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 I’d like to thank: Minh Le
Curious Borg Vladislav Kravtsov
Sushant Arora William Inabnett
Frank Walker Elad Avron And the rest of our 812 patrons for their
generous support. If you love what we’re doing and want to
get in on the action, head over to patreon.com/universetoday. 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? Let me know your thoughts in the comments. Once a week I gather up all my space news
into a single email newsletter and send it out. It’s got pictures, brief highlights about
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was aware that there are RTGs used in space but wasn't aware there were fission generators too.