The InSight lander will carry two major experiments, the Heat Flow and Physical Properties Probe (HP3) and the Seismic Experiment for Interior Structure (SEIS), to Mars, where a robotic arm will place them beside the spacecraft after landing. Further experimental components will remain attached to the lander itself to assist with the tasks involved in the experiments. There is also the Rotation and Interior Structure Experiment (RISE), which will record shifts in the frequencies of radio signals (Doppler effect) in order to measure tiny variations in the inclination of Mars’ rotational axis.
In many respects, the design of the lander is structurally similar to that of NASA's successful Phoenix lander , which touched down on 25 May 2008 near the Martian north pole and carried out experiments over a six-month period, using a scoop for the first time to detect water ice in the form of hoar frost. The aerospace company Lockheed Martin Space Systems began constructing the InSight lander on behalf of NASA in May 2014. The launch of InSight was originally planned for 2016, but NASA postponed it to 2018 due to a leak in a vacuum component of the SEIS seismometer. The main component of the lander is a platform measuring two metres across, to which most of the system components – the experiments in ‘travelling mode’, the antennas, the on-board computer, the thrusters, the propellant tanks and three telescopic legs – are attached. After landing, a robotic arm will unfold and lower the apparatus for the HP3 and SEIS experiments onto the Martian surface. Two solar panels are attached to the side of the platform. When these are deployed, InSight has a span of 6.1 metres. The spacecraft has a total mass of 360 kilograms.
Mars Cube One
One new feature of this mission is Mars Cube One (MarCO) – two small satellites. These are based on the CubeSat concept, in which cube-shaped modules with 10-centimetre-long sides are combined to form bigger satellites. Each of the two MarCOs consists of six CubeSat units. After leaving Earth orbit, they will detach from the InSight spacecraft and accompany the lander on its way to Mars, but following their own trajectories. On arrival, they will enter orbit around Mars. Their task is to maintain communications between InSight and NASA’s Mars Reconnaissance Orbiter (MRO) during InSight's arrival and landing – including the most critical phase of the mission (Entry, Descent and Landing; EDL) – while orbiting over the entry zone. This is the first time for small satellites to be used around another celestial body. The project has technology demonstrator status and is not critical to the success of InSight.
HP3 heat flow probe
The HP3 experiment, which was developed at DLR, consists of an instrument casing that houses the measuring cable and data cable for connection to the ground probe. A 'drilling rig' is mounted on the front of this housing, which contains the percussion hammer. At 40 centimetres long and 27 millimetres across, the 'Mole' penetrometer will use the hammer mechanism to bore up to five metres into the Martian soil. The centrepiece of the experiment is a measuring cable equipped with temperature sensors that will be pulled behind the penetrometer, while remaining connected to the casing on the surface, which serves as the Support System Assembly (SSA) or 'Sphinx'. The platinum temperature sensors will take readings every 35 centimetres to a depth of five metres every hour for one Martian year, which corresponds to approximately two Earth years. The sensors on the measuring cable provide measurements that are accurate to a few thousandths of a kelvin. Model calculations show that measurements must be taken for at least a few months to determine the heat flow from the interior of Mars. The 'brain' of the experiment – an electronics box – is located on the InSight lander, where it is protected from large temperature variations. This electronics box will be used to operate the experiment from the ground control centre at the DLR site in Cologne.
Ideally, after two months, the hammering mechanism will have reached a depth of five metres, where it will be able to record the heat flow without interference from the temperature variations at the surface. The minimum depth required for these conditions is three metres. But the Mole will not hammer continuously; it will do so for about a total of 10 days, penetrating the soil in 50-centimetre stages. After two-day cooling breaks, the progress of the drilling will be evaluated before an internal heater warms the outer shell of the Mole for 24 hours. Then, measurements will be made to determine how fast the heat from the metal of the Mole is released into the immediate environment around the drill. This will allow the thermal conductivity of the Martian soil – also known as the regolith – to be determined at the landing site, making it possible to estimate other properties such as porosity or solidity. An inclinometer will detect whether the borehole is running vertically into the ground or is deviating from a vertical path due to obstacles.
