Space is the worst.
It’s got hostile radiation, a total lack of atmosphere, near absolute zero temperatures,
problematic gravity wells, and worse.
In order to keep your spacecraft alive in that environment, you need electricity to
keep it warm.
Not to mention all the power to run scientific instruments and the transmitters to send that
data home.
Getting enough power in space is a big problem.
There are three ways of generating power in space: solar panels, radioisotope thermoelectric
generators, and nuclear fission reactors.
Each have their pros and cons.
Let’s start with solar panels.
This technology has been around for decades, and works by using photons from the Sun to
knock electrons free from atoms.
These electrons are harvested and provide electricity for a spacecraft to operate.
Solar panels need to be big to supply usable energy to a spacecraft.
The International Space Station’s eight solar arrays contain thousands of solar cells
and take up an area half the size of a football field.
Its arrays can produce up to 120,000 watts of energy, enough to power 40 homes.
Solar panels designed to fly to space are much more expensive than ground-based panels.
Because weight and volume on the spacecraft are at a premium, they use high-efficiency
cells.
Space agencies have pioneered many of the technologies used in solar panels.
The European Space Agency recently announced that they enabled a very thin solar cell,
just 0.1 millimeters thick, that provides 30 percent efficiency by sandwiching together
4 different layers of materials and can absorb wider wavelengths of sunlight.
The most efficient panels on the commercial market only convert 22.5 percent and most
are in the 15-17 percent range.
NASA’s Opportunity Rover, set down on the surface of Mars back in January, 2004.
It’s now spent more than 5,000 days exploring the Red Planet, searching for evidence of
past water.
At night, Opportunity experiences temperatures that go as low as minus 105 Celsius.
No matter what happens, Opportunity always needs to keep its batteries above -20 Celsius
when they’re supplying power to the rover, and 0 Celsius when they’re recharging.
In order to keep its electronics warm, Opportunity has 8 tiny pellets of decaying plutonium as
well as electrical heaters.
If Opportunity can’t get at least enough electricity to keep its batteries warm, it
can’t recharge, and it’ll die.
In the best conditions, its solar panels generate about 140 watts during the Martian day.
It needs about 100 watts if it needs to drive anywhere.
Dust falling on the solar panels was expected to reduce its power to below what it needed
to to keep operating within a couple of years, but surprisingly, dust devils have cleaned
off the panels and allowed it to keep going.
Eventually its batteries will run down, it won’t be able to keep itself warm enough
on a bitterly cold Martian night, and the rover will be lost for good.
Solar panels like this are equipped on many spacecraft throughout the inner Solar System,
where enough radiation from the Sun can be captured and used to power instruments, heaters,
and even ion engines.
While the solar energy at Earth is about 1300 watts per square meter, the intensity drops
to 1/25th, or 50 watts per square meter by the time you get to Jupiter.
This is all thanks to the inverse square law, where the intensity at any distance is equal
to the inverse square of that distance.
This is why NASA’s Juno spacecraft, is such a feat of power engineering.
The spacecraft is equipped with three solar panel arrays 9-meters long, covered with 18,698
separate solar cells.
If Juno was at Earth, it would be able to generate 14000 watts of electricity, but out
at Jupiter, it can only generate 500 watts.
This has given Juno the electricity it needs to run all its science experiments, no to
mention capturing these amazing images of Jupiter when it orbits the planet every 2
weeks.
But Juno is the first spacecraft to ever get this far from the Sun using solar panels.
The previous 8 spacecraft that ever got this far out have used radioisotope thermoelectric
generators.
Let’s go out the edge of the Solar System, to NASA’s twin Voyager spacecraft.
In order to just communicate with these probes from Earth, signals need to travel for almost
20 hours moving at the speed of light.
The Voyagers have the same problem as Opportunity, they need to keep themselves warm, but they’re
much farther from the Sun, and they need to have enough power to run their science instruments
and transmit their discoveries back to Earth.
Each Voyager is equipped with three radioisotope thermoelectric generators (or RTGs), which
are really just a chunk of Plutonium-238 which is slowly decaying.
This decay releases alpha particles, which bombard the surface of their container, heating
it up.
And this heat energy is converted into electricity.
At the beginning of the mission, the three RTGs supplied each Voyager with 470 watts
of electrical power.
The RTGs on board the Voyagers have been running for over 40 years now, but the amount of usable
heat is steadily decreasing.
It’s expected that they’ll get so low within the next decade that the spacecraft
won’t be able to power up their transmitters any longer.
RTGs provide ample power deep out in space, but they come with their own set of problems.
The first is the fact that they require a dangerous element like plutonium, strontium
or polonium.
They’re highly radioactive, and dangerous if they get released into the environment.
In fact, this was one of the reasons Cassini was crashed into Saturn, to minimize the chances
that its RTGs could harm life on one of its icy moons (not to mention infecting it with
its Earth bacteria).
