Philae (/ˈfaɪliː/ or /ˈfiːleɪ/) was a robotic lander built by the European Space Agency. It traveled with the Rosetta spacecraft until it separated to land on comet 67P/Churyumov–Gerasimenko, 10 years and 8 months after leaving Earth. On November 12, 2014, Philae landed on the comet, but it bounced after its anchoring harpoons did not deploy and a thruster meant to keep it attached failed. After two bounces, Philae made the first-ever "soft" (nondestructive) landing on a comet’s nucleus. However, the final landing placed it in a position and angle that made it difficult to use its instruments effectively.
Despite the landing challenges, Philae’s instruments captured the first images of a comet’s surface. Several of its tools performed the first direct analysis of a comet’s nucleus, sending data about the surface’s composition and gas escaping from beneath the surface. In October 2020, the scientific journal Nature published a report about Philae’s discoveries during its time on comet 67P/Churyumov–Gerasimenko.
On November 15, 2014, Philae entered safe mode, or hibernation, after its batteries ran out due to limited sunlight and an unexpected position at the landing site. Mission controllers hoped more sunlight might restart the lander. Philae occasionally communicated with Rosetta from June 13 to July 9, 2015, but contact was later lost. Its location was known within tens of meters but could not be seen. In September 2016, Rosetta’s photographs revealed Philae was lying on its side in a shadowed crack near a cliff. Knowing its location helped scientists better understand the images it sent. On September 30, 2016, the Rosetta spacecraft ended its mission by landing on the comet’s Ma’at region.
The lander is named after the Philae obelisk, which has a bilingual inscription and was used with the Rosetta Stone to help decipher Egyptian hieroglyphs. Philae was controlled from the Lander Control Center at DLR in Cologne, Germany, with support from the SONC at CNES in Toulouse, France.
Mission
Philae's mission was to land safely on a comet, stay attached to its surface, and send information about the comet's makeup. The Rosetta spacecraft and Philae lander were sent into space on an Ariane 5G+ rocket from French Guiana on March 2, 2004, at 7:17 UTC. They traveled for 3,907 days (about 10.7 years) to reach the comet named Churyumov–Gerasimenko. Unlike the Deep Impact probe, which intentionally hit the nucleus of comet Tempel 1 on July 4, 2005, Philae did not strike the comet. During a flyby of Mars on February 25, 2007, some of Philae's instruments were used for the first time without direct control from Earth. One camera system, called CIVA, took pictures while the Rosetta instruments were turned off, and another instrument, ROMAP, measured the magnetic field around Mars. Most other instruments required direct contact with the comet's surface to work and were not used during the flyby. Scientists expected the mission to last about four to five months after Philae landed.
The goals of the scientific mission have been summarised as follows:
Landing and surface operations
Philae stayed attached to the Rosetta spacecraft after meeting up with comet Churyumov–Gerasimenko on August 6, 2014. On September 15, 2014, the European Space Agency (ESA) announced "Site J" on the smaller part of the comet as Philae's landing spot. After a public contest in October 2014, Site J was renamed Agilkia in honor of Agilkia Island.
A series of four go/no-go checks were completed on November 11–12, 2014. One final test before Philae separated from Rosetta showed that the lander's cold-gas thruster was not working properly. However, the mission team decided to proceed with the separation because the issue could not be fixed. Philae detached from Rosetta on November 12, 2014, at 08:35 UTC SCET.
Philae's landing signal was received on Earth at 16:03 UTC, 28 minutes after it was sent. At the time, mission scientists did not know that the lander had bounced. It performed scientific measurements while moving away from the comet and returning, confusing the science team. Later analysis showed that Philae bounced twice.
Philae first touched the comet's surface at 15:34:04 UTC SCET. It then rebounded at 38 cm/s (15 in/s) and rose to about 1 km (0.6 mi) above the surface. If Philae had moved faster than 44 cm/s (17 in/s), it would have escaped the comet's gravity. After detecting the first landing, Philae's reaction wheel was turned off, which caused the lander to begin rotating every 13 seconds. During the first bounce, at 16:20 UTC SCET, Philae likely hit a raised area on the comet, slowing its rotation to once every 24 seconds and causing it to tumble. Philae landed a second time at 17:25:26 UTC SCET and bounced again at 3 cm/s (1.2 in/s). It finally stopped on the surface at 17:31:17 UTC SCET. The lander is in rough terrain, possibly in the shadow of a nearby cliff or crater wall, and is tilted at about 30 degrees. It is undamaged. Its final location was first determined using data from the CONSERT instrument and images from the Rosetta orbiter, and later confirmed by direct imaging from Rosetta.
Telemetry data showed that the first landing was softer than expected, the harpoons did not deploy, and the thruster did not fire. The harpoon system used 0.3 grams of nitrocellulose, a material shown in 2013 by Copenhagen Suborbitals to be unreliable in a vacuum.
Philae's primary battery was designed to power instruments for about 60 hours. ESA expected a secondary battery to be partially charged by solar panels on the lander. However, the limited sunlight at the landing site (90 minutes per 12.4-hour comet day) was not enough to keep Philae operating during this phase of the comet's orbit.
On the morning of November 14, 2014, battery power was estimated to last only until the end of the day. First, data was collected from instruments that did not require moving parts, which made up about 80% of the planned initial science observations. Later, the MUPUS soil penetrator and SD2 drill were deployed. Data from MUPUS, COSAC, and Ptolemy was returned, along with a final set of CONSERT data. In the evening, Philae was lifted 4 cm (1.6 in) and rotated 35 degrees to position its largest solar panel to capture more sunlight. Soon after, power dropped rapidly, and all instruments shut down. The data transmission slowed and stopped. Contact with Philae was lost on November 15 at 00:36 UTC.
