The days on Comet 67P are getting longer, but it has nothing to do with summer approaching as the nights are lengthening too. It turns out the comet being visited by the Rosetta spacecraft is spinning more slowly. The effect remains small, but it is increasing, and the fact that it is happening tells astronomers something they didn't know before about cometary dynamics.
Like other members of the solar system, comets spin around an axis. Most of those we have calculated have periods shorter than that of Earth, although Halley seems to be an exception. However, these measurements have had to be based on regular changes in brightness as measured from Earth, creating a fair degree of uncertainty.
Rosetta, on the other hand, has been able to track the timing of distinctive objects on the surface of 67P/Churyumove-Gerasimenko, establishing the period at 12.4 hours.
In September 2014, ESA researchers noticed that the period was slowing down at a rate of 33 milliseconds per day, despite having been longer the last time it approached the sun. In a talk at the Royal Aeronautical Society, flight director Andrea Accomazzo announced that this has now increased to almost a second a day.
The BBC reports Accomazzo saying, "OK, it's not going to slow down completely - but this gives you an order of magnitude for the accuracy we're now achieving with the navigation of the spacecraft around the comet.”
The Earth's spin is also slowing down, but for a very different reason. Our changes, which necessitate the occasional leap second, result from interactions with the moon. 67P's only satellite is Rosetta itself, whose mass is too small to make this difference.
Instead, the ESA attribute the changes to jets of gas coming off the comet as its ice sublimes. "The gas jets coming out of the comet - they are acting like thrusters and are slowing down the comet," Accomazzo said. This month Rosetta detected molecular nitrogen in the gas being released, a long sought-after goal for cometary research.
One consequence of these gasses is that Rosetta has had to back off into a more distant orbit, shifting from a distance of 30 kilometers (19 miles) to stretched out paths reaching 60 kilometers (37 miles) from the comet at their closest point. The jets are getting stronger and stronger," Accomazzo told BBC News. "To give you an idea, these gases come out of the comet for a few kilometres and are moving at 800 m/s. We definitely have to take this into account. We are a big spacecraft with 64 square metres of solar panels. We're like a big sail."
However, Accomazzo also revealed that the ESA is planning to make some more risky passes close to the comet, including one in July looking for Philae that could get within 20 kilometers of the surface (12 miles). While the longer day will not make a significant difference to Philae's chances of harvesting enough sunlight to recharge its battery, there is still a possibility that extra sunlight could start it working again, a prospect he rates as “50-50”.