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Tuesday, February 24, 2009

Do gravity holes harbour planetary assassins?

by Stuart Clark

NASA's STEREO spacecraft will search for objects trapped at the Lagrangian points (Image: Johns Hopkins University Applied Physics Laboratory / NASA)

NASA's STEREO spacecraft will search for objects trapped at the Lagrangian points (Image: Johns Hopkins University Applied Physics Laboratory / NASA)

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.

Fly-through zones

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.

L4 and L5 are particularly good vantage points from which to warn the Earth of incoming solar storms. "We talked at one stage about actually stopping the STEREO spacecraft when they got there, because you get about two or three days warning of a coming storm," says Michael Kaiser of the Goddard Space Flight Center in Greenbelt, Maryland, and STEREO's project scientist.

Ultimately the STEREO team discovered that it would take too much fuel to stop their spacecraft at L4 and L5. So they settled for a leisurely fly through instead, though they are still travelling too fast to get stuck. "These are big regions of space," says Kaiser, "It's going to take STEREO months to travel through them."

That's when it struck Richard Harrison of the Rutherford Appleton Laboratory in Oxfordshire, UK, and a member of the STEREO team, that the probes' cameras might be put to another use. He began to investigate the possibilities and realised that a pair of instruments known as heliospheric imagers could be used to search for asteroids. "They were not designed to do this work," says Harrison, who is the imagers' principal investigator. "It's an added bonus."

Even so, asteroid hunters face a painstaking task because a Lagrangian asteroid will appear as little more than a dot moving against a background of thousands of stars. Thankfully there is already a force of volunteers who scan the STEREO images via the internet for signs of near-Earth asteroids.

In addition to these efforts, Harrison is hoping to find some professional manpower for the L4 and L5 crossings. "The close-up investigation of L4 and L5 is completely new. That makes it something we should be driving," he says. In these cash-strapped times, however, that might be more easily said than done.

If investigators do find an asteroid in their sights, it will be worth their effort. "Wouldn't it be spectacular if we actually backed past an asteroid? Saw it come creeping into view around the camera," says Harrison.

They will be able to watch it and tell how it rotates from the variation in light it reflects from the sun. "We will be able to measure the distribution of any asteroids and dust in the Lagrangian points," says Harrison.

Armed with that information, we may be able to answer one of the most perplexing mysteries of the solar system: why Earth has such a large moon.

Most astronomers believe that the moon formed from the debris generated when a Mars-sized object struck the Earth a glancing blow about 4 billion years ago. Their problem is in understanding where the object came from.

Computer models show that incoming objects from elsewhere in the solar system would tend to strike the Earth with too much energy. Instead of creating the moon, they obliterate the Earth. So the impactor must have originated close by, the theory goes, where it could not accelerate too much before hitting.

Planetary leftovers

Another clue is that the moon contains the same abundance of oxygen isotopes as the Earth, hinting that whatever hit us must also have had the same isotope abundance. When astronomers look out into the solar system, to Mars for example, the isotope abundances are different. So this, too, hints that the impactor formed close by. But where?

What is puzzling is how an object could grow so close to the Earth and reach the size of Mars before a collision took place. Their mutual gravity should have pulled them together long before. Unless, says Gott, it formed at a Lagrangian point. "An object could sit at one of these stable points and just grow," he says.

Once it grew sufficiently large, gravitational interactions with other objects, such as Venus, could nudge it out of the Lagrangian point and onto a collision course with Earth.

"It would have the same oxygen isotopes as Earth, because it formed in the same region of the solar system," says Gott. Also, being in essentially the same orbit as Earth, the two planets would not be travelling at vastly different velocities when they collided (New Scientist, 14 August 2004, p 26).

Gott thinks that any objects still in L4 and L5 may be leftovers from the formation of that impacting body. "Let's say that you find a number of objects there. In that case, they would be great targets for a sample-return mission to see if they had the same oxygen isotope abundances as Earth," says Gott. If they do, Gott believes this strengthens the case for the Earth-impactor to have formed there.

In preparation for the crafts' arrivals at L4 and L5, Harrison is discussing with colleagues how best to manoeuvre the two probes for the optimal views. Strange as it sounds, the twin spacecraft do not look where they are going. Instead they fly facing backwards, with several of their electronic eyes pointed close to our planet, on the lookout for incoming solar storms. For the best views of the Lagrangian points, the spacecraft would have to be flipped over, so the heliospherical imagers point forwards while the other instruments remain trained on the sun.

The journey through L4 and L5 is potentially fraught with danger. Clouds of dust are thought to be trapped at these Lagrangian points and if the STEREO team is unlucky, a collision with a badly placed dust particle could be devastating. "If one hits us inside the camera, we are no more," says Chris Davis, part of the heliospheric imager team from Rutherford Appleton Laboratory.

The risk may be reduced by flipping the cameras back towards Earth as the spacecraft pass through the most dangerous spots. The rest of the STEREO team is confident that they will survive: ever since launch, the craft have been taking hits from dust that happens to lie along Earth's orbit. "It varies a lot, from a few, to a few thousand per day," says Chris St Cyr of the Goddard Space Flight Center, who heads the investigation to understand these dust events.

No one knows how many asteroids the STEREO probes will see. Weigert and colleagues have performed a number of computer simulations that showed how asteroids can be nudged from a Lagrangian point due to Venus's gravity; it can happen on a timescale of a million years or so. However, the same simulations showed that this works both ways, with asteroids being nudged into the Lagrangian points by Venus as well. These results, and the failure of telescopes to find a Lagrangian asteroid to date, have made Weigert cautious about the number and size of the asteroids he expects STEREO to find. "I think there may be a few asteroids, but not hundreds, and I'm thinking that they are less than a kilometre across. In the main asteroid belt, a typical asteroid is 100 kilometres across."

Such doubts do not concern Harrison. "Some think we will see something, others think we won't," he says. "But if we let this opportunity pass us by without even looking, we will regret it."

If we see a big asteroid there, it might be worth taking it out pre-emptively. And by that, I mean blowing it to pieces

Could another planet be stalking the Earth?

Most astronomers accept that the moon was formed when a Mars-sized planet crashed into Earth during its infancy. Some research suggests that this kamikaze planet formed at either the L4 or L5 Lagrangian point where gravity from the Earth and sun cancel each other out. So could some other threat be lurking there today?

"Absolutely not," says Paul Weigert at the University of Western Ontario, Canada. There are not enough dust particles or pebbles in the solar system.

It was a different story 4.5 billion years ago when the solar system was swathed in its birth clouds. The planets were forming from this mix of dust and gas, and L4 and L5 would have been prime places for matter to accumulate into larger bodies. "These days, there simply isn't enough material around to build anything that large," says Weigert.

OK, so nothing planet-sized. But Richard Gott of Princeton University reckons there may still be threatening asteroids. "If we see something big in there, it would be like a ticking time bomb," says Gott. That is because gravitational nudges from the other planets, particularly Venus, could pull the asteroid just far enough so that it escapes the Lagrangian point. Once outside this trap, it could easily fall into a collision course with Earth.

"If we see a big asteroid there, it might be worth taking it out pre-emptively," says Gott, "and by that I mean blowing it to pieces."

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