Spacecraft to study Trojan asteroids

By Tom Burns - Stargazing

The spacecraft Lucy is on its way. On Oct. 16, 2021, an Atlas V launch vehicle lifted Lucy into its 12-year, 4-billion-mile journey to explore the Jupiter trojan asteroids.

Like the henna-haired Lucy of old, Lucy “has some ‘splaining to do,’” this time about our solar system’s early days. (Those younger folks who do not understand the Lucy allusion above should ask their grandparents.)

Spacecraft Lucy is named after a significant hominid fossil that anthropologists have nicknamed Lucy. The spacecraft’s name is apt. Just as the fossilized Lucy has helped us understand our species’ origins, spacecraft Lucy’s data will increase our understanding of our solar system’s pre-planetary origins.

The best way to do that is to study Trojan asteroids, some of the strangest denizens of our solar system.

Most folks think of asteroids as the band of millions of objects in orbit around the sun between Mars and Jupiter. However, some belt asteroids have been dislodged from their orbits by the gravity of other solar-system bodies.

Some were ejected from the solar system entirely or settled into the outer solar system past Jupiter. Others careened into the inner solar system.

Fewer still have orbits that cross Earth’s path around the sun. They are called NEOs, or Near-Earth Objects. They are worth studying because a slight chance exists that one of them will collide with Earth in the distant future.

However, millions of asteroids don’t fit into any of those categories. The Jupiter Trojan asteroids, as they are called, orbit the sun, but they are not in the Asteroid Belt. Instead, they are in the same orbit as Jupiter.

They hover at two of Jupiter’s Lagrangian points, the places in Jupiter’s orbit where the sun’s gravity counterbalances Jupiter’s gravity. The two Lagrangian points in question are along Jupiter’s orbit — 60 degrees of arc in the direction of Jupiter’s orbit and 60 degrees behind Jupiter.

The L4 point precedes Jupiter, and the L5 trails it. Thus, the million-plus Jupiter Trojans traverse Jupiter’s path around the sun. A given Trojan takes about 19 years to orbit the sun, the same time it takes Jupiter.

The Jupiter Trojans are mostly small chunks of rock and metal, called planetesimals, that eventually clumped together to form planets. The Trojans are significant because they did not get a chance to clump.

The Lucy mission will last 12 years. The spacecraft won’t begin its actual scientific mission until 2025.

It will first extend its distance from Earth by returning to our planet’s proximity twice, once in 2022 and again in 2024. Each time it returns, it will slingshot around Earth and gain velocity, consequently coming closer each time to its asteroidal targets.

In 2025, it will pass close to and study the main-belt asteroid 52246 Donaldjohanson, named after the discoverer of the Lucy fossil.

Not until 2027 will Lucy reach its first target beyond the asteroid belt in the L4 Jupiter Trojan cloud. The asteroid 3548 Eurybates is only about 41 miles in diameter. However, it is notable because it is one of several known asteroids with a tiny moon scurrying around it.

From there, Lucy will visit Trojan asteroids at the L4 Lagrange point — 15094 Polymele, 11351 Leucus, and 21900 Orus in rapid succession. After those flybys, Lucy will return to Earth for another gravity boost in 2031.

In 2033, Lucy will finally complete its mission by passing near Trojan 617 Patroclus and its moon Menoetius at the L5 Lagrange point.

Where do all those strange names originate? The numbers indicate the order in which astronomers discovered the asteroids. For example, Giuseppe Piazzi discovered the first asteroid, 1 Ceres, on New Year’s Day in 1801. The discovery of 2 Pallas followed on March 28, 1802.

If I may say so, quite immodestly, Edward L. G. Bowell of Lowell Observatory discovered 10483 Tomburns on Sept. 4, 1983.

Astronomers thought the first few asteroids were planets, so they adopted the ancient tradition of identifying planets as Greco-Roman gods. (They knew better, of course. Come on. It was a tradition.)

Ceres was the Roman goddess who helped plants to grow. Pallas is short for Pallas Athena, the patron goddess of the ancient Greek city of Athens.

