Tag Archives: mars

An unexpected hazard of manned Mars exploration

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mars-surface

There are many risks associated with a manned mission to Mars. The journey itself would last between 150 and 350 days, and beside the risks associated with prolonged isolation and cramped conditions there is also the lack of real-time communication caused by the time taken for radio signals to travel the very large distances involved. Once arriving on Mars there is the presence of high levels of cosmic rays and ionising radiation to content with, all to be dealt with without proper medical facilities.

But a new paper* identifies a risk I hadn’t considered: asteroid impacts. Mars is much closer to the asteroid belt than Earth, and thus asteroid impacts are more frequent. The authors analyse the rate of crater formation on Mars and come up with a model that predicts the number of craters of a given diameter likely to be formed over a given period of time.

crater-impact-graph

Their model predicts that a one megaton (≈1 km crater) impact will occur once every 3.3 years, which would make spending any significant length of time on Mars quite hazardous. Mars’ atmosphere is much thinner than Earth’s, with an atmospheric pressure only 0.6% of ours, and so damage on the Martian surface is likely to be much more severe than for a similar impact on Earth.

* William Bruckman, Abraham Ruiz and Elio Ramos, “Earth and Mars crater size frequency distribution and impact rates: Theoretical and observational analysis”, arXiv:1212.3273.

Curiosity’s RAD750 radiation-hardened computer

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You can’t use just any computer on a Mars rover.

Two British Aerospace RAD750 single board computers, as used aboard the Curiosity rover.

Mars has no magnetic field and its atmosphere is very thin, about 0.6 kilopascals compared with Earth’s 101 kilopascals. This means that the surface of Mars is bathed in cosmic ray radiation, about 500 millisieverts per year according to an instrument aboard the Curiosity rover. This is about one thousand times the dose on Earth.

The charged particles that make up cosmic ray radiation smashing into electrons in electronic circuitry can knock them loose and cause noise and current spikes. This can turn a binary 0 into a binary 1 or vice versa (a “bit flip”) and thus any computer hardware travelling outside Earth’s protective bubble must be “hardened” to protect it from radiation.

There are a number of ways that hardware can be hardened:

  • By the use of physical shielding, such as lead or tungsten, designed to stop energetic particles from reaching components.
  • By replacing the semiconductor wafers on which chips are built with insulators such as sapphire (aluminium oxide). It is orders of magnitude harder to knock electrons loose from an insulator than from a semiconductor.
  • By replacing the Dynamic RAM (DRAM) used in regular computers with the bulkier and more expensive Static RAM (SRAM) that is less susceptible to bit flips.
  • By the use of error-correcting code in the computer’s code that checks for the damage (e.g. bit flips) caused by energetic particles.

The RAD750 single board computer manufactured by British Aerospace is a favourite of spacecraft designers, even at a cost of $200 000 per unit. The RAD750 has been used on board Curiosity, Juno, the Solar Dynamics Observatory, the Wide-field Infrared Survey Explorer, the Lunar Reconnaissance Orbiter, the Kepler habitable exoplanet observatory, the Fermi gamma-ray space telescope and the Mars Reconnaissance Orbiter.

Curiosity’s nuclear battery

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The Curiosity rover that is the main part of the Mars Science Laboratory mission is very different from its predecessors Sojourner and the twin rovers Spirit & Opportunity.

L-R: Spirit/Opportunity, Sojourner and Curiosity.

L-R: The wheels of Sojourner, Spirit/Opportunity and Curiosity.

Curiosity is nearly twice as long as Spirit/Opportunity and has more than five times the mass; at 2.1 metres in height it is taller than most of the people that built it.

For me, the most interesting difference between Curiosity and the other Mars rovers is its power source. Both Sojourner and Spirit/Opportunity were powered by solar cells but Curiosity is powered by a radioisotope thermoelectric generator (RTG), in particular the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) built by Pratt & Whitney’s Rocketdyne division.

Curiosity‘s RTG is the large unit attached to the rover’s rear.

The main problem with using solar cells for power is that the cells only work during daylight hours and don’t function well at high latitudes where there is less sunlight; Spirit/Opportunity‘s cells only worked at full strength for about four hours per day, producing about 900 watt hours (about 3.2 megajoules) per day at best. Mars is covered in fine dust and dust covering solar panels was a problem for the Spirit and Opportunity rovers, though this dust was occasionally blown away by high winds.

Spirit‘s solar panels before and after a “cleaning event”.

RTGs work via the Seebeck effect, where a difference in temperature between between the two junctions of a thermocouple cause an electric current to be produced. The heat source in an RTG is the decay of a radioactive isotope; in the case of most RTGs this isotope is plutonium-238 in the form of plutonium dioxide. Pu-238 is a nearly pure alpha emitter and therefore requires only minimal shielding.

A pellet of 238PuO2 glows red hot from internal radioactive decay.

The MMRTG uses 32 marshmallow-sized plutonium pellets and will initially produce about 125 watts of electrical power (from 2000 watts of thermal power), but this will drop off over time as the plutonium decays. The MMRTG will consistently produce about 2500 watt hours of electricity per day compared with Spirit/Opportunity‘s average of 600 Wh and this will enable Curiosity to operate in all seasons and at all times of day.

Curiosity’s MMRTG before installation.