Forgotten little rock

Little is known about Pluto, a small icy world on the edge of the solar system, and its moon Charon. The few pictures we have are blurry with little detail, and before Hubble we had even less than that. We do not know if Pluto is just a boring ball of rock and ice or whether it has a more interesting structure. We do not know the precise composition of the ice. We know that Charon is quite different from Pluto, its ice formed mainly of water whereas Pluto's ice seems to be a mix of various products. We have no idea why two bodies that are are so close would have such a big difference. No probes have been even remotely near Pluto. Things are about to change...

New Horizons

The New Horizons mission (also known as the Pluto-Kuiper Belt mission, replaces the now cancelled Pluto-Kuiper Express project and aims to answer those questions and more. With a bit of luck within just over 12 years we will have crystal clear snaps of Pluto and Charon as well as all kinds of data from the probe's numerous instruments. The project is being managed by the Johns Hopkins University Applied Physics Laboratory, with contributions from Ball Aerospace Corp., Stanford University, NASA's Goddard Space Flight Center and NASA's Jet Propulsion Laboratory.

Mission goals

(From the mission factsheet)

  • Global maps of Pluto and Charon at 40-km resolution
  • hemispheric maps at 1-km resolution
  • IR spectral maps at up to 7-km resolution and visible 4-colour maps at up to 3-km resolution
  • High-resolution terminator images at 100-m resolutions
  • UV and radio occultations of both Pluto and Charon
  • UV dayglow and nightglow spectra of Pluto's atmosphere
  • In situ measurements of energetic particles and the solar wind
  • Surface temperature maps at 50-km resolution
Additionally they hope to gather similar data at one or more Kuiper belt objects (if enough fuel is left).

Mission Timeline

Like most space exploration missions, there is a launch window not to be missed if one is to benefit from the gravitational field of large planets. The basic timeline (subject to change depending on the final choice of launcher) is :

  • January 2006: blast-off aboard a Delta IV or Atlas V rocket
  • February 2007: the probe takes a swing via Jupiter to boost its speed. The probe then enters a hibernation mode, waking once a week to confirm that all is OK and once a year to check all equipment.
  • February 2015: About 200 million kilometres from Pluto the spacecraft wakes up, just in time to take some pretty pictures
  • 14th July 2015: The actual flyby. The probe will pass within 11000 km of Pluto and 27000 of Charon. That's closer than the geostationary communications satellites that orbit our planet.

When the mission is over we will have detailed information about both this objects, and hopefully about some Kuiper belt objects. Observation of solar wind variations will tell us whether Pluto has a magnetic field or not. If it does, it will mean that Pluto is not a dead lump of rock and ice, that something may actually be happening in there. We may know a little more about where Pluto and its mysterious companion came from. And of course many many pictures.

Spacecraft characteristics

(pulled from the factsheet)

  • Wet mass at launch: 392 kg
  • Communications: X-band, 2.1m high gain antenna
  • Downlink from Pluto: 698 bits per second to 70m antenna
  • Propulsion: Hydrazine monopropellant
  • Margins: Power at Pluto 20%, Launch Dry Mass 30%
  • Power Supply: Radioisotope thermoelectric generator, producing 218W in 2017
  • Redundancy: All major electronics
    Star cameras
    Data Storage
    The star cameras are more important than they might sound. They are not just cameras that take pretty pictures of stars. Two fundamental pieces of information when you are a spacecraft are where you are, and which way you are pointing. This spacecraft uses cameras focussed on stars far enough away to appear motionless during the whole mission, to calculate its position. As you can guess, that makes them pretty important. Better have a few spare.

Slow downloads...

To finish off I'd like to point out one of the figures you may just have skimmed over. The downlink speed : 698 bits a second. That's 80 times slower than the 56k modem gathering dusk in my attic. I can just imagine some poor scientists huddled round a screen in a dark room (well ok, it's more likely to be a large mission control room with screens everywhere) watching an incredibly slow progress bar inching its way across the screen. You're entitled to wonder why we can't do better than that in this day and age. The main reason is that this data will be sent over more than 3,000,000,000 miles. When the signal reaches earth it will be faint and will have picked up a fair bit of noise on the way. The lower the frequency, the easier it will be to filter out the mess.

I just hope this project makes it and doesn't get cancelled or worse malfunction because of something silly like a loose nut on one of the rockets.
january 2003 issue of Science & Vie Junior

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