understand how the solar system was formed.

While the primary mission will be over after its final flyby, Lucy’s exploration will not be finished. In its final orbit, Lucy can continue exploring the trojan asteroids for many more years. NASA will most likely continue its mission until it runs out of fuel—or the agency loses communication with the spacecraft. Once that happens, Lucy will continue on a similar orbit for thousands or even millions of years.

In the future, Lucy’s sister mission, Psyche, will explore the metal asteroid 16 Psyche. Psyche will start its mission once its next launch window opens no earlier than 2023.

(Lead image: Artist’s impression of Lucy’s Earth gravity assist. Credit: NASA/SwRI)

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