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Comet's fizzing all over during closest approach to the Sun

Paul Sutherland, Feature writer
Aug 14, 2015, 20:01 UTC

Sen—The European Space Agency (ESA) has shared fresh images from Rosetta of Comet 67P/Churyumov-Gerasimenko to mark its closest approach to the Sun, an event known as perihelion.

A sequence of images was taken by the OSIRIS narrow-angle camera on August 12, just hours before perihelion, which show the comet fizzing with activity as it rotates. Another bright jet was observed during the sequence, just days after one which we reported earlier.

ESA celebrated perihelion with a public Google Hangout, bringing together several leading figures involved with the mission, and ably hosted by ESA space science editor Emily Baldwin. 

They told how the level of activity on 67P is now so high that Rosetta is having to keep at a safe distance where dust particles do not confuse its navigation system as it tries to lock onto the stars. Sylvain Lodiot, the spacecraft operations manager, said his team decides twice a week how best to steer Rosetta according to the prevailing conditions.

Though perihelion passed at 02:03 UTC on August 13, 67P is expected to keep fizzing for some weeks as freshly warmed comets tend to be at their most active after perihelion. Dr Holger Sierks, principal investigator for OSIRIS, speaking from the International Astronomical Union congress in Hawaii, said the amount of water being produced by the comet had increased by a factor of a thousand since August 2014. The equivalent of ten Olympic swimming pools of water a day are currently being released, carrying lots of dust. 

An animation made just hours before perihelion show widespread jetting across its surface. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Asked about a fracture that Rosetta observed on a close approach in the northern neck region of the comet, and measuring up to 2.4 meters wide, Dr Sierks said that region was now in shadow and Rosetta would not get to observe it again until spring next year. He said the chances of the comet breaking up were good, but believed there was only a 20 per cent possibility on the current orbit.

Dr Sierks also showed a remarkable sequence from July 30 where OSIRIS imaged a boulder-sized object being ejected fast enough from the nucleus to reach escape velocity and escape into space. The Rosetta observations did not allow the boulder’s distance to be accurately determined, so the object could be anything between one meter and 50 meters wide.

Sen asked Dr Sierks what other physical changes had been observed in the comet and whether he had been surprised by any seen.

He replied: “That’s actually a very good question, and it’s a fun question. We were surprised that when we arrived at the comet, its spin rate was changed (increased) by 20 minutes compared to its last orbit. That’s not uncommon—spin rate changes have also been observed in other comets—but 20 minutes was really at the upper end, the highest value ever observed.

A sequence of images showing a boulder being ejected from the comet's surface. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

“Now that we’ve stayed with the comet for quite a while, we saw the spin period slowing down again. It slowed down by 100 seconds—a good minute and a half up to May this year—and then it spun up again. So today, the latest info we have got, data from the flight dynamics team, shows that it has spun up by more than seven minutes again compared to the 12.4 hours (rotation rate) that we found on arriving in April last year.

“The nucleus is a body about 4 km in size and the spin rate is changing a whole lot.”

Meanwhile, the Rosetta team is still hopeful that contact can be renewed with the mission’s lander Philae. Operations manager Barbara Cozzoni, of the German Aerospace Centre (DLR), said that one of Philae’s receivers was broken and its two transmitters were not behaving as expected. The probe was designed so that if it did not manage to switch on a transmitter it would toggle between the two until one did switch on.

During the last communication, on July 9, the team found that, toggling between the two transmitters, it managed to switch one on 35 minutes after receiving Rosetta’s command. This meant they were unable to instruct Philae to carry out new science, but only to repeat what it did in November by sending commands to reactivate that sequence.

You can watch a replay of the Google Hangout below.