New day for comets

Photo of Comet 67P captured by Rosetta’s OSIRIS narrow-angle camera last August (European Space Agency)

Photo of Comet 67P captured by Rosetta’s OSIRIS narrow-angle camera last August (European Space Agency)

We’re suddenly very aware of comets. The spectacular Rosetta mission of the European Space Agency, launched way back in 2004, used three slingshot gravity assists from near-Earth fly-bys before it finally caught up to comet 67P this August while both were moving at 34,000 miles per hour. Orbiting the mountain-sized chunk of ice and dust, it has now dropped a lander onto its surface from a height of 14 miles.

A wild adventure! The minuscule gravity of the tiny 2.5-mile-wide comet was not enough to prevent the springy legs of the lander from making it bounce back up into space a half-mile. A thruster was supposed to fire to keep the lander glued to the surface, and two harpoons were supposed to moor it fiercely like a ship. None of those things worked. So, after bouncing skyward for two hours, it touched down again, this time sideways and in a bad place, shaded from the sunlight that its solar panels require.

We will see how much further useful science it produces. But you know what? It really doesn’t matter. The main Rosetta spacecraft still functions perfectly. From its perch just a few miles away, it will perform detailed spectral analysis of the comet’s escaping gases – cool stuff like formaldehyde and stinky hydrogen sulfide, along with copious water vapor – and its cameras will keep watching it.

Previously, we only cared about bright comets, which appear every 15 to 20 years on average. The most recent ones seen here were Comets Hale-Bopp and Hyakutake in the mid-1990s, and before that we had Comet West in 1976. Despite their sinister reputations for thousands of years, comets are both beautiful and fascinating. That brook in your neighborhood is probably made of 100 percent melted comets. Comets may even have brought the organic materials that served as life’s building blocks.

Each comet is an irregularly shaped chunk of dirty ice. They come in from a vast reservoir of comets far beyond Pluto. Their stretched-out, elongated orbits can bring them close to the Sun every few years like this one, or they may visit us only once every few thousand years, like Hale Bopp. As they near the Sun and some of their ice turns to steam, and imbedded dust and other debris get released, it all gets pushed back by the solar wind to form a million-mile-long tail. This is of course a comet’s most spectacular and distinctive feature.

But wouldn’t it be fun to have high-definition cameras at the comet itself during the months that it approaches the Sun? That’s what is finally happening. Between now and August 13, when Comet 67P reaches its perihelion, fissures will open, geysers of steam will erupt and the weird irregular surface will be transformed. Imagine being in a city as manhole covers blow into the air all around you. These violent vents on this comet currently release a gallon of water per second, but the insane activity will increase to 100 gallons per second in a few months. And we will be there watching it happen.

It’s the most exciting space probe since Cassini orbited Saturn ten years ago.

Want to know more? To read Bob’s previous “Night Sky” columns, visit our Almanac Weekly website at



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