The other problem is that that RTGs release radioactive particles that can interfere with
the spacecraft’s electronics, adding unnecessary noise to its data gathering.
For this reason, the RTGs are usually mounted on a boom far away from the spacecraft and
its instruments.
The Voyagers used a type of generator called the multihundred-watt radioisotope thermoelectric
generator.
Newer spacecraft like Galileo, Cassini and New Horizons used a modified design called
the General Purpose Heat Source, which could generate about 300 watts of power, using 7.8
kg of Plutonium-238.
Space missions built after 2010, like NASA’s Curiosity, use multi-mission RTGs.
Their heat source is plutonium-238 dioxide, generating 125 watts at the beginning of the
mission and falling to 100 watts after 14 years.
After the end of the Cold War, the US stopped producing plutonium-238, and ironically had
to start buying the material from Russia.
And then Russia stopped making it too.
With a dwindling supply, the US Department of Energy actually banned NASA from including
thermal RTGs in many missions.
This limited the amount of power they could produce, and pretty much ended science for
the outer Solar System.
Earlier this year, though, NASA announced that the ban was over, and upcoming mission
proposals for 2018 and beyond could include up to two multi-mission RTGs on a spacecraft.
Solar panels and decaying radioactive materials.
Is there anything else out there to supply power to a spacecraft?
There’s one more, nuclear fission reactors, and we’ll get to them in a second, but first
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We’ve talked about solar panels and nuclear batteries, but did you know there are more
than 30 spacecraft were launched with nuclear fission reactors?
One from the US, and the rest from the Soviet Union.
Space-based nuclear fission reactors are similar to the kinds of reactors used down here on
the surface to supply electricity.
They use uranium-235 as a fuel for a fission reaction, where the nucleus is split, releasing
energy.
A kilogram of uranium can provide as much energy as 3 million kilograms of burning coal.
The United States launched their SNAP-10A spacecraft in 1965, and it operated for 43
days before it stopped functioning.
It’s now in a slowly deteriorating orbit that’ll take another 3,000 years or so before
it crashes back to Earth.
Don’t worry, its nuclear material should be largely decayed by then.
During the space race, the Soviets equipped 31 of their RORSAT reconnaissance satellites
with BES-5 fission reactors.
These could generate 3,000 watts of usable electricity.
They also equipped two TOPAZ satellites that could produce 5,000 watts of electricity.
The Soviets had a few mishaps, though.
In 1977, they launched their Kosmos 954 spy satellite which had a BES-5 fission reactor
with 50 kg of uranium-235 on board.
The spacecraft suffered a series of malfunctions and they lost control of it.
Soviet officials assured the world that the spacecraft would burn up in the atmosphere
or hold together when it crashed into the ground.
Because it had been launched into an extreme 65-degree orbital inclination, it had the
potential to crash into almost any population center across the world.
When it came down, in 1977, the reactor came apart almost immediately as it struck the
atmosphere, coating itself in radioactive material.
Then it broke up into large chunks which fell to Earth over a 600-kilometer track in Canada’s
Northwest Territories.
Canada was only able to find, clean up and dispose of about 1% of the radioactive spacecraft
chunks, the rest is still out there.
Fortunately none of the other fission reactors are expected to return to Earth anytime soon.
Like SNAP-10A, they’ll take hundreds or thousands of years to re-enter the atmosphere.
Considering the dangers, no space agencies have launched spacecraft with fission reactors
in over 30 years, but it looks like the technology might return again soon.
NASA recently announced that they’re working on a new space-based fission reactor technology
called Kilopower.
They held a press conference in May, 2018 announcing that they’d completed their ground
tests of a new kind of fission reactor that could supply 1000 watts of electrical energy,
and up to 10000 watts for installations on the Moon, Mars or even for use in spacecraft.
The reactor consists of an enriched uranium core that’s undergoing fission decay.
Heat pipes extend out from the reactor and connect to Sterling engines which convert
the heat into electrical energy.
The whole system is self-regulating.
If the reactor overheats, the engines can draw off more power to cool it back down.
If it’s too cool, the core contracts, increasing the rate of fission again.
With the ground tests complete this year, NASA’s next step is to test them in space.
If all goes well, future Moon or Mars explorers will have all the power they’ll need to
survive on other worlds, run their science instruments, and transmit the results back
home.
NASA also thinks they could install these kilopower reactors onto spacecraft that use
ion engines, provide the electricity they would need for extended missions.
Solar panels, RTGs and fission reactors are the three methods that space agencies have
used to power their spacecraft.
And who knows, as new technologies are perfect down here on Earth like fusion reactors, maybe
someday we’ll see those implemented in space too.
Until then, getting anything done, especially anywhere far from the Sun is going to be incredibly
difficult and slow.
Like I said, space is the worst.
Have you got any ideas for space-based power systems?
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