617 Patroclus

617 Patroclus is a large binary Jupiter trojan asteroid. It is a dark D-type asteroid and a slow rotator, because its two parts take 103 hours to orbit each other. It is one of five Jupiter trojan asteroids that the Lucy space probe will visit, with a planned flyby in 2033.

Patroclus was discovered on 17 October 1906 by astronomer August Kopff at the Heidelberg Observatory in Germany. It was named after Patroclus from Greek mythology. Patroclus was the second trojan asteroid ever found and is special because it is the only one in the Trojan camp named after a Greek figure.

For a long time, Patroclus was thought to be one of the largest Jupiter trojans, about 150 km wide. But in 2001, it was discovered to be a binary asteroid made of two objects of almost the same size. The larger part, about 110–115 km across, keeps the name Patroclus. The smaller part, about 100–105 km across, is called Menoetius. This was the first time a binary trojan asteroid was found.

Orbit

Patroclus moves around the Sun in the area behind Jupiter, called the Trojan camp, named after one side of the famous Trojan War. It stays in a special spot called the L₅ Lagrangian point.

The asteroid travels at a distance of 4.5 to 5.9 AU from the Sun, completing one journey every 11 years and 11 months. Its path around the Sun is slightly stretched out and tilted at an angle of 22° compared to the plane where Earth and other planets move. People first saw Patroclus in November 1906 at the Heidelberg Observatory.

Binary system

In 2001, it was found that Patroclus is a binary system, meaning it has two parts that are almost the same size. It is one of six Trojan asteroids thought to have this kind of system. In 2006, scientists used special tools from the Keck telescope to learn more about how these two parts move.

The two parts go around each other every 4.3 days, about 680,000 meters apart. By looking at heat given off by the asteroid in 2000 and more recent observations, scientists think one part is about 122,000 meters across and the other is about 112,000 meters across.

On October 21, 2013, both parts passed in front of a star, allowing scientists to measure their sizes more accurately. The bigger part, still called Patroclus, is about 113,000 meters wide, and the smaller part, named Menoetius, is about 104,000 meters wide.

Physical characteristics

Several studies have measured how Patroclus spins, finding it takes about 103 hours to complete one rotation. This slow spin and small change in brightness suggest Patroclus has a rounded shape.

Measurements from space telescopes like IRAS and NEOWISE show Patroclus measures about 141 kilometers across and reflects very little sunlight, indicating a dark surface. Evidence suggests Patroclus may be made of icy materials similar to comets rather than rocky materials like most asteroids. Its low density of 0.88 g/cm³ supports this idea. Scientists think many Jupiter trojans may be ancient pieces of material captured by Jupiter’s gravity billions of years ago.

Exploration

The Patroclus–Menoetius system will be visited by the Lucy mission. This mission will fly by several asteroids, including many Jupiter trojans.

Name

See also: Patroclus § Pronunciation

This minor planet was named after the legendary Greek hero Patroclus. He was a friend of Achilles and played a role in the Trojan War. The name was suggested by an astronomer named Johann Palisa. The official naming was recorded in a book by Paul Herget in 1955.

The satellite Menoetius was named after Patroclus's father. It was found later and had a temporary name before getting its official one. Patroclus and Menoetius are special because they are named after Greek heroes, unlike most others in their group.