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- [Narrator] Some of the most daring
and exciting NASA missions exploring our solar system
have become a reality
thanks to a discovery made 200 years ago.
In 1821, a German scientist
discovered an unexpected property of physics,
metals and some compounds
are good conductors of electricity as well as heat.
The scientist, Thomas Seebeck,
found that when he combined two of these materials
and then applied heat to one end
while exposing the other end to cold,
a small electrical voltage was created across the materials.
It turns out that electrons in the materials
flow from the heat source, or the hot side,
to the colder side.
We now call this process of producing electricity
the Seebeck Effect.
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Today we use the term thermocouple
to describe the hundreds of small pairs of junctions
made from different conducting materials
that take advantage of this property.
We now know that the greater the difference in temperature
between the hot side and cold side,
the greater amount of power that can be generated.
Inside a Radioisotope Thermoelectric Generator, or RTG,
these thermocouples are strung together in a long series
so they can generate a steady current of electricity
that can provide robotic spacecraft
with the power they need to operate.
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Some of the specific materials used in RTGs
are selected because they can conduct electricity
as well as maintain a hot and a cold side.
Seebeck used copper and bismuth in his experiments.
Today, we use lead telluride and silicon germanium.
These materials are part of a family of compounds
known as semiconductors.
In the RTGs built by the U.S. Department of Energy
and used by NASA,
the hot side is provided by the heat energy
emitted from the natural radioactive decay
of plutonium dioxide.
The hot side of the thermocouples
can be more than 1,000 degrees Fahrenheit,
or 500 degrees Celsius.
The cold side faces out into space
or into the atmosphere of a planet or moon,
where the temperature can be well below zero.
In addition, since no moving parts are involved,
the thermoelectric effect is extremely reliable.
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Over the years,
the thousands of thermocouples in NASA's nuclear batteries
have performed without any noticeable failures
in all of the two dozen missions in which they've been used.
For example, NASA's two Voyager space probes,
powered by RTGs, have been carrying on steadily
since their launch back in 1977!
By using new, more Seebeck-friendly materials,
the RTGs in development by NASA's RPS Program
and its partners in industry
could be twice as efficient than those in use today.
The invisible process of energy transformation
from heat to electricity,
discovered by Seebeck two centuries ago,
continues to enable many
of the most amazing NASA missions and discoveries,
providing robotic spacecraft with the dependable power
they need to explore some of the harshest, darkest,
and dustiest places in the solar system.
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