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.

Springtime at Mars’ south pole

ESA’s Mars Express celebrates eight years in space with a new view of ice in the southern polar region of Mars. The poles are closely linked to the planet’s climate and constantly change with the seasons. Their study is an important scientific objective of the mission.

	Ulyxis Rupes in context

About two-thirds of the image is covered by part of the southern polar ice cap and other scattered ice deposits, near a feature known as Ulyxis Rupes. The left side of the image is dominated by the polar cap’s ice shield, which is covered by dark dusty material that hides the bright ices beneath.

At this location, further than 1000 km from the south pole itself, the ice is relatively thin: radar data indicate it is only about 500 m thick, whereas near the south pole it can reach more than 3.7 km.

Features near Ulyxis Rupes

However, on the north-facing cliffs the layers of ice and dust are discernible. These form part of the polar, layered deposits. The cliffs are often curved, which could mean that they are shaped by underlying impact craters.

The elevation of this region decreases markedly from south to north, dropping in steps by about 1500 m in total from left to right across the image.

	Elevation of Ulyxis Rupes

Just northward of the ice shield, about halfway across the image, there are large ice deposits that are heavily covered by overlying material blown into long dunes by the prevailing winds in this region. The orientation of the dunes suggests the wind must come predominantly from the northwest.

Ulyxis Rupes in high resolution

With increasing distance from the south pole, ice becomes confined to larger impact craters, such as the one in the top right of the image. These provide the best shelter. The ice itself is slightly offset towards the north because, with the sunlight coming from the north, the southern walls of the crater tend to warm up more, causing the ice to melt.

Ulyxis Rupes is a large cliff and is the only named feature in this image (‘rupes’ is the Latin term for cliff). With a length of 390 km and a height of up to 1 km, it is just visible at the top right of this image where it intrudes on the immediate left of the crater there.

	Ulyxis Rupes in perspective

Puzzling parallel structures in the martian dust can be seen in the bottom right quarter of the image. Although their origin is uncertain, it is possible that they are the result of underlying ice deposits, permanently frozen because they are protected by overlying dust and rocks.

The image was taken in January 2011, during the southern spring on Mars. At the moment it is summer there, but when the southern winter begins in March 2012, the temperatures will drop again and more ice will accumulate. Mars Express will be waiting.

Ulyxis Rupes in 3D

June 15th's Deep, Eastern Lunar Eclipse

During June 15th's total lunar eclipse, the lunar disk will spend
100 minutes completely inside Earth's umbral shadow.

We're in the midst of an interesting eclipse trifecta. Partial solar eclipses occur during the New Moons on June 1st and July 1st, which are sandwiched around a total lunar eclipse during the Full Moon on June 15th.

Unfortunately, none of these are really visible from North America — which is too bad, because this lunar eclipse should be a doozy. Those of you in Europe and elsewhere in the Eastern Hemisphere are in for a real treat.

The Moon will plunge deeply into Earth's shadow, passing almost directly through its center. Consequently, totality lasts a whopping 100 minutes — the longest umbral immersion since July 2000, and nearly 40 minutes longer than the well-observed lunar eclipse last December 21st.

The lunar disk first encroaches into the penumbra, Earth's partial shadow, at 17:25 Universal Time, but don't expect to see any dusky shading along its easternmost edge for at least 30 minutes afterward. The first nibble from Earth's deep umbra comes at 18:23 UT, with the black bite taking a full hour to creep steadily across the disk.

Totality begins at 19:22:30 UT and ends at 21:02:42 UT, with the moment of greatest eclipse at 20:12:37 UT.

The long duration of this event is due in part to Earth being near aphelion in its orbit and the Moon being near perigee in its orbit. Since the lunar disk passes 5.3 arcminutes north of the umbra's center, observers might see the northern limb appear a little brighter than the southern limb.

In any case, this has the makings of a very dark eclipse. Observers rate totality's darkness using a five-point scale developed by French astronomer André Danjon, ranging from L = 0 (nearly invisible) to L = 5 (bright copper-red or orange disk). (Click here to learn more about the Danjon scale and other eclipse tips.)

David Dunham points out that the darkened lunar disk makes it very easy to watch the Moon cover up stars along its path. "Especially good will be the occultation of 4.8-magnitude 51 Ophiuchi," he notes. "Perhaps a naked-eye event, it will be spectacular as seen with binoculars or any small telescope." Click here to get a full listing of stars to be occulted during the eclipse.

The total lunar eclipse on June 15, 2011, favors observers in the Eastern Hemisphere. Click on the map for a larger version.

As the map at right shows, the ringside seats for Wednesday's event will be centered around 50°E in longitude. This favors eastern Africa (near moonrise), Asia, and western Australia (near moonset). Those of you in Europe will see most everything, though the early stages occur before the Moon rises. Only northern Scotland and Scandinavia miss out — but, then again, they were favored for the partial solar eclipse on June 1st.

