Astronomy Without A Telescope – Solar Or RTG?


It used to be the case that if you wanted to send a spacecraft mission out past the asteroid belt, you’d need a chunk of plutonium-238 to generate electric power – like for Pioneers 10 and 11, Voyagers 1 and 2, Galileo, Cassini, even Ulysses which just did a big loop out and back to get a new angle on the Sun – and now New Horizons on its way to Pluto.

But in 2011, the Juno mission to Jupiter is scheduled for launch – the first outer planet exploration mission to be powered by solar panels. And also scheduled for 2011, in another break with tradition – Curiosity, the Mars Science Laboratory will be the first Mars rover to be powered by a plutonium-238 radioisotope thermoelectric generator – or RTG.

I mean OK, the Viking landers had RTGs, but they weren’t rovers. And the rovers (including Sojourner) had radioisotope heaters, but they weren’t RTGs.

So, solar or RTG – what’s best? Some commentators have suggested that NASA’s decision to power Juno with solar is a pragmatic one – seeking to conserve a dwindling supply of RTGs – which have a bit of a PR problem due to the plutonium.

However, if it works, why not push the limits of solar? Although some of our longest functioning probes (like the 33 year old Voyagers) are RTG powered, their long-term survival is largely a result of them operating far away from the harsh radiation of the inner solar system – where things are more likely to break down before they run out of power. That said, since Juno will lead a perilous life flying close to Jupiter’s own substantial radiation, longevity may not be a key feature of its mission.

Perhaps RTG power has more utility. It should enable Curiosity to go on roving throughout the Martian winter – and perhaps manage a range of analytical, processing and data transmission tasks at night, unlike the previous rovers.

With respect to power output, Juno’s solar panels would allegedly produce a whopping 18 kilowatts in Earth orbit, but will only manage 400 watts in Jupiter orbit. If correct, this is still on par with the output of a standard RTG unit – although a large spacecraft like Cassini can stack several RTG units together to generate up to 1 kilowatt.

So, some pros and cons there. Nonetheless, there is a point – which we might position beyond Jupiter’s orbit now – where solar power just isn’t going to cut it and RTGs still look like the only option.

Left image: a plutonium-238 ceramic pellet glowing red hot, like most concentrated ceramicised radioisotopes will do. Credit: Los Alamos National Laboratory. Right image: the Apollo 14 ALSEP RTG, almost identical to Apollo 13's RTG which re-entered Earth's atmosphere with the demise of the Aquarius lunar module. Credit: NASA

RTGs take advantage of the heat generated by a chunk of radioactive material (generally plutonium 238 in a ceramic form), surrounding it with thermocouples which use the thermal gradient between the heat source and the cooler outer surface of the RTG unit to generate current.

In response to any OMG it’s radioactive concerns, remember that RTGs travelled with the Apollo 12-17 crews to power their lunar surface experiment packages – including the one on Apollo 13 – which was returned unused to Earth with the lunar module Aquarius – the crew’s life boat until just before re-entry. Allegedly, NASA tested the waters where the remains of Aquarius ended up and found no trace of plutonium contamination – much as expected. It’s unlikely that its heat tested container was damaged on re-entry and its integrity was guaranteed for ten plutonium-238 half-lives, that is 900 years.

In any case, the most dangerous thing you can do with plutonium is to concentrate it. In the unlikely event that an RTG disintegrates on Earth re-entry and its plutonium is somehow dispersed across the planet – well, good. The bigger worry would be that it somehow stays together as a pellet and plonks into your beer without you noticing. Cheers.

NASA May Have to Revamp Science Plans Without RTGs

As if things weren’t tight enough at NASA, now the US House and Senate have decided to cut the funding to restart production of plutonium-238 (Pu-238), the power source for many of NASA’s robotic spacecraft. Under the Atomic Energy Act of 1954, only the US Department of Energy is allowed to possess, use and produce nuclear materials and facilities, and so NASA must rely on the DOE to produce these power sources and the fuel. A report by the National Research Council says “the day of reckoning has arrived” and that NASA has already been forced to limit deep space missions due to the short supply of Pu-238.

Pu-238 is needed for radioisotope thermoelectric generators (RTGs) that supply power for systems and instruments on spacecraft travel too far from the Sun to rely on solar energy or land on surfaces with long “nights.” For example, the Voyager spacecraft utilize RTGs and are still able to communicate and return science data after over 30 years of operation, and now are at the outer edges of our solar system.

Pellet of Pu-238.  RTGs are constructed using marshmallow-sized pellets of Pu-238. As it decays, interactions between the alpha particles and the shielding material produce heat that can be converted into electricity.
Pellet of Pu-238. RTGs are constructed using marshmallow-sized pellets of Pu-238. As it decays, interactions between the alpha particles and the shielding material produce heat that can be converted into electricity.

Pu-238 is expensive to produce, but it gives off low-penetration alpha radiation, which is much easier to shield against than the radiation produced by other isotopes.

Pu-238 does not occur in naturally, and the United States has not produced any since the late 1980s. It purchased Pu-238 for NASA missions from Russia during the 1990s, but those supplies reportedly are now exhausted. The NRC based its estimate of NASA’s Pu-238 requirements on a letter NASA sent to DOE on April 29, 2008 detailing space science and lunar exploration missions planned for the next 20 years.

The cost of restarting production appears to be the major reason for the cut, as estimates are it would cost at least $150 million.

The DOE requested $30 million in FY2010 to restart production, but the House cut that to $10 million when it passed the FY2010 Energy and Water appropriations bill (H.R. 3183) on July 17. The Senate went even further (S. 1436), completely cutting funds for restarting production of Pu-238.

Both the House and Senate Appropriations Committees complained that DOE had not explained how it would use the funds.

But if funds aren’t made available soon, NASA may have to revamp its plans significantly for the New Frontiers missions, lunar rovers, and other deep space missions. There are other isotopes that have been used in the past, such as strontium-90, but Pu-238 has been found to work the best. NASA has also solicited ideas for alternative power sources, as well.

Source: Space Policy Online