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Marvelous view ... and a mystery

The OSIRIS camera on the European Space Agency's Rosetta spacecraft captured

this view of Earth from 393,000 miles (633,000 kilometers) away on Thursday.

Europe’s Rosetta spacecraft is making its final flyby past Earth on its way to an asteroid and a comet – a close encounter that should yield beautiful pictures of our home planet, and perhaps the answer to a cosmic mystery as well.

Rosetta was launched five years ago and has already made two gravitational flybys past Earth, plus one past Mars. Friday's flyby represents the final boost, slingshotting the probe past the asteroid Lutetia for a quick look next year, and then pushing it along to the main event at Comet 67P/Churyumov-Gerasimenko in 2014.

When Rosetta arrives at its destination, it will send a small lander down to the comet's 2.4-mile-wide (4-kilometer-wide) icy nucleus and spend two years in orbit, studying Churyumov-Gerasimenko as it approaches the sun. Rosetta's 11 scientific instruments will record how the comet is transformed by the sun's warmth.

Time for more marvels

Rosetta has been snapping marvelous pictures of the sights it has encountered over the past five years, ranging from our round blue planet, the moon and a 3-D Mars to the asteroid Steins, a diamond-shaped space rock topped by a monster crater.

The current flyby has yielded yet another stunner: a view of a crescent Earth that highlights a slim slice of Antarctica and southern oceans, seen from 393,000 miles (633,000 kilometers) out. You can expect more pictures to be posted to the European Space Agency's Web site and the Rosetta blog as the encounter proceeds.

The climax comes around 2:45 a.m. ET (8:45 a.m. CET), when the spacecraft zooms past Earth at an altitude of 1,541 miles (2,481 kilometers), going 28,000 mph (45,000 kilometers per hour). Those are just rough estimates, of course. Mission controllers can monitor Rosetta's speed and location to an accuracy of millimeters. And that's where the mystery enters the picture.

Time for a mystery

For almost two decades, scientists have noticed slight anomalies in the effects of gravitational flybys on spacecraft speed. For example, when NASA's Galileo spacecraft swung past Earth in December 1990, controllers found that the probe picked up an extra 3.9 millimeters per second of speed over what was predicted by their calculations.

The NEAR Shoemaker asteroid probe got an extra boost of 13 millimeters per second during its own Earth flyby in 1998. But there were no anomalies noted outside the range of uncertainty during swingbys involving Cassini probe to Saturn and the Messenger probe to Mercury.

The discrepancies have been compared to the "Pioneer anomaly," another longstanding case in which the navigational numbers (this time involving the Pioneer 11 and 12 spacecraft) just didn't add up.

Rosetta itself registered an anomalous speed-up amounting to an extra 1.8 millimeters per second during its first Earth flyby in 2005 - but performed precisely as predicted during the second flyby in 2007. No one knows what will happen this time.

"It's a mystery as to what is happening with these gravity events," Trevor Morley, the Rosetta mission's lead flight dynamics specialist at ESA's European Space Operations Center, said in an ESA report on the anomalies. "Some studies have looked for answers in new interpretations of current physics. If this proves correct, it would be absolutely groundbreaking news."

The possible causes range from near-Earth tidal effects, atmospheric drag and radiation pressure to exotic concepts such as dark matter, dark energy or unexpected twists in the theory of relativity. One study suggests that relativistic time dilation would account for the discrepancies.  

Highly precise data will be recorded during the Rosetta flyby to determine whether the spacecraft speeds up or slows down more or less than predicted by the various theories. Ground stations in Australia, California and the Canary Islands are involved in the investigation.

"We are using as many ground stations as are available to maximize the amount of swingby data we record," Morley said. "The more data we get, the better the chance that we may eventually come up with an answer."

Update for 4:19 p.m. ET Nov. 13: The flyby went well, by all accounts, but it will take a while to analyze the precise velocity readings and determine whether or not Rosetta showed any of the mysterious anomalies this time around. The probe captured several more nice pictures of Earth. Check out the Rosetta blog for shots like this one of a cloud-covered North America, and these pictures of Earth's night side.

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