An illustration showing Trojan asteroid Polymele and the object orbiting it at a distance of around 200 kilometers. Image Credit: NASA's Goddard Space Flight Center.

NASA Spots Object Measuring 5km Orbiting Trojan Asteroid 200Km Away

NASA's Lucy spacecraft discovered that Polymele, the smallest Trojan asteroid among this group, is orbited by an object measuring 5 kilometers across. This "satellite" orbits the asteroid at an average distance of 200 kilometers.


Swarms of Trojan asteroids associated with Jupiter, remnants of the primordial material that formed the outer planets, are thought to have been formed more than 4 billion years ago. Two Trojan groups orbit the Sun, one leading Jupiter on its path, the other trailing behind. As a result of gravitational balancing acts between the Sun and Jupiter, the Trojans are located around the two Lagrange points. It is possible to decipher the solar system’s history by studying these primitive bodies.

A space mission called Lucy will be the first to study Trojans. This mission was named after Lucy, a fossilized ancestor of humanity whose skeleton shed light on human evolution. In the same way, the Lucy mission will revolutionize our understanding of planetary origins and solar system formation.


An odd companion

Polymele, Lucy’s smallest Trojan asteroid target, has its own companion, a strange object orbiting the asteroid from a great distance. The find was made by Lucy’s science team made on March 27.

When Polymele passed over the star on that day, the team could see the star blink out as it was briefly blocked by the asteroid.

Using 26 teams of professional and amateur astronomers across the path of the occultation, the Lucy team hoped to measure Polymele’s size, shape, location, and size with unprecedented precision.

It has been extremely successful in the past to carry out these occultation campaigns, giving the mission valuable information on its asteroid targets, but this day would be particularly exciting.

“The 14 teams who observed the star blinking out as it passed behind the asteroid were thrilled. Still, when we analyzed the data, we discovered that two of those observations differed from the others,” said Marc Buie, Lucy occultation scientist at Southwest Research Institute, in San Antonio.


Two objects, not one

Those two observers detected a mysterious object within 200 km (124 miles) of Polymele. “It must have been a satellite.”

Experts saying “it must have been a satellite” is kind of odd since what else could possibly be orbiting an asteroid at such a distance? A UFO, perhaps? (joke).

According to the occultation data, this satellite has a diameter of roughly 3 miles (5 km), orbiting Polymele, which at its widest point measures 17 miles (27 km). There was a distance of 125 miles (200 km) between the two bodies.

Until the team can determine the satellite’s orbit, it will not receive an official name.

Without the help of a fortuitously positioned star, Earth-based and Earth-orbiting telescopes cannot clearly see the satellite due to its close proximity to Polymele. It will be necessary to wait until Lucy approaches the asteroid in 2027 or until the team gets lucky with future occultation attempts.

Far, far away

When the observation was made, Polymele was 480 million miles (770 million km) away from Earth. Those distances would be like trying to locate a quarter on a sidewalk in Los Angeles from a skyscraper in Manhattan.

Asteroids hold crucial clues to deciphering the history of the solar system – perhaps even the origin of life. NASA is dedicated to solving these mysteries. According to the Lucy mission plan, one main belt asteroid and six Trojan asteroids would be visited, a previously unknown population of asteroids surrounding Jupiter.


Astroid Eurybates, one of the Trojan asteroids, has a small satellite discovered by the Hubble Space Telescope in January of 2021. As a result of this new satellite, Lucy will visit nine asteroids over the next 12 years.

“Lucy’s tagline started out: 12 years, seven asteroids, one spacecraft,” said Lucy program scientist Tom Statler at NASA Headquarters in Washington. “We keep having to change the tagline for this mission, but that’s a good problem to have.”

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Written by Ivan Petricevic

I've been writing passionately about ancient civilizations, history, alien life, and various other subjects for more than eight years. You may have seen me appear on Discovery Channel's What On Earth series, History Channel's Ancient Aliens, and Gaia's Ancient Civilizations among others.

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