Rossetta's comet is spinning down

Comet 67P as observed by Rosetta is very gradually spinning down. It takes 12.4 hours to complete one rotation but scientists have observed that this is extending by one second a day, in other words the comet is getting more active. In September last year, it was determined that the rotation period was extending by 33 milliseconds per day. Now withe comet moving closer to the sun and expelling a much larger amount of gas, this spin-down effect is increasing.

As the comet warms up its ice vaporises and throws jets of gas and dust. These gases expelled are acting like thrusters for the comet by slowing it down as said by flight director Andrea Accomazzo, who has been describing how his team has learnt to fly Rosetta around the comet with remarkable precision, at the Royal Aeronautical Society in London this week.

But how do navigators understand the comet's rotation?
By using a system of landmarks on the 67P being able to see and measure its rotation and trajectory, and sending this information to a model that helps plan the trajectory of the satellite.

During December and January, Rosetta was able to move within 30 km of Comet 67P and go into a gravitationally bound orbit. This is no longer possible and Rosetta has retreated. According to the ESA flight director "The aerodynamic effects are getting more and more important. The jets are getting stronger and stronger. To give you an idea, these gases come out of the comet for a few km and are moving at 800 metres per second. We definitely have to take this into account. We are a big spacecraft with 64 square meters of solar panels. We are like a big sail."

However ESA's is currently planning some closer flybys to try and discover Rosetta's lost landing probe, Philae.