Heat Flow and Physical Properties Package
Components of the HP3 heat flow probe
|Manufacturer||German Aerospace Center (DLR)|
|Instrument type||infrared radiometer|
|Function||Geophysics of Mars|
|Mission duration||2 years on Mars|
|Began operations||Landing: 26 November 2018|
|Mass||3 kg (6.6 lb)|
|Power consumption||2 watts|
|Spacecraft||InSight Mars lander|
|Launch date||5 May 2018, 11:05UTC|
|Rocket||Atlas V 401|
|Launch site||Vandenberg SLC-3E|
The Heat Flow and Physical Properties Package (HP3) is a science instrument onboard the InSight lander that features a probe to study the heat flow and other thermal properties of Mars. The probe is designed to penetrate 5 m below Mars' surface. In March 2019, HP3 successfully burrowed a few centimeters, but then became unable to continue. As of September 2019[update], work is underway to resolve the issue.
Referred to as a "self-hammering nail" and nicknamed "the mole", it was designed to burrow 5 m (16 ft) below the Martian surface while trailing a tether with embedded heat sensors to measure the thermal properties of Mars' interior, and thus reveal unique information about the planet's geologic history.
HP3 was provided by the German Aerospace Center (DLR), and the tractor mole portion of the instrument was designed by the Polish company Astronika and the Space Research Centre of the Polish Academy of Sciences under contract and in close cooperation with DLR.
The mission aims to understand the origin and diversity of terrestrial planets. Information from the HP3 heat flow package is expected to reveal whether Mars and Earth formed from the same material, and determine how active the interior of Mars is today. Additional science goals include determining the thickness of Mars' crust, the composition of its mantle, and thermal characteristics of the interior, such as the temperature gradient and heat flux.
Together with the seismometer, the mission will estimate the size of Mars' core and whether the core is liquid or solid. The vibrations generated by the mole will be monitored by SEIS to learn about the local subsurface.
In addition to the mole, HP3 includes an infrared radiometer (HP3-RAD) mounted to the landing platform, also contributed by DLR. The HP3 heat flow probe is made up of the following subsystems
- Support Structure (SS) a housing that includes:
- Infrared radiometer (HP3-RAD) for measuring surface temperature.
- Back end electronics (BEE) electronic control unit
- Mole penetrometer for burrowing beneath the surface
- TEM-A active thermal conductivity sensor
- STATIL tiltmeter for determining orientation and direction of the mole.
HP3 was conceived by Gromov V. V. et al. in 1997, and first flown as the PLUTO instrument on the failed 2003 Beagle 2 Mars lander mission. HP3 evolved further and it was proposed in 2001 for a mission to Mercury, in 2009 to the European Space Agency as part of the Humboldt payload onboard the ExoMars lander, in 2010 for a mission to the Moon, and in 2011 it was proposed to NASA's Discovery Program as a payload for InSight Mars lander, known at that time as GEMS (Geophysical Monitoring Station). InSight was launched on 5 May 2018 and landed on 26 November 2018.
Deployment and operation
The mole penetrator unit is designed to be placed near the lander in area about 3-m long and 2-m wide. The total mass of the system is approximately 3 kg (6.6 lb) and it consumes a maximum of 2 watts while the mole is active.
For displacement, the mole uses a motor and a gearbox (provided by maxon motor ag) and a roller that periodically loads a spring connected to a rod that functions as a hammer. After release from the cam, the hammer accelerates downwards to hit the outer casing and cause its penetration through the regolith. Meanwhile a suppressor mass travels upwards and its kinetic energy is compensated by gravitational potential and compression of a brake spring and wire helix on the opposite side of the mole.
The burrowing mole is a pointed cylinder with a smooth outer surface approximately 35 cm (14 in) in length and 3.5 cm (1.4 in) in diameter. It contains a heater to determine thermal conductivity during descent, and it trails a tether equipped with precise heat sensors placed at 10 cm (3.9 in) intervals to measure the temperature profile of the subsurface.
In principle, every 50 cm (20 in) the probe puts out a pulse of heat and its sensors measure how the heat pulse changes with time. If the crust material is a thermal conductor, like metal, the pulse will decay quickly. The mole is first allowed to cool down for two days, then it is heated to about 10 °C (50 °F) over 24 hours. Temperature sensors within the mole measure how rapidly this happens, which tells scientists the thermal conductivity of the soil. Together, these measurements yield the rate of heat flowing from the interior.
In March 2019, the HP3 began burrowing into the surface sand, but became stalled after several inches/cm by what was initially suspected to be a large rock. Further analysis and testing with a replica model on Earth suggested the problem may be due to insufficient friction. In June 2019, more evidence for this was revealed when the support structure was lifted off of the HP3 mole. The Martian regolith appeared to be compressed, leaving a gap around the probe. A proposal is in place to use the lander's robotic arm to press on the soil near the probe, to increase soil friction.
HP3-RAD Infrared Radiometer
The HP3 includes a separate infrared radiometer for measuring surface temperatures, contributed by DLR and based on the MARA radiometer for the Hayabusa2 mission. HP3-RAD uses thermopile detectors to measure three spectral bands: 8–14 μm, 16–19 μm and 7.8–9.6 μm. HP3-RAD has a mass of 120 g (4.2 oz) (about a quarter pound).
Background about infrared radiometers includes some important Mars science history. They were sent to Mars in 1969 as one of four major instruments on the Mariner 6 and Mariner 7 flyby spacecraft, and the observations helped to trigger a scientific revolution in Mars knowledge. The Mariner 6 & 7 infrared radiometer results showed that the atmosphere of Mars is composed mostly of carbon dioxide (CO2), and they were also able to detect trace amounts water on the surface of Mars.
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