Space Power

Scientists ponder the question, "What advances in power technology are required to send human and robotic explorers throughout the solar system?"

Beyond all the planets in our solar system in a cold, dark, empty region of space, Voyager 1 continues its 25-year journey of exploration. It's headed for the heliopause, that boundary where the Sun's influence ends and the dark recesses of interstellar space begin. From where Voyager sits, the Sun is merely the brightest star in the sky - seven thousand times dimmer than we see it from Earth.

Voyager doesn't have any solar panels; they wouldn't do any good so far from the Sun. The probe stays in touch by carrying its own power source, an early radioisotope thermoelectric generator (RTG), which converts the heat generated from the natural decay of its radioactive fuel into electricity. Its RTG will supply Voyager with electricity at least until 2020.

Space probes that travel much beyond Mars need more power than solar cells can provide. Another example is the Ulysses spacecraft. It was launched in October 1990 from the space shuttle on a mission to study the Sun's poles. To get above the Sun, Ulysses had to fly around Jupiter and slingshot out of the plane of the planets. Near Jupiter, the Sun's rays are 25 times weaker than near Earth. Solar panels large enough to catch this weak energy would have weighed 1,200 pounds, doubling the weight of the spacecraft and making it too heavy for booster rockets from the shuttle. Instead, Ulysses was equipped with an RTG weighing only 124 pounds. It easily powers all the probe's onboard systems, including navigation, communication and scientific instruments.

375 km above Earth's surface, solar arrays power the orbiting International Space Station.

A probe like Ulysses needs a couple of hundred watts of power to operate onboard systems. For comparison, the shuttle's onboard systems use 5 to 10 kilowatts (kW) of power, 50 times that. The International Space Station (ISS) uses 10 times more, or about 100 kW for onboard systems.

The ISS never leaves Earth orbit, which reduces the power it needs. Human missions beyond Earth's neighbourhood, however, will require power not only for onboard systems, but also for propulsion and for systems to support humans when they arrive wherever they're going. "To pursue ambitious human missions across the solar system, perhaps returning to the Moon, perhaps going on to Mars, will require hundreds to a thousand kilowatts on the surface and hundreds to thousands of kilowatts for transportation systems," says John Mankins, chief technologist for the Advance Systems Program at NASA headquarters. You can't just plug into the nearest electrical outlet, he added. You have to bring your own power source. Ideally, you'd like to find something that could provide power for both propulsion and operations.