European Space Agency scientists have determined that their robotic lander Philae is on the surface of comet 67P/Churyumov-Gerasimenko, but it is not anchored as it should be and is not sitting level. Philae may have come to rest on the steep rim of a crater after rebounding twice off the surface, following an initial touchdown near the centre of its planned landing site.
The first rebound may have pushed the washing machine-sized lander up to 3,280ft off the surface, and it was 2h before it touched down again. Following the second rebound, Philae came to rest after about 38min.
Now, teams at mission control in Darmstadt and the dedicated Rosetta mission lander control centre in Cologne are attempting to determine the lander’s location and orientation before activating any mechanical systems, for fear of upsetting the spacecraft. Philae, for example, features a drill designed to bring material from the comet into an onboard analysis laboratory.
The rebound may indicate that while the comet’s density is very low, its surface may be harder than was planned for. However, the lander engineering teams have expressed satisfaction with their contingency planning – the mission was launched a decade ago, with little idea about the composition of the target comet.
What is known so far is that the harpoon anchor mechanism did not fire on touchdown as expected, but had it functioned properly the Philae team believes it would have prevented the rebound. The comet is about 4km (3 miles) across – about the size of Mont Blanc – so it has very little gravity to hold Philae.
Now, Philae’s controllers face a race against time. The lander’s battery performance is critical to supporting scientific operations, including surface drilling. After about 60h the batteries will need recharging, but it is not yet clear how much sunlight will be available to the lander’s solar panels – which may be in the shade of features of the comet’s irregular surface.
In principle, Philae’s landing gear could be instructed to “hop”, possibly improving its orientation, but no such attempt will be made if it is deemed a risk to the mission. Also, even if Philae’s batteries were to run too low to sustain operations, the unit may be brought out of hibernation at a later time if they are recharged, even slowly, by the solar panels.
Regardless of what comes next for Philae, the lander is in contact with Rosetta whenever the orbiter comes over the comet’s horizon, so images and data are being returned to Earth. Indeed, mission controllers are still awaiting receipt of the high resolution images they need to better determine their next move.
The mission has in any case brought comparison to the Apollo Moon landings for technical prowess. In the 10 years since its launch, Rosetta has travelled 6.4 billion km through the solar system before arriving at the comet on 6 August 2014, and the soft landing by Philae was the first on a comet. During that 10-year trip to rendezvous with 67P, which spends much of its orbit around the Sun beyond Jupiter, Rosetta spent an extended time in hibernation. Effectively shut down and with no contact with its controllers, Rosetta relied on three gravity-assist fly-bys of Earth and one of Mars to reach the comet.
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 the comet throughout the remainder of 2015, as they head away from the Sun and activity begins to subside.
With the data gathered by Rosetta and Philae, scientists hope to reveal secrets of the origin of the solar system – and even of life on Earth, as comets are possibly the oldest fragments of the Sun’s early environment.