Exploring the red planet in leaps and bounds

Posted by mjs76 at Nov 22, 2010 09:00 AM |
Space scientists in Leicester are working on plans for a revolutionary new type of Martian rover – one that doesn’t have any wheels.
Exploring the red planet in leaps and bounds

image: NASA/ESA/Cornell/SSI

Ever since Lunokhod 1 landed on the Moon in November 1970, the accepted way to travel around celestial bodies has been big, chunky wheels. That first ever rover (there was supposed to be one in 1969 but the rocket failed) had eight wheels, each a sort of lightweight framework arrangement.

The Americans used four big, soft tyres on the astronaut-carrying Lunar Rovers which accompanied Apollos 15, 16 and 17 in 1971 and 1972. The following year, Lunokhod 2 was successfully deployed by the Russians, similar in design to its predecessor.

In December 1971, the Soviet space programme landed Mars 3 on the red planet several years before NASA got there. This lander carried the innovative Prop-M rover which, unlike the Lunokhods, was actually designed to ski across the Martian surface while tethered to its base station. Sadly we’ll never know whether that would have worked as, 14.5 seconds after touchdown, Mars 3 died and was never heard from again.

There was then a bit of a gap in planetary rover development; neither of the Viking landers, for example, could move an inch. But when NASA returned to Mars in 1997 with Mars Pathfinder they sent along Sojourner, a six-wheeled, untethered rover which proved a tremendous success, ranging up to half a kilometre from base at the terrifying speed of up to 36 metres per hour.

NASA followed Sojourner with the double whammy of Spirit and Opportunity in 2004, each of which had six aluminium wheels, controlled independently by six motors. (Here’s a terrific feature on NASA’s Mars Rover website, all about the development of Spirit and Opportunity’s wheels.)

Intended to operate for 90 sols (Martian days = 24hours, 39 minutes), Spirit lasted more than five years before becoming irretrievably stuck in soft soil. Plucky little Opportunity is still going and is currently midway through an epic, two-year journey to the giant crater Endeavour. And therein lies the problem with wheeled exploration. At an average speed of 100 metres per day, Opportunity is expected to take two Earth years to travel the 12km from Victoria crater to Endeavour. On Earth, that sort of journey would take less than 15 minutes in a car and frankly you could walk it in half a day.

Yes, there’s lots of stuff to explore along the way but on the other hand there’s no guarantee that the rover will still be working properly when it gets there. If it gets there. And there’s the rub: when you get a vehicle onto Mars, you want to examine as many different types of ground as possible but the ground where you land is likely to be very similar to the bit of ground next to it. Or think of it this way: if the Martians sent a wheeled rover to Earth and it landed in Hyde Park, their research would only be applicable to the bits of our planet that are covered in neatly mown lawns.

Which brings us to the paper by three staff from our Department of Physics and Astronomy, recently published in Proceedings of the Royal Society A, describing a propulsion system for a ‘Mars hopping vehicle’. The idea is that rather than trundling slowly across the surface, the vehicle would move from one site to the next in a big leap, using carbon dioxide extracted from the Martian atmosphere as a propellant. A 12km journey like Victoria to Endeavour could potentially be managed in a few weeks.

The advantages are enormous and obvious. To date, only tiny, tiny portions of the Martian surface (and subsurface) have been closely examined but a hopping vehicle could move quickly and easily from one location to another several kilometres away which would, crucially, be very different. Far more research could therefore be done during the lifetime of a vehicle. Cameras could operate during each leap to provide high resolution images of the ground being traversed (with the option to leap back later if something particularly interesting was spotted).

Big rocks and soft sand wouldn’t present a problem to journeys, although landing sites would of course need to be selected with care. But those landing sites could be in areas that wheeled rovers could never access, of which the most exciting are the floors of craters. These are, in effect, ‘excavated’ sites where subsurface material and rock strata – the Holy Grails of planetary geologists – have been exposed.

It really looks like hopping rovers are the future (perhaps working in collaboration with wheeled vehicles which do still have certain advantages).

