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Planet Pluto or Kuiper Belt King?
Planet Pluto or Kuiper Belt King?
24 Sep 2002
(Source: Johns Hopkins University Applied Physics Laboratory)

As astronomers continue a heated debate about whether Pluto should be yanked from the planetary lineup, researchers at the Applied Physics Laboratory are hoping to answer questions about the diminutive planet by sending a spacecraft to the far edge of the solar system.

Last December, NASA selected APL to build a spacecraft for and manage the New Horizons mission, which seeks to study Pluto and the recently discovered Kuiper Belt, a loose collection of icy-rocky bodies of which Pluto may be the largest. The mission could be key in classifying Pluto and in determining the significance of the Kuiper Belt, which Hopkins researchers suggest may be a third major region of the solar system.

If funding comes through for the project -- it was missing from President Bush's FY 2003 budget for NASA and would have to be restored by Congress this fall -- the long trip to Pluto could begin in 2006.

Because of Pluto's small size -- just two-thirds the size of our moon -- and tremendous distance from Earth, studying it remains difficult. Pluto orbits the sun at 60 astronomical units; one astronomical unit equals the distance from the Earth to the sun, or about 93 million miles. Even the Hubble Space Telescope can't discern anything on Pluto's surface. NASA sent two Voyager spacecraft in the late 1970s to explore the four giant gas planets closest to Pluto, but no spacecraft has been near enough to Pluto to answer some of the most basic questions.

The trip to Pluto will take 10 years. Once at Pluto, the craft's science goals include taking surface images at a resolution of about one-half mile. "This will allow us to distinguish major types of terrain," says APL's Andy Cheng, project scientist for New Horizons. Special instruments will gather information on the geology, interior makeup, and atmosphere of Pluto and its moon, Charon, before the next five-year leg of the journey to reach one or more Kuiper Belt Objects, or KBOs.

The discovery of the first KBOs in 1992 and the realization that Pluto shares an orbital timing with these bodies caused astronomers to revisit the issue of what constitutes a planet. Among those who believe that Pluto more correctly belongs to the group of KBOs is astronomer Paul Feldman, chairman of Hopkins' physics and astronomy department. "I would go along with the people who would remove Pluto from the list of planets," he says, "and then it becomes the largest of another class of objects."

But the issue is so heated among astronomers that the International Astronomical Union, which has authority over such matters, postponed indefinitely all discussions on Pluto and official planet definitions in January 1999. At that meeting, a proposal to grant dual status to Pluto as both a planet and a Kuiper Belt Object was debated but not adopted. "Many IAU members came down on the side of not changing Pluto's status, mainly for historical reasons," says planetary astronomer Keith Noll at the Space Telescope Science Institute in Baltimore, who participated in the IAU discussions. "In astronomy, we've inherited all sorts of things that if you were redefining now it would be different."

In the meantime, Cheng says that the discovery of the KBOs means we may need to redefine the solar system into three distinct regions: the terrestrial planets (Mercury, Venus, Earth, and Mars), the gas giants (Jupiter, Saturn, Uranus, and Neptune), and the Kuiper Belt, with Pluto the largest of potentially thousands of icy-rocky objects.

If funding is approved, the New Horizons spacecraft will be built at APL at a bargain price for such an ambitious mission -- just $488 million. Since smaller spacecraft tend to cost less to launch, the entire New Horizons spacecraft body is designed to fit beneath a 6-foot communications dish and will carry a number of miniaturized instruments weighing no more than a few pounds each. Further savings will be realized by using a conventional chemical propulsion engine to get the craft to Pluto.

The complicated maneuvers and distance of the mission combine to make New Horizons high-risk and serve as another reason to keep the costs down -- in case of failure. But mission director Robert Farquhar at APL asserts that all spacecraft systems will have backups. A Pluto enthusiast, Farquhar (now in his late 60s) plans to be on the job for the flyby of Pluto in 2016. "Mission [plans] cite that I may retire sometime during the cruise to Pluto, but I have no plans to retire. [It should say] 'Farquhar may expire sometime on the way out to Pluto,'" he says with a chuckle.

This fall, funding hurdles are eclipsing the risks of the mission itself. Unless Congress acts directly to restore money to NASA's budget for the mission, it will be shelved, although NASA insists a mission still could be funded as early as a year from now.

But a one-year delay at this juncture could mean losing the chance to study Pluto's atmosphere. Because Pluto's atmosphere collapses during its orbit as it reaches a certain distance from the sun, researchers believe a 2006 launch is the latest possible time to leave Earth; after 2016, they predict, it will be another 200 years before the atmosphere thaws and re-forms. "The timing is just too critical to horse around for a year," says Alan Stern, New Horizon's principal investigator and director of space studies at Southwest Research Institute in Boulder, Colorado.

Still, Stern and other mission researchers remain optimistic. "I think we're going to get the funding," Stern says, "and we're going to fly this mission."

On a more philosophical note, astronomers agree that the debate about Pluto and its similarities to the KBOs has been good for public understanding. "Planets vs. small bodies, and which bin they go in, may not be important to decide," says STScI's Noll. "It's educational for people to know that it isn't easy to define, and to talk about why."

Stern is among those already calling Pluto "King of the Kuiper Belt." "There are a number of strong arguments that we will find not only 'Plutos' in large numbers, but larger objects as well, farther out," says Stern."It's just the way the solar system formation worked -- it should have produced a lot of these objects."

-- Diana Whitman

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Last Updated: 11 May 2011