The year of living uncertainly

Life and death, doom and destruction - not the plot of a Hollywood thriller, but the main themes of the past year in science, and all with a single strand running through them: uncertainty.

However, the team funded by the US space agency Nasa speaking in August didn't sound that uncertain. Life on Mars? They were confident that there once was - based on an astonishingly detailed examination of a Martian meteorite, known as ALH 84001, discovered in Antarctica.

Unlike the cartoon image of a white-coated scientist squinting down a microscope and yelling, "Hey, come and look at these!", the Nasa work was the result of complex interdisciplinary research. True, it did involve a certain amount of squinting down (electron) microscopes, but there was also input from geologists, mineralogists, experts in the chemistry of meteorites, and biologists.

David McKay, who led the team based at Johnson Space Center, said: "There is not any one finding that leads us to believe that this is evidence of past life on Mars. Rather, it is a combination of many things that we have found." Those included the detection by a team at Stanford University of an apparently unique pattern of organic molecules (known as polycyclic aromatic hydrocarbons, or PAHs) and several unusual mineral phases that are known products of primitive microscopic organisms on Earth. "The relationship of all of these things in terms of location - within a few hundred thousandths of an inch of one another - is the most compelling evidence," he said.

But he was careful to round off his comments by saying, "What we have found to be the most reasonable interpretation is of such radical nature that it will only be accepted or rejected after other groups either confirm our findings or overturn them."

They've certainly been trying to do the latter. Since August, three papers in the journal which is the touchstone for meteorite scientists - Geochimica et Cosmochimica Acta - have offered inorganic explanations for the observations in ALH 84001. One paper suggests that the PAHs found in the meteorite in fact came from the surroundings, another that the microstructures in the meteorite could not have been made by organic action.

But as Ian Wright of the Open University said last week, as the latest doubts were raised, "My position on this swings from one day to the next. Some people are going to make it their life's work to prove or disprove this. The fact is, this is an incredibly complicated piece of rock. It confounds all of us because we can't tell a coherent story about it." Life on Mars? Don't be too sure.

However, in the case of mad cow disease, or BSE, and the fatal human illness Creuztfeldt-Jakob Disease (CJD), there is a coherent story. In March, the secretary of state for health, Stephen Dorrell, announced that the most likely cause of a new form of CJD, affecting people far younger than the usual disease, was exposure to BSE. So far, the toll is 13 dead from the "new variant" CJD, and two more confirmed cases still alive.

While it is worth noticing that this toll is lower than from the recent outbreak of E coli 157 in Lanarkshire, the key difference is that we know plenty about E coli 157, whereas by comparison we know hardly anything about a BSE-CJD link. We can prevent E coli infection by taking standard hygiene precautions with food preparation. But what should we avoid if we think that eating BSE-infected food leads to CJD? How dangerous is BSE-infected food? What constitutes a fatal dose - one bite, one burger, one meal? Indeed, which are the foods that put us at risk? The fact that CJD is irreversible and fatal makes us all the more keen to know those answers. The trouble is, nobody honestly knows.

The evidence for a link remains circumstantial rather than forensic; but some members of Seac, the government's advisory committee on BSE and CJD, are sure of it. The next problem is to forecast the epidemic, since thousands of infected cows were eaten by humans. The latest estimates, in a paper submitted to the medical journal The Lancet (but not yet published) suggests a peak in about seven years' time, affecting hundreds of people. Certainty? The only things we're certain of here is what we don't know.

As for doom, dinosaur-style, the portents looked as bad as at any time since the day 65 million years ago when a huge interplanetary rock crashed into the Yucatan peninsula. The evidence has been growing that there are huge pieces of rock floating about which could devastate the Earth; but, basically, we've got better at noticing them. The latest estimate is that there are more than 100,000 asteroids bigger than a football stadium floating "near" (within millions of miles) us. There were two "near-misses" in 1996 (both passing a few million miles away, which in celestial terms is no distance at all), and one notable impact, when in November a meteorite travelling at about eight miles per second - a modest 28,800 mph - crashed into a remote area of Honduras, leaving a 165ft crater and starting a fire which consumed several acres of a coffee plantation. Happily, nobody was hurt. But there was no warning; governments have been lukewarm about funding for "space radar" to detect threatening meteorites. And there's then the question of what you would do to deflect one. Again, there are multiple layers of uncertainty which science can do only a little to peel away.

Of course, the risk from meteorites pales a little compared to the damage we're able to wreak with our own ground-launched rockets, notably those which go wrong. In March, a one-ton oak-panelled (honestly) Chinese spy satellite veered out of orbit and splashed into the sea, though not before whistling over most countries in the world. And in November, the six-ton Mars 96 rocket, launched by Russia, also fell into the sea a day after taking off on a Mars mission.

Not that we can feel proud. The European Space Agency's Ariane 5 rocket blew up less than a minute after take-off, destroying the life work of some scientists whose experiments (to measure solar activity) were on board. The cause of the failure? A tiny software error in one of the engine controls.

So, it has been a year which has provided plenty of scientific advance - and yet reminded us each time that the corollary of science is uncertainty. As a certain TV programme's slogan notes, the truth is out there. Sometimes, though, it's damned elusive.