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New moon
New moon DETLEV VAN RAVENSWAAY/SCIENCE PHOTO LIBRARY
Earth may have many more moons than you think. Perhaps we should pay one a visit, says Stuart Clark
48 | NewScientist | 21 April 2012
A
LMOST a century ago, something strange split the sky across North America. On 9 February 1913, eyewitnesses reported dozens of burning fireballs cutting a swathe across the night sky. It was a display unlike any other meteor shower. Instead of shooting stars raining down in all directions, a train of bright fireballs moved slowly and deliberately over much of the continent. The first sighting was in Saskatchewan, Canada. Burning red-hot from its passage through the atmosphere and trailing streaks of vapour, the meteor train moved south-east, passing just a few kilometres north of New York and then out over the Atlantic Ocean. Final sightings of the spectacle came from Bermuda and a steamer ship near the equator. The distance between the first and last observing points was 9200 kilometres. To be seen over such an expanse, the meteors must have been in orbit around our planet. The conclusion was compelling: what people had seen that night was probably the break up of a small, previously undiscovered moon of Earth. We are now realising that the events of 1913 may not be unique. Computer models of asteroid orbits are showing that small space rocks a few metres across can lodge in Earth’s gravitational field if they stray too close. Only a tiny fraction of them break up and hit our planet. Most orbit unseen for months or years, somewhere beyond the moon, before slipping safely and silently back into deep space. But while they remain close, they are minimoons of Earth. Not only are they turning out to be more common than anyone thought, they could play a vital role in unravelling our solar system’s secrets. It is not unheard of for a planet to capture a small celestial object. Jupiter is a master of the art: it is 320 times more massive than Earth, and also orbits five times farther away from the sun. At that distance, the sun’s gravity is much weaker, so Jupiter can wrestle objects away from it and clutch its prey more tightly. Jupiter’s most notable recent catch was comet Shoemaker-Levy 9. The giant planet’s gravity subsequently pulled the comet to pieces and swallowed it in a series of spectacular explosions in July 1994. Thankfully for us, Earth’s gravity is
much weaker, meaning such violent acts are extremely rare. Most of the objects that do make it to Earth originate in the asteroid belt between the orbits of Mars and Jupiter. However, telescopes designed to identify asteroids that may one day smack into Earth have found growing numbers of objects in orbits across the solar system. Most are small, fragments of once larger objects that have broken up in collisions over the aeons. Any such small body that finds itself passing by on an orbit similar to Earth’s is likely to be snagged and yanked onto a course that takes it around our planet instead – if only for a short while.
Moon trackers Mikael Granvik at the University of Helsinki in Finland and his colleagues are among those dedicated to tracking down these celestial fly-by-nights. Calculations by Granvik’s group show that mini-moons are likely to be a few metres across and orbit slowly at up to 12 times the distance of the moon. The course they chart around Earth is a delicate one because of perturbations in the gravitational field from the sun and other planets (see “Mini-moon or quasi-moon?”, page 50). As a result, Granvik’s model predicts that most captured objects drift off again, spending on average just 9 months in orbit. Perhaps the biggest surprise, however, is that such temporary minimoons are common. “There is probably one up there right now,” says Granvik. Finding a mini-moon is no easy matter because their small size means they reflect little light. Present surveys, such as the one conducted with the Pan-STARRS telescope in Haleakala¯, Hawaii, are looking for potentially hazardous nearEarth asteroids. But they are not really powerful enough to search for metresized mini-moons. What Granvik and others do see often turns out to be space junk masquerading as small asteroids. “Out of six objects we have investigated, five have turned out to be upper stages of rockets,” says Paul Chodas at NASA’s Jet Propulsion Laboratory in Pasadena, California (see “Space junkie”, page 50). That leaves one. On 14 September >
21 April 2012 | NewScientist | 49
Discovered in 1986, asteroid Cruithne shot to fame when astronomers spotted that it takes a year to orbit the sun and never strays too far from Earth. This led to it being dubbed Earth’s second moon. But according to Mikael Granvik at the University of Helsinki in Finland, that is stretching things a little too far. Cruithne is an example of a quasi-moon – its path is dictated by the sun’s gravity rather than our planet’s. “If you took away the Earth, a quasi-moon’s orbit would not be affected,” says Granvik. In contrast, mini-moons are actually in Earth’s gravitational clutches – at least for a while. A mini-moon’s orbit is determined by a delicate balance between the gravitational fields of Earth, the sun and other celestial bodies. This is what makes them susceptible to falling into orbit around Earth in the first place, and then prone to drift off again.
