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Lucy in the sky: NASA launches probe to explore Trojan asteroids

October 16, 2021

— A NASA probe has embarked on a 12-year mission to visit a record number of small worlds, exploring for the first time a group of asteroids that may be the leftover remains from the creation of our solar system.

The spacecraft, named "Lucy," launched Saturday (Oct. 16) atop a United Launch Alliance (ULA) Atlas V rocket from Complex 41 at the Cape Canaveral Space Force Station in Florida. The 5:34 a.m. EDT (0934 GMT) liftoff began the mission's journey to the Trojans — two swarms of asteroids that lead and follow Jupiter in its orbit around the Sun.

"We're going to be visiting the most asteroids ever with one mission," said Cathy Olkin, deputy principal investigator for the Lucy mission at Southwest Research Institute, at a pre-launch press briefing. "We're going to be flying past seven Trojan asteroids [and one main belt asteroid] in this epic journey of nearly 4 billion miles."

Lucy was named after a fossilized hominid skeleton that was found in Ethiopia in 1974. Like its namesake, which provided new insights into humanity's evolution, the NASA mission anticipates that the Trojan asteroids — the "fossils" of our solar system — will advance what we know about the outer planets' origins.

Donald Johanson, the paleoanthropologist who is co-credited with the discovery of the Lucy fossil 47 years ago, was invited to see the spacecraft prior to its launch and was in Florida to see it leave Earth on Saturday.

"I don't think I'll ever be able to look up at Jupiter again the same way I did before," Johanson told about the connection between the mission and his discovery. "That, for me, is something that was utterly and totally unanticipated in my life."

NASA's Lucy mission launch. Click to enlarge and view video in pop-up window. (NASA)
Trojan tour

Before Lucy can reach its Trojan targets, it will first fly past Earth twice, gaining velocity and refining its trajectory each time. It will then travel through the main asteroid belt between Mars and Jupiter and visit its first asteroid in April 2025, a 2.5-mile-wide (4-km) rock named by the Lucy team "Donaldjohanson" after the fossil's co-finder.

The probe will then continue outwards to the leading swarm of Trojan asteroids, known as the "Greek camp" as most of its bodies are named after Greek heroes from around the time of the Trojan war. Lucy will fly by four of these "Greek" Trojans: Eurybates and its satellite Queta in August 2027, Polymele in September 2027, Leucus in April 2028, and Orus in November 2028.

Lucy will then return to Earth for another gravity assist and head out to the "Trojan camp" in the trailing swam. Here, in March 2033, Lucy will fly by the Greek "spies" Patroclus and its binary companion Menoetius.

"Those are the last two targets that we get to and they are among the most interesting, because when we look in the outer solar system beyond Neptune, we see a lot of these binary objects [but] we don't see so many in the inner solar system," said Keith Noll, Lucy project scientist at NASA's Goddard Space Flight Center. "What that tells us is that Patroclus and Menoetius likely survived getting transported there and then didn't get beat up too much."

"On the other hand, the very first target that we get to, Eurybates, is a target that we know has been involved in a massive collision, because we can see the debris from that collision in the orbits of the objects in the Trojan asteroids," said Noll.

"So that's a really good example of the power of what Lucy is able to do by having so many targets. We can construct all these comparisons between all the different varieties and the diversity that we see in the Trojans — the unexpected diversity — the different colors [and] the different collisional histories. It's really a repository of fossils, as we like to say, of things that happened at the earliest stages of solar system evolution," he said.

The flyby of Patroclus and Menoetius will complete Lucy's primary mission, but the probe will be in a stable orbit and will continue flying through the Trojan swarms for many years to follow.

...with diamonds

Lucy will come within 600 miles (965 km) of many of its targets while traveling at about 15,000 miles per hour (24,000 kph). To collect as much data as is possible, the mission's science instruments are attached to a pointing platform mounted to the 12.4-foot by 51.8-foot (3.8 by 15.8 m) spacecraft. The probe is powered by two 23.9-foot-diameter (7.3 m) solar arrays.

Lucy's instruments are L'Ralph, a color visible imager and infrared spectrometer; L'LORRI, a long-range reconnaissance imager, which will be used to capture the most detailed images of the Trojan surfaces; and L'TES, a thermal emission spectrometer, which is similar to the instruments flying on OSIRIS-REx and Mars Global Surveyor.

"Lucy's L'LORRI is the successor to an instrument that's currently flying in space on the New Horizons mission. It's also called LORRI and it's been flying in space for more than 16 years performing flawlessly with no degradation in its performance," said Hal Weaver, the principal investigator for L'LORRI at Johns Hopkins University Applied Physics Laboratory (JHU-APL).

Lucy's thermal emission spectrometer will help reveal the asteroids' composition and structure by measuring how well the bodies retain heat — and it will do so, in part, using diamonds.

"We are indeed taking diamonds into the sky," said Phil Christensen, principal investigation for the L'TES instrument at Arizona State University, referring to the song "Lucy in the Sky with Diamonds" by The Beatles."[The diamond] is a part of the instrument that breaks the light and sends it in two different directions."


A United Launch Alliance (ULA) Atlas V rocket lifts off with NASA's Lucy spacecraft from Complex 41 at Cape Canaveral Space Force Station in Florida on Saturday, Oct. 16, 2021. (NASA/Bill Ingalls)

NASA's Lucy spacecraft with its solar arrays folded for launch and high gain antenna attached is readied for flight. (NASA/Kim Shiflett)

Illustration of the Lucy mission's seven targets: the binary asteroid Patroclus/Menoetius, Eurybates, Orus, Leucus, Polymele, and the main belt asteroid DonaldJohanson. (NASA/GSFC)

Diagram of Lucy's orbital path. The spacecraft's path (green) is shown in a frame of reference where Jupiter remains stationary, giving the trajectory its pretzel-like shape. (SwRI)

Lucy "First to the Trojans" official mission logo. (NASA/SWRI/LM)

Artist's rendering of NASA's Lucy spacecraft flying by a Trojan asteroid in the same orbit of Jupiter around the Sun. (Southwest Research Institute)

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