Encounter with a distant stranger

Which is the farthest planet from the Sun? Although the answer given by all the textbooks is Pluto, this is not entirely true. For 20 years, during each 240-year circuit around the Sun, Pluto slips inside the orbit of Neptune and becomes the eighth planet from the Sun. This rare episode will come to an end in 1999, when Pluto will regain its status as the planetary outpost of the Solar System.

Such orbital idiosyncrasy is of great significance for scientists hoping to learn more about this peculiar world. At present, Pluto is relatively close, about 4.5bn kilometres from the Sun. However, its orbit is highly elliptical so that by the year 2113 it will have drifted out to almost 7.5bn kilometres - an increase of two-thirds in distance.

As if this remoteness was not enough, astronomers also have to contend with Pluto's tiny size. Just 2,300km across, Pluto is much smaller than our Moon. Not surprisingly, astronomers have struggled to piece together a coherent picture of this enigmatic object since its discovery in 1930. Another 48 years went by before the discovery of its moon, Charon.

Before the planet retreats into the depths of space and once more becomes a bleak, frozen ice ball, Nasa, the US space agency, is desperately attempting to put together a spacecraft reconnaissance mission. The current plan, dubbed Pluto Express, is to send two small spacecraft to the Pluto-Charon system. As both objects rotate once every 6.4 days, the second craft, targeted to arrive 3.2 days after the first, would be able to survey regions hidden in darkness during the initial approach.

In the Eighties, when Charon and Pluto began a series of mutual eclipses, astronomers were able to piece together the first maps of surface brightness. Evidence emerged of brighter polar regions, possibly covered in frost. Then, in 1988, came the revelation that Pluto had a thin atmosphere, probably composed of nitrogen or methane.

The latest breakthrough has been provided by the Hubble Space Telescope. From its advantageous viewpoint above Earth's turbulent atmosphere, pictures taken with European Space Agency's Faint Object Camera have observed details on the surface for the first time.

After computer processing, Hubble's snapshots, taken during one complete Plutonian rotation, reveal major variations in brightness across the entire planet. "I don't know anything in the outer Solar System that looks this complex," says team member Dr Alan Stern, of Southwest Research Institute in Boulder, Colorado. Apart from the previously known bright polar regions, the images show a "ragged" north polar cap bisected by a dark strip, a bright spot seen rotating with the planet, a cluster of dark spots and a bright linear marking.

Speculation is rife over what these markings might represent. Some may be topographic features such as basins and fresh craters. The most favoured explanation is dramatic seasonal changes resulting from Pluto's orbit. During the 200-year-long winter, gases freeze and settle on to the icy wastes. When the short warm season returns, these frosts probably turn back into a gas. As a result, some regions are bright like new snow while others resemble dirty snow. The brighter deposits are probably nitrogen frost deposited during the past few decades, while the grey areas may be coated in residues of hydrocarbons, where ultraviolet sunlight and cosmic rays have chemically altered the methane frost.

At present, Pluto is enjoying its short summer break. As the dirty ices are evaporated, the thin atmosphere swells and evolves. All too soon the winter will return and the gases will freeze out once more, forming a fresh, frosty coating.

Despite public support from the Nasa director, Dan Goldin, Pluto Express scientists at the Jet Propulsion Laboratory in Pasadena, California, are affected by the financial constraints on all space activities. The challenge is to present a plan that could be given the go-ahead by Congress in 1998 or 1999, allowing the launches to proceed in 2001, with Pluto flybys in 2012-2013. Current estimates put the cost of such a mission at around $300m.

In order to achieve this, planners are hoping to use lower-cost Russian rockets to boost the craft on their way. The spacecraft themselves will weigh no more than 100kg (220lb) yet have sufficient computer power to probe the planet in the visible, infrared, ultraviolet and radio regions of the electromagnetic spectrum. According to the Pluto Express preproject manager, Robert Staehle, both Pluto and Charon could be mapped with a global resolution of around 1km, with spot coverage revealing features as small as 100 metres across.

Does America still have the vision and purpose to grasp this rare opportunity? If Pluto Express remains grounded, several lifetimes will pass before a similar mission can be launched. The last time Pluto was this close to the Sun, King George II was on the throne and Britain still ruled the American colonies.