It seems unfair to call the Cassini space probe — celebrated in this week's issue (see pages 965–966 and 985–1005) — a relic of the past when it is still two years away from the encounter with Saturn that is its raison d'être.

Cassini is the most capable deep-space explorer ever launched, with optics and science instruments that are far superior to those on board its predecessors, such as the Galileo mission to Jupiter and its moons. The show from Saturn and its mysterious, cloud-covered moon Titan should be spectacular. And, unlike Galileo, Cassini isn't crippled by a broken antenna, so we should receive every byte of data that its design engineers planned for.

Still, the $3.5-billion, six-tonne Cassini is like a 1959 Cadillac: heavy, expensive and a thing of beauty — but outdated. We would do things differently today than we did in 1989, when the project left the drawing board.

Just how differently was hinted at earlier this month, when NASA announced its plan to invest substantially in space nuclear propulsion for the first time in years. This is great news for planetary science. New propulsion systems could dramatically cut travel times to the outer planets and enable an exciting new class of missions, such as placing an orbiter around Europa, the possibly life-bearing moon of Jupiter.

Caught between the old and new ways of doing business is NASA's proposed mission to Pluto, which, like Titan, is one of the Solar System's last bits of true terra incognita. The agency has been struggling for nearly a decade with the problem of how best to explore this distant world. Last year, at the US Congress's insistence, it chose a concept called New Horizons, which uses conventional rocket propulsion, for a launch in 2006. Now that plan is on hold, while NASA revamps its outer-planets programme to incorporate possible new propulsion technologies and waits for scientific advice on which planetary targets are the most attractive.

A mission to Pluto and the Kuiper Belt — the vast population of chunks of rock and ice that lies beyond the orbit of Neptune — is likely to be high on that list. But tempting though it may be to speed up the journey with nuclear rockets, it may ultimately make more sense to stay with the proposal already on the table. A revolutionary new propulsion system will take several years to develop and test in space. And while a nuclear rocket would be fast, reaching Pluto in as little as five years, it would not be cheap.

Meanwhile, the New Horizons team has cut its proposed travel time to under ten years, and would spend less than $500 million — well below the $650-million cap for the new breed of outer planetary missions. So while NASA should vigorously pursue the nuclear option, the best way to reach the last uncharted planet, at least in the short term, is probably still the old-fashioned way.