Philae has been useless for years. Its final transmission was acquired in July 2015; now, its unresponsive type rests underneath a rocky overhang on comet 67P/Churyumov-Gerasimenko.
But, even because the robotic lander lies in chilly repose, scientists have not given up on it. They’ve lastly discovered the place the place Philae bounced because it was coming in to land.
Right here, it scraped away a deep gouge within the comet’s floor rubble to disclose the ice beneath, gleaming brightly towards the darkish rock round it. That pristine ice had been hid for 4.5 billion years, because the comet fashioned within the earliest days of the Photo voltaic System.
Surprisingly, the ice is slightly gentle and fluffy. Comet 67P/C-G, formed a bit like a rubber ducky, actually is a grimy snowball – and that data might assist us plan future comet-landing missions.
Philae’s touchdown was considerably ignominious. It was launched from the Rosetta spacecraft on 12 November 2014, despite the fact that one in every of its thrusters was not operational. Then, when it hit touchdown website Agilkia, its anchoring harpoons did not hearth, and it bounced to a peak of a couple of kilometre (0.62 miles) earlier than coming again down. Then, it made contact and bounced once more. Philae lastly got here to a cease within the Abydos valley, the place it stays to today.
Scientists knew the place the primary level of contact was. After 22 months, they have been in a position to find Philae’s closing resting place. However the website of that intermediate bounce remained elusive.
“Philae had left us with one closing thriller ready to be solved,” mentioned astronomer Laurence O’Rourke of the European House Company.
“It was essential to search out the landing website as a result of sensors on Philae indicated that it had dug into the floor, probably exposing the primitive ice hidden beneath, which might give us invaluable entry to billions-of-years-old ice.”
The workforce began their search poring over information obtained by the Optical, Spectroscopic and Infrared Distant Imaging System (OSIRIS) aboard the Rosetta spacecraft. They have been trying particularly for vivid patches on 67P/C-G that may point out newly uncovered ice. Given the brightness variation on the comet’s floor, this was like on the lookout for a needle in a haystack.
The breakthrough got here after they studied information from the lander itself; or slightly, the ROsetta MAgnetometer and Plasma (ROMAP) instrument hooked up to the lander. ROMAP was designed to observe adjustments within the comet’s native magnetic discipline. However the instrument’s 48-centimetre (19-inch) magnetometer increase bent as Philae crashed.
This induced the increase to maneuver relative to the lander’s physique, from which the workforce might estimate the period of time Philae spent slamming into the ice. It additionally generated distinctive accelerometer measurements that described Philae’s movement.
The info from ROMAP have been correlated with information from the RCP magnetometer aboard Rosetta to trace down the exact location of the second bounce. It ended up being simply 30 metres (98 ft) away from the overhang.
Importantly, the workforce might lastly reconstruct how the bounce performed out.
Because it seems, Philae spent virtually two full minutes crashing across the bounce website, coming into contact with the comet a minimum of 4 occasions. At one level, it spent about three seconds sinking 25 centimetres (10 inches) into the comet. That lengthy period shouldn’t be uncommon for such a low-gravity surroundings as 67P/C-G, the researchers mentioned. All that clumsy rolling gave the positioning its new title.
“The form of the boulders impacted by Philae jogged my memory of a cranium when considered from above, so I made a decision to nickname the area ‘skull-top ridge’ and to proceed that theme for different options noticed,” O’Rourke mentioned.
“The suitable ‘eye’ of the ‘cranium face’ was made by Philae’s high floor compressing the mud whereas the hole between the boulders is ‘skull-top crevice’, the place Philae acted like a windmill to cross between them.”
When the workforce checked out photographs taken by Rosetta a number of months after the touchdown, there it was – a 3.5-square metre (37-square foot) patch of uncovered ice, brightly reflecting searing daylight, the place Philae had scraped towards the comet. It hadn’t been seen in photographs taken on the time of touchdown as a result of the area had been in shadow.
Finding out the sequence of occasions because the lander bounced – the contacts, the period, how deep Philae sank into the floor of 67P/C-G – ended up offering a brand new and surprising software for describing the density of the comet.
“The easy motion of Philae stamping into the facet of the crevice allowed us to work out that this historical, billions-of-years-old, icy-dust combination is very gentle – fluffier than froth on a cappuccino, or the froth present in a bubble bathtub or on high of waves on the seashore,” O’Rourke mentioned.
The info additionally allowed for an estimate of the comet’s porosity. The researchers discovered that roughly 75 p.c of 67P/C-G is empty house, in step with a earlier evaluation. That is similar to pumice. The rocks on the comet, the researchers mentioned, are in all probability extra like styrofoam than the heavy boulders they look like.
It is an interesting discovery. This sturdy-looking chunk of house rock (and ice) that has survived hanging across the Photo voltaic System for 4.5 billion years is barely holding itself collectively.
“The mechanical rigidity that holds the comet ice collectively on this chunk of mud is simply 12 pascal,” defined astronomer Jean-Baptiste Vincent of the DLR Institute of Planetary Analysis Institute in Germany. “That isn’t far more than ‘nothing’.”
This could possibly be very helpful data to remember for designing comet probes sooner or later.
The workforce’s analysis has been revealed in Nature.