CIS: Asteroid Found in Gravitational “Dead Zone”

[bystander]

Asteroid Found in Gravitational “Dead Zone”
Carnegie Institution for Science | 12 Aug 2010

There are places in space where the gravitational tug between a planet and the Sun balance out, allowing other smaller bodies to remain stable. These places are called Lagrangian points. So-called Trojan asteroids have been found in some of these stable spots near Jupiter and Neptune. Trojans share their planet’s orbit and help astronomers understand how the planets formed and how the solar system evolved. Now Scott Sheppard at the Carnegie Institution’s Department of Terrestrial Magnetism and Chad Trujillo have discovered the first Trojan asteroid, 2008 LC18, in a difficult-to-detect stability region at Neptune, called the Lagrangian L5 point. They used the discovery to estimate the asteroid population there and find that it is similar to the asteroid population at Neptune’s L4 point. The research is published in the August 12, 2010, online issue of Science Express.

Sheppard explained: “The L4 and L5 Neptune Trojan stability regions lie about 60 degrees ahead of and behind the planet, respectively. Unlike the other three Lagrangian points, these two areas are particularly stable, so dust and other objects tend to collect there. We found 3 of the 6 known Neptune Trojans in the L4 region in the last several years, but L5 is very difficult to observe because the line-of-sight of the region is near the bright center of our galaxy.”

The scientists devised a unique observing strategy. Using images from the digitized all-sky survey they identified places in the stability regions where dust clouds in our galaxy blocked out the background starlight from the galaxy’s plane, providing an observational window to the foreground asteroids. They discovered the L5 Neptune Trojan using the 8.2-meter Japanese Subaru telescope in Hawaii and determined its orbit with Carnegie’s 6.5-meter Magellan telescopes at Las Campanas, Chile.

“We estimate that the new Neptune Trojan has a diameter of about 100 kilometers and that there are about 150 Neptune Trojans of similar size at L5,” Sheppard said. “It matches the population estimates for the L4 Neptune stability region. This makes the Neptune Trojans in the 100-kilometer range more numerous than those bodies in the main asteroid belt between Mars and Jupiter. There are fewer Neptune Trojans known simply because they are very faint since they are so far from the Earth and Sun.”

The L5 Trojan has an orbit that is very tilted to the plane of the solar system, just like several in L4. This suggests they were captured into these stable regions during the very early solar system when Neptune was moving on a much different orbit than it is now. Capture was either through a slow, smooth planetary migration process or as the giant planets settled into their orbits, their gravitational attraction could have caught and “frozen” asteroids into these spots. The solar system was likely a much more chaotic place during that time with many bodies stirred up onto unusual orbits.

The region of space surveyed also included a volume through which the New Horizons spacecraft will pass after its encounter with Pluto in 2015.The work was funded in part by the New Horizon’s spacecraft mission to Pluto.

Detection of a Trailing (L5) Neptune Trojan

• Science Express (12 Aug 2010) DOI: 10.1126/science.1189666

Discovery of First Trojan Asteroid in a Stable Zone near Neptune
NAOJ Subaru Telescope | 12 Aug 2010

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New Horizons trajectory

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Asteroid Found in Gravitational “Dead Zone”

The region of space surveyed also included a volume through which the New Horizons spacecraft will pass after its encounter with Pluto in 2015.The work was funded in part by the New Horizon’s spacecraft mission to Pluto.

<<On August 12th, 2010, the Minor Planet Center announced the discover of the first known L5 Neptune trojan 2008 LC18. Neptune's trailing L5 region is very difficult to observe because it is along the line-of-sight to the the center of our galaxy, an area of the sky crowded with stars. It may be possible for the New Horizons spacecraft to investigate any L5 Neptune trojans discovered before it passes through this region of space in 2014 en route to Pluto.>>

<<New Horizons trajectory to Pluto passes near Neptune's trailing Lagrange point ("L5") which may host asteroids. If any asteroids are found to be close enough to be studied, observations may be planned. However, spacecraft passage near this Lagrange point comes shortly before the Pluto encounter. Depending on where the asteroid is along the spacecraft trajectory, New Horizons may not have significant downlink bandwidth, and thus free memory, for trojan asteroid data.>>

