Spacecraft on schedule to catch a comet for first time ever

Next week, for the first time ever, a robotic spacecraft is expected to catch a comet and go into orbit around it. The Rosetta spacecraft, operated by the European Space Agency, is right on schedule to do the most detailed examination of a comet ever attempted.

Physicist Paul Doherty at San Francisco's Exploratorium is watching closely to help the public understand this extremely difficult mission. "We have been really surprised recently as we get close to this comet, the telephoto cameras on the spacecraft have shown us, it's not what we expected," said Doherty.

Doherty is excited about strange images taken by Rosetta of Comet 67P/Churyumov-Gerasimenko. He said, "It's shaped like a duck! It's like two comets pasted together. "

Rosetta has been speeding into space for the past 10 years, and is about to catch Comet 67P 300-million miles from the sun. The space craft is now less than a thousand miles away and slowing down to maneuver into position.

According to Doherty, "On August 6, we really approach close to the comet, we'll be within tens of miles. Maybe from San Francisco to Palo Alto, we'll be that far away."

Rosetta will start to orbit the comet providing a view from above of the entire surface with high resolution pictures. Rosetta is carrying a small landing craft that will be deployed in November, with instruments to drill into the comet's surface. Doherty is thrilled. He said, "It will give us a real chance to find out what's this comet made of."

Now that scientists know there are actually two distinct parts of the comet, figuring out where to land is trickier than expected. "We can only land on one place, so which of the two will we land on? That is a big question now."

The comet is actually only a couple of miles wide and kind of fluffy. Doherty said is about the same density as a dry kitchen sponge.

The latest data from Rosetta shows the comet surface temperature is 94-degrees below zero, which is actually warmer than expected. Doherty explained, "The ices on the comet are turning into gas, and the gas is water vapor, and it's losing right now (the equivalent of) about two one liter water bottles of water every day."

An exhibit at the Exploratorium uses chunks of dry ice dropped into water to demonstrate comet behavior. It produces dramatic spinning and swirling patterns with gas jets shooting out. That is the kind of comet activity scientists hope to see for themselves in the coming months, along with more clues to understand how our solar system formed.

Doherty said he hopes, "We can find out where did the water ice on that comet come from? How does it match the water on earth today? How is it different?"