Stephan Ulamec, the German Aerospace Center's lander manager, stated that data from the SESAME instrument showed the first landing site had a thick layer of granular material covering a large amount of water ice. The ice was strong, and there was little cometary activity in that area. At the final landing site, the MUPUS instrument could not drill far into the surface, even with increased power. This area was found to be as hard as solid ice or pumice.
In the comet's atmosphere, the COSAC instrument detected molecules containing carbon and hydrogen. Soil elements could not be analyzed because the lander could not drill into the surface, likely due to hard ice. The SD2 drill completed steps to deliver a sample to COSAC, but no material entered the instrument.
During Philae's first landing, COSAC measured disturbed material at the bottom of the lander, while Ptolemy measured material at the top. Sixteen organic compounds were detected, four of which were discovered for the first time on a comet, including acetamide, acetone, methyl isocyanate, and propionaldehyde.
On June 13, 2015, at 20:28 UTC, ground controllers received an 85-second signal from Philae, relayed by Rosetta, indicating the lander was healthy and had enough battery power to leave safe mode. Philae sent data showing it had been operating before June 13, 2015, but could not contact Rosetta earlier. It reported using 24 watts of power at −35 °C (−31 °F).
A new contact between Rosetta and Philae was confirmed on June 19, 2015. The first signal was received on Earth at 13:37 UTC, and a second signal at 13:54 UTC. Each contact lasted about two minutes and provided additional status data. By June 26, 2015, there had been seven brief contacts between the lander and orbiter. Contacts occurred twice per Earth day, but their quality depended on Philae's antenna orientation and Rosetta's position around the comet. As the comet rotated, Philae was not always in sunlight, limiting power for communication. ESA controllers continued trying to establish a stable 50-minute contact.
If Philae had landed at the planned Agilkia site in November 2014, its mission might have ended in March 2015 due to higher temperatures from increased solar heating. As of June 2015, Philae's main remaining experiment was to drill into the comet's surface to study its chemical composition. On July 5, 2015, controllers sent commands to power up the CONSERT radar instrument, but received no immediate response. Confirmation came on July 9, when Philae transmitted data from the instrument.
After reawakening, data suggested Philae's systems were healthy. Controllers sent commands to Rosetta to adjust its orbit for better communication and science investigations. However, stable communication with Philae was difficult. Rosetta had to stay farther from the comet as it became more active. The last communication was on July 9, 2015, and controllers could not instruct Philae to perform new tasks. Philae did not respond to further commands,
Design
The lander was built to separate from the main spacecraft and fall from an orbit 22.5 kilometers (14 miles) high along a path determined by gravity. It was planned to land on the comet’s surface at a speed of about 1 meter per second (3.6 km/h; 2.2 mph). The lander’s legs were designed to reduce the force of the landing to prevent bouncing, as the comet’s escape speed is only about 1 m/s (3.6 km/h; 2.2 mph). The impact energy was meant to drive ice screws into the surface. Philae was also planned to fire a harpoon into the comet at 70 m/s (250 km/h; 160 mph) to secure itself. A thruster on Philae was meant to reduce bouncing during landing and minimize recoil from the harpoon. However, during the landing, the harpoons did not fire, and the thruster did not work, causing Philae to land multiple times.
Communication with Earth used the Rosetta orbiter as a relay to save power. The mission was planned to last at least one week on the surface, with the possibility of extending it for months.
The lander’s main structure is made of carbon fiber, shaped into a flat, stable base that supports scientific tools and connects parts. The lander weighs about 100 kilograms (220 pounds). Solar cells on its exterior generate power.
The Rosetta mission was originally planned to reach comet 46P/Wirtanen. However, a failure in a previous Ariane 5 rocket launch closed the window to reach that comet, so the mission’s target changed to comet 67P/Churyumov–Gerasimenko. The larger size of this comet and the higher landing speed required stronger landing gear.
Philae’s power system had two phases. In the first phase, it used battery power only. In the second phase, it relied on backup batteries recharged by solar cells.
The power system includes two batteries: a non-rechargeable battery providing power for the first 60 hours and a rechargeable battery powered by solar panels after the first battery is used. The solar panels cover 2.2 square meters (24 square feet) and were designed to produce up to 32 watts of power when 3 astronomical units (AU) away from the Sun.
The lander carried ten scientific instruments weighing 26.7 kilograms (59 pounds), which make up more than one-quarter of the lander’s total mass.
Analysis of comet
On October 28, 2020, it was reported that Philae found "low-strength primitive ice inside cometary boulders." This included water ice that is believed to have formed about 4.5 billion years ago. These discoveries happened mainly at the location of Philae's second landing on comet 67P/Churyumov–Gerasimenko. During this landing, the spacecraft made four separate contacts with the surface of two connected cometary boulders. Philae also drilled 25 centimeters (9.8 inches) into the ice of the comet's boulders.
Media coverage
The landing was widely shared on social media, with the lander having an official Twitter account that acted like a person. The hashtag "#CometLanding" became very popular online. A livestream of the control centers was created, and many official and unofficial events took place worldwide to watch Philae land on Churyumov–Gerasimenko. Each of Philae's scientific tools had its own Twitter account to share updates and discoveries.
Composer Vangelis created the music for three videos made by ESA to celebrate the first attempt to land softly on a comet by ESA's Rosetta mission.
On November 12, 2014, the search engine Google showed a special image of Philae on its homepage. On December 31, 2014, Google again displayed Philae as part of its New Year's Eve 2014 celebration.
Online comic writer Randall Munroe created a comic strip on his website, xkcd, that updated in real time on the day of the landing.