The Leicester team of Dr Hugo Williams, Dr Richard Ambrosi and Dr Nigel Bannister, who have collaborated with Astrium UK Ltd and the Center for Space Nuclear Research in Idaho, didn’t invent the idea of a Mars hopping vehicle. The concept dates back as far as 2000 when Robert Zubrin et al developed a working prototype (Word doc) of the ‘gashopper’ as part of a NASA research contract - with a helium balloon to simulate Mars gravity. The Leicester paper looks specifically at the design and materials that could be used for propulsion.

rocket schematic.gif
Schematic of the radioisotope thermal rocket motor concept

The basic principle behind hopper propulsion is that a ceramic or metallic ‘core’ is heated by some means and CO2 is collected from the atmosphere. When the vehicle is ready to hop, the CO2 is passed over the heated core, rapidly expanding as it does so, and is forced out of a nozzle below the craft, effectively acting like a rocket motor – but without causing any pollution, because the exhaust is just returning to the atmosphere from whence it came. And Newton’s Third Law does the rest.

The hopper would not be able to leap repeatedly across the surface like a giant, robotic kangaroo because after each landing it would need to remain in situ for at least a few days to collect more CO2. But useful scientific work could be done in each resting place on the way to the eventual destination. The question is (well, the questions are): what should the core be made of, what shape should it be and what is the best way to heat it?

Previous studies have generally concentrated on electric heating of the core, using energy derived from solar panels, but Williams et al have taken a different tack and examined the feasibility of a radioactive isotope as a heat source. They looked in particular at four factors:

  • A basic model for a thermodynamic ‘heat capacitor’ core; it was assumed that the radioisotope would be embedded within the core although it could be separate.
  • A comparison of a ‘channel’ core vs a ‘pebble-bed’ core; in other words, whether the CO2 would travel straight through holes in the core or permeate around spherical lumps of stuff. The latter option obviously offers more surface area for heat transfer but requires a greater volume for the same mass.
  • A look at how big this propulsion set-up would need to be and how that might affect the overall size of the hopper.
  • A comparison of possible materials to make the core out of.

The team’s baseline calculations used approximate figures which erred on the side of caution: safely generous in some parts, cautiously conservative in others. Assuming a hopper with a mass of 326kg plus 43kg of propellant and 4.22 kg of radioisotope (a mixture of plutonium oxide PuO2 and americium sesquioxide Am2O3), a standard hopping distance of one kilometre seems eminently feasible. As the paper points out: “even this conservative initial target represents an enormous increase in mobility over current rovers if it can be repeated approximately every few days.”

As to the actual material for the core, 29 substances were considered by plotting their specific heat capacity (the energy required to raise the temperature of a given mass by one degree) against their melting point on a graph, from which four were selected to be examined in detail: beryllium, boron carbide, silicon carbide and (by way of reference) an aerospace grade titanium alloy.

One essential factor to consider in any such design is safety and the protection which the core material would offer to the radioisotope. Just because it’s going to Mars doesn’t mean that the vehicle – which of course starts its journey on this planet - won’t have to meet the same exacting standards as all existing radioisotope power sources. In fact the hopper research is just part of a larger programme of space/nuclear work carried out by Leicester space scientists in collaboration with materials scientists in our Department of Engineering and at Nanoforce, a spin-out company owned by Queen Mary University of London. The safety of space-based nuclear power systems is very high priority and the techniques and processes being developed have benefits for terrestrial nuclear power too.

Calculating the ‘hopping coefficient’ (or Chop) for different configurations of vehicle, propellant and core showed that variations between the materials were in fact pretty small but that a pebble core offered about ten per cent better value than a channel core. So one of the paper’s key findings is that “Chop can, for practical engineering design purposes, be considered independent of material and initial core temperature” and is instead principally dependent on the shape and structure of the core itself.

Serious exploration of large swathes of Mars can only be achieved by travelling a bit faster than 100m per day, which is pretty much the limit for wheeled rovers. A one kilometre hop every week is not that much faster if you think about it but it allows much more time for soil sampling and other experiments which simply can’t be done while trundling along. A journey in leaps and bounds would brush aside the problems of large boulders, wide cracks and soft sand – and there is the potential to steadily increase the value of Chop for each generation of rovers to the point where a journey between craters could be made in days, not years.

Of course, there’s nothing entirely new in the Solar System. How did Captain Scarlet and his colleagues get around the Lunar surface on TV back in 1967? They used a Moonhopper...