Space junkie Paul Chodas studies near-Earth objects, and part of his job is to investigate the reports of possible mini-moons. Based at NASA’s Jet Propulsion Laboratory in Pasadena, California, Chodas’s latest case is asteroid 2010 KQ, discovered in May 2010 by the Catalina Sky Survey in Tucson, Arizona. Just a few metres across, it seemed a dead ringer for a mini-moon. Then infrared observations showed that its composition resembled no known asteroid type, but instead was reminiscent of metal rocket parts. Chodas provided the clinching evidence by showing that 2010 KQ had been very close to Earth in 1975, although its orbit is not known well enough to associate it with a specific rocket launch. He was also responsible for the calculations showing that another mini-moon, discovered in September 2002, was probably Apollo 12’s upper stage returning to Earth for a lap of honour. It had been left in a loose Earth orbit back in November 1969, had slipped into orbit around the sun, and been temporarily recaptured by Earth during 2002 and 2003.
50 | NewScientist | 21 April 2012
Bryan Versteeg/Spacehabs.com
Mini-moon or quasi-moon?
2006, the Catalina Sky Survey detected an object in Earth orbit. Designated 2006 RH120, it was calculated to have been captured by Earth in July of that year. As astronomers watched, it made three leisurely orbits over the following 12 months, one of them bringing it inside the orbit of the moon, and then drifted away again. This time everything fitted. To distinguish it from space junk, Chodas analysed the body’s orbit. Asteroids move primarily under the pull of gravity because they are dense, whereas space junk tends to be hollow and gets pushed around by the pressure of sunlight as well. The weight of evidence pointed to 2006 RH120 being an asteroid some 5 metres across. It is now moving away around the sun in a similar orbit to Earth. By 2017, it should be on the opposite side of the sun from us. Its return visit is likely to take place around 2028. Granvik suspects that astronomers have sighted other bona-fide mini-moons but have simply disregarded them. “When observers see an object in Earth orbit,” he says, “they tend to think it is just space junk and so throw the data away.” He is hopeful that new surveys will lead to the discovery of many more minimoons. The Large Synoptic Survey Telescope planned for completion on the slopes of Cerro Pachón in Chile, in 2019 is an 8.4-metre telescope that will survey the entire sky once a week looking for asteroids. It should be able to spot mini-moons easily and quickly. “If we do
start to routinely find mini-moons in the future, we will have the opportunity to study a population of small asteroids that we have not seen before,” says Granvik. “We could easily send spacecraft to them.” The scientific pay-off of such missions would be large indeed. Asteroids are the leftovers of planet formation. They are the fragments of rock and metal that never managed to coalesce into larger worlds. As such, they hold clues about the way the planets formed, such as the raw ingredients that went into those worlds – including the organic components that Earth managed to cook up into life.
