Mars the Record of Nuclear Power

Tonight — 10:31 Pacific Time Sunday, or 1:31 AM Eastern on Monday — the Curiosity rover will hopefully touch down safely on the surface of Mars.  The events actually take place about 14 minutes in advance, but we cannot know the results until the radio communications get from Mars to Earth. The vehicle is big. While the two famous rovers Opportunity and Spirit were roughly grocery-cart sized, this one is more like an automobile. It has tremendously greater science capability — and it is too big, and needs too much power, to operate from solar panels.  So it does not: Curiosity is nuclear-powered. This has been controversial. Almost US 30 space missions over the past half-century have carried aloft a nuclear electricity-generating mechanism, mostly using radioisotope thermal generators or RTGs. These devices used a relatively hot, short-half-life isotope of plutonium to not only provide electricity, but also to keep the spacecraft warm — a factor especially important when heading out away from the Sun.  (The Soviets/Russians have launched many times the nuclear power the US has — I understand that the ratio of missions is on the order of ten-to-one.)

Risks and Protests

But these nuclear power sources are controversial — there are protests at the launch, and many complaints about the possible harm. The complaints were large indeed when a 2.5 kilogram mass of plutonium burned up in the atmosphere —  and Curiosity has twice as much.

Cassini-Huygens, on a mission to Saturn, had more than 70 kilograms of plutonium 238 on board. This paper reviews the risks of that power source on the Cassini mission, which were much-talked-about at the time because Cassini initially went inward to Venus and then shot past the Earth to gain speed to get to the outer Solar System. Shooting past Earth at high speed with a nuclear payload concerned a number of folks, and represented a doomsday scenario for a few.

Many folks are still concerned about the use of plutonium, though the containers are so well-constructed that if a rocket explodes during launch or on ascent, the RTG capsule can be recovered intact and used again. (That’s actually happened.)

But there is a political component to this. Since the US shut down its ability to manufacture this plutonium isotope in 1988, we’ve been buying it from the Russians — and in recent years, they’ve stopped selling it to us. For this reason, and pushed by NASA’s super-green anti-nuclear leaders, NASA has tried to work their way out of using RTGs for the next big-planets missions. That comes as a cost — an internal NASA PowerPoint presentation shows that switching from radioisotope thermal generators to large solar power sails will cost a huge amount of payload capacity — about 1,000 pounds of science instruments and their support.

JUNO, outer planets, and technical challenges

The JUNO mission, en route to Jupiter now, is an example of the tradeoffs involved. It has 650 square feet of solar arrays, and no RTG. By the time it reaches Jupiter in four years, it will be generating only about 475 watts, and Jupiter’s harsh radiation environment is expected to degrade this to 420 watts in months. But, as this craft is also doing an Earth flyby, the solar power was forced onto the mission. The articles describe this as “NASA going green” and even note the “windmill shape” of the spacecraft.

There are big technical challenges using only solar for outer planets, as solar power at Saturn is at only two percent of the intensity as it is at Earth’s orbit, so solar sails must be comparatively gigantic to deliver tiny amounts of power.  In that PPT, they note that even a much larger solar sail array capable of generating 48,000 watts near Earth drops to 337 watts near Saturn even if they solve the LILT problems discussed in the document. For planets further out, the problem is much worse, and in all these cases the spacecraft must have better insulation as it does not have an RTG system to keep warm. This increases the stress on the electronics and other gear.

Budget restraints

So, barring political protests, nuclear RTGs still seem the way to go.  But — in order to make more, we need budget dollars to restart the controversial manufacturing process, and as this article points out, that’s not happening so far.

The potential failure of this mission, then, holds many potential threats for the future of space exploration — not just undermining NASA’s ability to mount such missions, but also killing budget dollars in several ways including the ability to power deep-space exploration.

For all these reasons, from the pure science to the future of man in space, I wish the NASA Curiosity team the best.  If you haven’t seen the video yet, an un-narrated one is here, with an overview in this article and details on NASA’s site here. The William Shatner narration, which I’ve not listened to, is here.  (Edit: I’ve just seen it. Not too bad.)

Also, live coverage is already happening on this uStream video feed.

===|==============/ Keith DeHavelle