IN AUGUST 1976, four years after the final mission, a 6-tonne spacecraft landed on the moon in the Mare Crisium. Over the course of a single day, it sent back spectacular pictures of the lunar surface, gathered rock samples and then blasted off for the journey home. Luna 24 was an uncrewed Russian sample-return mission, the last of only three to make it safely home.
In the bizarre logic of cold war politics, the Luna missions were seen as a defeat for Russia in the battle to reach the moon because they were mere robots. In reality, was a stupendous technical achievement. Luna 17 and 21 deployed the first automated extraterrestrial rovers, and they survived for months and travelled for several kilometres across the lunar surface. Luna 16, 20 and 24 are still the only robotic missions to have brought back soil samples from another world. Luna 24 drilled a 2-metre core sample, nearly as deep as the ones dug by Apollo astronauts. And that was 30 years ago. Since then, not a single spacecraft has landed intact on the lunar surface.
All that is about to change. Over the next 10 years or so, a new generation of moon missions is set to revolutionise our understanding of our nearest neighbour. With their modest budgets and limited payloads, these uncrewed spacecraft will have more in common with Luna than Apollo. The missions will be part of an international effort by , the , the , , and perhaps others, to deepen our understanding of the moon ahead of US plans to send humans back there by 2020.
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In contrast to the competitive space race of the 1960s, the return to the moon is billed as a highly cooperative venture. This March, for instance, NASA announced a proposal to link up the uncrewed landers of any nation that chooses to participate in the International Lunar Network (ILN). This network will span the lunar globe, listening, measuring and analysing the surface in unprecedented detail.
The moon still has many secrets to tell us. To start with, no one knows what the inner moon is made of. Does it have a solid metallic core, a molten core, or perhaps both? “We don’t actually know that the moon has a core at all,” says Maria Zuber, the principal investigator for a NASA-funded moon orbiter called the Gravity Recovery and Interior Laboratory (GRAIL), which will launch in 2011. In other words, the moon could just be rock all the way through.
“The return to the moon is billed as a highly cooperative venture”
GRAIL may be able to shed light on that puzzle. It will map the moon’s gravitational field with 1000 times the accuracy of any previous mission. Such knowledge will be essential for precision landings once astronauts start flying to the moon again. But the gravitational experiment also has a purely scientific purpose. If, for example, the moon has a solid inner core inside a liquid outer core, as Earth does, GRAIL should be able to detect the wobbling of that inner core and reveal its presence once and for all.
Even if GRAIL cannot detect the moon’s core directly, it might be able to determine the moon’s inner structure with the help of the seismic probes that will form part of the ILN. Lunar scientists have long dreamed of setting up a seismic network on the moon. “Seismology is a way of putting a stethoscope on a planet,” says Paul Spudis of the Lunar and Planetary Institute in Houston, Texas. “By hearing the way seismic waves propagate in a body, you can say a lot about what kinds of rocks the waves travel through, how they’re arranged, and what their density structure is. In fact, our knowledge of Earth’s interior primarily comes from its seismicity.”
The seismic network won’t be entirely novel. Four of the six Apollo missions carried seismometers that functioned perfectly until they were turned off in 1978. But seismometers are imperfect stethoscopes. They can only see about as deep as the distance between seismic stations. And because the Apollo seismometers were separated by just a few hundred kilometres, they could not provide answers about the lunar core. Nor could they really tell anything about the seismicity of the far side of the moon, where there were no sensors.
Despite its limitations, the Apollo network provided some tantalising scraps, such as the discovery of moonquakes and the fact that they come in two flavours: deep and shallow. The Apollo instruments recorded several thousand deep moonquakes, 700 to 1000 kilometres beneath the surface – much deeper than any earthquake. They appear to be caused by tidal deformation of the rock caused by Earth’s gravity squeezing and stretching the moon as it orbits, but no one knows exactly what’s happening. The stresses that we know about should not be large enough to cause rocks to break. Catherine Johnson of the University of British Columbia in Vancouver, Canada, who has re-analysed the Apollo data, suggests that a “phase transition” or change in the rocks’ structure at that depth might make them brittle. “But it could be something else we haven’t even thought of,” she adds. “We’re just grasping at what we know from Earth.”