Fly me to the moon Considering the great pains that planetary scientists have gone to in recent years to bring back a few specks of dust from an asteroid, a mission to a mini-moon would offer convenience and bounty beyond their wildest dreams. The Japanese space agency’s Hayabusa probe had already suffered its fair share of setbacks and delays when it finally blasted off towards asteroid Itokawa in 2003. It was hit by a violent solar storm that damaged its solar panels, reducing its on-board power and slowing the spacecraft down. On arrival at Itokawa in 2005, moves to stabilise the spacecraft failed and communications were lost during the attempted landing. On top of
Mini-moons orbiting Earth could bring asteroid mining closer to reality
one back to Earth for analysis,” says Granvik. of the inner solar system,” says Hartmann. Beyond the value of the science, there could The trouble is, of course, that such resources be other – more lucrative – rewards for bringing are not exactly easy to reach. Mini-moons a mini-moon down to Earth: precious metals. could change that. Although they are just a Asteroids come in three basic types. thousandth of the diameter of the asteroid M-types are largely metal and were once at the that Hartmann used in his 1994 example, hearts of now-shattered protoplanets. S-types they are far easier to reach. “Once Earth are stony asteroids but are noticeably rich in has captured these asteroids, they become metals such as iron, nickel and magnesium. accessible to us,” says Chodas, although he C-types are the most common and are remains unsure about the practicality of composed of elements in their average cosmic mining in space. abundances but without the hydrogen and Aerospace engineer Hexi Baoyin and his helium gases. Even though C-types are not colleagues at Tsinghua University in Beijing, notably enriched, they still contain enough China, are taking the idea further. Having precious metals to make them extremely independently identified how small asteroids valuable if they were brought to Earth. can be naturally captured by Earth, they have The last time people were talking about suggested that some closely approaching mining asteroids for mineral resources, the asteroids could be nudged into Earth orbit, chances are they were wearing a tank top and either by slamming projectiles into them or corduroy flares. It was all part of the Apollo-era optimism about living and working in space – ”Instead of taking years and it collapsed along with NASA’s budget to journey to an asteroid, sometime in the 1970s. Now the idea, unlike the tank top, is back in fashion. a spacecraft could be at a The reason for the renewed interest is the mini-moon within weeks” steady rise in the price of gold and other base metals during the past decade. Back in 1994, that, the sampling device did not work using more subtle methods like erecting solar William Hartmann at the Planetary Science correctly. Nevertheless, the spacecraft limped sails on them. This, they propose, could be one Institute in Tucson, Arizona, estimated that home and delivered its precious cargo of of the best ways to make mining near-Earth a 2-kilometre-wide asteroid would be worth asteroid dust in June 2010. asteroids possible (arxiv.org/abs/1108.4767). $25 trillion in metal and mineral resources Instead of a spacecraft taking months or As the world’s human population increases, (see diagram). That’s enough to pay off the years to journey into space to rendezvous with US’s $15 trillion national debt, use the loose so the demand for resources will grow. Some asteroids, it could be at a mini-moon after only change to settle up for Greece and still make of Earth’s resources are already expensive a few weeks, or even days, of travelling time. the investors very rich indeed. “I don’t see how to mine and this may make asteroids “At just a few metres across, they are small increasingly tempting targets – especially if you can look at any economic study of Earth enough that we could even bring a complete the cost of space missions drops dramatically and not think about the potential resources thanks to the efforts of private space companies such as SpaceX in Hawthorne, California. It is one of several companies in which the Asteroids could be a valuable source of metals. In 1994, William Hartmann at the Planetary Science Institute US government is investing to try to bring the estimated the value of a 2-kilometre-wide metal rich asteroid cost of launching each pound (0.45 kilograms) down to below $1000. Even so, Chodas is sceptical. “Getting the equipment to these asteroids is expensive, Value (2012) getting the minerals back is even more so,” Iron he says. “I’m not sure space travel will ever be cheap enough to make mining asteroids viable.” Value (1994) Composition Even if asteroid mining never takes off, minimoons are still a source of wonder. Why spend Nickel Platinum billions travelling to an asteroid when they are Cobalt US debt Nickel Platinum Iron gently knocking on our front door? Unseen Iron they may be, yet as we gaze out into the night Nickel Cobalt sky, there is every reason to think that they are up there, taking turns to orbit our planet, like Platinum celestial fruit just waiting to be picked. n
Cosmic cornucopia
Asteroid 1986 DA
$25 trillion
$87.2 trillion
$15.5 trillion
SOURCE: LONDON METAL EXCHANGE/PLATINUM TODAY
lt ba Co
30 trillion kg 3 x 1013 kg
