Thursday, November 20, 2014

The wrong kind of Bang

In science education and popularization, there is a delicate balance that must be struck between overcomplicating and oversimplifying. Insufficient simplification can result in overly obtuse deviation into secondary details, which confuse and distract the readers and derail the flow of the prose.  Excess simplification can be wrong.  And this is where the Medium article by Ethan Siegel of “Starts with a Bang” fame has ended up. 

Dr. Siegel argues that the recent Philae comet lander would have more successful if it had been powered with a 238Pu RTG device instead of solar panels.  However, his simplified argument ignores the reality of 238Pu fuel production, the definition of “we”, and the nature of comets.

238Pu is a byproduct of the nuclear arms race between the USA and the USSR. It is created by neutron activation of 237Np, which in turn is a byproduct of 239Pu production for nuclear weapons. With the nuclear arms deals of the 1980’s the superpowers stopped building nuclear weapons by the tens of thousands, and the cheap source of 237Np disappeared.  The USA stopped 238Pu production in 1988, all subsequent material has come from Russia, which has almost depleted its stockpiles.

This brings us to the definition of “we”.  As the battleground over which the USA and the USSR fought, Europe never developed its own mass nuclear warhead production facilities; the UK and French arms supplies are only a tiny fraction of the size of the 20th century superpowers.  As a result, Europe has never had its own large scale 238Pu production facilities. 

Philae was a European mission, not a USA or Russian one, so the ESA (European Space Agency) did not have access to 238Pu needed for RTG production.  NASA (USA) and the ESA (Europe) are separate space exploration entities, a point that was very unclear from this article’s frequent discussion of NASA and Philae.

Finally, RTG’s are hot, and comets are cold. The Philae lander was a very risky mission- there was a significant chance that it would not succeed at all, and in the end the lander ended up bounding off an unexpectedly hard surface several times before ending up on its side in a crater.

Comets, by definition, evaporate at low temperatures- this one is jetting out gasses despite being way out beyond the asteroid belt. So landing a heat-producing source on it, especially on a lander that ended up tipping over, would end up in a situation where the lander could drastically alter the local environment of the comet through thermal contact.  The whole point of the mission is to sample a comet in as pristine condition as possible, so potentially cooking the comet due to a landing mishap is not really a sensible design choice.

Dr. Siegel is correct that 238Pu is crucial for missions that operate beyond the orbit of Jupiter.  But the fuel used on previous missions was subsidized by the nuclear arms race.  It, and all the wondrous outer solar system exploration it allows, was an unintended byproduct of Mutually Assured Destruction, and the tens of thousands of nuclear weapons that policy produced.  Since the arms race ended, production of this isotope for the sole purpose of planetary exploration has been deemed too expensive to pursue by all the world’s governments.  Until we collectively decide to blow ourselves up again, this barrier to outer solar system exploration will continue.

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