The Back Page

Jupiter's Newest Satellite

Gabriel Spera

Peering through his homemade telescope nearly 400 years ago, Galileo first laid eyes on the four largest moons of Jupiter, now known as Io, Europa, Ganymede, and Callisto. His observations caused a notorious stir among his contemporaries, forcing a profound shift in the accepted model of the cosmos.

It's only fitting, then, that Galileo's namesake spacecraft should cause an equal sensation by indicating that these moons might hold vast saltwater oceans beneath their icy surfaces. Indeed, data from the Galileo craft suggest that liquid water on Europa made contact with the surface in geologically recent times and may still lie relatively close to the surface. If so, Europa could potentially harbor life.

Based on this possibility, NASA is developing ambitious plans for a new mission—the Jupiter Icy Moons Orbiter, or JIMO—that would orbit Callisto, Ganymede, and Europa to investigate their makeup, history, and potential for sustaining life. Sending a spacecraft halfway across the solar system is hard enough, but getting it into and out of three separate lunar orbits will be a tremendous feat, requiring a significant amount of energy. Thus, JIMO will be a new type of spacecraft, driven by nuclear-generated ion propulsion. The technology will be challenging, but the rewards will be significant: An onboard reactor could support an impressive suite of instruments far superior to anything that could be sent using traditional solar and battery power. It could even be used to beam power to a probe or lunar lander.

Aerospace has been lending its technical expertise to the JIMO project. For example, as part of the High-Capability Instrument Concept study, Aerospace helped develop a baseline design for a suite of instruments that can take advantage of the large power supply to achieve high sensitivity, spatial resolution, spectral resolution, duty cycle, and data rates. The candidate instruments included a visible and infrared imaging spectrometer, a thermal mapper, a laser altimeter, a multispectral laser surface-reflection spectrometer, an interferometric synthetic-aperture radar, a polarimetric synthetic-aperture radar, a subsurface radar sounder, and a radio plasma sounder. In addition to generating basic specifications for each instrument, Aerospace explored a number of design options to delineate critical trade-offs. Driving technologies for each instrument type were identified, as well as an estimate of the needed development time. The laser spectrometer, for example, is an entirely new instrument, and the multispectral selective-reflection lidar is based on capabilities that are available in the industry but do not exist in a single design.

Aerospace also performed the coverage analysis for JIMO, including verification of maximum revisit times for various inclinations and altitudes and access coverage for the entire moon of Europa. The Revisit program, a software tool developed by Aerospace, was used for the visualization and computation. Key results from the study included an analysis of the fields of view needed to achieve the desired mapping coverage. In some cases, the analysis prompted a change in sensor configuration to accommodate sunlight constraints. This analysis also helped define duty cycles that would reduce the amount of data being sent back to Earth without compromising overall performance.

In a related effort, Aerospace engineers analyzed the telecommunications needed for the return of data from the JIMO instruments—and derived a target specification of roughly 233 megabytes per second. Key considerations included loss of communication due to blockage from Jupiter, the sun, and the Jovian moons and the enormous amount of sensor data (even with onboard processing) that will need to be sent. Aerospace provided three system options: direct radio-frequency communication using a 3- or 5-meter dish at 35 gigahertz; laser communication using multiple lasers in the terahertz band; and radio-frequency communication via a relay satellite trailing JIMO.

One particular challenge facing JIMO is the harsh radiation environment. Jupiter has trapped proton and electron belts, much like Earth; however, the Jovian trapped electron environment is much more severe. Planning for this environment will require some new approaches because the most problematic particle around Jupiter is the high-energy electron—not the proton, which is the primary concern around Earth. Aerospace analyses indicate that the radiation challenges are not insurmountable: If commercial integrated circuits continue to evolve at their present rate, they should allow significant improvements in radiation hardness and better protection for both analog and digital flight electronics, including focal planes. Better inherent radiation resistance, along with proper shielding design, should allow JIMO to survive. Still, JIMO will need to overcome the data corruption that will occur as sensitive imagers and spectrometers attempt to collect data in the midst of this severe radiation.

As part of the conceptual mission studies, Aerospace performed independent cost estimates for various configurations and design iterations. The main trades consisted of varying power-conversion types, nuclear-reactor types, and power levels. The cost analysis emphasized technology forecasting, risk, radiation hardening, schedule penalty, calibration of the primary contractor's historical programs, safety specifications, and responsiveness to other program-management and engineering issues. After each design iteration, the Aerospace and contractor teams met to reconcile their cost estimates. This proved especially valuable because Aerospace was able to influence contractor cost estimates and, in certain cases, the contractor's cost methodology.

NASA hopes to launch JIMO early in the next decade—and it will probably take another six years to reach its destination. So, it will take some while before scientists crack the secrets of Jupiter's frozen moons. In the meantime, Aerospace will continue to support the program as needed, joining NASA and other organizations in honoring and advancing Galileo's great legacy.

To Summer 2004 Table of Contents