IT IS time to go Pluto, the only planet we haven鈥檛 sent a spacecraft to. That鈥檚 the message from scientists who have analysed data from the tiny planet when it moved in front of a nearby star last month.
The occultation revealed that Pluto鈥檚 atmosphere may be undergoing significant changes. But unless a mission is agreed soon, and launched within five years, we will miss our only opportunity for almost two centuries to get a close-up of the whole planet.
Planetary scientists have long been pressing for a mission to Pluto, but NASA and the Bush administration have been reluctant to foot the bill. Early last month, however, a report by the US National Research Council said that sending a probe to Pluto and the Kuiper Belt beyond should be a top priority.
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Pluto鈥檚 eccentric 248-year orbit is making any mission increasingly urgent, says the NRC. The planet is 4.5 billion kilometres from the Sun, and has been moving away steadily since 1989. Ice already covers much of the surface, and most of its thin atmosphere is expected to gradually freeze over. Also, the further Pluto gets from the Sun, the more its tilted polar regions will be hidden in shadows, making them difficult for a visiting probe to study. Each year, from 2015 to 2025, says the NRC report, 鈥渁bout 200,000 square kilometres of terrain will be lost to imaging and mapping.鈥�
Pluto鈥檚 atmosphere had been a complete mystery until 1988, when astronomers got their first glimpse as the planet moved in front of a star. Observations by Jim Elliot of Massachusetts Institute of Technology revealed that the atmosphere has a haze near the planet鈥檚 surface.
The occultation on 19 July has revealed further details, says Marc Buie of the Lowell Observatory in Arizona, who monitored the event. He says the atmosphere is already between 10 and 30 degrees cooler than the 100 kelvin recorded by Elliot. Buie also saw different changes in light intensity as the star slipped behind the planet, an indication that the atmosphere鈥檚 structure has changed.
However, other measurements suggest that Pluto鈥檚 surface is warming slightly rather than cooling as expected. Pluto should grow brighter as gases freeze on its surface, Buie says, but on average he found it is getting darker. The darker surface absorbs more solar energy, more than enough to offset the cooling of the receding planet.
The flood of new data has raised more questions, making it vital to send a probe soon. 鈥淓very time we visit a planet, it transforms our understanding,鈥� says Buie.
The chances are growing that he will get his wish. Two weeks after the NRC report, the Senate Committee on Appropriations gave NASA $105 million towards a Pluto mission.
A $488 million mission is now planned for launch in January 2006, says Alan Stern of the Southwest Research Institute in Boulder, Colorado. Depending on the launch vehicle chosen, says Stern, who is investigator for the mission, the probe will reach its destination in 2015 or 2016.