Using Cassini to Test Radioactive Decay Rate Variation

Artist impression of Cassini orbiting Saturn (NASA)

In a previous Astroengine article, I explored the possibility that the variation in radioactive decay rates may be synchronised with Earth’s orbital variations in distance from the Sun. Naturally, this would be a huge discovery, possibly questioning the fundamental law that nuclear decay rates are constant, no matter where the material is in the Universe. One of the conclusions in the original decay rate research suggested that we should attach a sample of a radioisotope onto an interplanetary mission far beyond the orbit of Earth. By doing this, the relationship between decay rates and distance from the Sun should become obvious, and terrestrial decay rate variations can be tested.

But wait a minute, let’s have a think about this. Haven’t we already sent radioactive material on board interplanetary missions? What about all that plutonium we use to power interplanetary probes like Voyager, Pioneer, Galileo or Cassini? Plutonium is pretty radioactive… isn’t it?

The decay rate of the radioactive isotope 32Si appears to corrolate with orbital distance from the Sun (Jenkins et al. 2008)

The paper entitled “Evidence for Correlations Between Nuclear Decay Rates and Earth-Sun Distance” by Jenkins et al. (2008) studied the link between nuclear decay rates of several independent silicon and radium isotopes. Decay data was accumulated over many years and a strange pattern emerged; radioactive decay rates fluctuated with the annual variation of Earth’s distance from the Sun (throughout Earth’s 365 day orbit, our planet fluctuates approximately 0.98 AU to 1.02 AU from the Sun).

For me, the biggest finding in this study was not that one sample’s decay rate fluctuated in concert with Earth’s orbit, it was that several independent samples of different types of radioisotope, generating both α and β radiation were correlated. Truly a ground-breaking discovery. The only thing needed was another test to see if this decay rate relationship extended beyond terrestrial samples. A follow-up to this study was suggested by Jenkins et al:

“Measurements on radioactive samples carried aboard spacecraft to other planets [which] would be very useful since the sample-Sun distance would vary over a much wider range.” – Jenkins et al (2008)

Well, wouldn’t it be nice if we could put a sample of radioactive material on a spaceship, send it away from the Sun, to see how the heliocentric distance affects the decay rate of the sample…? Are you thinking what I’m thinking?

One of Cassini's Radioisotope Thermoelectric Generators. An unlikely physics laboratory (NASA)

Enter the Radioisotope Thermoelectric Generator (RTG), used to power all space probes beyond the orbit of Mars… Flying through interplanetary space since 1961!

Indeed, space missions already use radioactive materials to provide energy to keep our intrepid robotic explorers alive when the Sun’s energy becomes too weak for solar panels to be practical. Is there some way we can gather decay rate data from long-term interplanetary missions such as Cassini?

Peter Cooper from the Fermi National Accelerator Laboratory in Batavia, Illinois, has just published a paper investigating the decay rates of the plutonium-238 RTG fuel. Cassini carries three RTGs (pictured), each weighing 7.7 kg. ²³⁸Pu is an α emitter with a half life of 87.7 years, an ideal radioisotope for long-term space missions.

Out of interest, another long-distance interplanetary mission, the Pluto New Horizons mission, will arrive at the dwarf planet and explore the Kuiper Belt from 2015 – that’s after nearly a decade of space travel. New Horizons also carries an RTG, currently delivering 300W of power to the craft; by the time it reaches Pluto it will deliver 200W after the gradual decay of the ²³⁸Pu pellets inside the RTG. Interestingly, New Horizons uses one of Cassini’s spare RTGs for all its power needs.

Cooper acquired Cassini RTG power output data from scientists at the Jet Propulsion Laboratory in California and plotted it along side Cassini’s distance from the Sun over a two year period. At launch, Cassini’s three RTGs generated approximately 13kW of heat, converting into 878W of electrical power. Two years later, the spacecraft was receiving around 815W of electrical power. This reduction in power output was due to the radioactive decay of the ²³⁸Pu isotope. Therefore, power output is related to the decay rate of the material inside the RTGs. Should there be any relationship between decay rates and heliocentric distance, some variation in the power decay rate should be evident during Cassini’s travels.

The decay of Cassini's RTG power over two years from launch. Blue diamonds: RTG power. Red line: Heliocentric distance (Cooper, 2008)

From launch, Cassini completed two gravitational assists around Venus and one around Earth before it journeyed to Jupiter on its way to Saturn. Cooper therefore analysed the variation in RTG power output over a heliocentric distance range of 0.7AU (at Venus) to 1.7AU (at Mars), looking out for any variation (see plot left).

So is there any relationship between plutonium decay rate and heliocentric distance?

In short: No.

It would appear from 0.6 AU to 1.7 AU, there is no variation in power output (and therefore decay rate) of plutonium with distance from the Sun.

How can the terrestrial decay rate variation (as reported by Jenkins et al.) be explained if the radioactive sample on board Cassini experienced no such variation (as reported by Cooper)? Perhaps it isn’t the distance from the Sun that influences decay rates. Could environmental variations (such as seasonal changes in air pressure/temperature) be to blame? Or could it be the orbital location (and not the heliocentric distance) that influences terrestrial changes in decay rates? For now, it seems, the jury is out…

Publication source: arXiv:0809.4248v1 [astro-ph]