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.
A probe like Ulysses needs a couple 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 neighborhood, 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.
Since Robert Goddard’s first test launch of a rocket in 1916, space missions
have used chemicals to get the acceleration needed to escape Earth’s
gravity. A rocket’s 5- to 15-minute burn sends the spacecraft towards its
destination; then it coasts the rest of the way unless it uses the gravity
of other planets for an additional boost. For Voyager, it took years to
reach Saturn and then the spacecraft was only able to spend days in the
Saturn system and only hours near the planet itself.
Mission planners would like to do better in the future.
From the perspective of the Exploration Office at the Johnson Space Center,
Jeff George sees “an evolving family of related power and propulsion
technologies” for the next wave of human exploration. The first likely
candidate is electric propulsion (EP). You don’t need as much thrust in
space as you do to escape Earth’s gravity, explains George, but you do need
to produce thrust using very little fuel because of weight restrictions.
Electric propulsion could provide fuel-efficient thrust after an initial
chemical boost into space.
Specific impulse–that is, the pounds of thrust produced per pound of
propellant used per second–is a measure of the efficiency with which a
system uses fuel to produce thrust. Higher is better. The space shuttle,
which stays near Earth, uses chemical propulsion with a specific impulse of
450-460 seconds or 450 pounds of thrust for a pound of propellant per
second. EP has 10 times the specific impulse of chemical propulsion and
potentially can go as high as 10,000 seconds.
EP got its first try in 1999 on Deep Space 1–a spacecraft that tested many
new technologies before it flew by comet Borrelly in 2001. Deep Space 1
needed 2.6 kW to power both its electric ion propulsion drive (pictured
left) and other onboard systems. The energy came from an innovative
collector consisting of advanced solar cells and a lens to concentrate
sunlight on the panels. Together they achieved a 23% efficiency in
converting sunlight to electricity compared with 14% efficiency for the
solar arrays on the ISS.
Building on the success of Deep Space 1, a new mission named “Dawn” will
leave Earth in 2006. Propelled by an ion engine with a specific impulse of
3100 seconds, Dawn will travel to Ceres and Vesta, two of the biggest
asteroids in the solar system. Although Ceres and Vesta lie farther from the
Sun than Mars does, the spacecraft will be able to draw all the power it
needs from 7.5 kW solar arrays.
Manned missions need more power. “The next step for a [human-crewed] Mars
mission,” says Jeff George, “is to step up to 5-10 megawatts of nuclear
power and then scale up the electric thrusters to megawatts per engine.”
Going from kilowatts to megawatts is not a simple problem. NASA is now
working on a 5-10 kW next-generation ion propulsion system. George envisions
small, nuclear-electric vehicles of 100-200 kW exploring the outer planets
as a pilot version of the megawatt scale they’d like to use for human
exploration.
To run a megawatt EP system, you need a source with both high energy and
high power. As John Cole, manager of the Revolutionary Propulsion Research
Project Office explained, “Energy is the most important factor, but power
(the energy released per unit time) determines acceleration.” So what source
provides enough power? “Nuclear has plenty of energy–and potentially plenty
of power, too,” Cole observes. “Solar panels provide insufficient power for
the entire vehicle to accelerate to levels that permit short trip times.”
Radioisotope power sources (like the RTGs onboard Voyager) give off a lot of
energy over a long period of time, but not a lot of power, only tens to
hundreds of watts. To get kilowatts to megawatts of power, you have to go to
nuclear fission, says Les Johnson, of NASA’s Advanced Space Transportation
Program.
Fission, in which a neutron splits an atom into two radioactive isotopes, is
the process nuclear power plants on Earth use to produce electricity.
“Bringing along a fission reactor on a spacecraft would be like bringing
along your own [mini] power plant,” says Johnson. A fission reactor is
capable of fueling high-performance electric propulsion beyond the inner
solar system. It is longer duration and power rich for performing
sophisticated scientific investigations, high-data rate communications, and
complex spacecraft operations.
