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Mirrors on the moon
”New analysis of the lunar samples has prompted us to rethink some key dates in the moon’s history” see from Earth? Can we put together a detailed history of the lava flows that formed the basalts of the lunar seas? Can we find any samples from deep inside the moon? These are all seen as powerful reasons for returning to the moon. The big picture requires more samples, more data and lots more context. “There’s no lack of targets and scientific questions,” says Gary Lofgren, curator of NASA’s lunar rock collection at Johnson Space Center in Houston. “It’s not just about the moon but about the solar system’s history. That’s the lesson that we have learned from Apollo.” ■ Dana Mackenzie is author of The Big Splat, or How Our Moon Came To Be (John Wiley & Sons) 32 | NewScientist | 11 July 2009
DAN LONG
the moon for chemical analysis and isotopic dating, we might never have made these key discoveries about our planet’s history. So do the Apollo rocks harbour any more secrets? All 2200 samples have been studied, and Randy Korotev, a lunar geochemist at Washington University in St Louis, Missouri, says this means there is unlikely to be anything worldshaking left to discover from them. They may yet hold some more subtle secrets, however. “We are constantly developing better tools and asking better questions,” Korotev says. In particular, the instruments for dating mineral samples have become more sophisticated, enabling researchers to determine the age of ever smaller samples, such as tiny mineral grains within a rock. In the past two years, these techniques have prompted a rethink of some key dates in lunar history. A team at the Swiss Federal Institute of Technology dated the formation of the moon’s magma oceans – and, by inference, the moon itself – to 20 to 30 million years later than we thought, to about 4.5 billion years ago (Nature, vol 450, p 1206). And Alexander Nemchin of Curtin University of Technology in Perth, Western Australia, with five colleagues, dated a lunar zircon at 4.417 billion years old, which pins down the likely time when the last of the magma oceans solidified (Nature Geoscience, vol 2, p 133). What the Apollo samples will never do is answer some of the remaining big-picture questions. What will we find on the far side – the half of the moon’s surface we can never
Mirrors on the moon Reflectors planted on the lunar surface may provide the first cracks in Einstein’s theory of gravity, says Stuart Clark EACH clear night when the moon is high in the sky, a group of astronomers in New Mexico take aim at our celestial neighbour and blast it repeatedly with pulses of light from a powerful laser. They target suitcase-sized reflectors left on the lunar surface by the Apollo 11, 14 and 15 missions, as well as by two Russian landers. Out of every 300 quadrillion (1015) photons that are sent to the moon, about five find their way back. The rest are lost to our atmosphere, or miss the lunar reflectors altogether. From this small catch, the team can assess the movement of the moon to an accuracy of a millimetre or two – a measurement so precise that it has the potential to show up any cracks in Einstein’s general theory of relativity. If that’s what it does, this lunar laser-ranging experiment will become Apollo’s greatest scientific legacy. Lunar laser ranging has a long history. “I wasn’t even born when the first reflectors were left on the moon,” says 39-year-old Tom Murphy from the University of California, San Diego, who heads the experiment at the Apache Point Observatory in Sunspot, New Mexico (pictured). In the mid-1960s, when NASA asked for suggestions for experiments that could be carried out on the moon, laser ranging was mooted but no one really knew what to do with it. There was a suggestion to look for gradual
changes in Newton’s gravitational constant, but this would have meant running the experiment for over 20 years – something no one was prepared to commit to. Then a young researcher called Ken Nordtvedt had an idea. Through a fiendish piece of mathematics, he showed that, with just a few years’ worth of data, lunar laser ranging could be used to test a cornerstone of general relativity known as the equivalence principle. It starts from the idea that a body has two kinds of mass. The first, called gravitational mass, is the mass that produces and feels the pull of gravity. The second is inertial mass, which describes how hard it is to move an object out of its current state of motion – or lack of it. The equivalence principle asserts that the two are exactly equal. The equivalence principle holds in general relativity, but in the mid-1960s, a rival theory developed by American physicists Carl Brans and Robert Dicke was gaining ground. By postulating a fifth force of nature, the Brans-Dicke theory of gravitation broke the equivalence principle and predicted a 13-metre perturbation in the moon’s orbit. Nordtvedt showed that analysing light signals reflecting from the moon could prove the existence of such a disturbance. Dicke was a member of the Apollo science advisory committee. He listened as the
astronauts complained that many of the proposed experiments were too fiddly to be performed when they were wearing their spacesuits. So he suggested that they simply set down some mirrors, angle them roughly at Earth and let astronomers do the rest. The Brans-Dicke theory became an early victim of laser-ranging’s success. The measurements were precise enough to show that gravitational mass and inertial mass are indeed equivalent, to an accuracy of one part in 1013. That severely constrains how strong a fifth force of nature could be. Still, new theoretical approaches to gravity such as string theory, and antigravity theories such as quintessence, all seem to imply that the equivalence principle must break. “We’re already in the regime where violations might be expected. Any push to greater accuracy is theoretically relevant,” says Murphy.
