ALMOST 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 mini-moons 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?"). 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 mini-moons 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 Haleakal, Hawaii, are looking for potentially hazardous near-Earth asteroids. But they are not really powerful enough to search for metre-sized 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
That leaves one. On 14 September 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 mini-moons. The Large Synoptic Survey Telescope planned for completion on the slopes of Cerro Pachn 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 that, the sampling device did not work correctly. Nevertheless, the spacecraft limped home and delivered its precious cargo of asteroid dust in June 2010.
Instead of a spacecraft taking months or years to journey into space to rendezvous with asteroids, it could be at a mini-moon after only a few weeks, or even days, of travelling time. "At just a few metres across, they are small enough that we could even bring a complete one back to Earth for analysis," says Granvik.
Beyond the value of the science, there could be other - more lucrative - rewards for bringing a mini-moon down to Earth: precious metals.
Asteroids come in three basic types. M-types are largely metal and were once at the hearts of now-shattered protoplanets. S-types are stony asteroids but are noticeably rich in metals such as iron, nickel and magnesium. C-types are the most common and are composed of elements in their average cosmic abundances but without the hydrogen and helium gases. Even though C-types are not notably enriched, they still contain enough precious metals to make them extremely valuable if they were brought to Earth.
The last time people were talking about mining asteroids for mineral resources, the chances are they were wearing a tank top and corduroy flares. It was all part of the Apollo-era optimism about living and working in space - and it collapsed along with NASA's budget sometime in the 1970s. Now the idea, unlike the tank top, is back in fashion.
The reason for the renewed interest is the steady rise in the price of gold and other base metals during the past decade. Back in 1994, William Hartmann at the Planetary Science Institute in Tucson, Arizona, estimated that a 2-kilometre-wide asteroid would be worth $25 trillion in metal and mineral resources . That's enough to pay off the US's $15 trillion national debt, use the loose change to settle up for Greece and still make the investors very rich indeed. "I don't see how you can look at any economic study of Earth and not think about the potential resources of the inner solar system," says Hartmann.
The trouble is, of course, that such resources are not exactly easy to reach. Mini-moons could change that. Although they are just a thousandth of the diameter of the asteroid that Hartmann used in his 1994 example, they are far easier to reach. "Once Earth has captured these asteroids, they become accessible to us," says Chodas, although he remains unsure about the practicality of mining in space.
Aerospace engineer Hexi Baoyin and his colleagues at Tsinghua University in Beijing, China, are taking the idea further. Having independently identified how small asteroids can be naturally captured by Earth, they have suggested that some closely approaching asteroids could be nudged into Earth orbit, either by slamming projectiles into them or using more subtle methods like erecting solar sails on them. This, they propose, could be one of the best ways to make mining near-Earth asteroids possible (arxiv.org/abs/1108.4767).
As the world's human population increases, so the demand for resources will grow. Some of Earth's resources are already expensive to mine and this may make asteroids increasingly tempting targets - especially if the cost of space missions drops dramatically thanks to the efforts of private space companies such as SpaceXMovie Camera in Hawthorne, California. It is one of several companies in which the US government is investing to try to bring the 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, getting the minerals back is even more so," he says. "I'm not sure space travel will ever be cheap enough to make mining asteroids viable."
Even if asteroid mining never takes off, mini-moons are still a source of wonder. Why spend billions travelling to an asteroid when they are gently knocking on our front door? Unseen they may be, yet as we gaze out into the night sky, there is every reason to think that they are up there, taking turns to orbit our planet, like celestial fruit just waiting to be picked.