Those of us stuck in North America won't see any of this eclipse by eyeball, but we'll be able to watch it vicariously thanks to the following live webcasts:

• AstroNation.net from Hessen, Germany
  
  
• Bareket Observatory from Maccabim, Israel
  
  
• ECA (Eclipse Chasers Athaenium) from Delhi, India
  

Later this year, on December 10th, most North Americans will have a chance to see a total lunar eclipse. Let's hope for clear weather!

Do Planets Outnumber Stars ?

An artist's portrayal of a rogue planet drifting alone through interstellar space, lit only by starlight.

Ask an astronomer how many stars populate the Milky Way, and the usual answer will be 200 to 400 billion. It's not that all those suns have actually been counted; instead, it's a statistical guesstimate based on the census in our immediate interstellar surroundings.

But a new study, published in today's issue of Nature, suggests that a complete census of "big bodies" drifting loose in our galaxy might actually total nearly one trillion — because Jupiter-mass "planets" in interstellar space might well outnumber the stars themselves.

The evidence for this sudden glut of planet-mass objects results from a dedicated search by two teams of observers: the Microlensing Observations in Astrophysics (MOA) Collaboration and the Optical Gravitational Lensing Experiment (OGLE) Collaboration.

In 2006-07, the MOA and OGLE teams used telescopes in New Zealand and Chile, respectively, to monitor the brightnesses of 50 million stars located in the huge stellar bulge surrounding the Milky Way's center. Instruments recorded the brightness of each star at least once per hour. After boiling down all that data, the teams found that 474 stars had briefly surged in brightness in a way that indicated gravitational lensing of their light by unseen foreground objects passing nearly front of them. During these incidental syzygies, the gravity of the foreground object bends and concentrates the light from the background star — an event known as microlensing.

Microlensing searches aren't new: they've long been used to search for massive dim or dark objects in the galaxy. But the MOA and OGLE teams found that 10 of these little surges lasted less than two days — too short to be caused by stars but just right for Jupiter-mass objects. Based on these statistics, the teams estimate that big planets must be far more common than believed and in fact must outnumber all the Milky Way's normal stars by about two to one.

Surprisingly, during these 10 brief events there were no corresponding lensing surges to betray the presence of nearby stars. So the observers conclude that these "Jupiters" must either be at least 10 astronomical units from their host stars (at least Saturn's distance from the Sun), or they are orphans drifting freely across interstellar space. They're more likely to be free-floaters, because previous direct-imaging searches found that giant planets rarely exist in very wide orbits.

"The implications of this discovery are profound," notes lensing specialist Joachim Wambsganss (Heidelberg University) in an accompanying Nature perspective.

Theorists are chuckling, "We told you so!" They've argued for years that the galaxy should teem with unbound planets. Some have proposed that objects with masses almost as low as Jupiter's form the way normal stars do, directly from collapsing clouds of gas and dust. Think of these as undersized brown dwarfs. Others point out that the chaos that seems to prevail in many just-formed solar systems must cause many close encounters among planets that yield "winners" (those that remain in orbit) and "losers" (those that get flung out of the system entirely).

Taken at face value, the MOA-OGLE statistics imply that most of the loose planet-mass objects aren't just low-mass stellar wannabes — there are too many of them. Instead, the researchers believe they're finding bodies that have been ejected from unstable planetary families — and, by extension, that planetary systems should be the norm, not the exception, for the Milky Way's hundreds of billions of stars.

This also implies that early chaos in planetary systems is common. Exoplanet researchers had already concluded that this is the case from the large number of explanets that have been left in highly eccentric orbits, which they could not have formed with.

The Four-Planet Dance of 2011

If you can find a spot with a completely unobstructed eastern horizon, you can watch an extraordinary sky show from late April 2011 through the end of May. Every morning just before sunrise, four planets combine to form fascinating and ever-changing patterns. This is the tightest grouping of bright planets that has occurred yet in the 21st century.

If you live in the Southern Hemisphere, you can watch the whole show with your unaided eyes, but you will need binoculars to appreciate it properly from mid-northern latitudes. Go outside 45 minutes before sunrise and scan the eastern horizon until you find a planet. You're sure to spot either Venus or Jupiter first, because these are by far the brightest of the four.

Venus appears at just about the same spot every morning in May — just 2° or 3° above the horizon 45 minutes before sunrise for observers at mid-northern latitudes, and rising 3° higher each 15 minutes after that. If you pay attention to its location, you can probably continue to see it without optical aid long after the Sun rises.

Jupiter is very low at the beginning of May, but it passes Venus on May 11th and ends the month more than 12° above the horizon 45 minutes before sunrise. So by mid-May, you're likely to spot Jupiter before Venus despite the fact that it's less than one-quarter as bright.

Mercury is the 3rd-brightest planet in the grouping, but it's five to ten times fainter than Jupiter, and quite low in the sky. So you're likely to need binoculars to spot it. It tracks Venus's motion, staying a few degrees to the lower left of the brighter planet throughout this period.

Mars is quite faint, just one-hundredth as bright as Venus. It starts May very low in the sky, but catches up with the Venus-Mercury pairing around mid-month.

A thin crescent Moon joins the show from April 29th to May 2nd and again on May 29-31.