WASHINGTON — After nearly nine-and-a-half years of flight, and an even longer effort to build it, NASA’s New Horizons spacecraft is poised to provide scientists with their first — and, perhaps for many decades, only — close-up look at the dwarf planet Pluto and its moons.
New Horizons will make its closest approach to Pluto on July 14, passing approximately 12,500 kilometers above the dwarf planet’s surface at 7:50 a.m. EDT. Less than 15 minutes later, it will pass within 29,000 kilometers of the largest of Pluto’s five moons, Charon.
The spacecraft is in good health, project officials said in recent updates, with no known issues that could affect the flyby. They have kept a close watch on any dust rings, moons or other debris in the vicinity of Pluto that could pose a risk to New Horizons and possibly force it to take a safer, but more distant, trajectory.
“We call this period of time where we’re doing the hazard search ‘seven weeks of suspense,’” the mission’s principal investigator, Alan Stern, said during a June 19 briefing, a play on the “seven minutes of terror” phrase NASA has used to describe spacecraft landings on Mars. The spacecraft’s camera, he said, has carried out one search a week of the space around Pluto, looking for previously unknown moons or rings.
Scientists are increasingly optimistic that there will be no need for New Horizons to divert. “We’re still not seeing anything, which is great from the perspective of danger to the spacecraft,” project scientist Hal Weaver of the Johns Hopkins University Applied Physics Laboratory said June 23. “So far, everything is looking great.”
While New Horizons will zoom past Pluto at about 50,000 kilometers per hour, the flyby will play out in slow motion here on Earth. The one-way light travel time from Pluto to Earth is nearly four-and-a-half hours, and New Horizons will not be in constant communication during the flyby. It will not be until about 9 p.m. EDT July 14 that the first flyby data are received on Earth.
Returning all of the flyby data will take more than a year, given the spacecraft’s distance and limited power. “The actual data rates are between 1 and 2 kilobits per second,” said Mark Holdridge, the New Horizons encounter mission manager. That is about the same data rate as a computer modem from a quarter-century ago.
The spacecraft will initially send low-resolution “browse” data, then later send the complete, uncompressed data. “The data will come off at a gradual rate over roughly 16 months,” he said.
That wait, though, will be relatively minor for a mission that took longer to get approved, built and launched than to fly from Earth to the outer solar system. In the early 1990s, the Jet Propulsion Laboratory studied a concept for a mission called Pluto Fast Flyby that would send one, or possibly two, small spacecraft past Pluto. That concept evolved by the late 1990s into a mission called Pluto-Kuiper Express, but NASA canceled it in 2000, citing cost growth.
Facing pressure from both the scientific community and Congress, NASA instead solicited proposals for Pluto mission concepts. It selected New Horizons in November 2001, later making it the first mission in its New Frontiers program of medium-sized planetary science missions that now includes Juno, currently en route to Jupiter, and the upcoming Osiris-Rex asteroid sample-return mission. New Horizons launched in January 2006.
The Pluto flyby will not necessarily be the end of the mission for New Horizons. The project has for years looked for small Kuiper Belt objects beyond Pluto within range of the spacecraft for a later flyby. There are currently two candidate objects for such a flyby, called PT1 and PT3, Alex Parker, a New Horizons science team member at the Southwest Research Institute, said during the June 19 briefing.
“The choice of [which Kuiper Belt] candidate will be NASA’s,” Stern said. “That’s a decision we’ll make with them in August.” If NASA does go ahead with a flyby of either object, the spacecraft could start maneuvers toward it in October, with a flyby in early 2019.
The Pluto flyby itself will be the first time a spacecraft has visited Pluto, but perhaps also the only one in the careers of most scientists involved in the mission. The time and expense of missions to the outer solar system makes a follow-up mission unlikely for decades. The planets Uranus and Neptune, which Voyager 2 flew by in 1986 and 1989, respectively, have yet to be visited again by spacecraft, with no missions to either world on the books for the foreseeable future.
Stern is aware of this once-in-a-lifetime opportunity to see a distant world for the first time. “There’s only one Pluto flyby planned in all of history,” he said in a mission update June 25, “and it’s happening next month!”