As is apparent to anyone who knows me well, astronomers had entirely run out of Greco-Roman gods by the time they got to 10483 Tomburns.

The first Jupiter Trojan, 588 Achilles, wasn’t discovered until 1906 by German astronomer Max Wolf. By that time, the catalog of Greco-Roman deities had been thoroughly exhausted.

By convention, astronomers started to name asteroids after women from Greco-Roman mythology in general. They made exceptions when the asteroids were in odd orbits. The Jupiter Trojans certainly filled the bill because they were outside the asteroid belt.

Astronomers decided to name Jupiter Trojans after figures in Homer’s Iliad, the story of the Greek besiegement of the city of Troy. By convention, the Trojan asteroids at the “Greek Camp” at L4 are all named after Greek warriors. The L5 Asteroids fall into the “Trojan Camp.”

Classical scholars will note a glaring inconsistency here. Above, I said that 617 Patroclus was at the L5 Lagrange point, in other words, in the Trojan Camp. But Patroclus was a Greek warrior.

Austrian astronomer Johann Palisa suggested Achilles, Patroclus, and Hektor as names for the first three Trojan asteroids discovered. Patroclus was the friend and companion of the mighty Greek warrior Achilles. Hektor, the leader of the Trojan forces, slew Patroclus. Afterward, Achilles killed Hektor to avenge the death of his friend.

Palisa’s suggestion came before the decision to name the asteroids by Greek and Trojan camps. After all, there were only three Trojans known, so there was no need for “camps.” Thus, Patroclus became a “spy” in the Trojan Camp — or a fly in the ointment if you happen to know the Iliad story.

For decades, the only Trojan asteroids discovered followed Jupiter’s orbital path. However, astronomers have begun to spot Trojans in co-orbits with other planets.

As one would expect from the proximity of Mars to the asteroid belt, the red planet has accumulated at least four Trojans, with several more suspected by astronomers. Astronomers have detected two Trojans co-orbiting with Uranus.

On October 16, 2004, astronomers at the Las Campanas Observatory in Chile detected the first Neptune Trojans. Since then, astronomers have spotted 27 more. Some astronomers believe that the number of large Neptune Trojans may exceed the number of large Jupiter Trojans.

For example, 385571 Otrera, the third Neptune Trojan discovered, is about 60 miles wide. Astronomers have provisionally named Otrera after one of the Amazons, an all-female tribe of warriors who fought with the Trojans in the Trojan war.

Our planet has a single Trojan. 2010 TK7 is only 1,000 or so feet wide. Located at Earth’s L4 Lagrangian point, it precedes Earth in its orbit.

Why are scientists convinced that the Trojans are fossils of our early solar system?

The asteroid belt was initially composed of planetesimal fragments that had not yet clumped together into planets. Newly formed planets like Jupiter, Saturn, and Neptune had unstable orbits, primarily because they exerted gravitational force on each other.

At that stage, Jupiter and Neptune must have wandered in and out of the asteroid belt, grabbing the lion’s share of planetesimals. About a million of them were captured at Jupiter’s L4 and L5 Lagrangian points.

Of the relatively few that remained in the asteroid belt, many clumped together by gravity to form an intermediate stage of planetary development called planetoids. At 588 miles wide, the dwarf planet Ceres is a prime example.

However, the intrusion of Neptune and Jupiter so depleted the asteroid belt that the formation of a full-fledged planet like Earth or Mars could not occur.

Meanwhile, the gravitational pull of nearby Jupiter disturbed the orbits of many of the planetoids. They collided with each other and other smaller bodies and shattered into myriad fragments.

Those fragments constitute much of the asteroid belt today. The original planetesimals are long gone, but caches remain at the Lagrangian points of Neptune and Jupiter.

We must go to those places if we want to understand the solar system’s origins. But Neptune is too far away, given current rocket technology, to make multiple visits.

The Lagrangian points along Jupiter’s orbit are the most accessible places to find planetesimals in quantity. They have been trapped there, more or less unchanged, for billions of years. It’s about time somebody paid them a visit.

By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.