HAL WEAVER hasn鈥檛 stopped kicking himself for over two years now. In July 2000 he was leading a team that had the Hubble Space Telescope trained on Comet Linear. The comet was just nearing the point of its closest approach to the Sun when Hubble looked away and missed the most spectacular moment of all: the comet鈥檚 dramatic break-up.
Linear鈥檚 disintegration came as a big surprise to cometary scientists 鈥� such an event was unprecedented. Though Weaver was able to turn his before and after shots into an intriguing paper for the journal Science, he was still annoyed. Great pictures or not, Hubble had failed to capture the process of the comet鈥檚 break-up.
However, Weaver might soon get another chance. NASA is planning to use a spacecraft called Deep Impact to probe Comet Tempel 1 in 2005. The mission鈥檚 impact may be deeper than anyone expects: a series of recent experiments suggests that comets are accidents waiting to happen, unexploded bombs set on a hair trigger. And detonation can occur with the tiniest impact from, say, a meteorite or a piece of space debris. Or a NASA spacecraft.
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Leaders of the Deep Impact mission would prefer that their craft didn鈥檛 destroy the comet, of course. When it meets Tempel 1 in 2005, Deep Impact will send back the very first images of the internal structure of comets. At the moment we know little more than that they are made of rock and ice. The plan is for the spacecraft to excavate a 20-metre deep crater in Tempel 1 and let us peer inside.
Unfortunately, this is exactly where the mission could run into big trouble. Dan Durda, an astronomer at the Southwest Research Institute, Boulder, Colorado, first began to wonder about the likely impact of the mission earlier this year. He had been working in Moffett Field, California, at NASA Ames鈥檚 vertical impact facility, a gun several metres long that helps researchers test the effects of high velocity impacts. Durda and George Flynn from the State University of New York at Plattsburgh were smashing up meteorites with the gun to find out what produced the space dust captured by interplanetary spacecraft. With some gun time to spare, they tried out a different experiment.
From computer simulations of asteroids in 1998 by Erik Asphaug鈥檚 team at the University of California, Santa Cruz, Durda knew impacts can have varying effects depending on the composition of the surface. Fairly dense asteroids seemed to sustain more serious crater damage than more porous ones. Durda decided there and then they would check out impacts on the most porous material he could think of: polystyrene.
In their first experiment, Durda and Flynn fired an aluminium pellet with a diameter of 3 millimetres into a 11.4- centimetre-diameter ball of polystyrene foam 鈥� styrofoam 鈥� at nearly 2 kilometres per second, about twice the speed of a rifle shot. They expected the pellet to tunnel straight through and fly out the other side, leaving the ball more or less intact. Astonishingly, it didn鈥檛. 鈥淭he ball completely exploded,鈥� says Durda.
Intrigued, his team tried as many ways as they could to get the polystyrene ball to explode. In the most extreme case they fired a tiny polystyrene pellet into it. Bizarrely, the ball blew apart. 鈥淲e were scratching our heads,鈥� Durda says.
Thinking about the energies involved, they concluded that heat from the collision wasn鈥檛 enough to trigger the explosion. And then it dawned on them that the gas gun fires into a depressurised chamber, creating a pressure imbalance between the air in the chamber and air pockets at atmospheric pressure inside the polystyrene balls. Essentially, they were like overblown balloons just waiting to pop.
Durda was well aware that styrofoam is a model for porous bodies such as low-density asteroids and comet nuclei. Realising how destructive a pressure imbalance could be, he thought immediately of the pressure building inside comets as the Sun heats the ice to a vapour.
If the Sun begins to melt ice trapped inside rocky pockets in the comets as they enter the inner Solar System, the ice heats up and vaporises, but the vapour can鈥檛 escape. The result? Huge strain inside the comet 鈥� it would be fit to burst. All that鈥檚 needed to set it off is an impact with anything that can do a little damage to the comet鈥檚 structure. An asteroid just one metre in diameter could do the trick, Durda says.
Deep Impact鈥檚 mission leader, Mike A鈥橦earn from the University of Maryland in Baltimore, first heard about Durda鈥檚 research on the astronomical grapevine. But when he saw Durda鈥檚 results for himself in September this year, he had no choice but to take it seriously. Why? Because the mission is planning to repeat Durda鈥檚 experiment, but on a grander scale. Deep Impact will fire a 350-kilogram copper 鈥渂ullet鈥� into the nucleus of Comet Tempel 1.
