Mission to Pluto Seeks to Uncover Scientific Mysteries
By John Johnson
Los Angeles Times
Astronomer Clyde Tombaugh could not have known how fitting it was to call his new planet Pluto, after the Roman god of the underworld.
In the 76 years since Tombaugh discovered the solar system’s ninth planet, Pluto has remained an enigma -- a shrouded phantom lurking in the dark recesses of the solar system.
Three billion miles from Earth, the diminutive ice world is so distant that even the Hubble Space Telescope can produce only a hazy image of an object resembling a chewed-on tennis ball.
Now, Pluto’s time to shine has finally arrived.
On Tuesday, the National Aeronautics and Space Administration is scheduled to launch its $700-million New Horizons spacecraft from Cape Canaveral in Florida on a nearly nine-year journey to the ninth planet.
It will be the space agency’s first spacecraft since 1977 to be dispatched to an unexplored planet, and will complete NASA’s grand tour of all nine planets in the solar system.
“This is the capstone of the missions to the planets that NASA has led since the 1960s,’’ said Alan Stern, an astrophysicist with the Southwest Research Institute in Colorado, who is the principal investigator for the New Horizons mission.
NASA Administrator Michael D. Griffin declared: “In that sense, this mission truly marks the end of the beginning.’’
For planetary scientists, understanding distant Pluto with its unique composition of rock and ice is key to answering questions about how planets formed and why they formed where they did.
“Pluto is a treasure trove of scientific information waiting to be discovered,’’ said Andrew Dantzler, director of NASA’s Solar System Division.
New Horizons, designed and built at the Johns Hopkins University Applied Physics Laboratory, also will broaden scientists’ understanding of the distant Kuiper Belt, located beyond Neptune at the fringes of the solar system. At one time thought to be a sparse junkyard of castoff planetary parts, the Kuiper Belt is now thought to contain as many as 100,000 miniature worlds, making it more crowded than the asteroid belt between Mars and Jupiter.
And what lies beyond?
New Horizons will try to find out.
Since Pluto’s discovery, the planet’s story has been one of enduring mystery.
After the discovery of Neptune in 1846, astronomers believed that odd perturbations in the orbits of Uranus and Neptune could not be entirely explained except by the presence of yet another massive planet somewhere beyond Neptune. They named that body Planet X.
The astronomical mystery became a cause celebre. “I believe in the new planet,’’ Mark Twain wrote. “I hope it is going to be named after me; I should just love it if I can’t have a constellation.’’
Twain was long dead by the time the answer came.
As it turned out, much of what astronomers thought they knew about Planet X was wrong. It wasn’t big. In fact, it would be smaller than seven moons in the solar system. And it didn’t cause the perturbations in Uranus’ and Neptune’s orbits that were the primary reason for believing there was a ninth planet.
Scientists eventually determined the perturbations were the result of inaccurate observations.
Tombaugh didn’t know any of this in 1929, when he began scouring the night sky for Planet X. He used the 13-inch telescope at the Lowell Observatory in Flagstaff, Ariz., where he worked as a research assistant.
For months, he snapped pictures of the sky and compared the pictures to see if anything was moving, which would be evidence of a relatively close object orbiting the sun. The task was daunting. Each photographic plate contained at least 40,000 stars.
The name Pluto came from 11-year-old Venetia Burney in Oxford, England. Her reasoning was that it must be very dark where Pluto was.
She was correct. From the surface of Pluto, the sun is just another star in a perpetual night.
The more scientists learned about Pluto and its neighborhood, the stranger it seemed.
For one thing, the planet’s orbit is highly eccentric. From 1979 to 1999, it was closer to the sun than Neptune. At other times during its 248-year journey around the sun, it’s much farther away.
It also rotates in the opposite direction from most of the other planets. Its eccentric orbit means Pluto is likely to have the most complex seasons of any planet, even if its surface temperature never gets much above minus 387 degrees Fahrenheit.
Pluto belongs to its own planetary class, distinct from the inner rocky planets (Mercury, Venus, Earth and Mars) and the outer gas giants (Jupiter, Saturn, Uranus and Neptune).
Some scientists no longer believe Pluto is a planet, saying it’s more like a comet or asteroid.
That argument gained weight last year, when a team led by California Institute of Technology astronomer Michael Brown announced the discovery of an object in the Kuiper Belt larger than Pluto.
The find suggested that either the new object, which Brown nicknamed Lilah after his daughter, was the solar system’s 10th planet, or Pluto was just another Kuiper Belt object.
So far, Pluto has held onto its status as a planet, if for no other reason than tradition.
Stern considers Pluto’s status “a settled matter. It passes the test of a planetary body.’’
It has at least three moons, one of which, Charon, is half the size of Pluto itself. Named for the mythological boatman who ferried the souls of the dead across the river Acheron to Hades, Charon is the largest moon in the solar system when compared to its host planet.
The International Astronomical Union has been considering a new definition of a planet. The final tally could range from eight, if Pluto is demoted, to 23, if all the large objects orbiting the sun, including the asteroid belt, are counted.
Stern, a member of the Astronomical Union working group, said he believed Pluto would remain a planet.
Pluto’s exact composition is unknown, but it is thought to be made up of about 70 percent rock and 30 percent water ice.
Pluto also has polar caps. Earth’s atmosphere has one gas that forms ice caps at the poles: water vapor. Pluto has three: nitrogen, carbon monoxide and methane. Its thin atmosphere bleeds into space at its closest point to the sun, but freezes and collapses to the surface at its farthest point.
New Horizons will be the fastest NASA spacecraft ever, traveling at 36,000 mph after separating from its giant Atlas 5 rocket. It will cross the moon’s orbit in just nine hours, compared with the three-day journey of Apollo 11. It will then use Jupiter’s gravity to slingshot itself into the outer solar system, reaching a top speed of 47,000 mph.
Even at those speeds, the spacecraft isn’t scheduled to reach Pluto until 2015 at the earliest.
The piano-sized craft is designed to use minimal power -- about the same amount as two 100-watt lightbulbs. After sling-shotting away from Jupiter, the craft will go into hibernation. It is to awaken once a year to check its systems.
After traveling billions of miles, New Horizons must thread a needle 186 miles wide to approach Pluto. Swinging over the small planet at a distance of 6,200 miles, it will have just one Earth day to get the best pictures. The onboard cameras will be able to resolve features as small as 80 feet across.
One challenge facing NASA controllers is the time it will take to communicate with the spacecraft once it reaches Pluto -- about four hours and 25 minutes, one way. At those distances, New Horizons will be well into its observations before controllers find out whether they are receiving good data.
New Horizons’ trajectory will take it between Pluto and Charon. Besides visible-light cameras, the craft will employ infrared cameras, a mass spectrometer and an ultraviolet spectroscope. The instruments may also get a glimpse of Pluto’s two other moons, which were discovered last year by the Hubble Space Telescope. So far, they are simply known as P1 and P2.
“After leaving Pluto, it will go out into the Kuiper Belt,’’ said Glen Fountain of the Applied Physics Laboratory.