<<L4 and L5 are sometimes called triangular Lagrange points or Trojan points. The name Trojan points comes from the Trojan asteroids at the Sun–Jupiter L4 and L5 points, which themselves are named after characters from Homer's Iliad (the legendary siege of Troy). Asteroids at the L4 point, which leads Jupiter, are referred to as the 'Greek camp', while at the L5 point they are referred to as the 'Trojan camp'. These asteroids are (largely) named after characters from the respective sides of the Trojan War.>>

* The Sun–Earth L4 and L5 points lie 60° ahead of and 60° behind the Earth as it orbits the Sun. They contain interplanetary dust.

* The Earth–Moon L4 and L5 points lie 60° ahead of and 60° behind the Moon as it orbits the Earth. They may contain interplanetary dust in what is called Kordylewski clouds.

* The Sun–Jupiter L4 and L5 points are occupied by the Trojan asteroids.

* The Sun-Neptune L4 and L5 points have Trojan objects.

* Saturn's moon Tethys has two much smaller satellites at its L4 and L5 points named Telesto and Calypso, respectively.

* Saturn's moon Dione has smaller moons Helene and Polydeuces at its L4 and L5 points, respectively.

* One version of the giant impact hypothesis suggests that an object named Theia formed at the Sun–Earth L4 or L5 points and crashed into the Earth after its orbit destabilized, forming the moon.>>

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The green arrow shows the 2008 LC18 Trojan object.

The other bright objects are stars in the Milky Way.
Credit: Scott Sheppard

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2008 LC18, the first Trojan asteroid discovered in Neptune's L5 point
Aug. 13, 2010 | 08:52 PDT | 15:52 UTC

Discovery images of Neptune Trojan 2008 LC18 Credit: Carnegie Institution of Washington / National Astronomical Observatory of Japan

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<<Congratulations to Scott Sheppard and Chad Trujillo for identifying the first known L5 Trojan asteroid of Neptune! This story is not just interesting because it is a first-of-its-kind discovery, but because of the tricky way that the astronomers went about searching for it, and because of the collateral benefits that their search will have for the New Horizons mission.

Yikes! It seems ridiculous to go searching for faint bodies in this region of space. Another thing that doesn't help is that Neptune's L5 region is really huge -- about 1500 square degrees on the sky -- because bodies in stable orbits at L5 may librate up to 30 degrees from the L5 point over time. But Sheppard and Trujillo came up with a tricky plan for locating sparse objects against such a dense star field: they took advantage of naturally occurring dust clouds that obscure part of that star field. The dust clouds are beyond the solar system, so any moving bodies would appear in the foreground, in front of the obscuring dust clouds.

So Sheppard and Trujillo used images from a wide-field sky survey to locate dust clouds, finding 19 square degrees of them in the L5 region. They finally located a single asteroid. Next comes the fun of the statistics of small numbers. If there is one L5 Trojan in 19 square degrees, how many should there be total? They estimate that there should be about 150 -- give 100 or take 200 -- Neptune L5 Trojans brighter than 24th magnitude (which corresponds to a diameter of 80 kilometers). Other statistics of small numbers based on the six known L4 Trojans produced an estimate of 250 give or take 100 L4 Trojans. Within the (admittedly quite large) uncertainties of these small-number statistics, the two numbers match. Which is good news!

Why do we care about Neptune Trojans? First of all, their existence and the characteristics of their orbits (especially, how inclined the orbits are) provide important conditions on models for how the solar system formed -- especially now, when most people seem to agree that the outer planets did not form where they are now, that they migrated into their current positions. Such migration would have brought the giant planets into gravitational interaction with smaller bodies lying in the way; the smaller bodies' orbits are profoundly influenced by the motion, the speed of that motion, and the final positions of the planets. (That's why, for instance, Pluto orbits the Sun twice for every three times Neptune makes the trip; it was shoved into such a resonant orbit by Neptune's migration, much as the Neptune Trojans were picked up and carried into those positions as Neptune moved outward.)>>