Millimetre precision To that end, ground-based improvements in technology have allowed the team to move from a precision of a few centimetres to just a few millimetres. The trouble is that the analysis has not kept pace with the wealth of measurements. At the millimetre scale, there are a number of new effects to deal with, such as solar radiation pressure, which pushes the moon’s entire orbit from its calculated path by about 4 millimetres. These must all be included in the analysis. On top of this, general relativity has to be gone through with the mathematical equivalent of a fine-tooth comb to determine whether there are subtleties that have been neglected so far, yet are relevant at millimetre scales. “The important thing is that we are collecting data. That is money in the bank to us,” says Murphy. “The analysis will follow once we are satisfied with our new moon model.” As for that proposed 20-year observation of Newton’s gravitational constant, which sounded so impossible in the 1960s, Murphy’s team have pinned down any changes in the constant to less than one part in 1012 per year. This has provided another powerful constraint on new physics and cosmological theories. “It’s amazing to think that it’s been such a technical success and that we can keep pushing the technology to greater accuracy,” says Nordtvedt, now emeritus professor at Montana State University in Bozeman. “I would not have counted on a 40-year observation. It is a tremendous bonus.” ■ Stuart Clark’s latest book is Galaxy (Quercus)
Another small step Astronaut, cosmonaut or taikonaut? Greg Klerkx asks whose footprints will be next
I
N JANUARY this year, the Indian space probe, Chandrayaan-1, embarked on something of a nostalgia tour. Over the course of four days, the craft flew high above the lunar surface, methodically mapping the landing sites of all six crewed Apollo missions that touched down on the moon. The purpose, according to P. Sreekumar of the Indian Space Research Organisation, was to confirm the Apollo missions’ findings about the moon’s surface and rocks. Yet it may have had an additional motive: India is among a handful of countries determined to land a human on the moon. The architects of the cold war-era space race could hardly have imagined the number of nations that would follow their lead. India, Japan, China and the European Space Agency have added to the traffic in lunar orbit, with most of these missions carrying scientific instruments from other countries. Four of the six members of this lunar explorer’s club – the US, Russia, China and India – have professed an intention to send people to the moon. Which country, if any, will succeed?
