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It will be the deepest hole ever hammered into another celestial body using manmade technology. During the NASA InSight mission, the Heat Flow and Physical Properties Package (HP³), the Mole, which was developed and built by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) will penetrate up to five metres deep into the Martian soil to measure the temperature and thermal conductivity of the substrate materials there. This glimpse of the interior of the Red Planet will help us to better understand the formation and evolution of Earth-like bodies. The landing is scheduled to take place at 20:53 CET on 26 November 2018 on the Elysium Planitia plain, a bit north of the Martian equator. As the hour approaches, excitement is mounting over where exactly the landing probe will touch down within the 140-kilometre-long landing ellipse and where an appropriate spot for the historic Mars experiment might be found at the landing site. Until now, heat flow measurements have only been performed on the Moon, as part of the Apollo 17 mission, during which the astronauts Eugene Cernan and Jack Schmitt used a hand-powered drill to bore up to three metres into the surface.

"Our Mars mole is doing well on board InSight on its final stretch to Mars," says the Principal Investigator of the HP³ experiment, Tilman Spohn of the DLR Institute of Planetary Research in Berlin. "Checks during the cruise phase did not reveal any irregularities. Now we are eagerly yet confidently awaiting the landing just a few days from now."

Spohn and a number of other DLR scientists will be at the Jet Propulsion Laboratory (JPL) on 26 November, which runs the mission for NASA. Also present will be scientists from the DLR Microgravity User Support Center in Cologne (MUSC), which is directly involved with the mission operations team that is controlling the mole. "From the landing and the choice of a definitive site on the surface all the way through to the successful penetration of the Martian soil, we will be working directly with our US colleagues at the control centre in Pasadena, California," says HP³ Operations Manager Christian Krause.

The deployment of the HP³ experiment is now planned for early January 2019. At this point, the penetrometer will gradually hammer down to a depth of five metres in 50-centimetre progressions over three phases that will be repeatedly interrupted by measurements and checks. It will use a fully automatic electrically powered hammer mechanism and pull a tether fitted with measuring sensors behind it along into the Martian soil. "As we do not know what surprises – hard rocks, for instance – may await us underground, we will proceed with great caution and are planning to reach our target depth within several weeks, in March 2019," adds Spohn. Another part of the HP³ experiment involves an infrared radiometer, which will be activated shortly after the landing and will measure the surface thermal radiation yielding the temperature of Mars from the lander platform. Together, these datasets will make it possible to draw conclusions about the planet's heat flow.

Landing in a 'car park'

In order to mitigate the risks to the mission, the engineers and scientists have chosen a landing area southwest of the large Elysium volcanic complex, on the Elysium Planitia plain, which is largely flat and free of large stones and rocks. The deliberate monotony of this landing site, essential for conducting measurements beneath the surface of Mars, has led NASA researchers to describe the landing, in tongue-in-cheek fashion, as taking place on a large 'parking lot'. Yet even here, precise analysis of landing site will be necessary from the very start. Once this has been done, a robotic arm will set the seismometer SEIS (Seismic Experiment for Interior Structure), built under the lead of CNES by an international consortium to which DLR has also contributed, down on the surface. The seismometer will record waves that originate from marsquakes and meteorite impacts and propagate through the planet. As a next step, the HP³'Mole' experiment, which was developed and built at DLR, will be placed on the surface. The InSight landing platform also carries the US experiment RISE (Rotation and Interior Structure Experiment), which will record fluctuations in the Mars' rotation axis.

Braking manoeuvre at 1500 degrees Celsius

Before the measurements can begin, InSight must first endure the roughly seven-minute critical Entry, Descent and Landing phase (EDL). On 26 November 2018 at 20:47 CET, the probe will enter the Martian atmosphere at a shallow angle. In the process, the protective shield will heat up to around 1500 degrees Celsius within three minutes. The friction with the Martian atmosphere will slow down the probe until a parachute opens 12 kilometres above the ground, allowing the lander to slowly float down to the planet's surface. Upon separation from the parachute at an altitude of 1200 metres, InSight will be decelerated by its descent engines. Contact will be maintained with the space probe during its flight to Mars and over the course of the mission via the 70-metre antennas of NASA's Deep Space Network in California, Australia and Spain. The NASA Mars Reconnaissance Orbiter and Mars Odyssey 2001 probes will fly over the InSight landing site twice per Martian day and will serve as relay stations for communications with Earth.

Looking at Mars to discover how Earth was formed

InSight is a stationary geophysical observatory, and the only one of its kind in the history of Solar System research. Its main scientific task is to investigate the interior and composition of our neighbouring planet, examining the evolution, structure and physical properties of the crust, mantle and core, thus enabling us to draw conclusions about the formation and evolution of terrestrial bodies. Mars is the perfect destination, as it is relatively easy to reach and makes an ideal comparison object to Earth.

The processes that took place after the formation of a metal core inside Mars and in the overlying rock mantle and crust likely slowed down faster than they did on Earth. As such, the fingerprints of the processes that once formed the core, mantle and crust of Earth-like planets may have been better preserved on Mars to this day. Researchers are thus hoping to gain a deeper understanding of what occurred from the formation of Earth up to the evolution of life, also comparing these processes with those that occurred on its closest neighbours, Mercury, Venus and Mars, as well as looking at rocky planets orbiting other stars. The researchers are curious as to whether there is still a hot, molten core at the centre of Mars – as there is on Earth – and whether this is completely molten, unlike on Earth, as some theories suggest. Tilman Spohn, who developed this theory together with fellow scientists from the US in the 1990s, maintains that "This model would easily explain the lack of a magnetic field on Mars today".

The HP³ experiment on the NASA InSight mission

The InSight mission is being conducted by the Jet Propulsion Laboratory (JPL) in Pasadena, California, on behalf of NASA's Science Mission Directorate. The InSight mission is part of the NASA Discovery Program. DLR is contributing to the mission with its HP³ (Heat Flow and Physical Properties Package) experiment. The DLR Institute of Planetary Research, which was responsible for developing the experiment in collaboration with the DLR institutes of Space Systems, Optical Sensor Systems, Space Operations and Astronaut Training, Composite Structures and Adaptive Systems, System Dynamics and Control, and Robotics and Mechatronics, is leading the experiment. The industry partners Astronika, CBK Space Research Centre, Magson and Sonaca are also involved. The Space Research Institute at the Austrian Academy of Science and the University of Kaiserslautern are scientific partners for the project. HP³ is operated by the DLR Microgravity User Support Centre (MUSC) in Cologne.

Detailed information on InSight and the HP³ experiment are available on DLR's dedicated mission site: www.dlr.de/en/insight. For mission updates follow @NASAInSight on Twitter.