After traveling nine years across more than 3 billion miles of space, a spacecraft the size of a grand piano is about to give humanity its first high-resolution view of Pluto, which is about two-thirds the size of our moon.
Nobody knows what the rendezvous will reveal. Pluto’s icy surface may resemble an extreme version of Antarctica, with snow-capped mountains, steep crevasses and towering ice cliffs. The dwarf planet could be surrounded by rings of tiny ice particles, like its giant neighbor Neptune. There may even be evidence that once, long ago, there was an ocean beneath the frozen crust of its largest moon, Charon.
When it comes to Pluto, nothing is certain.
“Our knowledge of Pluto is quite meager,” said planetary scientist Alan Stern, the principal investigator for the NASA mission known as New Horizons. “It is very much like our knowledge of Mars was before our first mission there 50 years ago.”
New Horizons is poised to change all that. Sunday, the spacecraft’s long-range cameras began snapping pictures of Pluto and its moons against a backdrop of stars. New Horizons has been taking detailed measurements of the dust and charged particles in the dwarf planet’s environment since mid-January.
More data will be collected during the months leading up to the mission’s big moment this summer: A close approach on July 14 that will take the spacecraft just 7,700 miles from Pluto’s surface.
From that distance, New Horizons will be able to determine what the dwarf planet is made of, create temperature maps of its multi-colored surface, and look for auroras in its thin atmosphere. Scientists and the public will see the first high-definition images this summer.
Until now, the best pictures astronomers have managed to get consist of a few hazy pixels that were captured by the Hubble Space Telescope more than a decade ago. The resolution is so poor that if you looked at a comparable image of Earth, you wouldn’t be able to distinguish the continents from the seas.
The instruments on New Horizons will take images so detailed that if they were pictures of Los Angeles, they would show individual runways at Los Angeles International Airport, said Stern, who is based at the Southwest Research Institute in Boulder, Colo.
“What I’m most looking forward to is taking this point of light and transforming it into a planet,” he said.
The existence of a planet beyond Neptune was first hypothesized in the early 20th century after scientists noticed what they thought were disturbances in the orbits of Neptune and Uranus. Those wobbles turned out to be measurement errors, but decades of searching for the elusive “Planet X” led astronomers to Pluto in 1930.
Despite its great distance and diminutive size, scientists have been able to glean a remarkable amount of information from the anemic data gathered so far. By watching Pluto’s movements across the night sky, they deduced that it takes 248 Earth years to make one trip around the sun. Because Pluto’s brightness oscillates in a regular pattern, they think it makes a complete rotation on its axis every 6.4 Earth days.
Astronomers also noted that Pluto ventures far above and below the paths of the major planets in our night sky, leading them to conclude that its orbital plane has a distinctive tilt.
Close observations have revealed that Pluto has at least five moons – the biggest being Charon, which is about the size of Texas. After watching how Pluto’s gravity affects the movement of these moons, scientists have a sense of what the dwarf planet’s mass and volume might be and how much of it is made of rock and ice.
By examining the sunlight that reflects off Pluto through a prism, astronomers have been able to detect frozen methane, nitrogen and carbon monoxide on its surface. They’ve also determined that water ice appears to be absent.
Astronomers can even get a rough approximation of the temperature on Pluto’s surface by using large telescopes to look at the radiation emitted from its surface after it travels feebly across billions of miles of space.
“It is amazing what scientists can squeeze out of pathetic data,” said Hal Weaver, a planetary scientist at Johns Hopkins University and the project scientist on New Horizons.
But in the last few decades, scientists have hit a wall.
“At some point you get the maximum amount of information out of the data that you can, and the only way to advance your understanding is to send a spacecraft out,” said Richard Binzel, a professor at Massachusetts Institute of Technology and co-investigator on New Horizons.
NASA has considered going to Pluto many times over the last 25 years, but three previous missions – Pluto Fast Flyby, Pluto Express and Pluto Kuiper Express – were shelved or canceled. New Horizons got the green light in 2001 with a relatively low budget of $700 million.
“It is going to be a huge advance over anything we’ve done so far with telescopes on the ground,” said University of California, Los Angeles astronomer Dave Jewitt.
The mission got a boost from the 1992 discovery by Jewitt and his former graduate student Jane Luu that Pluto was not alone in the distant band of the solar system now known as the Kuiper Belt. More than 1,500 Kuiper Belt objects have been found so far – a cosmic zoo of bodies that vary in size, color and composition.
Occasionally, these bodies get knocked out of their distant orbits and come zooming to the inner solar system, ejecting gas and dust as they encounter the sun’s warmth for the first time. These are known as the short-period comets.
A handful of spacecraft have flown to these comets, including the European Space Agency’s Rosetta orbiter. But New Horizons’ visit to Pluto will provide the first glimpse of a Kuiper Belt object in its native habitat.
Pluto is the largest known member of the Kuiper Belt, but not by much. The dwarf planet Eris is close enough in size that astronomers briefly thought it might be larger, though that is no longer the case.
Pluto was still considered a full-fledged planet when New Horizons blasted off from Earth in 2006, but it was demoted to dwarf planet a few months later. The International Astronomical Union, which makes such determinations, said Pluto didn’t make the cut because it wasn’t hefty enough to prevent similar-sized objects from forming in its section of the solar system.
This indignity has not stopped the New Horizons scientists from describing their mission as one of planetary exploration.
“We will find out if it has enough mass that we think it deserves to be in the planet category,” said Weaver, who helped find four of Pluto’s five confirmed moons. “For now, I think calling it a dwarf planet still makes it a planet. Is a Chihuahua any less of a dog because it is small?”
Even if Pluto turns out to be smaller than astronomers anticipate, Binzel said he won’t be disappointed.
“There is nothing about the quest for knowledge about Pluto that has anything to do with its label,” he said.
New Horizons will spend most of 2015 collecting data from Pluto, its moons and its local area. Scientists anticipate that it will take until the fall of 2016 for the spacecraft to deliver its trove of data back to Earth.
By then, New Horizons may be on its way to visit other objects in the Kuiper Belt, if NASA opts to extend the mission. Scientists have already identified two candidates, each about the size of Orange County, that they would like to study once the primary mission is over.
“They are another billion miles further out, and it would take us until 2019 to get there,” Stern said. But astronomers don’t want to miss this chance to visit objects that have been in a deep freeze since the dawn of the solar system.
“The spacecraft is healthy and full of fuel,” he said. “The instruments are approved to go further.”
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