A nother express has come off the rails, and may be years late in reaching its destination. It is not a train, but Nasa's long-planned Pluto-Kuiper Express probe.
The only planet yet to be visited by a spacecraft, and so the one we know least about, is Pluto. This is hardly surprising. We've known of it for but a fraction of a Pluto year. Since it was discovered in 1930 Pluto has traversed little more than a quarter of its loop around the Sun, a complete orbit taking 248 of our years.
Tiny Pluto - only two-thirds the size of our Moon - plods around the frigid depths of space, never coming within 2.7 billion miles of the Sun. But it's not alone. In 1978 a moon of Pluto was discovered, and named Charon. Unlike most moons it is large compared to its parent. Pluto is about 1,475 miles across, Charon a little more than half that, at 750 miles. Astronomers tend to think of the two as a binary planet, taking a little over six days to pirouette around each other.
More recently, a host of other cosmic lumps have been found in their vicinity. Since 1992 astronomers have spotted more than 300 minor planets, typically 100 miles or so in size, slowly circuiting the sun beyond Neptune. This is called the Kuiper belt, after the Dutch-American astronomer who suggested its existence in 1950. The plan was for the Pluto-Kuiper Express (PKE) space probe to fly past Pluto, then visit several Kuiper belt objects.
Express seems a misnomer when interplanetary journeys can take decades. Nasa's Voyager 2 probe, launched in 1977, arrived at Neptune in 1989. Even that required boosts from successive fly-bys of Jupiter, Saturn and Uranus. The plan is to launch PKE in 2004. An encounter with Jupiter in 2006 will use its gravitational slingshot action to throw the satellite on to a high-speed trajectory reaching Pluto in 2012.
That plan has its drawbacks. When it gets to Pluto, PKE will be travelling at 12 miles per second, and so will whiz by, providing only a series of snapshots rather than a prolonged period of data collection. It is not feasible to give PKE an adequate rocket booster to stop it at its destination.
The other problem is that Jupiter has to be in the right place if PKE is to use its gravity to accelerate. Jupiter will be positioned correctly until 2015 at the earliest. PKE would then reach Pluto in 2022. The researchers, who have invested a decade in planning PKE, would by then be superannuated.
PKE's timetable is set by celestial mechanics. If it does not fly in 2004, it may be left standing for a dozen years. Someone might invent a novel propulsion technique that will work far from the Sun, but nothing could be cheaper than a free gravity assist from Jupiter in 2006.
This seems a compelling case, if the last unexplored planet is to be reconnoitred in the near future. Enter the reality of limited budgets, and vested interests. The Nasa hierarchy says the agency cannot afford the $600m PKE would cost to build for a 2004 launch. Instead the order has been given to concentrate on a mission to Europa, one of the four large moons of Jupiter.
Here critics think that Nasa is bowing too low to public opinion, which is bedazzled by the prospect of life elsewhere. Detractors say astrobiology is the only science with nothing to study, although it hits the headlines frequently, such as in 1996 when Nasa announced the possible identification of microfossils in a meteorite from Mars.
Why Europa? It is largely made of ice. Many scientists think it likely that Europa has an ocean beneath the frozen crust now being inspected by the Galileo satellite, which has spent the past five years orbiting Jupiter, returning detailed images of the planet and its moons. The sine qua non of life on Earth is liquid water. If Europa has that, plus the plethora of organic chemicals we find everywhere, then some form of life is a possibility, even if it's only microbial.
The Europa Orbiter mission will carry a radar system capable of penetrating the ice, indicating the existence of that ocean, if it's there. But new engineering technologies are needed, and their development has already caused the launch date to slip from 2003 to 2006.
To keep even to that delayed timetable, Nasa chiefs decided that PKE must be dropped, deferring arrival at Pluto until 2020 or so. This provoked a storm of protest. The Planetary Society, the worldwide association of space exploration enthusiasts, collected 10,000 letters of support for PKE within a fortnight.
PKE advocates say that Nasa cannot be that hard up if it has just found $200m for an additional rover vehicle to be sent to Mars in 2003, for which there is limited scientific priority but huge public interest. They also argue that the Europa Orbiter could be launched any year, whereas PKE has to go in 2004.
Would it be that bad if PKE were delayed a decade or so? It was a surprise when, in the 1980s, Pluto was recognised to have a tenuous atmosphere, largely composed of methane. At that time Pluto was closest to the Sun (and hence the hottest) it ever gets, having dipped within Neptune's orbit from 1979 until 1999.
As Pluto recedes, the atmosphere is expected to freeze out. This also explains why the planet is relatively bright, for its tiny size: we think it is largely coated with methane snow, replenished each orbit, and that frosting reflects much of the sunlight reaching it.
This provides another timetable. Soon that gaseous shroud will be no more, and PKE was designed specifically for atmospheric investigations. As one PKE advocate put it: "The last time Pluto was where it is right now, George Washington was chopping down a cherry tree. It will be more than two hundred years before we can do what we can do today with Pluto."
The time to build a PKE to reach Pluto in 2012 is right now. A few years back, when astronomers discussed if Pluto should be regarded as a fully-fledged planet, there were howls of protest from the public, and the plan to demote it was abandoned. Perhaps the imperative to visit Pluto within our lifetimes, will also put enough pressure on Nasa to put the Express back on the rails.
Duncan Steel works at the University of Salford. His new book Target Earth (Time Life) will be published shortly.