Shallow moonquakes are even more of a puzzle. They are very rare; the Apollo seismometers recorded 28 in eight years, with magnitudes up to 5.8. That would be strong enough to cause significant structural damage to any man-made base stationed on the moon, especially because the rigidity of the crust makes moonquakes last longer than earthquakes. “The energy [of the moonquake] is not damped,” says Clive Neal of the University of Notre Dame, Indiana. “To keep a lunar base airtight when it’s shaking for 10 minutes, you’ll need a lot of duct tape.”
Basically, we know nothing about shallow moonquakes. No one knows what sets them off, nor whether they occur 200 kilometres beneath the surface or 1 kilometre, nor whether they are randomly distributed or tend to happen in certain places – which would obviously make them a bad site for building a moon base. The new seismic network should help to answer these questions. “Improving on the Apollo measurements is like big-game hunting from a helicopter,” says Bruce Banerdt of the Jet Propulsion Laboratory in Pasadena, California. “It’s so easy that it’s not much of a sport.”
Another item on the “to-do list” for the lunar network is to measure the flow of heat coming from inside the moon. This will reveal how rapidly the satellite cooled down after it formed and how much heat is being generated by radioactive elements in its interior. Such information will help the interpretation of the seismic findings, because the velocity of seismic waves depends on rock temperature. It would also help nail down what the moon is made of. If the composition turns out to be quite different from Earth’s, it would throw a spanner in the works because the moon is supposed to have formed from the debris of a collision between Earth and a Mars-sized planet.
Setting up a network of heat-flow sensors could be tricky because they need to be buried deep in the lunar soil. Apollo astronauts had hoped to drill holes 3 metres deep but were shocked at how hard-packed the regolith was and were unable to reach that depth. If it’s hard for an astronaut, it will be even harder for a robot – but, as Luna 24 showed, perhaps not impossible.
A third riddle that the ILN could address has to do with the moon’s magnetism. Unlike Earth, the moon has no magnetic field. Yet NASA’s Lunar Prospector, which orbited and was deliberately crashed into the moon in the late 1990s, discovered large patches of magnetised material on the surface. How it got there remains a mystery. Is the field a remnant of an earlier era when the moon had a global magnetic field? Or was the material somehow magnetised by a meteorite impact? Lunar soil is known to contain microscopic grains of iron. An impact could have melted or vaporised the iron and produced a transient magnetic field that became permanently imprinted on the iron grains when they cooled. These imprinted magnetic fields could help to shield a future moonbase from the solar wind and protect electronic systems and astronauts from being bombarded by charged particles, as the Earth’s field does for us.
For all these reasons the nodes of the ILN will probably include a seismometer, a heat-flow probe and a magnetometer. However, many details remain to be ironed out. First, there is the question of location. To work well, a seismic network would need to have at least six stations, distributed widely over the moon, with some on the far side, some on the near side, and some near the poles. Second, all of the stations have to be working at the same time, which doesn’t sound like a problem until you realise that they are likely to arrive on the moon in piecemeal fashion, launched by different space agencies over a period of years. They will need to have a lifetime at least that long to operate as a network.
An even more daunting problem is providing power for the stations during the long 14-day lunar nights, when solar power is not available (see “Got any spare plutonium, pal?”). If half the nodes are offline at any given time because they lack power in the darkness, the network would be crippled. To make matters worse, the stations on the far side will have no direct line of sight to Earth, so they will need an orbiting satellite to act as a communications relay. This poses a classic chicken-and-egg conundrum: do you put the satellite up first, when there are no stations for it to communicate with, or do you set up the science nodes first, when they have no way of reporting their results? Solving this question will require detailed planning between the space agencies interested in sending probes.