Stuart Clark is a consultant to New Scientist. His new novel is The Sensorium of God (Polygon) 21 April 2012 | NewScientist | 51
Earth may have many more moons than you think. Perhaps we should pay one a visit, says Stuart Clark
48 | NewScientist | 21 April 2012
A
LMOST a century ago, something strange split the sky across North America. On 9 February 1913, eyewitnesses reported dozens of burning fireballs cutting a swathe across the night sky. It was a display unlike any other meteor shower. Instead of shooting stars raining down in all directions, a train of bright fireballs moved slowly and deliberately over much of the continent. The first sighting was in Saskatchewan, Canada. Burning red-hot from its passage through the atmosphere and trailing streaks of vapour, the meteor train moved south-east, passing just a few kilometres north of New York and then out over the Atlantic Ocean. Final sightings of the spectacle came from Bermuda and a steamer ship near the equator. The distance between the first and last observing points was 9200 kilometres. To be seen over such an expanse, the meteors must have been in orbit around our planet. The conclusion was compelling: what people had seen that night was probably the break up of a small, previously undiscovered moon of Earth. We are now realising that the events of 1913 may not be unique. Computer models of asteroid orbits are showing that small space rocks a few metres across can lodge in Earth’s gravitational field if they stray too close. Only a tiny fraction of them break up and hit our planet. Most orbit unseen for months or years, somewhere beyond the moon, before slipping safely and silently back into deep space. But while they remain close, they are minimoons of Earth. Not only are they turning out to be more common than anyone thought, they could play a vital role in unravelling our solar system’s secrets. It is not unheard of for a planet to capture a small celestial object. Jupiter is a master of the art: it is 320 times more massive than Earth, and also orbits five times farther away from the sun. At that distance, the sun’s gravity is much weaker, so Jupiter can wrestle objects away from it and clutch its prey more tightly. Jupiter’s most notable recent catch was comet Shoemaker-Levy 9. The giant planet’s gravity subsequently pulled the comet to pieces and swallowed it in a series of spectacular explosions in July 1994. Thankfully for us, Earth’s gravity is
much weaker, meaning such violent acts are extremely rare. Most of the objects that do make it to Earth originate in the asteroid belt between the orbits of Mars and Jupiter. However, telescopes designed to identify asteroids that may one day smack into Earth have found growing numbers of objects in orbits across the solar system. Most are small, fragments of once larger objects that have broken up in collisions over the aeons. Any such small body that finds itself passing by on an orbit similar to Earth’s is likely to be snagged and yanked onto a course that takes it around our planet instead – if only for a short while.
Moon trackers Mikael Granvik at the University of Helsinki in Finland and his colleagues are among those dedicated to tracking down these celestial fly-by-nights. Calculations by Granvik’s group show that mini-moons are likely to be a few metres across and orbit slowly at up to 12 times the distance of the moon. The course they chart around Earth is a delicate one because of perturbations in the gravitational field from the sun and other planets (see “Mini-moon or quasi-moon?”, page 50). As a result, Granvik’s model predicts that most captured objects drift off again, spending on average just 9 months in orbit. Perhaps the biggest surprise, however, is that such temporary minimoons are common. “There is probably one up there right now,” says Granvik. Finding a mini-moon is no easy matter because their small size means they reflect little light. Present surveys, such as the one conducted with the Pan-STARRS telescope in Haleakala¯, Hawaii, are looking for potentially hazardous nearEarth asteroids. But they are not really powerful enough to search for metresized mini-moons. What Granvik and others do see often turns out to be space junk masquerading as small asteroids. “Out of six objects we have investigated, five have turned out to be upper stages of rockets,” says Paul Chodas at NASA’s Jet Propulsion Laboratory in Pasadena, California (see “Space junkie”, page 50). That leaves one. On 14 September >
21 April 2012 | NewScientist | 49
Discovered in 1986, asteroid Cruithne shot to fame when astronomers spotted that it takes a year to orbit the sun and never strays too far from Earth. This led to it being dubbed Earth’s second moon. But according to Mikael Granvik at the University of Helsinki in Finland, that is stretching things a little too far. Cruithne is an example of a quasi-moon – its path is dictated by the sun’s gravity rather than our planet’s. “If you took away the Earth, a quasi-moon’s orbit would not be affected,” says Granvik. In contrast, mini-moons are actually in Earth’s gravitational clutches – at least for a while. A mini-moon’s orbit is determined by a delicate balance between the gravitational fields of Earth, the sun and other celestial bodies. This is what makes them susceptible to falling into orbit around Earth in the first place, and then prone to drift off again.
