Earth's Traveling Companion

Earth's Trojan asteroid
Not much to look at, the asteroid 2010 TK7 nonetheless represents Earth's first Trojan asteoid. NASA's WISE spacecraft captured the view at top in October 2010 at the infrared wavelength of 4 microns. Then, in April 2011, a follow-up image was recorded by the Canada-France-Hawaii Telescope in Hawaii. M. Connors & P. Wiegert (top); C. Veillet (bottom)

There's something deeply intriguing about the interplanetary objects known as Trojan asteroids.

The great French dynamicist Joseph-Louis Lagrange predicted in 1772 that small bodies might be sharing Jupiter's orbit, in gravitationally stable sweet spots (now called Lagrange points) located ahead of and behind the planet by 60°. But it wasn't until 1906 that the first of these, 588 Achilles, was spotted. Today more than 4,800 Jupiter Trojans are known, with roughly two-thirds in the preceding "Greek camp" (L4) and a third in the trailing "Trojan camp" (L5).

Within the past two decades, astronomers have found four Trojan asteroids sharing the orbit of Mars and seven accompanying Neptune. They've looked for companions to Earth as well, but the geometry is all wrong: Earth's Trojans would spend most of their time in the daylight sky.

But the odds tipped back in observers' favor with the 2009 launch of NASA's Wide-field Infrared Survey Explorer (WISE), which recorded big swaths of sky 90° away from the Sun. Late last year, Canadian astronomers Martin Connors (Athabasca University) and Paul Wiegert (University of Western Ontario) picked through the spacecraft's scans and identified one object, designated 2010 TK7, that seemed to have an Earthlike orbit. Follow-up was needed, but that wasn't possible until this past April, when it was swept up by two observers in Hawaii.

Their suspicions confirmed, Connors, Wiegert, and Christian Veillet (Canada-France-Hawaii Telescope) report the discovery in July 28th's Nature.

This little body is tied to Earth's preceding Lagrange point. But if you're imagining it circling the Sun in lock step with our planet, think again. The orbit of 2010 TK7 is distinctly eccentric (0.19) and inclined (21°). In fact, it's never actually at L4. Instead, it vacillates widely — almost wildly — in a 400-year-long epicyclic pattern that at times brings it relatively near Earth (though still many times the Moon's distance) and at others places it on the far side of the Sun from us, near the L3 point.



Orbit of asteroid 2010 TK₇
The small asteroid designated 2010 TK7 is locked in an orbital resonance with Earth. This plot shows the range of separation between the asteroid and our planet over a 400-year period. The red line is its average orbit, which is pinned to the L4 Lagrange point that precedes Earth by 60°.

Earth's little buddy is so wide ranging that it might even occasionally spend some time resonating around the distant L3 point. In fact, gravitational influences from Jupiter make the orbit chaotic, and there's no way to know with certainty where 2010 TK7 was or will be when its orbit is tracked for more than 10,000 years.

Unfortunately, even though Earth probably has other Trojans in its entourage, WISE won't be able to see them. The spacecraft ran out of its cryogenic coolant last October, and on February 17th principal investigator Ned Wright sent a command to turn off WISE's transmitter for good. Word is that the spacecraft will remain in hibernation, awaiting a possible wake-up call in the future.

Massive Meteorite Found in China

Xinjiang metoerite
Chinese researchers measure a huge iron meteorite found in a remote mountainous region in July 2011. The oblong metallic object has an estimated mass of 25 tons or more.

As the meteorite specialist for the Beijing Planetarium, Baolin Zhang gets all kinds of unusual reports — like the dramatic (but ultimately specious) tale of a peasant woman who recently found a blue-ice "meteorite" in her yard.


Map of China's Xinjiang region
Although the exact location of the newly found meteorite has not been announced, its general location is the mountainous border region of China's Xinjiang Uyghur province.

But credible reports of a massive, oddly shaped and colored stone in the remote Altai Mountains of Xinjiang Uygur province (in northwest China) got his attention. So earlier this month he assembled a small team to check it out firsthand. The trek was cold and arduous, involving a rented jeep, borrowed horses, and even a camel to cross rugged terrain and rivers still swollen with snowmelt.

On the afternoon of July 16th, after reaching a mountainous crest 9,500 feet (2,900 m) up Zhang and his team finally spotted their objective: a large dark-brown stone jutting from the ground. It took only moments for him to realize what they'd found. "This is a huge iron meteorite," he exulted as cameras recorded the scene.

