Exploration of Pluto
The exploration of Pluto began with the arrival of the New Horizons probe in July 2015, though plans for such a mission had been in place for many decades. There are no plans as yet for a follow-up mission.
Early mission proposals
Pluto presents significant challenges for spacecraft because of its small mass and great distance from Earth. Voyager 1 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, resulting in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto. No serious attempt to explore Pluto by spacecraft occurred until the last decade of the 20th century. In August 1992, JPL scientist Robert Staehle telephoned Pluto's discoverer, Clyde Tombaugh, requesting permission to visit his planet. "I told him he was welcome to it", Tombaugh later remembered, "though he's got to go one long, cold trip". By the mid-1990s, NASA was considering a Pluto Express mission of twin spacecraft that would fly by Pluto in 2013, possibly even with Russian participation, providing "Drop Zond" probes that would return images before colliding with it. Despite this early momentum, in 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle delays.
Proposed exploration (2003)
A Pluto orbiter/lander/sample return mission was proposed in 2003. The plan included a twelve-year trip from Earth to Pluto, mapping from orbit, multiple landings, a warm water probe, and possible in situ propellant production for another twelve-year trip back to Earth with samples. Power and propulsion would come from the bimodal MITEE nuclear reactor system.
After an intense political battle, a revised mission to Pluto, dubbed New Horizons, was granted funding from the US government in 2003. New Horizons was launched successfully on 19 January 2006. The mission leader, S. Alan Stern, confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.
New Horizons captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager. The images, taken from a distance of approximately 4.2 billion kilometers, confirmed the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper belt objects. In early 2007 the craft made use of a gravity assist from Jupiter.
On 4 February 2015, NASA released new images of Pluto (taken on 25 and 27 January) from the approaching probe. New Horizons was more than 203,000,000 km (126,000,000 mi) away from Pluto when it began taking the photos, which showed Pluto and its largest moon, Charon. On 20 March 2015, NASA invited the general public to suggest names for surface features that will be discovered on Pluto and Charon. On 15 April 2015, Pluto was imaged showing a possible polar cap. Between April and June 2015, New Horizons began returning images of Pluto that exceeded the quality that the Hubble Space Telescope could produce.
Pluto's small moons, discovered shortly before and after the probe's launch, were considered to be potentially hazardous, as debris from collisions between them and other Kuiper belt objects could have produced a tenuous dusty ring. If New Horizons had travelled through such a ring system, there would have been an increased risk of potentially disabling micrometeoroid damage.
New Horizons had its closest approach to Pluto on 14 July 2015—after a 3,462-day journey across the Solar System. Scientific observations of Pluto have begun five months before the closest approach and will continue for at least a month after the encounter. New Horizons used a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition and analyze Pluto's neutral atmosphere and its escape rate. New Horizons also photographed the surfaces of Pluto and Charon.
Photographs of Pluto taken on 14 July 2015 taken 15 minutes after New Horizon's closest approach, from a distance of 18,000 kilometers and sent to Earth on 13 September 2015 show a near-sunset on Pluto with details of the surface and a haze in the atmosphere.
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