Spacecraft instruments are selected to meet a mission's science goals. On New Horizons, for example, NASA set out a list of things it (and the planetary science community) wanted to know about Pluto: What is its atmosphere made of, and how does it behave? What does the surface of Pluto look like? Are there big geological structures? How do particles ejected from the Sun (known as the solar wind) interact with Pluto's atmosphere?
The New Horizons team selected instruments that not only would directly measure NASA's items of interest, but also provide backup to other instruments on the spacecraft should one fail during the mission.
The payload includes seven instruments:
Ralph's main objectives are to obtain high resolution color maps and surface composition maps of the surfaces of Pluto and Charon. The instrument has two separate channels: the Multispectral Visible Imaging Camera (MVIC) and the Linear Etalon Imaging Spectral Array (LEISA). A single telescope with a 3-inch (6-centimeter) aperture collects and focuses the light used in both channels.
MVIC operates at visible wavelengths - using the same light by which we see - and has 4 different filters for producing color maps. One filter is tailored to measure the methane frost distribution over the surface, while the others are more generic and cover blue, red and near-infrared colors, respectively. MVIC also has two panchromatic filters, which pass essentially all visible light, for when maximum sensitivity to faint light levels is required. In all cases, the light passes from the telescope through the filters and is focused onto a charge coupled device (CCD). (Although the MVIC CCD is a unique, sophisticated device, virtually all consumer digital cameras use CCDs.)
LEISA operates at infrared wavelengths (it uses heat radiation), and its etalon acts like a prism to bend different wavelengths of light by different amounts so that each wavelength can be analyzed separately. Since quantum physics teaches us that different molecules emit and absorb light at different wavelengths, analysis of the different components of the light by LEISA can be used to identify the unique "fingerprints" of these molecules. LEISA will be used to map the distribution of frosts of methane (CH4), molecular nitrogen (N2), carbon monoxide (CO), and water (H2O) over the surface of Pluto and the water frost distribution over the surface of Charon. LEISA data may also reveal new constituents on the surfaces that have not yet been detected.
Alice is an ultraviolet imaging spectrometer that will probe the atmospheric composition of Pluto. A "spectrometer" is an instrument that separates light into its constituent wavelengths, like a prism, only better. An "imaging spectrometer" both separates the different wavelengths of light and produces an image of the target at each wavelength.
Alice has two modes of operation: an "airglow" mode, which allows measurement of emissions from atmospheric constituents, and an "occultation" mode, when either the Sun or a bright star is viewed through the atmosphere producing absorption by the atmospheric constituents. The Alice occultation mode will be used just after New Horizons passes behind Pluto and looks back at the Sun through Pluto's atmosphere.
REX is an acronym for "radio experiment," - it is really just a small printed circuit board, containing sophisticated electronics, integrated into the New Horizons radio telecommunications system. All communication with New Horizons, including the downlink of science data, takes place through the radio package, which makes it critical to mission success.
Using an occultation technique similar to that described above for the Alice instrument, REX can be used to probe Pluto's atmosphere. After New Horizons flies by Pluto, its 83-inch (2.1-meter) radio antenna will point back at Earth. On Earth, powerful radio transmitters in NASA's Deep Space Network (DSN) will point at New Horizons and send radio signals to the spacecraft. As the spacecraft passes behind Pluto, the atmosphere bends the radio waves by an amount that depends on the average molecular weight of the gas in the atmosphere and the atmospheric temperature. REX will record the detected radio waves and send the data back to Earth for analysis.
REX also has a "radiometry" mode, which will measure the weak radio emission from Pluto itself. When this radiometry measurement is performed looking back at Pluto following the flyby, REX data can be used to derive a very accurate value for Pluto's nightside temperature.
The instrument that provides the highest spatial resolution on New Horizons is LORRI - short for Long Range Reconnaissance Imager - which consists of a telescope with a 8.2-inch (20.8-centimeter) aperture that focuses visible light onto a charge coupled device (CCD). LORRI has a very simple design; there are no filters or moving parts. Near the time of closest approach, LORRI will take images of Pluto's surface at football-field sized resolution, resolving features approximately 100 yards or 100 meters across.
The Solar Wind Analyzer around Pluto (SWAP) instrument will measure charged particles from the solar wind near Pluto to determine whether Pluto has a magnetosphere and how fast its atmosphere is escaping.
Another plasma-sensing instrument, the Pluto Energetic Particle Spectrometer Investigation (PEPSSI), will search for neutral atoms that escape Pluto's atmosphere and subsequently become charged by their interaction with the solar wind.
The last scientific instrument on New Horizons is an Education and Public Outreach project. The Student Dust Counter (SDC) will count and measure the sizes of dust particles along New Horizons' entire trajectory, which covers regions of interplanetary space never before sampled. Such dust particles are created by comets shedding material and Kuiper Belt Objects colliding with one another. The SDC is managed and was built primarily by students at the University of Colorado in Boulder, with supervision from professional space scientists.