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.
