'Scientific wonderland' expected as spacecraft approaches Pluto
As the New Horizons spacecraft approaches Pluto for a flyby, astronomers are about to get their first close look at the planet and its moons.
A planet redder than Mars? Rivers of liquid neon flowing across the surface? A subsurface ocean? A binary planet in a solar system filled with soloists?
"It sounds like science fiction," notes planetary scientist Alan Stern.
But it's not. It's Pluto, or at least inferences or speculations about Pluto astronomers have drawn from their ever-shifting vantage point, currently some 3 billion miles away.
In less than 100 days, a half-ton spacecraft the size of a baby grand piano will dramatically shrink that distance to just over 6,000 miles in a flyby that will give humanity its first up-close look at Pluto and its moons.
So many questions surround the system that it presents "a scientific wonderland," says Dr. Stern, lead scientist for the $700-million NASA mission, known as New Horizons. At the same time, the system formed in a poorly known region known as the Kuiper Belt. Pluto and moons Charon, Kerbos, Hydra, Styx, and Nix are ambassadors for the belt's myriad inhabitants. These include objects comparable in size to Pluto – all relics from the early phases of planet formation in the solar system.
"Pluto is like a mini-planetary system. The architectural details of this system are fascinating," says William McKinnon, a researcher at Washington University in St. Louis and a member of the New Horizons science team. "They'll tell us about Pluto's origin and ... they'll tell us about the conditions in the ancient Kuiper Belt where Pluto formed."
On one level, New Horizons caps a 50-year quest to tour the planets beyond Mars. First envisioned in the mid-1960s, the grand tour of flybys morphed into the Voyager 1 and 2 missions. Launched in 1977, these provided the first stunning, close-in observations of Jupiter, Saturn, Uranus, and Neptune. Orbiters to Jupiter and Saturn for more-detailed studies would follow. Pluto, however, was struck from the tour's TripTik.
Now, 26 years after Voyager 2's Neptune fly-by, New Horizons is set to introduce a new generation to the awe of seeing new worlds for the first time as something more than remote dots of light.
New Horizons is doing more than putting the wraps on a 20th-century odyssey, however. Astronomers are discovering planets and planet candidates by the thousands orbiting other stars, as well as debris disks orbiting stars at distances comparable to the Kuiper Belt's distance from the sun.
Yet "at least in our lifetimes I don't believe we'll be able to look at exoplanets like we're going to look at Pluto," says Jim Green, who heads NASA's planetary-science division in Washington. New Horizons and other studies of the Kuiper Belt will help astronomers to interpret what they see in other planetary systems.
After observing Pluto for more than 80 years, researchers have assembled the outlines of a story for the Pluto-Charon system.
Charon is not so much a moon as it is a planetary companion, some researchers say. At some 750 miles across, Charon is about half as large as Pluto and has about 12 percent of Pluto's mass. Charon's size makes it the largest moon in the solar system relative to the size of the object it's orbiting.
Moreover, in a planet-moon system, the two orbit a common center of mass. For a typical planet-moon system, however, the center of mass falls within the interior of the planet. Charon and Pluto orbit a center of mass that falls between the two objects. Charon may be less a moon and more a companion dwarf planet.
Given the sizes, masses, and densities of the two, Charon most likely formed from a high-speed collision between Pluto and another Kuiper Belt object. This also provides the simplest pathways for forming the other four moons.
For instance, modeling studies by Scott Kenyon at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and colleague Benjamin Bromley at the University of Utah point to a formation process that, initial collision aside, mirrors that of planets forming around a star.
They found that following the impact, debris would have gathered in a disk around Pluto and Charon. Collisions within the disk would eject some debris out of the system and break up the rest. As the disk spreads out, collisions would become less frequent. Eventually the environment would become stable enough to allow moons to form through accretion. If the material in the disk was smaller than a couple of football fields across, moons ranging in size from 12 to 50 miles across could form within 1,000 to 10,000 years after the initial collision that formed Charon.
One of New Horizon's goals is to hunt for any additional moons as well as any extended debris disk or rings that could remain.
The collision that formed Charon also could have led to the presence of an internal ocean on Pluto, at least early in its history, according a study by Amy Barr Mlinar, a senior scientist with the Planetary Science Institute, based in Tucson, and Geoffery Collins, a planetary scientist at Wheaton College in Norton, Mass.
The collision itself would have generated substantial amounts of heat. Changes over time in the rotation rates and orbital distances of the two objects as Charon slowly moved away from Pluto would have set up stresses in each body that would have led to continued, frictional heating. For Pluto, this heat could have sustained a layer of liquid water beneath its icy crust.
Evidence of that stress may appear in the location and orientation of faults on the surfaces of Pluto and Charon, unless processes at the surface have since erased them.
As for the possibility that Pluto is a crimson-tinged orb, the evidence comes from lab experiments involving chemistry that sunlight triggers between the nitrogen compounds, methane and other simple organic molecules, and carbon monoxide bound up in the icy crust.
Using a mix of these molecules at temperatures comparable to those on Pluto's surface and irradiated by artificial sunlight dimmed to match the amount of light reaching Pluto, researchers have found that the three combine into more-complex molecules that have a reddish hue.
To tackle these and other questions, New Horizons is carrying seven instruments that represent the most capable suite of tools ever mustered for a fly-by mission, according to the science team.
They began gathering information about Pluto's general environment in January. This past week, the team released the mission's first color image of Pluto and Charon – a still-fuzzy duo that will sharpen as New Horizons hurtles toward its July 14 flyby at a pace that covers nearly a million miles a day.
One sign that the excitement is building: Like expectant parents, scientists are trying to figure out what to name the features they discover – from craters, chasms, or ice volcanoes on the surfaces of Pluto and Charon to any additional moons they may find.
The New Horizons crew has teamed up with NASA, the International Astronomical Union (IAU), and the SETI Institute to open the naming process to the rest of the world. Through April 24, people can vote for their favorite monikers at http://www.ourpluto.org/. The researchers gathered an initial set of names from the history of exploration, mythology related to the underworld, and from films, TV shows, and novels related to exploration and travel. People also are free to suggest additional names. Once data from the flyby are beamed back to Earth, the science team will figure out which category of names fit with which type of feature or satellite they discovery.
Others have suggested similar approaches, with names for some types of features more closely tied to astronomers who made significant contributions to the study of Pluto, such as its discoverer Clyde Tombaugh, as well as Gerald Peter Kuiper, or Fred Whipple.
Be careful what name you pick; you'll have to live with it, cautions Eric Mamajek, an astronomer at the University of Rochester. With help from astronomy students, he and 10 colleagues from the US, Mexico, and France shipped a proposed naming scheme to the IAU early last year and posted the proposal on line two weeks ago.
"The science community will be stuck with surface feature and satellite names after they've been proposed and adopted by the IAU, so it is worth a bit of discussion beforehand," he writes in an email. "The naming should be 'beautiful.' "