Pluto is so far away (4.8bn km) and so small (about two-thirds the size of the Earth’s moon) that we’ve never had a good look at it, not even with the Hubble space telescope. In Hubble images, Pluto has always been a tiny, pixelated blob. Until now.
A Nasa spacecraft, New Horizons, is bearing down on the dwarf planet at 5,100km/h. The robotic probe, which weighs half a tonne and is shaped like a vacuum cleaner attachment, will fly past Pluto, its cameras and instruments ravenously gobbling data, at 11.49am GMT on 14 July.
That, at least, is what we can expect to happen given the current trajectory of New Horizons and the laws of physics. But this is not a mission free of hazard. A craft travelling so fast – New Horizons is the fastest spaceship ever launched from Earth – can be disabled by a collision with something as small as a grain of rice.
Pluto had been left out in the cold for decades as Nasa probes explored larger and flashier planets. Recently, it endured a downgrade among astronomers, who declared that it wasn’t a full-blown planet at all. But it’s definitely something intriguing – easily the most famous of the small, icy worlds that inhabit the exurbs of the solar system.
“We are running the anchor leg in a 50-year exploration of the planets,” says Alan Stern, the principal investigator – leader – of the New Horizons mission. “I tell people, this is it, it’s the last picture show, it’s the last train to Clarksville. Better watch!”
That’s vintage Stern: he is a tireless promoter of New Horizons. You could call him “mercurial” if that weren’t inappropriate for a planetary scientist focused on Pluto.
In a world in which scientists tend to speak in jargon, and where project managers default to bureaucratese, Stern, 57, a former Nasa associate administrator who is now at the non-profit Southwest Research Institute in Boulder, Colorado, speaks in tweets, news alerts, soundbites, headlines and pull-quotes.
“There could be boulder-sized particles strewn all over the place,” Green said.
The scientists think New Horizons will probably sail through the Pluto system without any nasty collisions. Probably. “My biggest worry is something we haven’t thought of,” Stern said.
In the early 20th century, there were only eight known planets, but astronomer Percival Lowell believed there must be a ninth, “Planet X,” which had perturbed the orbit of Neptune. Lowell died before he could find it, and astronomers today question his calculations.
In 1930, Clyde Tombaugh, an astronomer working at the Lowell Observatory in Flagstaff, Arizona, stumbled on an object beyond the orbit of Neptune that was moving with respect to the background stars. Tombaugh’s object became known as Pluto, for the Roman god of the underworld, with a nod, in the first two letters, to Percival Lowell.
Its status as a planet became controversial a decade ago after astronomers discovered that the solar system’s outer regions are crowded with icy worlds. Amid a raging debate, the International Astronomical Union reclassified Pluto as a “dwarf planet.”
Nasa’s Jim Green is dismissive of the controversy: “That’s nomenclature. To me, that’s unimportant. What’s important is that this is a body well worth going to. It represents a brand-new frontier.”
Does Alan Stern think Pluto is still a legitimate, no-qualifiers “planet”? “Of course I do!” he said. “It has all the attributes of a planet. Screw the astronomers! Would you go to a podiatrist for brain surgery? They don’t know what they’re talking about!”