December 4th: Comet Day

It's Comet Day. Although no explanation is given as to why this should be, there is mention of a hen in Rome which laid an egg on this date in 1680, which bore an image of a comet which was not seen by humans until December 16. For comet day, here are some facts about the recent European Space Agency mission to land a spacecraft on a comet. You'll know that the landing craft Philae travelled on a spacecraft called Rosetta, and landed on the comet in November, but here are 10 things you might not know.

Picture: European Space Agency

1. It took 10 years to get there. An Ariane 5G+ rocket carrying the Rosetta spacecraft and Philae lander launched from French Guiana on 2 March 2004.
2. The comet is called Comet 67P/Churyumov–Gerasimenko.
3. The original plan was to rendezvous with a different comet, 46P/Wirtanen. However, there was a problem with the rocket launcher, which meant ESA lost the launch window for reaching that one.
4. The spacecraft was named after the Rosetta Stone, which helped decipher Egyptian hieroglyphics. The Philae obelisk also had inscriptions in more than one language, and the lander is named for that.
5. ESA announced in September that the landing site would be "Site J". They held a competition to give it a proper name, and duly named it Agilkia after Agilkia Island, an island in the River Nile and the present site of an Ancient Egyptian temple complex of Philae.
6. Just before detachment, one of Philae's thrusters was found to be faulty. However, as it wasn't going to be possible to repair it, the mission proceeded anyway.
7. Philae actually landed on the comet three times, having bounced twice. The first bounce lasted two hours, and the second lasted seven minutes. So it isn't at Agilkia, but at an unknown site.
8. ESA had hoped that Philae's battery life would be 60 hours, but that depended on it absorbing sunlight at Agilkia and recharging. It ended up in a place which doesn't get so much sunlight and so the battery died on 15 November - but not before a load of data was transmitted back.
9. It is possible that in August next year, when the comet is closer to the Sun, Philae may reawaken as its solar panels get more light.
10. Philae has its own Twitter account, @Philae2014.