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, NASA 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 said: "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, should also broaden scientists' understanding of the distant Kuiper Belt, located beyond Neptune at the fringes of the solar system. Once 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, there are seven moons in the solar system that would be larger than the planet. 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 lowly 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.
Tombaugh's discovery won him world renown -- and presented a puzzle. What to call the new object? Early suggestions included Atlas, Artemis, Vulcan and Minerva.
The name Pluto came from 11-year-old Venetia Burney in Oxford, England. Her reasoning was that it must be very dark where the planet 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; they say it's more like a comet or asteroid.
That argument gained weight last year, when a team led by Caltech 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. 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 with 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% rock and 30% water ice.
Despite its small size, Pluto is one of the brightest objects in the solar system. Its primary color is red; scientists think the brightest patches are exotic ices such as methane, nitrogen, ethane and carbon monoxide.
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 would be the fastest NASA spacecraft ever, traveling at 36,000 mph after separating from its giant Atlas 5 rocket. It is to cross the moon's orbit in nine hours, compared with the three-day journey of Apollo 11. It would then use Jupiter's gravity to slingshot itself into the outer solar system, reaching a top speed of 47,000 mph. The maneuver would shave three to five years off the trip to Pluto.
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 slingshotting away from Jupiter, the craft is to go into hibernation, awakening once a year for about 50 days 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 would have just one Earth day to get the best pictures. The onboard cameras should be able to resolve features as small as 80 feet across.
One challenge facing NASA controllers is the time it would take to communicate with the spacecraft once it reaches Pluto -- about four hours and 25 minutes, one way. At those distances, New Horizons would be well into its observations before controllers find out whether they are getting good data.
New Horizons' trajectory would take it between Pluto and Charon. Besides visible-light cameras, the craft would 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.
There, the craft could encounter mysteries almost as interesting as those of Pluto itself.
Computer models show the ancient Kuiper Belt was probably 10 times larger than it is today. Some planetary scientists believe the belt is the remains of a giant 10th planet that was torn apart by the gravitational pull of the outer planets and battered to bits by other space rocks.
New Horizons could send back data on Kuiper Belt objects for up to a decade, until its plutonium power source runs down.
Eventually, it will pass into the emptiness of interstellar space.
(BEGIN TEXT OF INFOBOX)
On a mission
It should take the New Horizons spacecraft about nine years to travel the 3 billion miles to Pluto. Once there, it is to gather data on the planet and its moon Charon before heading toward the Kuiper Belt -- a ring of small objects orbiting the sun.
If the spacecraft lifts off within the first three weeks of its launch window:
Jan. 17-Feb. 14: Launch from Florida
February 2007: The spacecraft gets a solid gravity boost from Jupiter, saving up to three years of flight time.
March 2007-June 2015: The spacecraft spins slowly in a state of hibernation, signaling once a week and conducting instrument calibrations about 50 days a year.
July 2015: The spacecraft encounters Pluto and Charon, passing within 6,000 miles of Pluto.
2016-20: With NASA's approval, the spacecraft will be directed toward one or more Kuiper Belt objects beyond Pluto.
PEPSSI: Plasma-sensing instrument for measuring particles escaping from Pluto's atmosphere
SWAP: Plasma-sensing instrument for measuring the properties of the solar wind at Pluto
Lorri: Telescopic camera that provides the highest-resolution imaging of the surface
SDC (Student Dust Center): Built and operated by students to measure dust peppering the spacecraft during its voyage
Ralph: Spectrometer that measures atmospheric composition and temperature
Alice: Ultraviolet spectrometer for measuring gas composition
REX: Measures atmospheric composition and temperature
Pluto, named after the Roman god of the underworld, is the ninth planet from the sun.
Date of discovery: 1930
Mean distance from the sun: 3.66 billion miles
Equatorial diameter: About 1,450 miles
Surface gravity: About 6% of Earth's
Mean surface temperature: Minus 387 degrees Fahrenheit
Length of day: 6.4 Earth days
Length of year: 248 Earth years
Sources: NASA/JPL, Johns Hopkins University Applied Physics Laboratory. Graphics reporting by Julie Sheer