Tuesday, July 14th promises to be a major day in the annals of space exploration, as the deep space probe New Horizons hurls through the Pluto-Charon system, making its closest approach to both, allowing us to gain our best views yet of this binary pairing of dwarf worlds and their little nest of moonlets.
The mission is already fast approaching the 10th anniversary of its launch (January 19th, 2006), with the overall mission (from inception to the present day) already almost 15 years old – although the planning for a Pluto mission goes back a lot further than that. Getting to the Pluto-Charon system has been a remarkable feat.
Originally, Voyager 1 had been provisionally scheduled to make a Pluto flyby as a part of its half of the “grand tour” of the solar system, using its encounter with Saturn to swing the probe on to a rendezvous with Pluto in 1986. In the end, Saturn’s Mighty moon Titan was considered a more valuable target for study, and the laws of celestial mechanics meant that a study of Titan and a swing-by of Saturn suitable to send the mission on to Pluto were mutually exclusive.
In the 1990s various missions to Pluto were proposed, ranging in size from the huge Mariner II mission, utilising an update on NASA’s veritable Mariner class probes, weighing two tonnes, down to the tiny Pluto 350, a comparatively tiny vehicle massing just 350 kilogrammes (772 pounds). These evolved, through short-lived programmes such as the Pluto Fast Flyby mission and the Pluto-Kuiper Express mission to eventually become New Horizons in 2001, a mission conceived and operated by the Applied Physics Laboratory, which often operates in partnership with NASA’s Jet Propulsion Laboratory.
At launch, New Horizons became the fastest spacecraft ever launched, using an Atlas V booster with no fewer than five strap-on solid rocket boosters. In addition, a high-powered third stage was used to boost it directly onto a solar escape trajectory – something which required the vehicle to achieve a velocity of over 16 kilometres per second (56,000 km/h or 37,000 mph) following launch. To put that in perspective, such was New Horizons’ velocity that it had passed beyond the orbit of the Moon (an average of 384,400 km / 238,900 miles from Earth) less than nine hours after launch.
Just under 3 months after launch, and travelling at over 21 kilometres a second, (76,000 km/h; 47,000 mph), New Horizons passed beyond the orbit of Mars, heading onwards for Jupiter, and a manoeuvre referred to a gravity assist.
Reaching the Jovian system in September, 2006, New Horizons was able to stretch its scientific legs, when it started observing Jupiter and its moons from a distance of 291 million kilometres (181 million miles). Over the next 6 months, the craft continued to close on Jupiter, gathering a huge amount of data along the way to add to our understanding of the biggest planet in the solar system, its complex weather systems and atmospheric composition, and its ever-growing system of smaller moons, many of which perform a vital role is “shepherding” Jupiter’s thin ring system.
This was the first real opportunity to observe Jupiter and its moons since the end of the Galileo mission in 2003, and New Horizons did so spectacularly well, passing within 2.3 million kilometres of the planet and using its gravity to further increase its speed by 14,000 km/h (9,000 mph), shortening the journey time to Pluto by some 3 years.
Following the Jupiter mission, the vehicle went into a hibernation mode, allowing it to reduce the power drain on its nuclear “battery”, the radioisotope thermoelectric generator (RTG) which provides the vehicle with all its electrical power (and which itself was the back-up unit for the Cassini mission which is still in operation around Saturn, 18 years after its launch).
During the vehicle’s hibernation, things were changing with regards to Pluto. Until the 1990s, it had always been classified as a planet – albeit one with an unusual orbit, which is both sharply inclined to the plane of the ecliptic in which the other planets of the solar system orbit, and highly elliptical, bringing it closer to the Sun than Neptune during certain periods.
Both of these factors, coupled with Pluto’s relatively small size, suggested that it was more of a “captured” object from the Scattered Disc, a region of the Solar System between Neptune and the Kuiper Belt that is sparsely populated by icy minor planets (Pluto’s orbit around the Sun actually sits within the Scattered Disc).
In 2005, while New Horizons was sleeping, astronomers at Mount Palomar Observatory imaged Eris, a Scattered Disc object, complete with a moon of its own (Dysnomia), which is some 27% more massive than Pluto. This discovery, coupled with the fact that the Scattered Disc may be the home of other objects of similar size, caused the International Astronomical Union to officially define the term “dwarf planet” in 2006, and downgrade Pluto’s status to match – although not without a certain amount of controversy and protest.
Continue reading “Space Sunday: Pluto – the history of a brief encounter”
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