The New Space Environment

NASA's "faster, better, cheaper" concept radically changed experimental space science - and fostered the development of one of the most efficient plasma sensors ever to fly.

By David T. Young, Ph.D., and John J. Hanley

Dr. David T. Young, an Institute scientist in the SwRI Instrumentation and Space Research Division, has expertise in electron and ion optics, ion mass spectrometry, and space plasma physics. He is PEPE principal investigator and currently a lead co-investigator for NASA's Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission. Young will join the faculty at the University of Michigan at Ann Arbor as professor of space sciences in September 1999.

John J. Hanley, a senior research engineer in the Instrumentation and Space Research Division, wrote the real-time flight software for PEPE. Since its launch aboard Deep Space 1, he has remained involved in mission operations and planning for the instrument.

In the early part of this decade, engineers in the Instrumentation and Space Research Division of Southwest Research Institute (SwRI) were at work on the largest, most complex space plasma instrument ever flown: the Cassini Plasma Spectrometer (CAPS) - a 23-kilogram instrument that detects and analyzes plasma (electrons and ions) found throughout the solar system. CAPS, begun in 1990 under SwRI leadership, was launched on the Cassini spacecraft in October 1997 and is now on its way to Saturn to image the planetary system and probe its rings, moons, and magnetosphere.

Despite CAPS' success, the authors realized early on that those larger instruments, although highly capable, were probably not the wave of the future. Therefore, in 1993 SwRI supported an internal research project to build the Miniaturized Optimized Smart Sensor (MOSS), a plasma sensor roughly the size of a coffee mug.

The development of MOSS was a challenge, but it worked. The instrument, about the size of a 4-inch cube, led to new particle optics, signal acquisition electronics, and high-voltage power supplies (Young et al., 1998). In early 1996, MOSS also helped SwRI get a foot in the door with NASA's New Millennium Program, sponsor of the Deep Space 1 (DS1) mission. The DS1 spacecraft would incorporate the first ion propulsion engine (see Risky Business) and use it to reach an asteroid and possibly two comets - challenging targets for a new space plasma sensor - which could give researchers insight to the composition of those bodies.

At NASA's invitation, SwRI collaborated with the Los Alamos National Laboratory (LANL) to combine MOSS with time-of-flight mass spectrometry similar to that of CAPS. Ion mass spectrometry is critical to understanding the chemical composition, and hence the behavior and origins, of space plasma. CAPS measures the energy and, via the time-of-flight method, the velocity of ions entering the instrument. From this, the mass of the ions can be determined. The new instrument, the Plasma Experiment for Planetary Exploration (PEPE), would be a much smaller but still highly capable version of CAPS. Because CAPS itself is a state-of-the-art plasma spectrometer comprising three separate sensors, it was necessary to dig deep into new technologies to dramatically reduce CAPS' 23-kilogram mass, 2.5-cubic-foot volume, and 21 watts of electrical power.

A Boeing Delta II rocket propels Deep Space 1 following liftoff from Cape Canaveral Air Station on October 24, 1998. As the first flight in NASA's New Millennium Program, the spacecraft has validated 12 new technologies, including PEPE, for scientific space missions of the next century.

New technologies

The team began designing PEPE to be equipped with a new system of charged particle optics based on MOSS and CAPS. With LANL handling the time-of-flight optics, SwRI designed a new optical system that, unlike either of its predecessors, could "see" both ions and electrons simultaneously, could scan the field of view of both, and could measure both their energies and directions of arrival - all without any moving parts.

PEPE combined many of the CAPS elements into a single sensor, allowing reductions in both size and complexity. The new optics eliminated a large stepping motor that rotates CAPS to give it a clear view of about 50 percent of space at any time - a necessary feature aboard Cassini given that it is a three-axis stabilized spacecraft. PEPE's scanning optics cover an 80 percent field of view at a considerable savings in cost, mass and power, and complexity. This combination of optics had never been tried before. LANL also reduced the size and complexity of the time-of-flight mass spectrometer optics and electronics while maintaining sufficient mass resolution to identify the complex chemical ingredients found in cometary atmospheres.

Although nearly identical in function, the Cassini Plasma Spectrometer (CAPS, at left) and the Plasma Experiment for Planetary Exploration (PEPE) vary dramatically in size, weight, and power usage. By applying several new technologies, SwRI engineers were able to make significant reductions - important for weight-frugal NASA launches. PEPE has only 25 percent of the mass and 12 percent of the volume of CAPS, and uses 50 percent of the power.

The new optical design required significant changes in the mechanical and electrical architecture of MOSS. The team used new tools and manufacturing techniques to meet the approaching launch date. A new data system, based on field-programmable gate arrays, was designed to handle not only PEPE requirements, but also possible applications to instruments used onboard future missions. PEPE requires seven high voltage power supplies, two of them operating at 15,000 volts. By employing new circuit topologies and components, power supplies of the type used in CAPS were reduced to the size of a deck of playing cards and integrated directly into the sensor unit without the need for bulky cables and connectors. To further reduce the weight of the instrument, SwRI engineers and machinists developed new methods to machine complex optical components out of exotic plastic materials and then plate their surfaces with metals and other agents needed to produce efficient anti-scattering optical surfaces.

The PEPE team achieved its goals. The instrument has about 25 percent of the mass, 12 percent of the volume, and uses 50 percent of the power of the CAPS instrument, at about 25 percent of the cost.

Developed in cooperation with the Los Alamos National Laboratory, PEPE deflects ions and electrons through a system of optics that measures their energy, direction of arrival, and mass. This cutaway view shows (from top) the electron sensor, deflection electrodes, and the ion time-of-flight sensor. The lower portion contains five boards of high-speed digital electronics.

Epilogue

The long hours of effort paid off when, almost one year after Cassini, the DS1 spacecraft with PEPE onboard lifted off from Cape Canaveral on October 24, 1998, on a Delta II rocket.

By January, the PEPE team was taking advantage of a favorable alignment of the DS1 and Cassini spacecraft. The team successfully measured and compared the plasma at both locations, though the craft were nearly 40 million miles apart.

On July 29, 1999, DS1 and PEPE will fly past Asteroid 1992 KD at 34,000 miles per hour at a distance of under 15 kilometers. If the rendezvous is successful, PEPE will become the first instrument to directly identify electrons and ions originating from the surface of an asteroid. For ions, the lowest detectable density is about 1 ion per 100 cubic centimeters, and for electrons it is 10 times less. This could yield valuable new data on the makeup of these enigmatic bodies remaining from the early violent era of solar system formation.

At roughly the same time, the team will also propose to NASA the construction of four advanced PEPE-like instruments as part of the Auroral Multiscale MIDEX (AMM) mission led by the Johns Hopkins University Applied Physics Laboratory. Should the AMM mission be selected late this fall, the Institute will have the opportunity to produce an even more capable and smaller sensor that, with its larger siblings, will continue the exploration of the plasma universe well into the next millennium.

Reference

D.T. Young, R.P. Bowman, R.K. Black, et al., "Miniaturized Optimized Smart Sensors (MOSS) for Space Plasma Diagnostics," American Geophysical Union Monograph Series, Vol. 102, 313-316, 1998.
Captions

Published in the Summer 1999 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Martinez.

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