NASA prepared to launch an unmanned, piano-sized probe that will fly by Pluto, the solar system's last unexplored planet, and also study a mysterious zone of icy objects that surrounds the frosty planet at the outer edges of the planetary system.
The scheduled launch of the New Horizons spacecraft yesterday afternoon, and a successful, nine-year journey to Pluto, would complete an exploration of the planets started by NASA in the early 1960s with unmanned missions to observe Mars, Mercury and Venus.
"What we know about Pluto today could fit on the back of a postage stamp," said Colleen Hartman, a deputy associate administrator at NASA. "The textbooks will be rewritten after this mission is completed."
Pluto is the only planet discovered by a US citizen, though some astronomers dispute Pluto's right to be called a planet. It is an oddball icy dwarf unlike the rocky planets of Mercury, Venus, Earth and Mars and the gaseous planets of Jupiter, Saturn, Uranus and Neptune.
"My dad would be absolutely thrilled to see this," said Annette Tombaugh-Sitze, whose father, astronomer Clyde Tombaugh, discovered Pluto in 1930.
Members of the Tombaugh family, including Tombaugh's 93-year-old widow, Patricia, planned to witness the launch at the Cape Canaveral Air Force Station.
Pluto is the brightest body in a zone of the solar system known as the Kuiper Belt, which is home to thousands of icy, rocky objects, including tiny planets whose development was stunted for unknown reasons midway through the planetary creation process.
Scientists believe studying those "planetary embryos" can help them understand how planets were formed.
"Something, and we don't understand what ... stopped that process of growth and left us with this fantastic relic, this forensic evidence of planets that were arrested in the midstage of growth," said Alan Stern, the US$700 million mission's principal investigator.
The New Horizons spacecraft will lift off on an Atlas V rocket and speed away from Earth at 58,000kph, the fastest spacecraft ever launched. It will reach Earth's moon in about nine hours and arrive in 13 months at Jupiter, where it will use the giant planet's gravity as a slingshot, shaving five years off the 4.8 billion-kilometer trip.
Some NASA safety managers had raised concerns about a fuel tank similar to the one expected to be used on the rocket during launch since the test tank had failed a pressure evaluation at the factory.
The decision ultimately was made to fly since the flight tank was in pristine condition and had no signs of any defects like the ones found on the tank that had undergone "brutal" tests, said NASA launch director Omar Baez.
The launch had drawn protests from anti-nuclear activists because the spacecraft will be powered by 11kg of plutonium, which will produce energy from natural radioactive decay.
NASA and the US Department of Energy have put the probability of an early-launch accident that could release plutonium at 1 in 350. The agencies have brought in 16 mobile field teams that can detect radiation and 33 air samplers and monitors.
"Just as we have ambulances at football games, you don't expect to use them, but we have them there if we need them," NASA official Randy Scott said.