From the Sphinx, a second data cable maintains contact with the computer on board InSight, from which measurement data are transmitted to an orbiter travelling around Mars, and from there to Earth. The experiment also includes an infrared radiometer that measures the daily temperature trend at the landing site over the course of a Martian year. DLR's Microgravity User Support Centre (MUSC) in Cologne will operate HP3. The DLR Institute of Planetary Research, which was responsible for developing the experiment in collaboration with the DLR Institutes of Space Systems (Bremen), Optical Sensor Systems (Berlin), Space Operations and Astronaut Training (Cologne facility), Composite Structures and Adaptive Systems (Braunschweig), System Dynamics and Control (Oberpfaffenhofen), and Robotics and Mechatronics (Oberpfaffenhofen), is directing the experiment. The industry partners were Astronika (Warsaw), Magson (Berlin) and Sonaca (Berlin).
SEIS is a seismometer that will measure ground movements on Mars at different frequencies using six sensors, three short period sensors (SP) and three very broadband sensors (VBB). The instrument was developed by a consortium led by the French space agency (CNES). Germany provided the levelling system (LVL) for SEIS, which was developed and built at the Max Planck Institute for Solar System Research in Göttingen.
A seismometer records ground vibrations. On Earth, massive vibrations can be triggered by earthquakes or explosions. The quake waves then travel across the planet and, if the source is strong enough, may be detected all over the world. Minor vibrations are familiar in everyday life, caused by passing trains or, more rarely, underground cavities collapsing, as occasionally happens with abandoned mines. Their measurement uses the principle of the inertia of masses – every mass endeavours to maintain its current state of motion. Only when a force acts on the mass does it change its state of movement, for instance by accelerating or decelerating. If the suspension point of a pendulum is suddenly moved to the side, the pendulum mass will initially remain where it is, and then follow the movement. Scientists are hoping to record seismic waves on Mars triggered by marsquakes due to tectonic activity, or by meteorite impacts.
The centrepieces of the SEIS experiment are two sets of three extremely sensitive suspended masses that register even the smallest movements of the Martian surface. The movement of the masses is recorded electronically. A feedback mechanism attempts to keep the mass stationary, and the power required to do so serves as the measured variable. Such devices, referred to as null instruments, allow highly sensitive measurements. Motors that are able to tilt the system in all directions by small increments ensure that SEIS is perfectly levelled. The biggest challenge to performing consistently reliable seismic measurements on Mars comes in the form of the large differences in temperature between night and day, and from summer to winter. Due to the fact that materials expand when heated and contract when cooled, SEIS is equipped with a sophisticated thermal control system that comprises several insulating layers, like a Matryoshka doll. These layers compensate for external temperature differences, so that stable measurement conditions prevail within the instrument. A hemispherical dome consisting of several individual layers protects SEIS against the effects of the Martian wind and the dust transported by it.
The RISE experiment, devised by the NASA Jet Propulsion Laboratory, records shifts in the X-band radio frequencies (microwaves, 7–11 gigahertz) in order to measure tiny changes in the inclination of Mars’ rotational axis, which in turn indicate variations in homogeneity within the planet. The experiment will complement data recorded by the Viking lander in the 1970s and the Mars Pathfinder mission (1997), which provided important indications of the size of the Martian core. The third dataset provided by InSight is intended to help characterise the nutation of Mars’ axis of rotation. Nutation is a ‘wobble’ of the planet’s rotational axis caused by the inhomogeneous distribution of mass within the planet. It is a deviation from the circle that the Mars’ axis ‘traces’ in the sky. The orientation of this axis changes over long periods due to the influence of massive celestial bodies such as the Sun and neighbouring Jupiter. The circular motion of the orientation of the axis of rotation is referred to as precession. Nutation adds a small sinuous variation to the precession.
A camera attached to the telescopic arm will take 3D colour images of the area immediately around the landing site, in particular the sites for the deployment of HP3 and SEIS, and will be used to monitor the activities involved in the experiments.