Space junkie Paul Chodas studies near-Earth objects, and part of his job is to investigate the reports of possible mini-moons. Based at NASA’s Jet Propulsion Laboratory in Pasadena, California, Chodas’s latest case is asteroid 2010 KQ, discovered in May 2010 by the Catalina Sky Survey in Tucson, Arizona. Just a few metres across, it seemed a dead ringer for a mini-moon. Then infrared observations showed that its composition resembled no known asteroid type, but instead was reminiscent of metal rocket parts. Chodas provided the clinching evidence by showing that 2010 KQ had been very close to Earth in 1975, although its orbit is not known well enough to associate it with a specific rocket launch. He was also responsible for the calculations showing that another mini-moon, discovered in September 2002, was probably Apollo 12’s upper stage returning to Earth for a lap of honour. It had been left in a loose Earth orbit back in November 1969, had slipped into orbit around the sun, and been temporarily recaptured by Earth during 2002 and 2003.
50 | NewScientist | 21 April 2012
Bryan Versteeg/Spacehabs.com
Mini-moon or quasi-moon?
2006, the Catalina Sky Survey detected an object in Earth orbit. Designated 2006 RH120, it was calculated to have been captured by Earth in July of that year. As astronomers watched, it made three leisurely orbits over the following 12 months, one of them bringing it inside the orbit of the moon, and then drifted away again. This time everything fitted. To distinguish it from space junk, Chodas analysed the body’s orbit. Asteroids move primarily under the pull of gravity because they are dense, whereas space junk tends to be hollow and gets pushed around by the pressure of sunlight as well. The weight of evidence pointed to 2006 RH120 being an asteroid some 5 metres across. It is now moving away around the sun in a similar orbit to Earth. By 2017, it should be on the opposite side of the sun from us. Its return visit is likely to take place around 2028. Granvik suspects that astronomers have sighted other bona-fide mini-moons but have simply disregarded them. “When observers see an object in Earth orbit,” he says, “they tend to think it is just space junk and so throw the data away.” He is hopeful that new surveys will lead to the discovery of many more minimoons. The Large Synoptic Survey Telescope planned for completion on the slopes of Cerro Pachón in Chile, in 2019 is an 8.4-metre telescope that will survey the entire sky once a week looking for asteroids. It should be able to spot mini-moons easily and quickly. “If we do
start to routinely find mini-moons in the future, we will have the opportunity to study a population of small asteroids that we have not seen before,” says Granvik. “We could easily send spacecraft to them.” The scientific pay-off of such missions would be large indeed. Asteroids are the leftovers of planet formation. They are the fragments of rock and metal that never managed to coalesce into larger worlds. As such, they hold clues about the way the planets formed, such as the raw ingredients that went into those worlds – including the organic components that Earth managed to cook up into life.
