THEY ARE the stealth bombers of the Solar System. Almost invisible in their pitch-black coats, they can come at us from any direction, giving almost no warning. Mostly they pass us by, but one day, one of these marauders will be right on target. And when it does it will devastate the Earth.
These bringers of doom are known as long-period comets. Because they have the advantage of surprise, it might seem almost impossible to defend our planet against them. But there is hope. A NASA study, to be made public this summer, outlines an array of defensive measures that could prevent many catastrophic collisions. Meanwhile, Finnish astronomers have hit upon a new kind of early warning system for comets.
It is common knowledge that cosmic collisions have taken a toll on our planet. An impact at the end of the Cretaceous, about 65 million years ago, is widely believed to have finished off the dinosaurs. More recently, in 1908, a comet fragment or asteroid is thought to have exploded above the remote Siberian town of Tunguska. The blast killed reindeer and levelled trees within a 20-kilometre radius. And then there was the great comet crash of 1994, when Shoemaker-Levy 9 smashed into Jupiter. The event, recorded by dozens of telescopes and spacecraft, seemed to bring home a feeling of global vulnerability.
Advertisement
How great are the risks? Estimates vary, but roughly speaking we can expect a 100-metre body to hit the Earth once in a thousand years. That would be big enough to cause massive regional damage, perhaps killing millions of people if it hit a densely populated area. A 1-kilometre comet or asteroid, which could be large enough to wipe us out, might hit us once in a hundred thousand years. Robert Gold, of Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, puts it starkly. 鈥淭he greatest natural threat to the long-term survivability of mankind is an asteroid or comet impact with the Earth.鈥� But asteroids and comets present very different threats. Asteroids are dense chunks of rock or metal that inhabit the inner Solar System, with orbits that mostly lie near the ecliptic-the plane of the planets (see Diagram). That makes them relatively easy to spot. Astronomers can gradually catalogue all these objects, and then project their orbits far into the future. If we find that an asteroid is due to hit Earth in, say, twenty years, we鈥檒l have plenty of time to do something about it.
Comets are another matter. Astronomers believe that these 鈥渄irty snowballs鈥� start out wandering in a vast swarm of icy bodies surrounding the Solar System, called the Oort Cloud. Every now and again something disturbs part of the cloud, sending a few lumps of ice plummeting inwards. After many trips around the Sun, comets eventually get captured within the inner Solar System, they become 鈥渟hort-period comets鈥�, in orbits that are almost as easy to predict as those of asteroids.
The wild-cards are the long-period comets, with orbits that take thousands of years to complete. They travel at about 50 kilometres per second, compared with 10 to 20 kilometres per second for asteroids, and on average they鈥檙e larger. So even though they probably account for only about ten per cent of Earth impacts, according to Gold, they鈥檙e more like 40 per cent of the total threat to humanity.
Worst of all, long-period comets are frighteningly difficult to detect. In the outer Solar System, they are effectively invisible-there is nothing to see but a tiny nucleus covered in a tarry substance about as bright as charcoal. Only when a comet draws closer to the Sun does it become brilliant. Sunlight evaporates its ices, which are ionised by the solar wind. The ions glow, forming the comet鈥檚 coma and tail.
What鈥檚 more, the job of spotting long-period comets is left to amateur astronomers, who scan the morning or evening skies with wide-field telescopes or jumbo binoculars. It鈥檚 hardly a systematic search plan. So if one of these objects is Earth-bound, we鈥檒l have very little time to react once we see it: anywhere from a couple of years down to a few months. This reaction time is crucial.
But governments and space agencies are beginning to take the threat seriously. Last year, NASA鈥檚 Institute for Advanced Concepts asked Gold to study cosmic collisions and find ways to fight the threat. At a meeting in Poland this summer he will present phase 1 of his investigation.
Gold鈥檚 report proposes a three-part defence system: a set of orbital telescopes, dubbed Sentry, designed to identify and track threatening objects; a set of spacecraft, called Soldier craft, deployed to intercept an incoming body; and an Earth-based control centre to oversee the network. The system could be built within the next 10 to 40 years.
Space sentry
The three Sentry telescopes would each be similar to the Hubble Space Telescope, scanning visible and infrared wavelengths. Ideally they would orbit at the same distance from the Sun as Venus, a vantage that would let them see asteroids whose orbits take them close to the Sun. Sentry would monitor the whole sky, allowing it to hunt comets as well as asteroids. The telescopes would also see long-period comets on the far side of the Sun, giving up to nine months鈥� more warning than we have now. This could turn 鈥渁n uncorrectable disaster into a potentially correctable one鈥�, says Gold.
