Waiting For A Signal From Philae

Artist’s impression of the Rosetta orbiter deploying the Philae lander to comet 67P/Churyumov–Gerasimenko. Image Credit:  ESA–J. Huart, 2013

Artist’s impression of the Rosetta orbiter deploying the Philae lander to comet 67P/Churyumov–Gerasimenko.
Image Credit: ESA–J. Huart, 2013

March 17, 2015 – With Philae now receiving twice as much solar energy as it did last November when it finally came to rest in a shaded spot on Comet 67P, the communication unit on the Rosetta orbiter has been switched on to call the lander. Although it is probably still too cold for the lander to wake up, the prospects will improve with each passing day.

Several conditions must be met for Philae to start operating again. First, the interior of the lander must be at least at –45ºC before Philae can wake up from its winter sleep. At its new landing site – Abydos – only a little sunlight reaches Philae, and the temperatures are significantly lower than at the originally planned landing location. In addition, the lander must be able to generate at least 5.5 watts using its solar panels to wake up.

As soon as Philae registers that it is receiving more than 5.5 watts of power and its internal temperature is above –45ºC, it will turn on, heat up further and attempt to charge its battery.

Once awakened, Philae switches on its receiver every 30 minutes and listens for a signal from the Rosetta orbiter. This, too, can be performed in a very low power state.

It could be that the lander has already woken up from its winter sleep some 500 million kilometres away from Earth, but does not yet have sufficient power to communicate with Rosetta, which relays Philae’s signal back to Earth. Philae needs a total of 19 watts to begin operating and allow two-way communication.

Between March 12 and 20, the Rosetta orbiter is transmitting to the lander and listening for a response. The most likely time for contact is during the 11 flybys where the orbiter’s path puts it in a particularly favourable position with respect to the lander during comet ‘daytime’ – when Philae is in sunlight and being supplied with power by its solar panels. Communication will be attempted continuously because Philae’s environment could have changed since landing in November 2014.

Rosetta was launched on March 2, 2004 and was reactivated in January 2014 after a record 957 days in hibernation. The mission consists of an orbiter and lander. Its objective since arriving at comet 67P/Churyumov-Gerasimenko on August 6, 2014 has been to study the celestial object up close in unprecedented detail. The orbiter will continue tracking the comet’s changes as it sweeps past the sun.

The comet will reach its closest distance to the Sun on 13 August 2015 at about 185 million km, roughly between the orbits of Earth and Mars. Rosetta will follow it throughout the remainder of 2015, as they head away from the Sun and activity begins to subside.

On November 12, 2014 the European Space Agency’s Rosetta mission successfully landed on the surface of comet 67P/Churyumov-Gerasimenko. Descending at a speed of about 2 mph (3.2 kilometers per hour) the lander, called “Philae,” first touched down and its signal was received at 9:03 a.m. MDT.

Partially due to anchoring harpoons not firing, and the comet’s low gravity (a hundred-thousand times less than that of Earth), Philae bounced off the surface and flew up to about six-tenths of a mile (1 kilometer) both above the comet’s surface as well as downrange.

At 10:53 a.m. MDT, almost two hours after first contact, Philae again touched down. A second, more modest bounce resulted, again sending it airborne. Philae’s third contact with the comet’s nucleus was the charm. At 11 a.m. MDT, the Rosetta mission’s Philae lander became the first spacecraft to soft-land on a comet.

Rosetta mission controllers believe Philae alighted in a hole, or crevice, about six feet (two meters) in diameter and six feet (two meters) deep and that it is lying on its side. The solar panels are only being illuminated for 1.5 hours of each 12-hour comet day, which is much less than they need to power the lander. However, as the comet gets closer to the sun, more sunlight will reach the lander, and the lander should wake up and begin communicating with Rosetta.

Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta’s Philae lander is provided by a consortium led by the German Aerospace Center, Cologne; Max Planck Institute for Solar System Research, Gottingen; French National Space Agency, Paris; and the Italian Space Agency, Rome. JPL, a Division of the California Institute of Technology, Pasadena, manages the U.S. contribution of the Rosetta mission for NASA’s Science Mission Directorate in Washington. JPL also built the MIRO and hosts its principal investigator, Samuel Gulkis.

The Southwest Research Institute (San Antonio and Boulder), developed the Rosetta orbiter’s IES and Alice instruments, and hosts their principal investigators, James Burch (IES) and Alan Stern (Alice).