Somewhere out there, halfway between the third and fourth planets from the Sun, lies a fabulous creature that devours robots. Invisible to everything but the imagination, the existence of this monster has been inferred rather than confirmed. It's a myth, of course, but even in the scientific community, a myth can stem from fact. And the fact in this instance is that more than half of the spacecraft sent to Mars have failed, and nobody knows why.
The beast has actually acquired a name. Some time ago, an unnamed Nasa scientist decided to call it the Great Galactic Ghoul. In the corridors of the Jet Propulsion Laboratory, cartoon visions of this creature have appeared on noticeboards, some of them doodled by the greatest of engineering minds of our time. The Ghoul has been openly blamed for countless rocket failures, and for those of us who believe in space exploration, there is every reason to be frightened. Because right now, Nasa's latest creation, named Phoenix, is heading for the Red Planet at 13,000 miles per hour.
The Phoenix spacecraft set out from Earth in August last year and is due to land on Mars's northern polar icecap on Sunday. If it survives the journey through the planet's atmosphere, it will attempt a touchdown using retro-rockets, which fire in reverse to bring the spacecraft gently down to the planet's surface. This technique was successfully used by Nasa's Viking landers, but it is far from risk-free. As recently as 1999, Nasa attempted to use retro-rockets on its Mars Polar Lander, only to watch the spacecraft vanish without a trace. The chances of this happening with Phoenix are not insignificant. As Nasa's Doug McCuistion puts it: "Not since the 1970s have we had a successful powered landing on this unforgiving planet."
Part of the problem here is that robots on Mars are too far from the Earth to land by radio control. Instead, they are designed to be intelligent, but they lack the brain power to avoid unforeseen boulders. The proposed landing zone for Phoenix is an ellipse of soil about 62 by 12 miles in area, and if it comes down with one of its three footpads on a large boulder (and there are five million large boulders in the ellipse), then a multimillion-dollar probe will be lost to sheer bad luck. In the past few months, Nasa has worked hard to minimise this risk, scouting for a rock-free landing zone from orbit, but it cannot guarantee safety. As David Spencer of the Jet Propulsion Laboratory acknowledges: "Rocks and slopes represent the most significant threat to a successful mission."
In fact, some of the most successful probes to the surface of Mars barely fired retro-rockets at all. Robots are much sturdier than you might think, and engineers at the JPL have managed to land three of them on Mars using air bags. When the air bag method was first suggested, it was laughed off. But the idea gained ground and was successfully used in 1997 by the technology demonstrator Mars Pathfinder, and again in 2004 by Nasa's remarkable Spirit and Opportunity rovers. Inspired by this example, the British decided to use air bags for their Beagle 2. But no signal was ever received from Beagle 2, and the £22m probe is presumed to have gone astray.
Assuming Phoenix does make it to the surface, it will then attempt to analyse the ice and rocks in this region. It's long been recognised that the icecaps on Mars contain "dry ice", which is frozen carbon dioxide, but recent data from orbiting probes has suggested that water ice may exist here too. The only way to be sure is to examine the ice close up, and Phoenix is carrying an on-board laboratory to perform this task.
The existence of water is regarded as an essential prerequisite to the existence of life on Mars, and while finding an indigenous life form now seems unlikely, any future explorers from Earth would be eager to exploit this resource. The chances of human beings successfully colonising Mars will soar if we don't have to take water with us. The chemical formula for water is H2O. Not only would water be useful for farming and drinking, it can be broken up into its chemical constituents, hydrogen (H2) and oxygen (O2), by electrolysis. Hydrogen and oxygen are the raw material for rocket fuel.
In the days of the Soviet Union, the Russians liked to embarrass the Americans by putting men and machines into space just ahead of Nasa. Quite apart from such high-profile successes as Sputnik and Yuri Gagarin, they repeatedly made it to the hellish surface of Venus. And yet, not one Soviet probe to Mars fully succeeded.
This wasn't for lack of trying. In 1971, Russia's Mars 3 spacecraft made it on to the surface of the Red Planet, beating all rivals to the punch, but the probe shut down 20 seconds after reaching the surface. It is believed to have landed in a sandstorm. Nonetheless, the Americans were annoyed that the Russians had got there first.
The US engineers were also convinced that their superior computers would give them the edge, and so began to launch their own spacecraft to Mars, only to discover similar problems. As recently as 1999, the highly acclaimed Mars Climate Orbiter crashed into the Martian atmosphere because one of their design teams had done its calculations in feet and inches, while the other used metres. Neither team had bothered to tell the other what units they were using. Aside from sounding outrageously silly, this case is unusual in that it's reasonably obvious what went wrong.
In contrast, many of mankind's failures to get to Mars remain a mystery. A machine that enters the Martian atmosphere has to pass through the searing heat caused by friction against the atmosphere, and at this point, all radio communication breaks down. None of the robots sent to the Red Planet has taken a black box of the kind used in the terrestrial airline industry. When they crash, we never really know why, and for some at the JPL, the Ghoul is as good an explanation as any.
At this point, you could be forgiven for feeling surprised. Technology is constantly improving, and it's been a long time since the 1960s. Surely we ought to be getting better at space exploration? Well, in some respects we are. Wander through any council estate in Britain and you'll see thousands of people picking up satellite channels using a device the size of a frying pan on their chimney. These dishes are receiving more than a hundred high-resolution channels, and the spacecraft they're speaking to are a hundred times further out than the space shuttle has ever gone.
In spite of this, they work very well. It's true that Mars is further away than a telecommunications satellite, but it's true also that Nasa has now returned images from such faraway places as Jupiter, Saturn and Uranus. Incredibly, we have even put a probe down on Titan! These sorts of places are 10 times further out than Mars and yet, in many ways, they seem to be far more amenable to visitors.
It was perhaps with this in mind that the Americans decided to change their tactics and put a cap on the cost of their spaceships. Where once they would have spent $1bn on one robotic probe, they now build four at about $250m each. Why? So that when the Ghoul devours two of them, the other two might still get through. It's a cynical tactic, but it seems to be working. For all their disappointments, scientists now have some excellent spacecraft on or around the planet, and the data streams are awesome.
If the Phoenix lander succeeds, then the images it returns may prove to be some of the most fantastic in the history of science. But don't get excited until you actually see them on your television screen. The Great Galactic Ghoul is out there, and the damned thing's always hungry.
Of Mars and men
* Mars is the most visited planet in the Solar System
* The USA has sent 20 missions (six have been lost)
* Russia (as the USSR) has sent 18 (only two worked well); Japanone (which got lost en route)
*Europe has sent one (Beagle 2 crashed, the orbiter worked well)
* The average total or partial failure rate is around 60 per cent
* Mariner 9 – 1974 (USA)
* Viking 1 and 2 – 1976 (USA)
* Spirit and Opportunity – 2004 (USA)
* Pathfinder – 1997 (USA)
* Mars Express Orbiter (European) – 2003
* Mariner 8 – crashed into the sea (USA)
* Nozomi (Japan) – got lost en route
* Mars Polar Lander – 1998/99 (USA) reached the planet, landed but never transmitted from the surface
* Ditto Beagle 2 – 2003 (UK/ESA)
* Mars Climate Orbiter – 1999 (USA) crashed due to a metric/imperial units conversion error