Some of these partners have well-developed plans. According to senior research coordinator Bernard Foing, the European Space Agency (ESA) has been developing a set of miniaturised instruments for 10 years which will fly on ESA’s ExoMars mission in 2013. It is a virtual blueprint for what an ILN node should look like, as it sports a seismometer, a heat-flow probe and a magnetometer. The same package could go to the moon as part of ESA’s proposed Moon Next mission. The UK is also studying the feasibility of a mission called MoonLITE, which could land three or four small science stations on the moon, according to David Parker, the director of space science at the British National Space Centre in London. “The MoonLITE orbiter could offer a data-relay link as well,” Parker adds, although the details have yet to be finalised.
The sudden surge in planned missions is starting to change the landscape of lunar science. There are basically two things that are sure to attract scientific interest: money and information. And the money is beginning to flow. In April, NASA cut the ribbon for a new Lunar Science Institute in Moffett Field, California, which will orchestrate lunar science in the US and eventually in other countries as well. The institute will make awards totalling about $5 million this year, supporting as many as 50 lunar scientists by the end of 2008.
As for the data, that will soon start flooding in from orbiters such as the Japanese Kaguya mission and the Chinese Chang’e which both launched last year, the Indian Chandrayaan-1 that will launch in the next few months, NASA’s Lunar Reconnaissance Orbiter launching in November and then GRAIL in 2011. Then it’ll be the turn of the landers such as those that will be part of the ILN. By mid-decade, the Lunar Science Institute will be a busy place indeed.
Don’t think planetary scientists haven’t noticed. At this year’s Lunar and Planetary Science Conference in League City, Texas, the lunar science sessions were packed. “Back in 1987, you could have fitted them in someone’s garage,” says Neal. “If you had told me that we would someday have two lunar sessions in the biggest room in the conference centre, and have standing room only, I would have laughed.” Thirty years on, the descendants of Luna 24 are about to make an impact.
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Got any spare plutonium, pal?
Building an International Lunar Network comprising uncrewed landers packed with scientific instruments may be the obvious next step towards understanding our nearest neighbour. But the devil is in the detail. And there is no detail more devilish than the question of how to keep the planned science outposts running during the long lunar night.
Nuclear power may not be an option. The world is running out of plutonium, which has been the power source for space missions from Apollo to Cassini. Plutonium does not occur naturally; it is a by-product of nuclear-weapons production and certain kinds of nuclear reactors. Hardly any is being made any more since the end of the cold war and improvements in the design of reactors. “Plutonium is a pretty nasty element, and you don’t want to have it around if you don’t have a use for it,” says Bruce Banerdt of the Jet Propulsion Laboratory in Pasadena, California.
NASA has already committed the last of its plutonium to existing missions, so there is none left to power the ILN stations. The job of finding more has fallen to James Green, the director of the Planetary Sciences Division at NASA. Green is negotiating with Russia, which has 10 kilograms left. NASA is planning to buy 5 kilograms this year and 5 kilograms next year.
It is not clear how much is needed for a single ILN station; probably only a few grams if the only objective is to keep the instruments warm, but more to actually power them. To convert heat to electricity, NASA is developing Stirling engines that would be four to five times as efficient as its current radioisotope thermal generators and so would use far less plutonium.
In the meantime, development of NASA’s first ILN outpost is proceeding on the assumption that they will not be nuclear-powered. The options are limited: solar power during the day, perhaps batteries or fuel cells or turning the machines off at night. However, stations near the poles have the advantage of near-continuous sunlight, and may be able to run on solar power alone. That is one reason why the first two ILN outposts will be placed at the poles.
According to Paul Spudis of the Lunar and Planetary Institute in Houston, Texas, the scramble for plutonium is the first harbinger of a problem that will only get worse – how to power future lunar bases. “Ultimately, if people are going to live on other worlds, they will need nuclear reactors,” he says. “It’s the best source of reliable, continuous power that we know of.”
