After a trip of nearly three billion miles, NASA’s New Horizons spacecraft, our first probe to Pluto, will on Wednesday start sending back up-close pictures of the dwarf planet.
The craft was sent into our Solar System's obsidian void on January 19, 2006 – back when Pluto was still considered a planet in its own right. Later that very year the International Astronomical Union altered the small world's status to that of a dwarf planet, after finding several larger bodies in our system.
New Horizons slept through that entire controversy as it barreled out towards its target. It spent more than two thirds of the flight in hibernation – being woken up periodically to check it was still up to the job of studying Pluto.
New Horizons will make its closest pass of the dwarf planet on July 14. In the meantime we’ll be getting early images of Pluto, and data on how it interacts with its moons.
The spacecraft carries seven instruments: two infrared and ultraviolet spectrometers, along with a couple of particle spectrometers, a small color camera and a high-resolution telescopic one, and a space-dust detector.
NASA fitted New Horizons [PDF] with a radiation-hardened, 32-bit 12MHz Mongoose V processor, a close cousin of the MIPS R3000 CPU – variants of which have appeared in various electronics over the years, including the original Sony PlayStation.
The probe has 8GB of solid-state memory to record data; this is split into two blocks, one for backup. It communicates with Earth via a 30cm (12in) medium-gain dish antenna and a large 2.1m (83in) high-gain system.
All this is driven by a plutonium-powered radioisotope thermoelectric generator capable of putting out 240W at 30V. But that drops by about 3.5W per year as the plutonium dioxide decays.
Tribute to Clyde Tombaugh
New Horizons also carries a very special cargo. Strapped down within the hull is a container holding an ounce of the ashes of Clyde Tombaugh, the American astronomer who discovered Pluto in 1930, along with the following inscription:
Interned herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system's 'third zone'. Adelle and Muron's boy, Patricia's husband, Annette and Alden's father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906–1997).
As New Horizons screams along its Plutonian path at more than 30,000 miles per hour, scientists have been concerned that they might lose the spacecraft to unidentified objects around the icy body. So boffins have been using the Hubble space telescope and other 'scopes to snatch glimpses of Pluto to check for smaller moons that may have been missed, and the results have been surprising.
Nearly 50 years after Pluto was discovered, its moon Charon was first spotted. Since then four other moons have been found and named. In the process, New Horizons’ course was tweaked in 2010 to keep it out of harm’s way.
Communicating with the probe isn't quick or especially easy, though. With the distances involved, it takes more than four hours to get a command from Earth to the spacecraft. That also cuts down on bandwidth, and at this far out, the best data speed possible is about 700 bits per second. NASA estimates it will take at least nine months to download all the data and imagery collected from Pluto.
The spacecraft has four main thrusters for propulsion, and 12 smaller units that control its attitude and spin. These are useful, but they won’t be enough to slow the spacecraft when it gets to its primary target.
In July the probe will get to within 10,000km (6,200 miles) of the ball of ice and rock, and within 27,000km (17,000 miles) of Charon. It will then rapidly head out into the Kuiper belt, a zone of frozen rocks and planetoids that encircles the outer Solar System.
Three Kuiper belt objects have been identified for further study, including one icy rock about 20 miles across. NASA is calculating the burn times for Deep Horizon's main thrusters once the probe is past Pluto so that it can reach these cosmic boulders; the plucky nuclear-powered mini-lab could stumble across something interesting in early 2019.
After that then, who knows? The probe will continue on its trip until it hits something and could still be shooting through the stars long after humanity has given up the ghost. ®