Eyes wide open
At a mean distance from the Sun of 5.9 billion kilometres, some 39.5
times further away than the Earth, where mean temperatures are only
33K (minus 240°C), one might expect these worlds to be inert and frozen, cratered relics bearing mainly the scars of the violent early days
of solar system formation. However, previous missions, from Voyager to
Cassini, have taught us that the outer solar system is far more exotic
and active than we would ever have imagined. In particular, Voyager
2’s 1989 flyby of Triton, Neptune’s largest satellite, with its interesting
chemistry, atmosphere and nitrogen ‘geysers’, hinted at exciting things
to come from New Horizons. This was especially so as Triton was thought
possibly to be a near ‘twin’ of Pluto.
Accordingly, New Horizons was equipped with a suite of sophisticated
scientific instruments, so as to have its eyes (and other senses) as wide
open as possible for the encounter. Previous missions helped inform the
choice and design of data-collecting instruments. For instance, as Dr.
Young points out, ‘One of the things we learned from flying Voyager at
Triton was that, boy, if you want to learn what things are made of you really need the infrared!’ So, as well as a telescopic, high resolution camera (LORRI), the spacecraft’s ‘Ralph’ instrument was both a visible and
infra-red wavelength imager and spectrometer, while the ‘Alice’ detector operated in the ultraviolet. At such great distances from the Sun, the
instruments were designed to function in the cold conditions and low
light levels at Pluto and in the Kuiper Belt beyond.
Asking the right questions – New Horizons’ science payload
The New Horizons science payload consists of seven
instruments – three optical instruments, two plasma
instruments, a dust sensor and a radio science receiver
and radiometer. It was designed to investigate the
global geology, surface composition and temperature
of Pluto and its moons, as well as the dwarf planet’s
atmospheric characteristics – pressure, temperature,
structure, escape rate -, and interactions with the solar
wind and space environment.
Needing to operate so far from the Sun, the whole
payload is powered by a radioisotope thermoelectric generator (RTG), running on a total of less than 28
watts. In order to include a maximum of science, but
with as little payload weight as possible, ‘it represents’
to quote the New Horizons team, ‘a degree of miniaCredit: NASA/JHUAPL/SwRI
turization unprecedented in planetary exploration.’
• Long Range Reconnaissance Imager (LORRI) – telescopic camera to obtain high resolution geological data
• Ralph - a visible and infrared imager/spectrometer for colour, compositional and thermal mapping.
• Alice – an ultra-violet imager/spectrometer to analyse the structure and composition of Pluto’s
atmosphere, as well as searching for atmospheres around Charon and other KBOs.
• Radio Science Experiment (REX) – radiometer to measure composition and temperature of Pluto’s atmosphere
• Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) – energetic particle spectrometer
to measure the composition and density of ions escaping from Pluto’s atmosphere
• Solar Wind Around Pluto (SWAP) – solar wind and plasma spectrometer to measure
Pluto’s interaction with the solar wind and the escape-rate of its atmosphere
• Venetia Burney Student Dust Counter (SDC) – a student experiment to measure space
dust impacts during New Horizons voyage. Named after the person who, as a child,
suggested the name Pluto after its discovery by Clyde Tombaugh in 1930.
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