The Soldiers, meanwhile, would be kept ready to launch, perhaps in orbit around Venus, from where they would be able to use planetary 鈥済ravity-boosts鈥� to reach their targets more quickly. Their job would be to nudge any threatening object away from a collision course with Earth. The movie Armageddon took some liberties with this technique-splitting the asteroid would be more dangerous than pushing it off course-but they were right about the need for nukes. The simplest method would be to explode a nuclear bomb above the object鈥檚 surface, blasting material off the body to deliver a kick.
The earlier an Earth-bound object is discovered and reached, the smaller the nudge required. Take a body 100 metres in diameter. If intercepted a year ahead of impact, it could be deflected with a tiny nuclear explosion, equivalent to just 100 tonnes of TNT.
As the size of the object increases, so does the required warning time and payload. For a 鈥済lobal killer鈥� much more than a kilometre across, a Soldier craft would need to intercept the object more than a year ahead of time, even if armed with a multi-megaton bomb. Given the time taken for a Soldier to reach the object, that means a need for even earlier detection. Gold suggests a grander Sentry scheme, using many telescopes based as far out as the orbit of Jupiter.
Chase the comet
In the hunt for wayward comets, any new detection tool is welcome. The latest comes from a seemingly unlikely source. The SOHO satellite, launched in 1995, was designed to study the Sun, its atmosphere and the solar wind, but a team of Finnish and French scientists has noticed that the spacecraft is also a surprisingly adept comet spotter. This week, they report finding 18 comets on images captured by SOHO between late 1995 and mid-1998 (Nature, vol 405, p 321). These images were produced by the spacecraft鈥檚 SWAN (Solar Wind Anisotropies) detector, which monitors a type of ultraviolet radiation known as Lyman alpha emission. This radiation is characteristic of hydrogen ions, which make up much of the solar wind and, as it happens, the coma and tail of a comet.
Most of the comets seen by SOHO, had already been detected by other means. But one of them, a long-period comet dubbed C/1997 K2, was a SOHO original. 鈥淥ur work shows that one of the bright comets of 1997 passed by unnoticed,鈥� says Teemu M盲kinen of the Finnish Meteorological Institute in Helsinki. It would have been a little too faint to see with the naked eye.
Because it wasn鈥檛 designed explicitly for the purpose, SWAN is not a good instrument for detecting comets, M盲kinen says. But the principle is sound. 鈥淚f we had some kind of super-SWAN-a similar instrument but with much higher resolution-it would be the ultimate comet chaser,鈥� he says.
The ideal Lyman-alpha detector would be space-based, like Gold鈥檚 proposed Sentry telescopes. He suggests placing it at one of the Lagrangian points-stable points where the gravitational tug from the Sun and the Earth cancel each other out. SOHO sits at the L1 Lagrangian point on the sunward side of Earth, orbiting about 1.5 million kilometres closer to the Sun than Earth鈥檚 own orbit.
Michael A鈥橦earn of the University of Maryland in College Park agrees that the Lyman alpha method could be an important technique for finding comets. Optical detectors need ultra-high resolution to pick out a distant comet鈥檚 tiny visible-light image, but the Lyman alpha emissions from a comet cover a much larger swathe of sky. With a detector sensitive to this radiation, you can use a much lower spatial resolution, says A鈥橦earn. This makes it easier to search large areas of the sky.
There is a catch, though. Lyman alpha detectors lock in on the glowing ions that surround a comet鈥檚 nucleus, and this glow only exists when the comet is within about 3 astronomical units of the Sun (1 AU is the radius of the Earth鈥檚 orbit, or about 150 million kilometres). SWAN registered Comet Hale-Bopp when it was 2.9 AU out, roughly twice the distance of the orbit of Mars. That was a year after Alan Hale and Thomas Bopp had seen it through small optical telescopes, when it was still beyond the orbit of Jupiter, at a distance of more than 5 AU.
But Hale-Bopp was a colossus, far brighter than most comets. So a tool that can help astronomers find small comets out to 3 AU could be a life-saver. Depending on its orbit, a comet at that distance would be from a few months to about a year from impact. If it were small enough-no more than a few hundred metres across-we might be able to defend ourselves with a system like Gold鈥檚.
The question is, will anyone put up the money needed to realise Gold鈥檚 scheme? At the moment, we don鈥檛 even know how much it would cost, because NASA isn鈥檛 prepared to fund a second phase to the study, to look at engineering feasibility and prices. If tens of thousands of dollars seems like too much, who will spend hundreds of billions?