In all likelihood, a JIMO-type mission would be powered by a cluster of ion engines that would take full advantage of the 100 kilowatts of power produced by the nuclear reactor. One option would be to line them up in an array along a boom. “Is it going to be three thrusters, five thrusters, or 10 thrusters?” Jankovsky asks. At the moment no one knows, because no one knows for sure how much thrust a single ion engine will be able to produce.
But there’s little doubt that in terms of mission design, the combination of nuclear power and ion propulsion offers a sharp departure from the past. Prometheus and JIMO could open up whole new types of missions for exploring the solar system. “For example, if one were to go to Saturn with a follow-on to Cassini, you could envision the possibility of [placing] landers down on the solid parts of [Saturn’s moon] Titan,” Greeley says. Multiple atmospheric probes might plunge into Saturn or Neptune to assess the planet’s chemical makeup. All this would be possible because nuclear electric propulsion lets mission designers devote more of their budgeted mass to scientific instruments and less to fuel.
Designers could also forgo gravity-assist fly-bys to gain additional delta-V. The way it works now, scientists have to wait for the planets to get in a certain alignment before launching to the outer solar system via Jupiter. A delay on the ground can result in the narrow launch window being missed, and thus a wait for months or sometimes even years for the next alignment.
Beyond the maneuverability of the spacecraft, the extra power from nuclear reactors offers other advantages. “We want to have long-lived landers on the polar areas of Mars,” Greeley says. Scientists suspect that if Mars has traces of life, past or present, they might be found at high latitudes. Exploring these areas hasn’t been possible because they are in shadow much of the time, which rules out solar power. Scientists also would like to drill into the Martian ice caps, but that too requires extra power, says Greeley.
At Jupiter, JIMO will take the study of Europa and the other icy moons to a new level. Galileo’s instruments were passive: They soaked up whatever feeble light and other radiation was reflected from the moons and converted them into images. With JIMO, scientists will be able to beam powerful radar signals at the moons, and the returning signals will be used to generate pictures, measure the altitude of varous features, and even see beneath the ice. Another idea is to melt the ice with a laser, making it possible to determine its chemical makeup. “That is the breakthrough—to get to active sensing of the outer planets,” says Ray Taylor, NASA’s overall system engineer for Project Prometheus and another Navy nuclear propulsion veteran.
In February, JIMO managers at JPL briefed U.S. spacecraft manufacturers on the conceptual design for the mission. NASA then awarded contracts to Lockheed Martin, Boeing, and Northrop Grumman to have them investigate designs for the Jupiter spacecraft. Newhouse wants the contractors to feel free to brainstorm. He warns against assuming that thrusters based on the DS1 technology will be the only answer. Another proposed ion thruster developed in Russia, for example, uses as its fuel the metallic element bismuth, which can be stored as a solid. At this point almost any propulsion design is still on the table.
For that reason, Newhouse says it would be foolhardy now to attempt a cost prediction for JIMO. “Come back in two years and I’ll tell you the cost,” he says. Everyone agrees that the technology will cost billions, however. “This could be the biggest procurement since the space station in terms of dollar value, so we have to do it right,” Newhouse says. It’s even possible, he concedes, that the contractors will tell him he’s asking for the impossible. Then again, that kind of advice would not have deterred Prometheus.