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*****以下は関連記事（New Scientist 誌）
Do gravity holes harbour planetary assassins?
THEY are the places gravity forgot. Vast regions of space, millions of kilometres across, in which celestial forces conspire to cancel out gravity and so trap anything that falls into them. They sit in the Earth's orbit, one marching ahead of our planet, the other trailing along behind. Astronomers call them Lagrangian points, or L4 and L5 for short. The best way to think of them, though, is as celestial flypaper.
In the 4.5 billion years since the formation of the solar system, everything from dust clouds to asteroids and hidden planets may have accumulated there. Some have even speculated that alien spacecraft are watching us from the Lagrangian points, looking for signs of intelligence.
Putting little green men to one side for the moment, even the presence of plain old space rocks would be enough to keep most people happy. "I think you certainly might find a whole population of objects at L4 and L5," says astrophysicist Richard Gott of Princeton University.
After nearly a century of speculation, we are on the verge of finding out what they are hiding once and for all. Later this year, two spacecraft that spend their lives studying the sun will begin their slow journeys through L4 and L5.
Space scientists plan to use instruments on board NASA's STEREO probes A and B to search for celestial objects becalmed at the Lagrangian points. What they find could hugely enhance our view of how the solar system formed, tell us more about the colossal impact that formed the moon, and warn us if another major collision is on the cards.
The Lagrangian points were first discovered in 1772 by the mathematician Joseph-Louis Lagrange. He calculated that the Earth's gravitational field neutralises the gravitational pull of the sun at five regions in space, making them the only places near our planet where an object is truly weightless.
Of the five Lagrangian points, L4 and L5 are the most intriguing. They are the only ones that are stable: while a satellite parked at L1 or L2 will wander off after a few months unless it is nudged back into place, any object at L4 or L5 will stay put due to a complex web of forces. Lying 150 million kilometres away, along the line of Earth's orbit, L4 circles the sun 60 degrees in front of our planet while L5 lies at the same angle behind (see diagram).
Evidence for such gravitational potholes appears around other planets too. In 1906, Max Wolf discovered an asteroid outside of the main belt between Mars and Jupiter, and recognised that it was sitting at Jupiter's L4 point. Wolf named it Achilles, and so began the tradition of naming these asteroids after characters from the Trojan wars.
The realisation that Achilles would be trapped in its place and forced to orbit with Jupiter, never getting much closer or further away, started a flurry of telescopic searches for more examples. There are now more than 1000 asteroids known to reside at each of Jupiter's L4 and L5 points.
Searches for "Trojan" asteroids around other planets have met with mixed results. Saturn seemingly has none, and only in the last decade have Trojans been found at Neptune. Naturally, astronomers have often wondered about asteroids at Earth's L4 and L5 points.
The trouble is that our L4 and L5 points are not easy to see from the ground. They appear to lie close to the sun, so by the time night falls, the trailing L5 region is low in the sky and setting fast. On the other side of the sky, the preceding L4 point rises in darkness but the dawn is hot on its heels.
That didn't prevent Paul Weigert at the University of Western Ontario in Canada and his colleagues from conducting a number of searches in the 1990s with the Canada-France-Hawaii telescope on Mauna Kea, Hawaii. It was a tough job because L4 and L5 appear wider in the sky than the full moon so a large number of observations would be needed to search them thoroughly. Alas, Weigert and colleagues came up empty-handed as their search wasn't detailed enough.
More recently, automated asteroid searches, such as the Lincoln Near Earth Asteroid Research project, have begun to creep closer to the Lagrangian points in their nightly robotic scans of the sky, but at this stage no Lagrangian asteroids have been identified. "The field has languished because we are all waiting for somebody to see something," says Weigert.
NASA's STEREO spacecraft could change everything - even though they were never designed to look for asteroids. Launched in 2006, one of the twin STEREO probes was placed ahead of Earth, the other behind. Tracing Earth's orbit, STEREO A gradually outpaces the Earth while its sister ship, STEREO B, trails ever further behind. From these two vantage points, the spacecraft monitor the region of space directly between the Earth and the sun, looking for solar storms that can wreak havoc with electrical equipment on satellites and on Earth.
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