That’s a pretty good resume for fission, but it still doesn’t pass John
Cole’s test. Cole set himself the requirement of getting humans to the outer
planets in a year and back in a year. Nuclear fission has enough energy, but
not enough power to provide the acceleration needed. NASA is designing a
300-kW flight configuration system using nuclear fission. But to meet Cole’s
test, “one needs a very high specific power, power per unit mass vehicle
three orders of magnitude better than what we’ve currently planned for
nuclear fission.” For that, you have to step up to nuclear fusion–the same
process that powers the Sun and stars.
Fusion, which releases energy by combining rather than splitting atoms,
could in principle supply gigawatts of clean power. However, fusion
propulsion systems as we understand them today would be very big, requiring
a vehicle the size of the space station or Battlestar Galactica, weighing
hundreds of tons–although the size might come down with research.
Fusion engines would be very efficient fuel burners with a specific impulse
of 100,000 seconds. “Though we couldn’t do it in 10 years, if we could
launch a fusion propulsion system 10 years from now, we could send a vehicle
out to catch Voyager and bring it back,” says Cole. That kind of power and
speed shortens the time that astronauts would be exposed to harmful cosmic
radiation and the bone loss that comes from prolonged weightlessness.
Perhaps there’s something even better than fusion: A thruster powered by
matter-antimatter annihilation would have a specific impulse of 2,000,000
seconds, according to Cole.
It sounds like science fiction, but researchers are learning to create and
store small amounts of antimatter in real-life labs. A portable
electromagnetic antimatter trap at Penn State University, for example, can
hold 10 billion antiprotons. If we could learn how to use such antimatter
safely, we could impinge some on a thin stream of hydrogen gas to create
thrust. Alternatively, a little antimatter could be injected into a fusion
reactor to lower the temperatures needed to trigger a fusion reaction.
“Propulsion isn’t the only reason to go nuclear,” notes Colleen Hartman,
director of solar-system exploration at NASA headquarters. “Onboard systems
benefit, too. The excess power is like getting the Las Vegas strip instead
of a single light bulb. It gives you greater communication and mission
flexibility.”
The Mars Smart Lander and Mobile Laboratory, slated for launch as early as
2009, will be upgrading from solar to nuclear power: “Putting nuclear power
on board will extend the mission from 3-6 months [with solar power] to 5
years [with radioisotope power],” says Ed Weiler, head of the Space Science
Enterprise at NASA headquarters. “It will enable the rover to drive to a
location rather than having to land there. The bandwidth for data
communication goes way up, and the rover can work 24 hours a day. Everything
increases by a factor of 10 when you add an RTG to a mission.”
Scaling up from the Mars Lander to a human mission on Mars requires more
power–about 30 kW to heat and cool a human habitat, run computers and
lights, make oxygen, recycle water and recharge the rovers, says Jeff
George. For a long mission “we don’t have the kind of energetics where you
can dash back home [in case of trouble],” adds Gary Martin, assistant
associate administrator for Advanced Systems in NASA’s Office of Space
Flight. “You’re building things that have to be ultra reliable,
self-healing, and autonomously sense when they’re hurt.” Broken parts will
have to be made or repaired on site: you can’t bring spare parts.
Power-intensive processes like making parts or producing propellant for
leaving Mars would be another 60 kW, according to George.
In the end, one power source does not fit all needs. Looking at the big
picture, John Mankins says “we need very high-efficiency, high-power
electric propulsion for interplanetary travel; we need reliable and
affordable high-energy chemical propulsion systems for going up and down
from planetary surfaces; and we need to be able to store chemical or solar
power in order to live and work on the surface. Robots could use
radioisotope power; and there’s reactor power and wireless beaming to
consider as well.”
The choices are many, yet one thing is clear: Wherever we go in space and
whatever we do there, we’ll need more power.
