ORNL has lead roles in a new NASA program to build a nuclear reactor power system that will enable future missions to our solar system's outer planets.
Exploring planets beyond Mars will require a power source different from those now deployed in American spacecraft. Radioisotope thermal generators and solar energy cannot meet the challenges posed by proposed missions to the cold, dark regions of our solar system. ORNL engineers are convinced nuclear fission power will.
"To understand why nuclear reactors are needed in space, we must understand the size of our solar system and the distances involved in interplanetary space exploration," says Sherrell Greene of ORNL's Nuclear Science and Technology Division. "A radio signal requires 35 minutes to travel from Earth to Jupiter and 5 hours is required for a signal to travel from Earth to Pluto. The distance to Jupiter from Earth is 4 times the distance between Earth and the sun."
Because these distances are so large, the intensity of sunlight shining on each planet is greatly reduced as we travel outward from Earth toward the edge of the solar system. The intensity of the sun's light shining on Jupiter is only 1/27 th its intensity on Earth.
"There isn't much light getting out to Jupiter," Greene says. "It's cold and dark there. For spacecraft carrying scientific instruments beyond Mars, solar energy is not an option, and command and control are more complicated."
The traditional approach of mounting solar cells on unmanned spacecraft works well for voyages to Venus, Mercury, and Mars. However, beyond Mars this approach is not practical because the sunlight's intensity is so low that the space probe cannot capture enough solar energy without huge, unwieldy arrays of photovoltaic cells.
"As we move beyond Mars, we need an alternative source of electrical power," Greene explains. "Radioisotope thermal generators are a very good option for providing low levels of electrical power.
None of the space probes, such as Voyager, Galileo,and Cassini, has had even 1 kilowatt (1 kW) of power. Most have had only a few hundred watts of power. The beautiful photos of the planets and the data collected there were obtained with less power than is required to blow-dry your hair.
"The National Aeronautics and Space Administration (NASA) has done marvelous things with one kilowatt, but NASA has just about exhausted its options for solar system exploration with current power sources. The bulk of the solar system simply cannot be explored in any meaningful way unless we employ nuclear reactors in space."
NASA desires to go to different planets (and their moons) with more robust spacecraft that can maneuver around moons, collect more data, and communicate the information to Earth more quickly than can be done with current technologies. More electricity is required to operate science packages; power command, control, and telemetry systems; and provide electric propulsion. According to Greene, these needs can be met only by using spacecraft powered by nuclear reactors.
"The future of science in space depends on the successful deployment of space-based reactor power systems," Greene says. "On Earth knowledge is power, but in space, power is knowledge. More power means more destinations, more observations, more knowledge, and quicker data return."
Exploring Jupiter's Moons
Greene leads a team that is working on a NASA-approved mission called JIMO, for Jupiter Icy Moons Orbiter. The unmanned spacecraft, designed to do science, will require 100 kW of electrical power to move out to Jupiter, spiral into the huge planet, and then fly out and maneuver in orbit around three moons—Callisto, Gannymede, and Europa. Next to Mars, solar system scientists believe Europa to be the most desirable destination in our search for life, because this moon is thought to host a vast, ice-covered ocean. JIMO will be the first space probe to orbit around and collect data from three moons.
JIMO will be powered by a reactor—the second reactor launched into space by the United States. The first, called SNAP10A, orbited Earth in 1965. Since that time, the Russians have launched more than 30 Earth-orbiting space reactors, mostly for military purposes, in reconnaissance satellites. JIMO would fly in the early part of the next decade. The JIMO mission is led by NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, and is funded by NASA's Office of Space Science. The Office of Nuclear Energy has been the lead DOE organization, but officials recently announced that the National Nuclear Security Administration (NNSA) Office of Naval Reactors will eventually take over as DOE's supporting agency.
Two NASA centers are supporting JPL—Marshall Space Flight Center and Glenn Research Center. Also collaborating in this project are two DOE labs— Los Alamos National Laboratory and ORNL.
Greene attributes ORNL's success in becoming a supporting organization for JIMO partly to the existence in Oak Ridge of hundreds of old files and documents. Some were written in the 1950s for ORNL's Aircraft Nuclear Propulsion Program, which spawned nuclear and materials technologies that formed the backbone of today's nuclear enterprise. Fortunately, these ORNL documents, including Robert W. Bussard's 1953 visionary report on "Nuclear Energy for Rocket Propulsion," were not destroyed.
"Our ability to locate, secure, access, and digest those documents has contributed greatly to our success in bringing this work to ORNL," Greene says. "ORNL is a major element of our nation's 'institutional memory' in the space nuclear power arena. We're working hard to help ensure successful implementation of our nation's next nuclear-powered space mission."
Lab's Lead Roles
ORNL has several important roles on the government's team. The Laboratory has the lead in developing advanced materials and refractory metal technologies needed to enable the reactor system to operate at the high temperatures required to minimize its size and mass. ORNL also is coordinating the development of the autonomous reactor control, strategies, and control-system concepts required to enable the JIMO reactor to deal with its environment and changing power demands without human intervention (and the associated time delays). ORNL's third lead role is to provide nuclear safety assurance for the space probe, both in flight and on the ground. "ORNL will estimate the probability and consequences of various potential accidents involving the reactor and recommend approaches to ensure safe performance of the mission," Greene says.
ORNL also will guide the design of the reactor's nuclear shield, which protects the spacecraft's electronics, controls, and science packages from radiation emanating from the reactor. "The government will develop its own design for this complicated shield so it can be a 'smart buyer' of shield components," Greene explains.
ORNL has the lead role in estimating the cost and schedule for the JIMO program's nuclear part. The Laboratory also has a support role for the JIMO mission: providing technical support and implementation for National Environmental Policy Act compliance activities in building the JIMO spacecraft on the ground."We're excited about the roles ORNL is playing in advancing knowledge of our solar system," says Greene, "Space really is the fission frontier!"
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