The copper bullet will hit the comet with the equivalent of nearly 5 tons of TNT, gouging out a crater the area of a football field but seven storeys deep. The main aim is to delve beneath the surface of the comet and get a good look at its internal structure. But A鈥橦earn says 鈥渃atastrophic fragmentation鈥� is on his list of things the mission might achieve. 鈥淚t鈥檚 around fifth out of seven,鈥� he says. Although he believes the chance of that happening is low 鈥� just a few per cent 鈥� it remains a possibility. This mission may be the first that manages to destroy a celestial object.
Which would be far from an ideal outcome, but not necessarily a disaster, A鈥橦earn says. The spacecraft will be 500 kilometres from the comet at the time of disintegration and should be able to watch from a safe distance. Not that there鈥檚 no danger. Large grains surrounding the nucleus in the comet鈥檚 inner coma 鈥� dust that was part of the comet but is released when ice melts and boils on the surface 鈥� could shoot out with some power. 鈥淎ny of these could possibly hit the spacecraft in a vital spot with enough energy to get through the shielding on the spacecraft and damage us,鈥� A鈥橦earn admits. That鈥檚 why they are planning to send as much observational data as possible back to Earth in real time.
But A鈥橦earn believes the possibility of destroying Tempel 1 is remote. It鈥檚 a relatively inactive comet in a slow short orbit, he says, and seems to have shed most of its volatile material already.
Of course, people were saying much the same about Comet Linear 鈥� when it suddenly brightened and blew apart. Hal Levison, also at the Southwest Research Institute, thinks Durda is on the right track with his ideas about comet explosion; they certainly fit well with astronomical observations, he says.
For a start, Levison has worked out that 99 per cent of the comets in our Solar System seem to go suddenly and inexplicably missing (see 鈥淟ost in space鈥�). And there鈥檚 an intriguing disparity between the types of comets that hang around and those that disappear.
A high proportion of the 鈥渓ong period鈥� comets have gone missing. These come in very fast towards the Sun from far away, and heat up quickly. Hence they have less time to vent their gases safely and are more likely to become dangerously pressurised and break up.
The comets that never stray far out into the Solar System, on the other hand, seem less prone to rapid disappearance. Images taken by SOHO, a NASA-ESA spacecraft, revealed more than 500 Sun-grazing mini-comets, thought to be fragments of larger comets. Being so close to the Sun, these comets stay at a fairly constant high temperature rather than being subjected to sudden 鈥� and catastrophic 鈥� heating. And so they tend to break up gently over many orbits.
Durda is the first to admit that we can鈥檛 make firm predictions about the likely effects of NASA鈥檚 probe on Tempel 1 because no one knows how the internal structure of a comet actually relates to a polystyrene ball. But a spectacular explosion remains a tantalising possibility. Weaver can鈥檛 wait: a total destruction would be 鈥渧ery appealing,鈥� he says, although he鈥檚 not convinced it will happen. 鈥淕ene Shoemaker was always mortified that we were going to go out there and possibly destroy a comet. But I think a lot of us are just hoping to learn something.鈥� Weaver will definitely be watching; he thinks we should use every piece of equipment we can to observe Deep Impact鈥檚 effect on Tempel 1. 鈥淢aybe some of them will be bust, but we may be pleasantly surprised,鈥� he says. Whatever happens, this time he鈥檒l know not to look away 鈥� even for a moment.
Lost in Space
Once they get into a near-Earth orbit, comets ought to return at regular intervals over thousands of years. But it seems they don鈥檛. Orbital calculations show most comets astronomers spot are on their first pass through the inner Solar System.
Until very recently, astronomers thought they understood why all the comets were disappearing from view. Once the Sun has evaporated all their ice, the comets are left as sad, dead nuclei that Earthbound astronomers simply can鈥檛 see. The more times a comet flies through the inner Solar System and heats up, the less likely astronomers are to see it.
It鈥檚 a plausible idea, but last year Hal Levison and colleagues at Southwest Research Institute in Boulder, Colorado, proved it wrong. They were inspired by the recent discoveries of two near-Earth asteroids that appeared to be moving in comet-like orbits. The objects were perfect candidates for dead cometary nuclei. If the standard picture of comets was right, the inner Solar System ought to be teeming with rocks like these.
Levison analysed the data from surveys of near-Earth objects to work out how many of these potential dead comets would be picked up. He then compared that with estimates of the number of dead comets that should be orbiting. His results suggest that as many as 99 per cent of the comets are missing 鈥� not just dead and invisible, but gone completely.
Whether or not they exploded is still an open question, however; if they did, then surely we鈥檇 have a sky full of firecrackers? Levison admits he still doesn鈥檛 have all the answers, and is working to pin down the statistics on disappearance more accurately.
But whatever happens to comets after their first pass through the Solar System, he says, it鈥檚 a catastrophic 鈥� and widespread 鈥� phenomenon.