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The planets were literally aligned on Monday for NASA's celebration of its Voyager mission to the outer planets and its upcoming New Horizons mission to Pluto and beyond.
Monday marked 25 years since the Voyager 2 probe made its flyby of the planet Neptune, and the day also marked a milestone for the New Horizons spacecraft, which was launched eight and a half years ago.
"Today we crossed the orbit of Neptune ... outbound for Pluto to make a little bit of history," said Alan Stern, a planetary scientist at the Southwest Research Institute who heads up the $728 million New Horizons mission. New Horizons is due to fly past Pluto and its moons next July 14, on Bastille Day.
Stern said he considered the flyby to be on a par with Voyager's encounter with Neptune in 1989, in that it would mark the first close-up view of a new frontier in the solar system. To mark the occasion, the New Horizons team released a picture of Neptune and its largest moon, Triton, taken in July from a distance of nearly 2.5 billion miles (4 billion kilometers).
The planet and moon look like little more than specks in New Horizons' picture — and so do Pluto and its largest moon, Charon, which are still about 240 million miles (385 million kilometers) away. But the controversial dwarf planet is due to loom much larger in the coming months.
By next May, the views from New Horizons' high-resolution LORRI camera should outdo the Hubble Space Telescope, Stern said. And during next July's 30,000-mph flyby, the pictures of Pluto are expected to be far better than anything Voyager sent back from Neptune.
During a news briefing at NASA Headquarters in Washington on Monday, Stern paid tribute to Voyager's project scientist, Ed Stone. "We stand on the shoulders of giants, giants like Ed Stone and his Voyager science team, who pioneered how to do the exploration of the deep outer solar system," he said.
Stern also praised the more than 2,000 members of what he called the "New Horizons Corps of Discovery." Thanks to technological advances in the past 25 years, New Horizons' mission cost is about a fifth of Voyager's price tag, Stern said. He noted that the piano-sized probe's seven scientific instruments weigh less than the camera on NASA's Cassini orbiter, and that they draw "less than one-half the power of a 60-watt light bulb." The power is provided by a plutonium-fueled generator that's expected to run for decades.
Focusing on the solar system's icy ring
New Horizons is the first mission to focus on the Kuiper Belt, a ring of icy material that lies more than 2.7 billion miles (4.5 billion kilometers) from the sun.
In preparation for the flyby, the New Horizons team has been using Hubble to survey Pluto's vicinity for moons and debris, as well as for potential targets beyond Pluto in the Kuiper Belt. The effort has turned up four new moons, plus several prospects for a follow-up flyby. Stern said it would take several months to determine whether any of the Kuiper Belt targets were within the spacecraft's fuel range.
New Horizons will be programmed to take pictures of Pluto and all of its five known moons, with the tiny moon Nix looming as a particularly promising target. The probe's instruments will also analyze Pluto's ultra-thin atmosphere and watch for phenomena ranging from planetary rings to clouds and ice geysers. Although there are no pictures that show Pluto's surface in detail, scientists expect it to have the weird-looking "cantaloupe terrain" that Voyager's cameras saw on Triton 25 years ago.
"This is just mouth-watering," Stern said. "It's a scientific wonderland that we look forward to exploring."
Veterans of the twin Voyager missions said they were looking forward to the Pluto flyby as well. "The one thing I'd be surprised about at Pluto is if we weren't truly surprised," said Jeffrey Moore, a planetary scientist at NASA Ames Research Center.
Stern said there'd be booklets and "Plutopalooza" party kits to publicize New Horizons' mission. He promised that during the height of the mission, pictures from Pluto would be posted to the Web on the same day they're received. "We're going to take you along on all of this journey," he said.
Revisiting the question of Pluto's planethood
This week marks yet another anniversary for Pluto's fans and detractors: Eight years ago, the International Astronomical Union voted to reclassify the icy world as a dwarf planet, and declared that such bodies were not planets per se because they did not "clear the neighborhood around their orbits."
Pluto was once seen as a misfit in the solar system, but astronomers have found more objects like it in recent years. So far, the IAU has designated four other worlds as dwarf planets: Eris, Makemake and Haumea, which are beyond Pluto's orbit, plus Ceres in the main asteroid belt. Stern said at least eight more objects (Quaoar, Sedna, Orcus, Ixion, Varuna, 2002 AW197, Pallas and Vesta) may well be round enough to fit the IAU's dwarf-planet definition.
"More dwarf planets than all the giants and terrestrials combined," he noted. "You might ask yourself, who are the misfits now?"
During the briefing, a NASA moderator read out a Pluto-centric question submitted via Twitter by William Shatner, who portrayed Captain Kirk in the popular "Star Trek" TV series and several movies.
"It's silly that it comes down to whether you think a dwarf planet is a planet or not. That's kind of semantics," Voyager veteran David Grinspoon, a researcher at the Planetary Science Institute, replied.
The University of Colorado's Fran Bagenal jumped in, saying, "Dwarf planets are planets — come on!"
Grinspoon (who has criticized the IAU's ruling in the past) noted that Pluto would probably pass the "Captain Kirk Test" for planethood — that is, a judgment by Kirk and Spock that what they were seeing on the Starship Enterprise's viewscreen was a planet rather than something else. But he said the semantics weren't as important as the prospect of learning about a new planetary frontier.
"We're exploring this new realm of the solar system that we haven't explored before, the realm of dwarf planets," Grinspoon said. "Dwarf planets are this amazing new kind of body that we're about to learn a lot more about. Take us into orbit, Captain. Beam us up to Pluto."