If technology were the only factor, this new race would be America’s to lose. NASA’s Constellation programme, unveiled in 2006, has the ultimate goal of establishing a permanently inhabited base on the moon. To do this, the programme includes a design for the Orion crew capsule with up to six seats, the Altair lunar lander, and a new booster system called Ares, which uses components from both the Apollo-era Saturn rocket and the space shuttle. There are two versions of Ares: Ares I, designed to transport crew and the Orion capsule, and the more powerful Ares V, designed to transport supplies and the Altair lunar lander. Under current plans, a crewed lunar mission is set for 2020. First, Ares I would be launched into Earth orbit, where the Orion capsule containing the crew would detach. The capsule would then rendezvous and link up with the Altair lander once it had reached Earth orbit, before heading to the moon. All is not well with Constellation, however. NASA’s 2010 budget continues to support the programme, though looming cuts could see the Ares >
The US flag hasn’t stopped other nations lusting after lunar glory
NASA
Only a tiny fraction of this laser light will make it back from the moon
11 July 2009 | NewScientist | 33
DAN LONG
the moon for chemical analysis and isotopic dating, we might never have made these key discoveries about our planet’s history. So do the Apollo rocks harbour any more secrets? All 2200 samples have been studied, and Randy Korotev, a lunar geochemist at Washington University in St Louis, Missouri, says this means there is unlikely to be anything worldshaking left to discover from them. They may yet hold some more subtle secrets, however. “We are constantly developing better tools and asking better questions,” Korotev says. In particular, the instruments for dating mineral samples have become more sophisticated, enabling researchers to determine the age of ever smaller samples, such as tiny mineral grains within a rock. In the past two years, these techniques have prompted a rethink of some key dates in lunar history. A team at the Swiss Federal Institute of Technology dated the formation of the moon’s magma oceans – and, by inference, the moon itself – to 20 to 30 million years later than we thought, to about 4.5 billion years ago (Nature, vol 450, p 1206). And Alexander Nemchin of Curtin University of Technology in Perth, Western Australia, with five colleagues, dated a lunar zircon at 4.417 billion years old, which pins down the likely time when the last of the magma oceans solidified (Nature Geoscience, vol 2, p 133). What the Apollo samples will never do is answer some of the remaining big-picture questions. What will we find on the far side – the half of the moon’s surface we can never
Mirrors on the moon Reflectors planted on the lunar surface may provide the first cracks in Einstein’s theory of gravity, says Stuart Clark EACH clear night when the moon is high in the sky, a group of astronomers in New Mexico take aim at our celestial neighbour and blast it repeatedly with pulses of light from a powerful laser. They target suitcase-sized reflectors left on the lunar surface by the Apollo 11, 14 and 15 missions, as well as by two Russian landers. Out of every 300 quadrillion (1015) photons that are sent to the moon, about five find their way back. The rest are lost to our atmosphere, or miss the lunar reflectors altogether. From this small catch, the team can assess the movement of the moon to an accuracy of a millimetre or two – a measurement so precise that it has the potential to show up any cracks in Einstein’s general theory of relativity. If that’s what it does, this lunar laser-ranging experiment will become Apollo’s greatest scientific legacy. Lunar laser ranging has a long history. “I wasn’t even born when the first reflectors were left on the moon,” says 39-year-old Tom Murphy from the University of California, San Diego, who heads the experiment at the Apache Point Observatory in Sunspot, New Mexico (pictured). In the mid-1960s, when NASA asked for suggestions for experiments that could be carried out on the moon, laser ranging was mooted but no one really knew what to do with it. There was a suggestion to look for gradual
changes in Newton’s gravitational constant, but this would have meant running the experiment for over 20 years – something no one was prepared to commit to. Then a young researcher called Ken Nordtvedt had an idea. Through a fiendish piece of mathematics, he showed that, with just a few years’ worth of data, lunar laser ranging could be used to test a cornerstone of general relativity known as the equivalence principle. It starts from the idea that a body has two kinds of mass. The first, called gravitational mass, is the mass that produces and feels the pull of gravity. The second is inertial mass, which describes how hard it is to move an object out of its current state of motion – or lack of it. The equivalence principle asserts that the two are exactly equal. The equivalence principle holds in general relativity, but in the mid-1960s, a rival theory developed by American physicists Carl Brans and Robert Dicke was gaining ground. By postulating a fifth force of nature, the Brans-Dicke theory of gravitation broke the equivalence principle and predicted a 13-metre perturbation in the moon’s orbit. Nordtvedt showed that analysing light signals reflecting from the moon could prove the existence of such a disturbance. Dicke was a member of the Apollo science advisory committee. He listened as the
astronauts complained that many of the proposed experiments were too fiddly to be performed when they were wearing their spacesuits. So he suggested that they simply set down some mirrors, angle them roughly at Earth and let astronomers do the rest. The Brans-Dicke theory became an early victim of laser-ranging’s success. The measurements were precise enough to show that gravitational mass and inertial mass are indeed equivalent, to an accuracy of one part in 1013. That severely constrains how strong a fifth force of nature could be. Still, new theoretical approaches to gravity such as string theory, and antigravity theories such as quintessence, all seem to imply that the equivalence principle must break. “We’re already in the regime where violations might be expected. Any push to greater accuracy is theoretically relevant,” says Murphy.