Fly me to the moon Considering the great pains that planetary scientists have gone to in recent years to bring back a few specks of dust from an asteroid, a mission to a mini-moon would offer convenience and bounty beyond their wildest dreams. The Japanese space agency’s Hayabusa probe had already suffered its fair share of setbacks and delays when it finally blasted off towards asteroid Itokawa in 2003. It was hit by a violent solar storm that damaged its solar panels, reducing its on-board power and slowing the spacecraft down. On arrival at Itokawa in 2005, moves to stabilise the spacecraft failed and communications were lost during the attempted landing. On top of
Mini-moons orbiting Earth could bring asteroid mining closer to reality
one back to Earth for analysis,” says Granvik. of the inner solar system,” says Hartmann. Beyond the value of the science, there could The trouble is, of course, that such resources be other – more lucrative – rewards for bringing are not exactly easy to reach. Mini-moons a mini-moon down to Earth: precious metals. could change that. Although they are just a Asteroids come in three basic types. thousandth of the diameter of the asteroid M-types are largely metal and were once at the that Hartmann used in his 1994 example, hearts of now-shattered protoplanets. S-types they are far easier to reach. “Once Earth are stony asteroids but are noticeably rich in has captured these asteroids, they become metals such as iron, nickel and magnesium. accessible to us,” says Chodas, although he C-types are the most common and are remains unsure about the practicality of composed of elements in their average cosmic mining in space. abundances but without the hydrogen and Aerospace engineer Hexi Baoyin and his helium gases. Even though C-types are not colleagues at Tsinghua University in Beijing, notably enriched, they still contain enough China, are taking the idea further. Having precious metals to make them extremely independently identified how small asteroids valuable if they were brought to Earth. can be naturally captured by Earth, they have The last time people were talking about suggested that some closely approaching mining asteroids for mineral resources, the asteroids could be nudged into Earth orbit, chances are they were wearing a tank top and either by slamming projectiles into them or corduroy flares. It was all part of the Apollo-era optimism about living and working in space – ”Instead of taking years and it collapsed along with NASA’s budget to journey to an asteroid, sometime in the 1970s. Now the idea, unlike the tank top, is back in fashion. a spacecraft could be at a The reason for the renewed interest is the mini-moon within weeks” steady rise in the price of gold and other base metals during the past decade. Back in 1994, that, the sampling device did not work using more subtle methods like erecting solar William Hartmann at the Planetary Science correctly. Nevertheless, the spacecraft limped sails on them. This, they propose, could be one Institute in Tucson, Arizona, estimated that home and delivered its precious cargo of of the best ways to make mining near-Earth a 2-kilometre-wide asteroid would be worth asteroid dust in June 2010. asteroids possible (arxiv.org/abs/1108.4767). $25 trillion in metal and mineral resources Instead of a spacecraft taking months or As the world’s human population increases, (see diagram). That’s enough to pay off the years to journey into space to rendezvous with US’s $15 trillion national debt, use the loose so the demand for resources will grow. Some asteroids, it could be at a mini-moon after only change to settle up for Greece and still make of Earth’s resources are already expensive a few weeks, or even days, of travelling time. the investors very rich indeed. “I don’t see how to mine and this may make asteroids “At just a few metres across, they are small increasingly tempting targets – especially if you can look at any economic study of Earth enough that we could even bring a complete the cost of space missions drops dramatically and not think about the potential resources thanks to the efforts of private space companies such as SpaceX in Hawthorne, California. It is one of several companies in which the Asteroids could be a valuable source of metals. In 1994, William Hartmann at the Planetary Science Institute US government is investing to try to bring the estimated the value of a 2-kilometre-wide metal rich asteroid cost of launching each pound (0.45 kilograms) down to below $1000. Even so, Chodas is sceptical. “Getting the equipment to these asteroids is expensive, Value (2012) getting the minerals back is even more so,” Iron he says. “I’m not sure space travel will ever be cheap enough to make mining asteroids viable.” Value (1994) Composition Even if asteroid mining never takes off, minimoons are still a source of wonder. Why spend Nickel Platinum billions travelling to an asteroid when they are Cobalt US debt Nickel Platinum Iron gently knocking on our front door? Unseen Iron they may be, yet as we gaze out into the night Nickel Cobalt sky, there is every reason to think that they are up there, taking turns to orbit our planet, like Platinum celestial fruit just waiting to be picked. n
Cosmic cornucopia
Asteroid 1986 DA
$25 trillion
$87.2 trillion
$15.5 trillion
SOURCE: LONDON METAL EXCHANGE/PLATINUM TODAY
lt ba Co
30 trillion kg 3 x 1013 kg
Stuart Clark is a consultant to New Scientist. His new novel is The Sensorium of God (Polygon) 21 April 2012 | NewScientist | 51