An instrument called SESAME-CASSE aboard Rosetta’s 100-kg Philae lander has recorded the sound of touchdown – in the form of vibrations detected in the soles of the lander’s feet – as it first came into contact with the comet. Before going into the sleep mode, the lander has also been able to collect the ambient gases of the comet, observe the environment around it, detect organic molecules and conduct some other experiments. In addition, Rosetta’s Plasma Consortium has uncovered a mysterious ‘song’ that the comet is singing into space.
The Philae lander at work on Comet 67P/Churyumov-Gerasimenko. Image credit: ESA / AOES Medialab.
“Our data record the first touchdown and show that Philae’s feet first penetrated a soft surface layer, possibly a dust layer several centimeters thick, until they hit a hard surface – probably a sintered ice-dust layer – a few milliseconds later,” said Dr Klaus Seidensticker, a Rosetta team member and researcher with the DLR Institute of Planetary Research.
“Data from the SESAME-DIM instrument meanwhile suggest that current cometary activity at the final landing site is low, while preliminary data from SESAME-PP are consistent with a large amount of water ice under the lander.”
The data suggest that the surface of the comet is significantly structured, mixing soft and hard aspects.
“At the moment, we are supporting the effort to reconstruct the flight path of the lander after first touchdown, collecting all available data across the various instruments,” Dr Klaus said.
“This is important for SESAME, especially CASSE, as we need to know the speed, impact angle, and rotation rate before the first touchdown, but also the final landing place.”
Before going into hibernation on 15 November, Philae was able to conduct other experiments and return the data to Earth.
An instrument called the Multi-Purpose Sensors for Surface and Subsurface Science (MUPUS) began observing the environment around the comet once the lander was released from the orbiter on 12 November. After the first touchdown recorded by Philae its harpoons and ice screws did not deploy as planned and the lander subsequently rebounded, experiencing two further touchdowns.
Because part of the MUPUS package was contained in the harpoons, some temperature and accelerometer data could not be gathered. However, the MUPUS thermal mapper worked throughout the descent and during all three touchdowns.
At Philae’s final landing spot, the MUPUS probe recorded a temperature of minus 153 degrees Celsius (minus 243.4 degrees Fahrenheit) close to the floor of the lander’s balcony before it was deployed. Then, after deployment, the sensors near the tip cooled by about 10 degrees Celsius (18 degrees Fahrenheit) over a period of roughly half an hour.
“We think this is either due to radiative transfer of heat to the cold nearby wall seen in the Rosetta images or because the probe had been pushed into a cold dust pile,” said Dr Jörg Knollenberg, instrument scientist for MUPUS at DLR. “The probe then started to hammer itself into the subsurface, but was unable to make more than a few mm of progress even at the highest power level of the hammer motor.”
Looking at the results of the thermal mapper and the probe together, the scientists have made the assessment that the upper layers of the comet’s surface consist of dust of 10–20 cm thickness, overlaying mechanically strong ice or ice and dust mixtures.
At greater depths, the ice likely becomes more porous, as the overall low density of the nucleus – determined by instruments on the Rosetta orbiter – suggests.
Philae’s Sampling, Drilling and Distribution (SD2) subsystem was activated towards the end of the surface operations that Philae performed on the comet.
Its goal was to drill into the surface in order to collect and deliver samples to the COSAC and Ptolemy instruments inside the lander.
The SD2 team said that the drill was deployed as planned, extending 46.9 cm below the balcony of the lander and 56.0 cm from its reference point.
“It was then retracted to the reference position, the carousel turned in a way that the sampling tube was in front of the right oven, the discharge operation from the sampling tube to the oven was completed, and the carousel rotated in a way that that oven was positioned at COSAC’s location,” said Prof Amalia Ercoli Finzi, the SD2 principal investigator and a researcher with the Politecnico di Milano, Italy.
Although the ovens worked correctly, the scientists do not yet know how much – if any – material was actually delivered to the ovens by SD2, or whether the instruments sampled dust or gas that entered the chamber during the touchdown.