Millimetre precision To that end, ground-based improvements in technology have allowed the team to move from a precision of a few centimetres to just a few millimetres. The trouble is that the analysis has not kept pace with the wealth of measurements. At the millimetre scale, there are a number of new effects to deal with, such as solar radiation pressure, which pushes the moon’s entire orbit from its calculated path by about 4 millimetres. These must all be included in the analysis. On top of this, general relativity has to be gone through with the mathematical equivalent of a fine-tooth comb to determine whether there are subtleties that have been neglected so far, yet are relevant at millimetre scales. “The important thing is that we are collecting data. That is money in the bank to us,” says Murphy. “The analysis will follow once we are satisfied with our new moon model.” As for that proposed 20-year observation of Newton’s gravitational constant, which sounded so impossible in the 1960s, Murphy’s team have pinned down any changes in the constant to less than one part in 1012 per year. This has provided another powerful constraint on new physics and cosmological theories. “It’s amazing to think that it’s been such a technical success and that we can keep pushing the technology to greater accuracy,” says Nordtvedt, now emeritus professor at Montana State University in Bozeman. “I would not have counted on a 40-year observation. It is a tremendous bonus.” ■ Stuart Clark’s latest book is Galaxy (Quercus)
Another small step Astronaut, cosmonaut or taikonaut? Greg Klerkx asks whose footprints will be next
I
N JANUARY this year, the Indian space probe, Chandrayaan-1, embarked on something of a nostalgia tour. Over the course of four days, the craft flew high above the lunar surface, methodically mapping the landing sites of all six crewed Apollo missions that touched down on the moon. The purpose, according to P. Sreekumar of the Indian Space Research Organisation, was to confirm the Apollo missions’ findings about the moon’s surface and rocks. Yet it may have had an additional motive: India is among a handful of countries determined to land a human on the moon. The architects of the cold war-era space race could hardly have imagined the number of nations that would follow their lead. India, Japan, China and the European Space Agency have added to the traffic in lunar orbit, with most of these missions carrying scientific instruments from other countries. Four of the six members of this lunar explorer’s club – the US, Russia, China and India – have professed an intention to send people to the moon. Which country, if any, will succeed?
If technology were the only factor, this new race would be America’s to lose. NASA’s Constellation programme, unveiled in 2006, has the ultimate goal of establishing a permanently inhabited base on the moon. To do this, the programme includes a design for the Orion crew capsule with up to six seats, the Altair lunar lander, and a new booster system called Ares, which uses components from both the Apollo-era Saturn rocket and the space shuttle. There are two versions of Ares: Ares I, designed to transport crew and the Orion capsule, and the more powerful Ares V, designed to transport supplies and the Altair lunar lander. Under current plans, a crewed lunar mission is set for 2020. First, Ares I would be launched into Earth orbit, where the Orion capsule containing the crew would detach. The capsule would then rendezvous and link up with the Altair lander once it had reached Earth orbit, before heading to the moon. All is not well with Constellation, however. NASA’s 2010 budget continues to support the programme, though looming cuts could see the Ares >
The US flag hasn’t stopped other nations lusting after lunar glory
NASA
Only a tiny fraction of this laser light will make it back from the moon
11 July 2009 | NewScientist | 33