November 18, 2005: When your computer behaves erratically,
mauls your data, or just "crashes" completely, it
can be frustrating. But for an astronaut trusting a computer
to run navigation and life-support systems, computer glitches
could be fatal.
Unfortunately,
the radiation that pervades space can trigger such glitches.
When high-speed particles, such as cosmic rays, collide with
the microscopic circuitry of computer chips, they can cause
chips to make errors. If those errors send the spacecraft
flying off in the wrong direction or disrupt the life-support
system, it could be bad news.
Right:
The humans inside this spacecraft aren't the only ones who
need protection from space radiation; their computers do,
too. [More]
To
ensure safety, most space missions use radiation hardened
computer chips. "Rad-hard" chips are unlike ordinary
chips in many ways. For example, they contain extra transistors
that take more energy to switch on and off. Cosmic rays can't
trigger them so easily. Rad-hard chips continue to do accurate
calculations when ordinary chips might "glitch."
NASA
relies almost exclusively on these extra-durable chips to
make computers space-worthy. But these custom-made chips have
some downsides: They're expensive, power hungry, and slow
-- as much as 10 times slower than an equivalent CPU in a
modern consumer desktop PC.
With
NASA sending people back to the moon and on to Mars--see the
Vision for Space Exploration--mission planners would love
to give their spacecraft more computing horsepower.
On
the surface of the moon or Mars, explorers could use fast
computers to analyze their data right after collecting it,
quickly identifying areas of high scientific interest and
perhaps gathering more data before a fleeting opportunity
passes. Rovers would benefit, too, from the extra intelligence
of modern CPUs.
Using
the same inexpensive, powerful Pentium and PowerPC chips found
in consumer PCs would help tremendously, but to do so, the
problem of radiation-induced errors must be solved.
This
is where a NASA project called Environmentally Adaptive Fault-Tolerant
Computing (EAFTC) comes in. Researchers working on the project
are experimenting with ways to use consumer CPUs in space
missions. They're particularly interested in "single
event upsets," the most common kind of glitches caused
by single particles of radiation barreling into chips.
Left:
Space radiation hits a computer chip, an artist's concept.
Credit: ESA.
Team
member Raphael Some of JPL explains: "One way to use
faster, consumer CPUs in space is simply to have three times
as many CPUs as you need: The three CPUs perform the same
calculation and vote on the result. If one of the CPUs makes
a radiation-induced error, the other two will still agree,
thus winning the vote and giving the correct result."
This
works, but often it's overkill, wasting precious electricity
and computing power to triple-check calculations that aren't
critical.
"To
do this smarter and more efficiently, we're developing software
that weighs the importance of a calculation," continues
Some. "If it's very important, like navigation, all three
CPUs must vote. If it's less important, like measuring the
chemical makeup of a rock, only one or two CPUs might be involved."
This
is just one of dozens of error-correction techniques that
EAFTC pulls together into a single package. The result is
much better efficiency: Without the EAFTC software, a computer
based on consumer CPUs needs 100-200% redundancy to protect
against radiation-caused errors. (100% redundancy means 2
CPUs; 200% means 3 CPUs.) With EAFTC, only 15-20% redundancy
is needed for the same degree of protection. All of that saved
CPU time can be used productively instead.
Right:
EAFTC computers in a space-ready flight chassis. Credit: NASA/Honeywell.
"EAFTC
is not going to replace rad-hard CPUs," cautions Some.
"Some tasks, such as life support, are so important we'll
always want radiation hardened chips to run them." But,
in due course, EAFTC algorithms might take some of the data-processing
load off those chips, making vastly greater computer power
available to future missions.
EAFTC's
first test will be onboard a satellite called Space Technology
8 (ST-8). Part of NASA's New Millennium Program, ST-8 will
flight-test new, experimental space technologies such as EAFTC,
making it possible to use them in future missions with greater
confidence.
The satellite, scheduled for a 2009 launch, will skim the
Van Allen radiation belts during each of its elliptical orbits,
testing EAFTC in this high-radiation environment similar to
deep space.
If
all goes well, space probes venturing across the solar system
may soon be using the exact same chips found in your desktop
PC -- just without the glitches.