<!-- end infobox -->
Philae ( or ) was a robotic European Space Agency lander that accompanied the Rosetta spacecraft until it separated to land on comet 67P/Churyumov–Gerasimenko, ten years and eight months after departing Earth. On 12 November 2014, Philae touched down on the comet, but it bounced when its anchoring harpoons failed to deploy and a thruster designed to hold the probe to the surface did not fire. although the lander's final, uncontrolled touchdown left it in a non-optimal location and orientation.
Despite the landing problems, the probe's instruments obtained the first images from a comet's surface. Several of the instruments on Philae made the first in-situ analysis of a comet nucleus, sending back data regarding the composition of the surface and outgassing from the subsurface. In October 2020, scientific journal Nature published an article revealing what Philae had discovered while it was operational on the surface of 67P/Churyumov–Gerasimenko.
On 15 November 2014 Philae entered safe mode, or hibernation, after its batteries ran down due to reduced sunlight and an off-nominal spacecraft orientation at the crash site. Mission controllers hoped that additional sunlight on the solar panels might be sufficient to reboot the lander. Philae communicated sporadically with Rosetta from 13 June to 9 July 2015, but contact was then lost. The lander's location was known to within a few tens of metres but it could not be seen. Its location was finally identified in photographs taken by Rosetta on 2 September 2016 as the orbiter was sent on orbits closer to the comet. The now-silent Philae was lying on its side in a deep crack in the shadow of a cliff. Knowledge of its location would help in interpretation of the images it had sent. On 30 September 2016, the Rosetta spacecraft ended its mission by crashing in the comet's Ma'at region.
The lander is named after the Philae obelisk, which bears a bilingual inscription and was used along with the Rosetta Stone to decipher Egyptian hieroglyphs. Philae was monitored and operated from DLR's Lander Control Center in Cologne, Germany, supported by the CNES's SONC in Toulouse, France.
Mission
thumb|thumbtime=10|[[:File:Landing on a Comet - The Rosetta Mission.webm|Video report by the German Aerospace Centre about Philae landing mission. (10 min, English, in 1080p HD)]]
Philae mission was to land successfully on the surface of a comet, attach itself, and transmit data about the comet's composition. The Rosetta spacecraft and Philae lander were launched on an Ariane 5G+ rocket from French Guiana on 2 March 2004, 07:17 UTC, and travelled for 3,907 days (10.7 years) to Churyumov–Gerasimenko. Unlike the Deep Impact probe, which by design struck comet Tempel 1's nucleus on 4 July 2005, Philae is not an impactor. Some of the instruments on the lander were used for the first time as autonomous systems during the Mars flyby on 25 February 2007. CIVA, one of the camera systems, returned some images while the Rosetta instruments were powered down, while ROMAP took measurements of the Martian magnetosphere. Most of the other instruments needed contact with the surface for analysis and stayed offline during the flyby. An optimistic estimate of mission length following touchdown was "four to five months".
Scientific goals
The goals of the scientific mission have been summarised as follows: <blockquote>"The scientific goals of its experiments focus on elemental, isotopic, molecular and mineralogical composition of the cometary material, the characterization of physical properties of the surface and subsurface material, the large-scale structure and the magnetic and plasma environment of the nucleus. In particular, surface and sub-surface samples will be acquired and sequentially analyzed by a suite of instruments. Measurements will be performed primarily during descent and along the first five days following touch-down. "</blockquote>
Landing and surface operations
thumb|Depiction of Philae on Churyumov-Gerasimenko
Philae remained attached to the Rosetta spacecraft after rendezvousing with Churyumov–Gerasimenko on 6 August 2014. On 15 September 2014, ESA announced "" on the smaller lobe of the comet as the lander's destination. Following an ESA public contest in October 2014, was renamed Agilkia in honour of Agilkia Island.
A series of four go/no-go checks were performed on 11–12 November 2014. One of the final tests before detachment from Rosetta showed that the lander's cold-gas thruster was not working correctly, but the "go" was given anyway, as it could not be repaired. Philae detached from Rosetta on 12 November 2014 at 08:35 UTC SCET.
Landing events
thumb|left|[[Rosetta (spacecraft)|Rosetta signal received at ESOC in Darmstadt, Germany (20 January 2014)]]
Philae landing signal was received by Earth communication stations at 16:03 UTC after a 28-minute delay. Unknown to mission scientists at that time, the lander had bounced. It began performing scientific measurements while slowly moving away from the comet and coming back down, confusing the science team. Further analysis showed that it bounced twice. The probe rebounded off the comet's surface at and rose to an altitude of approximately . After detecting the touchdown, Philae reaction wheel was automatically powered off, resulting in its momentum being transferred back into the lander. This caused the vehicle to begin rotating every 13 seconds. Philae touched down a second time at 17:25:26 UTC SCET and rebounded at . Its final location was determined initially by analysis of data from CONSERT in combination with the comet shape model based on images from the Rosetta orbiter, and later precisely by direct imaging from Rosetta. that the harpoons had not deployed, and that the thruster had not fired. The harpoon propulsion system contained 0.3 grams of nitrocellulose, which was shown by Copenhagen Suborbitals in 2013 to be unreliable in a vacuum.
Operations and communication loss
thumb|300px|Philae intended landing site Agilkia (Site J)
The primary battery was designed to power the instruments for about 60 hours.
On the morning of 14 November 2014, the battery charge was estimated to be only enough for continuing operations for the remainder of the day. After first obtaining data from instruments whose operation did not require mechanical movement, comprising about 80% of the planned initial science observations, both the MUPUS soil penetrator and the SD2 drill were commanded to deploy. Subsequently, MUPUS data as well as COSAC and Ptolemy data were returned. A final set of CONSERT data was also downlinked towards the end of operations. During the evening's transmission session, Philae was raised by and its body rotated 35 degrees to more favourably position the largest solar panel to capture the most sunlight in the future. Shortly afterwards, electrical power dwindled rapidly and all instruments were forced to shut down. The downlink rate slowed to a trickle before coming to a stop. Contact was lost on 15 November at 00:36 UTC.
The German Aerospace Center's lander manager Stephan Ulamec stated: