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Rosetta rides the night train, drops within 12.4 miles of comet

For the past couple weeks Rosetta has been orbiting Comet 67P Churyumov-Gerasimenko from a distance of 18.6 miles (30 km) and returning crisp, detailed images like the one above using only its navigation camera. Tomorrow the spacecraft will...

For the past couple weeks Rosetta has been orbiting Comet 67P Churyumov-Gerasimenko from a distance of 18.6 miles (30 km) and returning crisp, detailed images like the one above using only its navigation camera. Tomorrow the spacecraft will drop even lower to 12.4 miles (20 km) as it  alters its orbit for a look at the comet's nightside.

Through a series of thruster firings, Rosetta has been changing the angle of its orbit above the comet's day-night terminator to capture views of areas during their "morning hours" and later during local afternoon.

Maneuvers began on September 24th to direct the probe around the comet's backside to study the temperature properties of the shadowed regions at the same time lowering its orbit to 12.4 miles (20 km). The spacecraft itself will remain in sunlight - like the space station crossing the sky at night - but the "ground" below will be in darkness. Rosetta has been descending since Wednesday and will reach the 12.4 mile mark as its orbit is "circularized"tomorrow, September 29. After a week at 12.4 miles, a decision will be made as to whether it's safe to proceed to just 6.2 miles (10 km).

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Assuming Rosetta continues to take photos during its nightside excursion, we should soon have much better pictures of the jets of vapor and dust it's been geysering into space. Up till now, the craft has only photographed the comet with the sun at its back, a perspective that makes for brilliant images of the 2.5-mile-wide nucleus. The much fainter jets, made of fine particles, barely show. But seen from behind with the nucleus blocking the sun, the jets should really pop.

The same principle's at work when you can see your breath on a cold winter day - the cloud of vapor (condensing water) is far brighter with the sun shining behind it than coming over your shoulder.

Looking more closely at the photos, we see lots of depressions. Most of them likely formed through surface collapse as buried ice vaporized in the solar heat or when gas trapped beneath the surface was expelled, leaving the crust unsupported.

There may also be impact craters too, though that scenario's less likely given that any older impacts, like those seen on just about every planet and moon in the solar system, would have been erased from comet activity over the aeons.

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