Trojan Asteroid pile-up in the orbit of Mars

The paths traced by all seven Martian Trojans around L4 or L5 (crosses) in a frame rotating with the average angular speed of Mars (red disk) around the Sun (yellow disk).

A full revolution around the corresponding Lagrange point takes approximately 1,400 years to complete. 

The dotted circle indicates the average distance of Mars from the Sun.

The orbit of the planet Mars is host to the remains of an ancient collision that created many of its Trojan asteroids, a new study has concluded.

It paints a new picture of how these objects came to be and may even hold important lessons for deflecting asteroids on a collision course with our own planet.

Apostolos Christou

The findings were presented at the annual Meeting of the Division for Planetary Sciences of the American Astronomical Society in Denver last week, by Dr. Apostolos Christou, a Research Astronomer at the Armagh Observatory in Northern Ireland, United Kingdom.

Trojan asteroids, or "Trojans," move in orbits with the same average distance from the Sun as a planet.

This may seem as a precarious state to be in, as eventually the asteroid either hits the planets or is flung, by the planet's gravity, on an entirely different orbit.

But solar and planetary gravity combine in such a way as to create dynamical "safe havens" 60 degrees in front and behind the planet's orbital phase.

The special significance of these, as well as three other similar locations in the so-called three-body problem, was worked out by 18th century French Mathematician Joseph-Louis Lagrange.

In his honour, they are nowadays referred to as the Lagrange points.

The point leading the planet is referred to as L4; that trailing the planet as L5.

Although not all Trojans are stable for long periods of time, almost 6,000 such objects have been found at the orbit of Jupiter and about 10 at Neptune's.

Those are believed to date from the solar system's earliest times when the planets were not yet at their present orbits and the distribution of small bodies across the solar system was very different than observed today.

Of the inner planets, only Mars is known to have stable, long-lived, Trojan companions.

The first, discovered back in 1990 near L5 and now named Eureka, was later joined by two more asteroids, 1998 VF31 also at L5 and 1999 UJ7 at L4.

In the first decade of the 21st century, observations revealed them to be a few km across and compositionally diverse.

A 2005 study led by Hans Scholl of the Observatoire de Cote d'Azur (Nice, France) demonstrated that all three objects persist as Mars Trojans for the age of the solar system, putting them on a par with the Trojans of Jupiter.

In that same decade, however, no new stable Trojans were discovered, which is curious if one considers the ever-improving sky coverage and sensitivity of asteroid surveys.

Christou decided to investigate. Sifting through the Minor Planet Center database of asteroids, he flagged six additional objects as potential Martian Trojans and simulated the evolution of their orbits in the computer for one hundred million years.

He found that at least three of the new objects are also stable. He also confirmed the stability of an object originally looked at by Scholl et al., 2001 DH47, using a much better starting orbit that was available at the time.

The result: the size of the known population has now more than doubled, from 3 to 7.

Left (same as Figure 1): The paths traced by all seven Martian Trojans around L4 or L5 (crosses) in a frame rotating with the average angular speed of Mars (red disk) around the Sun (yellow disk).

A full revolution around the corresponding Lagrange point takes approximately 1,400 years to complete. The dotted circle indicates the average distance of Mars from the Sun.

Right: Detail of left panel (demarcated by the dashed rectangle) showing the motion, over 1,400 years, of the six L5 Trojans: 1998 VF31 (blue), Eureka (red), and the objects identified in the new work (amber).

Note the latter's similarity to the path of Eureka. The disks indicate the estimated relative sizes of the asteroids.