atw International Journal for Nuclear Power | 04.2020

atw Vol. 65 (2020) | Issue 4 ı April
Space: Final Frontier – Always Nuclear
Dear reader, In these days, when our society is confronted globally with the challenges and the management of the
Corona pandemic, the focus of this editorial should not be on controversial issues concerning the earth, but rather look
beyond into the vastness of space.
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If mankind embarks on a journey into space or sends
satellites on their way into the cosmos, the question of a
suitable energy supply arises – the effort to navigate and
leave the Earth's gravitational field is left out of the equation
– which makes a space mission possible. If astronauts are
involved, sufficient heat, cooling and breathing air must be
provided, among other things. If they are unmanned
satellites, the only essential thing is to provide energy for the
technical, i.e. electrical, systems.
There are basically three forms of nuclear energy
available today for space missions:
The first is indirect use. The nuclear fusion reactor, the
sun, provides the radiation energy – light, which is converted
into electrical energy in photovoltaic cells. The basic
principles of photovoltaics have been known for a very long
time. The basis, the photoelectric effect, was discovered in
1839. However, the technical breakthrough came more than
100 years later, when the U.S. satellite Vanguard I was
equipped with photovoltaic cells in addition to a fuel cell in
1958. Among other things, these made it possible to operate
the satellite for almost six years.
The second form is the installation of a nuclear fission
reactor:
The first nuclear reactor for energy supply was launched
into orbit on 4 April, 1965 by the U.S. Air Force with an Atlas
launcher and the Snapshot technology satellite. The aim of
the mission was, on the one hand, to test a nuclear reactor in
a satellite and, on the other, to test the function of an ion
engine. The reactor was derived from the SNAP – System
for Auxiliary Power Program of the U.S. Atomic Energy
Commission. The actual reactor core had a weight of 290 kg,
a volume of about 16 l and gave off a thermal output of
about 30 kW during operation. The chain reaction was
controlled by four externally arranged, semi-cylindrical
neutron reflectors made of beryllium. The heat was removed
from the reactor by an alloy of sodium-potassium (NaK)
and converted into electric current in thermocouples
with a maximum output power of 0.5 kilowatts (kW). The
temperature difference between the NaK coolant and the
surrounding space was the driving force. Due to a malfunction
in the satellite electronics, the mission was aborted after
43 days and the reactor was shut down. While for the USA
the energy supply with nuclear reactors in space did not play
a role in later years, the Soviet Union had launched almost
40 Radar Ocean Reconnaissance Satellites into orbit, which
were equipped with uranium reactors of the designation
BES-5 and Topas. The military satellites, also known as
Cosmos, were able to monitor ship movements with active
radar from low orbit and therefore had to be supplied with a
powerful energy source, i.e. a nuclear reactor, due to their
high energy requirements.
In early March 2020, there was a surprising success story
from space, from Mars. The Rover Curiosity, which has been
active on our neighbouring planet for more than seven
years, provided the largest panorama to date with a
resolution of 1.8 gigapixels – current digital cameras provide
single images with a range of around 25 megapixels. The
image was assembled from more than 1000 single images
and shows the surroundings on the slope of the mountain
Aeolis Mons. The images were taken between November 24
and December 1, 2019, when no further experiments or
activities were scheduled for Curiosity. Curiosity had landed
on Mars in 2012. The search for traces of earlier life and
basic environmental conditions on Mars are among the
mission's objectives. The energy supply for the rover is
provided by radionuclide batteries which, unlike in previous
missions, are independent of weather conditions and also
ensure constant, stable thermal conditions for the rover's
systems. Radionuclide batteries are the third option for
energy supply in space.
The Curiosity mission is based on initial considerations in
2003 and a National Academy document entitled “New
Frontiers in the Solar System: An Integrated Exploration
Strategy”. The mission was launched on 26 November 2011
on board an Atlas V rocket. Nine months later, the rover
landed on Mars in August 2012 and started its experiments.
Another important aspect of the rover is its mobility. This
means that it is not tied to its landing point for carrying out
experiments, but can travel to points that seem particularly
suitable for investigations. By the beginning of 2020,
Curiosity was thus able to cover a distance of around 22 km
and to convince with impressive photos of the surface of
Mars in particular. The Curiosity mission is also a
technological success. The mission was originally planned
for two years, but was extended until today – seven and a
half years on Mars – due to the reliability of the rover's
technology, including the energy supply, and the scientific
results.
Radionuclide batteries, or RTGs (radioisotope thermoelectric
generators) for short, are a very reliable and compact
option for energy supply. The basis is the conversion of
thermal energy from the decay of radio active isotopes in a
thermoelectric element into electrical energy. Since the
half-life period can be used to adjust the temporal availability
of the heat source and RTGs do not need any moving
parts, they are very reliable. Although the mass-to-power
ratio is worse than that of nuclear reactors, their simple
design is an advantage for missions without the possibility of
on-site maintenance.
The first known RTGs for space missions were tested
under the NASA SNAP program in 1958. In 1961, SNAP-3
was the first application in space. NASA has documented
27 missions with RTGs, including one in the Apollo program,
which used nuclear energy to power a measuring instrument
on the moon. Furthermore the use in space missions
of ESA, China and Russia or the former Soviet Union is
known but not documented in detail.
Two other missions, Voyager 1 and 2 probes, have also
brought terrestrial nuclear energy technology outside the
solar system. In 2012 and 2018, respectively, the spacecraft
launched in 1977 left our closer space environment. These
missions were originally planned for four years; the power
supply was designed from the outset on the basis of RTGs, as
the power of photovoltaic cells beyond the orbit of Mars is
not sufficient and, in addition, they would degrade too
quickly in the radiation belt that would then follow.
It remains interesting with nuclear energy in space – let's
observe this with common sense from Earth and stay health,
Yours
Christopher
Weßelmann
– Editor in Chief –
EDITORIAL
Editorial
Space: Final Frontier – Always Nuclear