Los Alamos Science Instruments to Fly on Cassini

Los Alamos National Laboratory

John Gustafson, 505-665-9197 / [email protected]

LOS ALAMOS SCIENCE INSTRUMENTS TO FLY ON CASSINI

LOS ALAMOS, N.M., Oct. 8, 1997 -- Los Alamos National Laboratory scientists led the development of two scientific sensors that will provide key measurements of the space environment around Saturn when the Cassini spacecraft reaches the ringed planet in 2004.

One sensor, the ion beam spectrometer, takes advantage of space technology that Los Alamos first developed and flew in the 1970s; the other, the ion mass spectrometer, features a completely new design that will enable mission scientists to sort out the composition of the rings and moons orbiting within Saturn's magnetic influence.

The two sensors are part of the Cassini Plasma Spectrometer, or CAPS, a microwave oven-sized unit that is one of 12 scientific instruments on the two-story-tall Cassini spacecraft. Cassini, a joint effort of the National Aeronautics and Space Agency and the European Space Agency, is scheduled for launch Oct. 13.

"This spacecraft is huge," said Dave McComas, leader of Los Alamos' Space and Atmospheric Sciences Group and of Los Alamos' Cassini involvement, noting that CAPS alone weighs about 50 pounds, a sizeable amount for any space instrument.

Scientists from the Department of Energy's Los Alamos laboratory also developed and manufactured the heat sources for Cassini's radioisotopic thermal generators, which use heat from the decay of plutonium-238 to generate electricity, and other heat sources that warm critical spacecraft components. Los Alamos developed heat sources for Mars Pathfinder, Pioneer, Voyager, Ulysses and other space missions as well.

Nothing like Los Alamos' ion mass spectrometer has flown in space before, though the technique behind it has been used in ground-based applications. The Los Alamos team expanded the existing technique by designing their instrument to capture ions from all directions.

"We realized that if you could combine high mass resolution and high sensitivity with viewing in three dimensions," McComas said, "you could address the many scientific issues involved in studying the ions trapped within Saturn's magnetosphere," the region of space dominated by the planet's magnetic field.

The rings and moons of Saturn, orbiting within the magnetosphere, shed atoms and molecules that create a rich population of electrically charged particles, or ions, trapped by the magnetic field.

Schematically, the mass spectrometer consists of an entry, a passageway in which the ion's energy is measured and a chamber for mass analysis that's about the size and shape of a coffee can. As an ion enters the cylindrical chamber, it passes through a microscopically thin foil of carbon -- only a few dozen atoms thick -- which starts a clock that times the ion's flight through the cylinder and also provides information for determining the direction from which the ion entered the mass spectrometer.

The ion dives into the cylinder until the repelling force of an electric field permeating the cylinder overwhelms the ion and kicks it back, as if a spring attached to the ion had been stretched to its limit. The ion's return stops the clock measuring its flight. The time it takes for the ion to "bounce" within the cylinder yields a very accurate measurement of its mass.

Using information from other particles that don't "bounce" gives the mass spectrometer improved sensitivity. The Los Alamos mass spectrometer, unlike others, provides both high mass resolution and high-sensitivity measurements in a single package. It also has low electrical power requirements and is compact, lightweight and sufficiently robust to survive Cassini's launch and long space journey.

The ion beam spectrometer is a modern rendition of a design that flew on the first International Sun-Earth Explorer spacecraft in the 1970s; it features innovations that improve its performance over the 1970s' model. "To fit into Cassini's original envelope we had to come up with the design for a beam spectrometer that required the absolute minimum in mass and power," McComas said.

The ion beam spectrometer consists of two eight-inch-diameter, nested hemispheres of aluminum, each machined so thin they are like paper. Three openings about the size of a coin slot on a vending machine allow ions to enter at the base of the hemispheres. An electrical voltage forces ions of the right energy to travel a curved route between the hemispheres to a detector on the far side.

Tuning the electrical voltage lets the spectrometer sample for ions of different energies or speeds. Timing the arrival of ions that reach a detector as the spectrometer scans the sky provides data that can be decoded to reveal the spatial distribution of the incoming ions, especially intense, narrow beams of particles like those that stream along planetary magnetic field lines and cause phenomena such as auroras.

The two Los Alamos sensors will measure the solar wind -- a million-mile- per-hour gale of atomic particles that streams into space from the sun's outer atmosphere -- during part of Cassini's voyage to Saturn; measure the interaction of the solar wind with Saturn's magnetosphere; search for beams of ions within the planet's magnetosphere; and sample atmospheric constituents from Titan, Saturn's largest moon, when Cassini swoops past it.

These measurements will not only allow scientists to determine the origins and fates of various ions but also identify physical processes taking place on the surfaces of the moons and within the rings of the Saturnian system.

"That's why the ion mass spectrometer was developed," said McComas. "The different bodies orbiting Saturn have different compositions and we can use the IMS data to figure out which bodies various ions came from. Saturn's magnetosphere is like a soup with lots of different ingredients from different places, and the trick is to figure out the sources of those ingredients."

Cassini measurements also will let space scientists engage in "comparative planetology" to better understand the interaction of the sun and planets and apply what's learned to Earth. "As scientists it's fun to learn more about the solar system, but what we learn can also help us understand space storms that knock out power grids and satellites and even affect the electrical energy surrounding Earth that degrades structures such as pipelines," McComas said.

Dave Young of the Southwest Research Institute in San Antonio, Texas, is the principal investigator for the Cassini Plasma Spectrometer, which incorporates an electron spectrometer, power supplies, controlling electronics and motors along with the Los Alamos sensors.

CAPS, like the rest of the Cassini mission, is truly an international experience. The CAPS instrument development team includes representatives from the Mullard Space Science Laboratory and Rutherford Appleton Laboratory in England; the Centre de Etude des Environnements Terrestre et Plainetaires in France; the Norwegian Defense Research Establishment; the Technical Research Centre of Finland; and NASA's Goddard Spaceflight Center, in addition to Los Alamos and SWRI.

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.

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