Over the next 5 years NASA has in the planning stages (i.e., the mission is being funded and is under development) about 20 space science missions. None of those missions require the use of radioisotope power sources and 2 may require small Radioisotope Heater Units (RHUs) like the ones used for the Mars Pathfinder rover. Additionally, about 30 other potential missions are under study and may be selected for development. There are two categories of study: "Advanced Study" means NASA generally accepts the concept, however, detailed spacecraft and mission design (and sometimes specific funding approval) are needed before development can begin. Currently there are 3 missions under advanced study that may require radioisotope power systems and RHUs, and 2 missions that may need RHUs for heating purposes. "Conceptual Study" means the mission is an idea that might be proposed by or to NASA but has not been selected for advanced study. Currently there are 5 conceptual missions that may require radioisotope power systems and RHUs, and 1 that may require RHUs for heating purposes. Any mission implementation is dependent on a decision to proceed after the required environmental review process is completed.

Below are some examples of missions that will be able to use arrays of solar cells and batteries to enable the missions.

• Deep Space-1 (Planned to launch in July 1998): Validate advanced spacecraft and instrument technologies (including ion propulsion and high efficiency solar arrays).
• Stardust (Planned to launch in February 1999): Collect interstellar dust and cometary material by flying by a comet and returning the sample to Earth.
• Space InfraRed Telescope Facility (Planned): Collect information about the early universe using infrared imaging & spectroscopy.
• Mars 2001 Orbiter (Planned): Globally map elemental composition of the surface.
• Mars 2003 Orbiter (Adv. Study): Establish a sustained communications and navigation capability at Mars

Sometimes it is not possible to use arrays of solar cells for space missions. This is especially true when a mission is constrained by one or more of the factors below:

• too far from the sun to make use of solar power
• in a space radiation environment too harsh to allow sustained use of solar cells (e.g., very near the Sun)
• landing near a planet's poles where solar illumination is insufficient
• in night environments with time frames beyond practical battery capacity
• on a dust- or cloud-enshrouded world, or in a subsurface application, where the use of solar power is impractical or impossible, where the use of solar power is impractical or impossible

When arrays of solar cells and batteries are not feasible, other power designs are needed. Similar to past missions to the outer solar system, some future explorations may require radioisotope power systems to generate electricity for the scientific instruments and for the spacecraft or lander. For example, the recent Cassini mission to Saturn needed three RTGs to power the scientific instruments and the Saturn Orbiter itself. Examples of future missions, which may require the use of radioisotope power systems are:

• Pluto/Kuiper Express (Adv. Study): Map the surface and characterize the atmospheres of Pluto and its moon Charon.
• Europa Orbiter (Adv. Study): Study Europa (a moon a Jupiter) in search of possible liquid water oceans beneath the surface ice.
• Solar Probe (Adv. Study): Study the origin of the solar wind.
• Interstellar Probe (Conceptual Study): Characterize interstellar dust and gas at 900 million miles from the sun and beyond.
• Europa Lander (Conceptual Study): Study the seismology and possibly penetrate the ice crust to reach a liquid water ocean.
• Io Volcanic Observer (Conceptual Study): Extensive study of Io's (a moon of Jupiter) surface and volcanic activity.
• Titan Organic Explorer (Conceptual Study): Use landers or aerobots to investigate the surface and chemistry of Titan's (a moon of Saturn) atmosphere.
• Neptune Orbiter (Conceptual Study): Extensive study of Neptune's system.

The RTG design used on the Cassini mission generates electrical power by converting the heat from the natural decay of plutonium through the use of solid-state thermoelectric converters. It is about 113 cm (about 44 in) long and about 43 cm (about 17 in) in diameter, and contains about 10.8 kg (about 24 lb) of plutonium dioxide. RTGs have been designed to contain their plutonium dioxide in the event of launch or reentry from Earth orbit accident. NASA is working with the Department of Energy to identify power requirements of future spacecraft, and to design smaller and more efficient power systems. These power systems may only need to carry about 2-3 kg (about 4-7 lb) of nuclear material for power generation.

Some future missions may require the use of RHUs in order to keep a spacecraft, lander, or rover electrical components warm enough to function. For example, the 1997 Mars Pathfinder rover, Sojourner, used three RHUs to keep its electronics from freezing during the cold Martian nights. RHU's provide about one watt of heat, derived from the radioactive decay of plutonium. They are cylindrical in shape, about 2.5 cm (about 1 in) in diameter and about 3.2 cm ( about 1.3 in) long, and contain about 2.7 g (about 0.1 oz) of plutonium dioxide. RHU's have been designed, built, and tested to contain their plutonium dioxide even if they were to be exposed to an accident during launch or reentry from Earth orbit. Missions that may require the use of RHUs include:

• Mars 2001 Lander/Rover (Planned): Explore a site for biologic or prebiologic processes and store samples for possible return to Earth by Mars Sample Return mission.
• Mars 2003 Lander/Rover (Planned): Akin to Mars 2001 Lander/Rover, but would explore a different site.
• Mars Sample Return (Adv. Study): Mission to Mars that could return to Earth samples of Martian surface material and atmosphere for analysis.
• Deep Space-4 (Adv. Study): Demonstrate advanced spacecraft technologies by landing a probe (Champollion) on a comet nucleus and possibly returning a sample to Earth.
• Jupiter Deep Probe (Conceptual Study): Study Jupiter's composition and atmospheric structure at multiple locations.

For more information about NASA's space science enterprise, visit the NASA home page at:http://www.hq.nasa.gov/office/oss/osshome.htm

Office of Space Science Public Affairs Office: 202/358-1547