MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) became the first spacecraft to orbit the planet Mercury on March 18, 2011. It was sent to answer very fundamental questions about the innermost planet and its harsh environment, e.g., What is Mercury's geologic history? What is the state of Mercury's core? What are the sources and sinks for its atmosphere? Sending a spacecraft so close to the Sun is not easy. In fact, prior to MESSENGER, Mercury had only been visited during the three flybys of the Mariner 10 spacecraft in the mid-1970's. Launched in 2004, it had taken 7 years and 6 gravity assist maneuvers for MESSENGER to begin orbiting Mercury, flying by the Earth, Venus (twice), and Mercury itself (3 times). In order to complete the long journey, MESSENGER carried over 500 kg of fuel, more than half of the spacecraft mass at launch. As a consequence of Mercury's proximity to the Sun, MESSENGER has a large sunshade to protect the instruments from solar radiation, which is 10 times more intense at Mercury than at Earth. The spacecraft is outfitted with a full complement of scientific instruments: two high resolution cameras to photograph the surface, several spectrometers to measure the elemental composition of the planet's crust, a laser altimeter to measure topography, a magnetometer to measure Mercury's intrinsic magnetic field, and two instruments to measure high-energy ions in its space environment.
The Solar and Heliospheric Research Group contributed one of those instruments, the Fast Imaging Plasma Spectrometer (FIPS). FIPS was designed and built here at the University of Michigan in conjunction with the Space Physics Research Laboratory. When compared with its predecessors, FIPS is much smaller, lighter, and uses much less power, properties which were required to fit aboard the MESSENGER spacecraft. Its primary function is to measure ions from Mercury itself, created from the ionization of the planet's very thin atmosphere or via collisions of micrometeroids and high energy ions from the solar wind with its surface. FIPS can measure ions ranging from hydrogen through iron on the periodic table and at energies up to 13 keV, which, for protons, corresponds to speeds of up to 3.4 million miles per hour. FIPS uses several key technologies to make these measurements, such as a precisely designed ion path including a 15-kV acceleration region and an electrostatic mirror, microchannel plate detectors capable of detecting single electrons, and fast electronics which can measure times of flight for these ions down to 1 ns.