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.
“This” – he pauses dramatically – “is a moment. People should watch it. They should sit their freakin’ kids down and say, think about this technology. Think about people who worked on this for 25 years to bring this knowledge… It’s a long way to go to the outer edge, the very edge of the solar system.”
The headquarters of the New Horizons mission is the Johns Hopkins University Applied Physics Laboratory, in Laurel, Maryland. A mural outside the New Horizons mission control room reads “The Year of Pluto.”
Even at the speed of light, a one-way message to or from New Horizons requires a 4.5-hour trip. When sending a message, APL technicians have to aim where the spacecraft will be 4.5 hours in the future, the way a clay-pigeon shooter aims in front of the target.
Many people at APL have been with New Horizons since the launch in 2006. “We know this spacecraft very well. It’s our baby,” said Alice Bowman, the mission operations manager. “It went through its toddler stage where it was a little ornery.”
The spacecraft’s main computer rebooted itself occasionally. Her team has had to upload corrective software. Even with advanced technology, she said, “There’s always risks involved when you’re sending 1s and 0s across billions of miles of space.”
Stern keeps on display in his office the August 1970 issue of National Geographic describing a future “grand tour” of the planets by robotic spacecraft. An article foretold a visit to Pluto in 1986. Nasa’s two Voyager spacecraft carried out the grand tour, flying past Jupiter and Saturn, with Voyager 2 going all the way to Uranus and Neptune, but the best route for studying Saturn and its huge moon Titan didn’t permit Voyager 2 to visit Pluto, Stern said.
Moreover, Nasa didn’t realise, when it put together the Voyager programme, that there are thousands of icy objects beyond Neptune’s orbit – a region dubbed the Kuiper Belt. The two Voyager spacecraft turned off their cameras once they reached that region of supposedly empty space, said Nasa director of planetary science Jim Green.
“We believe there are tens of thousands of Kuiper Belt objects,” Green said.
The scientists behind New Horizons say the mission will help us understand the origin and evolution of the solar system.
Already, the little spaceship’s long-range camera has detected intriguing patterns on the surface of Pluto that scientists cannot easily explain. The tiny world may have mountains and valleys, possibly frozen methane lakes or even a liquid water ocean far beneath the frozen surface. Or maybe Pluto has no topography at all and is just a smooth ball covered in a deep layer of nitrogen slush.
Stern said he began pushing for a Pluto mission in 1988. Nasa considered numerous proposals before deciding, in 2001, to go with the New Horizons mission, which called for a relatively inexpensive, no-frills spacecraft using off-the-shelf technology.
The probe rode into space one afternoon in 2006 on an Atlas V rocket, reaching record-breaking velocity. Before the day was out, it had flown past the orbit of the moon.
A gravity boost from Jupiter shortened the Pluto trip by three years. “You had to hit a little keyhole in space near Jupiter,” said project scientist Hal Weaver.
New Horizons will come within 12,550km of Pluto’s surface at its closest approach. Because of the way the instruments and the antennae are configured, New Horizons cannot observe Pluto and simultaneously transmit data to Earth. Normally, it does one or the other. But the flyby is so fast, and such a precious opportunity, that the spacecraft will focus entirely on Pluto during the 14 July encounter, at the cost of leaving the scientists and engineers back home in suspense.
Scientists and prominent officials will have a countdown to the flyby, and they will celebrate at 11.49am GMT, but they won’t know for a while whether the spacecraft survived the encounter. They’ll get a simple batch of data – a health report about 11 hours later. A very bad scenario would involve red alarms going off.
“The worst case is we don’t hear at all,” Stern said.
His team came up with 249 contingencies,or problems, ranging from power brownouts on the spacecraft to a catastrophic fire at mission control. The biggest concern has been debris around Pluto.
Pluto has five moons, the largest of which, Charon, is half Pluto’s size. The four smaller moons of Pluto don’t have much of a gravitational field. When a rock from space strikes one of these moons, debris hurtles into orbit around Pluto and Charon.
“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!”
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