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Trojan asteroid 2010 TK7

Trojan asteroid 2010 TK7 (circled in green) orbits the Sun ahead of the Earth. This single frame was taken by NASA's Wide-field Infrared Survey Explorer, or WISE. The majority of the other dots are stars or galaxies far beyond our Solar System.

  • Trojan asteroid found orbiting ahead of Earth
  • 80 million kilometres from our planet
  • No danger of it colliding with us

ASTRONOMERS STUDYING OBSERVATIONS taken by NASA’s Wide-field Infrared Survey Explorer (WISE) mission have discovered the first known ‘Trojan’ asteroid orbiting the Sun along with Earth.

Trojans are asteroids that share an orbit with a planet near stable points in front of or behind the planet. Because they constantly lead or follow in the same orbit as the planet, they never can collide with it.

In our Solar System, Trojans also share orbits with Neptune, Mars and Jupiter. Two of Saturn’s moons even have Trojans.

Hard to see

Scientists had predicted Earth should have Trojans, but they have been difficult to find because they are relatively small and appear near the Sun from Earth’s point of view.

“These asteroids dwell mostly in the daylight, making them very hard to see,” said Martin Connors of Athabasca University in Canada, lead author of a new paper on the discovery in the July 28 issue of the journal Nature.

“But we finally found one, because the object has an unusual orbit that takes it farther away from the Sun than what is typical for Trojans,” added Connors. “WISE was a game-changer, giving us a point of view difficult to have at Earth’s surface.”

Artist's impression of the WISE space telescope

Artist's impression of the WISE space telescope

Thousands of asteroids found

The WISE telescope scanned the entire sky in infrared light from January 2010 to February 2011.

Connors and his team began their search for an Earth Trojan using data from NEOWISE, an extension to the WISE mission that focused in part on near-Earth objects, or NEOs, such as asteroids and comets.

NEOs are bodies that pass within 45 million kilometres of Earth’s path around the Sun.

The NEOWISE project saw more than 155,000 asteroids in the main belt between Mars and Jupiter, and more than 500 NEOs, including 132 that were previously unknown.

Unusual orbit

The team’s hunt resulted in two Trojan candidates. One, called 2010 TK7, was confirmed as an Earth Trojan after follow-up observations with the Canada-France-Hawaii Telescope in Hawaii.

The asteroid is roughly 300 metres in diameter. It has an unusual orbit that traces a complex motion near a stable point in the plane of Earth’s orbit, although the asteroid also moves above and below the plane.

The object is about 80 million kilometres from Earth.

The following video shows how the asteroid continually loops above and below Earth’s orbital plane, while always remaining ahead of our planet:

No danger to Earth

The asteroid’s orbit is well-defined, and for at least the next 100 years it will not come closer to Earth than 24 million kilometres.

“It’s as though Earth is playing follow the leader,” said Amy Mainzer, the principal investigator of NEOWISE at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Earth always is chasing this asteroid around.”

A handful of other asteroids also have orbits similar to Earth. Such objects could make excellent candidates for future robotic or human exploration.

Asteroid 2010 TK7 is not a good target because it travels too far above and below the plane of Earth’s orbit, which would require large amounts of fuel to reach it.

More information: WISE mission

Adapted from information issued by NASA / JPL-Caltech / UCLA / Paul Wiegert, University of Western Ontario.

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Neptune’s dead zone is not empty

Neptune and some of its moons

Neptune and some of its moons. Stable points along Neptune's orbit, called Lagrangian points, have been found to harbour asteroids that probably were captured as they wandered through.

  • Stable points where gravity from Sun and a planet balance out
  • Asteroids and other material can gather in these points
  • First asteroid has been found in one of Neptune’s stable points

There are places in space where the gravitational tug between a planet and the Sun balance out, allowing other smaller bodies to remain stable there. These places are called Lagrangian points.

Three Lagrangian points are found inline with a planet and the Sun—one beyond the planet (called L2), one between the planet and the Sun (L1), and one on other side of the Sun (L3)

There are also two stable points along the planet’s orbit, but at an angle of 60 degrees to the Sun. These are the L4 and L5 points (see the diagram at right).

Diagram showing Neptune's Lagrangian points

Neptune's Lagrangian points, where the gravitational tug of the planet and the Sun balances out. Asteroids had been found at L4, and now one has been found at L5.

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.

Scott Sheppard at the Carnegie Institution’s Department of Terrestrial Magnetism and Chad Trujillo at the Gemini Observatory have discovered the first Trojan asteroid, 2008 LC18, in the difficult-to-study L5 point at Neptune.

They used the discovery to estimate the asteroid population there and suggest that it is similar to the asteroid population at Neptune’s L4 point.

“The L4 and L5 Neptune Trojan stability regions lie about 60 degrees ahead of and behind the planet, respectively,” explains Sheppard. “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 centre of our galaxy.”

This means that it is very hard to pick out a faint asteroid from amongst the myriad stars in the background.

Silver lining to these dark clouds

So the scientists devised a unique observing strategy. They used images from a digitised all-sky survey to identify places in the stability regions where dust clouds in our galaxy blocked out the background stars, making it easier to spot the foreground asteroids.

Discovery images of the L5 trailing Neptune Trojan 2008 LC18

Discovery images of the L5 trailing Neptune Trojan 2008 LC18, taken at the Subaru telescope on June 7, 2008. The Trojan is seen moving from right to left near the centre of the image. Each image is separated by about one hour in time. The background stars are stationary.

They discovered the L5 Neptune Trojan using the 8.2-metre Japanese Subaru telescope in Hawaii and determined its orbit with Carnegie’s 6.5-metre Magellan telescopes at Las Campanas, Chile.

“We estimate that the new Neptune Trojan has a diameter of about 100 kilometres 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 100-km-wide Neptune Trojans more numerous than similar-sized bodies in the main asteroid belt between Mars and Jupiter.”

Probably captured

There are fewer Neptune Trojans known simply because they are very faint since they are so far from the Earth and Sun.”

The Trojan 2008 LC18 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.

Adapted from information issued by Carnegie Institution / NASA, ESA, E. Karkoschka (University of Arizona), and H.B. Hammel (Space Science Institute, Boulder, Colorado).

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