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New Horizons gets "all clear" for July 14 Pluto flyby

After an exhaustive search for heretofore unseen rings, small moons or other space debris, senior managers concluded Wednesday that NASA's New Horizons spacecraft, hurtling toward a July 14 flyby of Pluto at more than 30,000 mph, can safety stay on its current course without undue fears of a mission-ending impact.

"We're breathing a collective sigh of relief knowing that the way appears to be clear," Jim Green, NASA's director of planetary science, said in a statement. "The science payoff will be richer as we gather data from the optimal flight path, as opposed to having to conduct observations from one of the back-up trajectories."

Using the spacecraft's Long Range Reconnaissance Imager, or LORRI, engineers have been studying the space around Pluto and its retinue of five moons since mid May, on the lookout for any potential hazards. At New Horizons' velocity, even impacts with tiny bits of debris could prove catastrophic.

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The hazards analysis team was formed in 2011 after the Hubble Space Telescope discovered a fourth moon orbiting Pluto, a total that since has increased to at least five. The concern is that the gravity of the smaller moons would be too weak to capture debris kicked up by impacts, leaving possible clouds of rocky dust along New Horizon's path.

Playing it safe, engineers developed an alternative "safe haven" trajectory, a slight detour that would have carried New Horizons farther from Pluto during its upcoming flyby to avoid any potential debris. A decision on whether to fire the spacecraft's thrusters to change course had to be made this week.

In the latest set of LORRI images, taken June 22, 23 and 26, Pluto, its large moon Charon and the smaller moons Nix, Hydra, Kerberos and Styx can be seen, but there are no signs of any faint rings or other obvious debris.

New color pictures of Pluto and its large moon Charon were released July 1, revealing evenly spaced Missouri-size dark spots along the dwarf planet's equator. NASA

The camera is sensitive enough to detect objects 15 times dimmer than Styx, Pluto's faintest known moon, and engineers concluded that if any rings are present, they are reflecting less than one 5-millionth of the sunlight reaching the outer solar system.

In short, no obvious threats could be seen.

"Not finding new moons or rings present is a bit of a scientific surprise to most of us," Alan Stern, the mission's principal investigator, said in the NASA statement. "But as a result, no engine burn is needed to steer clear of potential hazards. We presented these data to NASA for review and received approval to proceed on course and plan. We are 'go' for the best of our planned Pluto encounter trajectories."

John Spencer, leader of the hazard analysis team, said the suspense -- "at least most of it" -- is now behind the New Horizons team as the spacecraft races toward its once-in-a-lifetime encounter with the famous dwarf planet.

"As a scientist I'm a bit disappointed that we didn't spot additional moons to study, but as a New Horizons team member I am much more relieved that we didn't find something that could harm the spacecraft," he said. "New Horizons already has six amazing objects to analyze in this incredible system."

Adding to the margin of safety, New Horizons will fly through a zone "swept out" by Charon, which orbits in gravitational lockstep with Pluto, completing one revolution every 6.4 days.

"As it turns out, where we're going is a place called the Charon instability strip where the big moon Charon plows through and carves out a clean place every 6.4 days," Stern said in an earlier interview. "Looking back on it, although we chose it for other reasons, (we) couldn't have picked a safer place to go according to everything we know today."

If all goes well, New Horizons will race past Pluto at a distance of about 7,750 miles a few seconds before 7:50 a.m. on July 14. Because of Pluto's 3-billion-mile distance, the spacecraft must carry out its observations autonomously, following a detailed command sequence to aim itself and its instruments at Pluto, Charon and the other moons.

Timing is critical. For all of the observations to work out, New Horizons must fly through an imaginary targeting box measuring 60 by 90 miles within about 100 seconds of the ideal time. And it must do so at the end of a 3-billion-mile voyage spanning nine-and-a-half years.

Late Monday, New Horizons carried out a 23-second thruster firing, the third and final planned burn to fine tune the spacecraft's preferred trajectory. It was only the ninth trajectory correction maneuver, or TCM, carried out since the spacecraft was launched in January 2006.

The firing increased the probe's velocity by about one-half mile per hour. Without it, New Horizons would have been 114 miles off target for critical observations of Pluto's atmosphere. The observations require the spacecraft, flying into and out of Pluto's shadow, to capture radio signals from Earth after they pass through the dwarf planet's atmosphere. Similar observations are planned for Charon.

The thruster firing Monday "was perfectly performed by the spacecraft and its operations team," said Stern. "Now we're set to fly right down the middle of the optimal approach corridor."

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