Because Philae was not anchored to the comet surface, it is also possible that, if the drill touched a particularly hard surface material, it moved the lander instead of drilling into the surface.
Furthermore, the SD2 instrument lacks dedicated sensors to determine whether or not the surface has been reached, whether a sample was then collected in the sample tube, or if it was then discharged into the oven.
But other instruments aboard the lander can help understand what actually happened. For example, the downward-looking ROLIS camera obtained two images of the surface under the balcony, one before and one after the lander’s main body was lifted and rotated. Because of those movements, the SD2 ‘footprint’ may be included in those images and thus may be able to provide visual evidence that the drill interacted with the surface.
“As far as we can see at the moment, SD2 and COSAC telemetry cannot reliably discern between lack of sample and insufficient gas generation from it. A CIVA-MV/MI image would have been needed for this purpose, which was not available for the first sample,” Prof Finzi said.
Meanwhile, COSAC’s analyses on the data acquired from its surface measurements are on-going.
But it is apparent that COSAC already ‘sniffed’ the comet’s atmosphere during the first touchdown, detecting organic molecules.
The Ptolemy instrument is also reported to have successfully collected the ambient gases of the comet.
Analysis of the spectra and identification of the molecules detected by both instruments are continuing.
In addition, Rosetta’s suite of five instruments collectively called the Plasma Consortium uncovered a mysterious ‘song’ that the comet is singing into space.
The instruments are designed to study a number of phenomena, including: the interaction of the comet with the solar wind, a continuous stream of plasma emitted by the Sun; changes of activity on the comet; the structure and dynamics of the comet’s tenuous plasma ‘atmosphere’, known as the coma; and the physical properties of the cometary nucleus and surface.
The ‘song’ is in the form of oscillations in the magnetic field in the comet’s environment. It is being sung at 40-50 millihertz, far below human hearing. To make the music audible to the human ear, the frequencies have been increased by a factor of about 10,000.
With the Philae lander’s mission complete, Rosetta will now continue its own exploration, orbiting the comet during the coming year.
“With lander delivery complete, Rosetta will resume routine science observations and we will transition to the comet escort phase. This science-gathering phase will take us into next year as we go with the comet towards the Sun, passing perihelion, or closest approach, on 13 August 2015, at 186 million km from our star,” said Dr Andrea Accomazzo, the Rosetta flight operations director.
From next week, Rosetta’s orbit will be selected and planned based on the needs of the scientific sensors. After arrival on 6 August 2014, the orbit was designed to meet the lander’s needs.
On 3 December, the craft will move down to height of 20 km for about 10 days, after which it will return to 30 km.
“With the landing performed, all future trajectories are designed purely with science as the driver,” said Dr Laurence O’Rourke and Dr Michael Küppers from the Rosetta Science Operations Center near Madrid, Spain.
“The desire is to place the spacecraft as close as feasible to the comet before the activity becomes too high to maintain closed orbits. This 20 km orbit will be used by the science teams to map large parts of the nucleus at high resolution and to collect gas, dust and plasma at increasing activity.”
Planning the science orbits involves two different trajectories: ‘preferred’ and ‘high-activity.’
While the intention is always to fly the preferred path, Rosetta will move to the high-activity trajectory in the event the comet becomes too active as it heats up.
“This will allow science operations to continue besides the initial impact on science planning that such a move would entail.”
Dr Matt Taylor, Rosetta project scientist, added: “science will now take front seat in this great mission. It’s why we are there in the first place!”
“When solar heat activates the frozen gases on and below the surface, outflowing gas and dust particles will create an atmosphere around the comet’s nucleus, known as the coma.”
Rosetta will become the first spacecraft to witness at close quarters the development of a comet’s coma and the subsequent tail streaming for millions km into space.
The spacecraft will then have to stay further from the comet to avoid the coma affecting its orbit.
In addition, as the comet nears the Sun, illumination on its surface is expected to increase. This may provide sufficient sunlight for the Philae lander, now in the idle mode, to reactivate, although this is far from certain.
Early next year, the orbiter will be switched into a mode that allows it to listen periodically for beacon signals from the surface.