Based on the size of the oblong portion above ground, 7.5 feet (2.3 m) long and about half as wide, Zhang thinks its mass is roughly 25 tons — and it could perhaps top 30 tons. Such an enormous find would rank as one of the largest meteorites known, perhaps even surpassing China's current record-holder, the 28-ton Armanty iron, found in the same region in 1898.

Apparently the big stone's existence has been well known among locals for decades. A few scrawls of graffiti have been cut into the exterior, which also bears "saw marks" that expose the interior. As Zhang reports, "The surface was shiny silver, and I can clearly see exposed not only the iron-nickel composition but also the unique grid lines," called a Widmanstätten pattern, that are common among iron meteorites.

Interestingly, the meteorite is wedged beneath an even larger granite slab, and apparently both were dragged to their current locations long ago by glaciers. It's not yet clear when or how the massive Xinjiang stone will be excavated — though this would seem too magnificent a prize to simply leave in place. The Armanty iron is on display outside the Xinjiang Geology and Mineral Museum in Urumqi, the region's capital city.


Graffiti in Xinjiang meteorite
A dozen names, some dating to 1980, are carved into the Xinjiang meteorite.

Conceivably, the Xinjiang and Armanty meteorites are part of the same fall; tests should soon establish whether they are siblings or just happen to be enormous unrelated hunks of meteoritic metal that fell to Earth from interplanetary space.

Dawn Arrives at Vesta

After gently cruising through interplanetary space for over four years, Dawn, NASA’s asteroid probe, will enter orbit around asteroid 4 Vesta at 1 a.m. EDT on July 16th. The arrival marks the beginning of a yearlong study of the second-largest object in the belt of rocky bodies between Mars and Jupiter.

Yesterday NASA released an image of Vesta taken on July 9th, when Dawn was only 26,000 miles (42,000 km) from the asteroid. As chief engineer Marc Rayman noted earlier this month, the spacecraft's destination looks, "wrinkled, ancient, wizened, with a tremendous amount of character that bears witness to some fascinating episodes in the solar system's history."

Launched on September 27, 2007, Dawn carries high-resolution cameras, spectrometers, and other instruments to investigate the true nature of two alien worlds: Vesta and 1 Ceres. After exploring Vesta for a year, Dawn will set sail for Ceres in late 2012. Scientists believe that these two objects, which formed early in the life of the solar system, carry important clues to the formation of the terrestrial planets.

What we know about Vesta is fascinating but incomplete. Images from the Hubble Space Telescope reveal that the object was pummeled early in the history of the solar system. Radioisotope studies of meteorites presumed to be fragments of Vesta show evidence that the object accreted in a span of 5 to 15 million years, possibly in the same manner as the terrestrial planets. Then it got hot enough (due to the decay of radioisotopes in its interior) to melt at least partially . Researchers believe that Vesta might have an iron-nickel core and a metal-rich mantle beneath its rocky exterior.

Beginning tomorrow, dynamicists will use measurements of Vesta's influence on the craft's motion to determine the asteroid’s mass and deduce the distribution of mass in its interior. And as Dawn dips closer and closer to Vesta’s surface to make these measurements, the spacecraft will also send home some of the best pictures of the asteroid.

Initially the craft will hover about 9,000 miles from the surface. Then, in mid-August, Dawn’s ion propulsion system will reduce the separation to about 2,800 miles, and then to only 110 miles early next year. The resolution of the pictures will increase from about 500 m per pixel in late July to about 30 m per pixel in 2012.

Dawn’s science team has extended the scope of the mission, and is even planning to look for moons.

Observing Vesta

Fortunately for observers, the Dawn's arrival happens at a time when Vesta is readily visible through binoculars and telescopes. It now shines in Capricornus at magnitude 6.0, almost as bright as it ever gets. The only bad news is that the just-past-full Moon is also in Capricornus, brightening the sky tremendously.

For Northern Hemisphere observers, Vesta climbs to a point fairly low in the southeast by midnight, rising higher in the hours before dawn. You can pinpoint its location using our finder chart. Of course, even the most powerful backyard telescope will only reveal Vesta as a simple point of light — and the spacecraft is far too small to spot across such a huge distance.