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Comment by blakecaldwell | Hacker News Reader

It kills me we're not gonna orbit Pluto.

In the press conference, the PI was asked about a future mission. He said that he has thought about orbiters and landers and considers the Pluto system to be a valuable science target. But he said that we need to study the New Horizons data for some years, to better design the next mission. At this time, they wouldn't know what the best instruments to send would be. Also, it is far from clear how to send a mission to Pluto that takes a reasonable amount of time to get there (9 years, in the case of New Horizons) but can also stop when it gets there. Cassini took almost seven years to reach Saturn and the orbital insertion burn was something like ninety minutes! I'm not sure how fast it was going at the time, compared to New Horizons at Pluto, but the point is, if you want to get there quickly, you go fast, but then it's hard to stop.

> At this time, they wouldn't know what the best instruments to send would be.

There's an interesting BBC documentary series on the planets, called The Planets, that has an episode covering the Voyager probes. They had to do some quick redesigns on them after Pioneer 10 and 11 visited the outer planets and found fun stuff like radiation and unexpected rings. They said that if they hadn't had the earlier probes, Voyager would have been fried when it got to the system.

I second that recommendation, and it's currently on Netflix. It's a good documentary series. A little long in the tooth now, but only for the entirely positive reason that we've done a fair bit more planetary science since it was made.

There is a mission study which says it's feasible to orbit Pluto. http://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Plut... . It requires 4 RTGs, nuclear electric propulsion, and 18 years of travel time.

You might be dead by the time that actually happens, so better to be killed by sorrow of only getting fly-by data than the agony of having only what Hubble and its successors can resolve.

It can't actually happen. NASA only has enough Pu-238 for three more RTGs.

They can use Americium-241, it's not quite as good, but it has some advantages, plus it's in much larger supply.

One can just imagine the complaints about the name of that fuel...

It seems like just the ticket to get a certain contingent on board. "The noble probe will remain stout and true; in the freezing depths of space it will be warmed by the Americium it carries in its heart."

Why can't they get more?

I might be mistaken, (and someone please correct me if I'm wrong) but IIRC Plutonium is a side-product of the enrichment of weapons-grade Uranium. I think the non-proliferation of nuclear weapons treaty might impose some barriers on this process, and we're no longer in the Cold War, so it's kinda hard to get Plutonium.

Uranium enrichment is actually a physical rather than nuclear process, and thus it cannot produce other elements such as Plutonium. However once you have enriched enough uranium to create an operating reactor, its spent fuel will contain a few percent Plutonium. The problem is that only a few percent of that will be Pu-238, and separating that from the other four Pu isotopes is incredibly difficult, that is to say, economically intractable.

Therefore, the Pu-238 used in RTGs is likely produced through another process, neutron capture in Np-237. Np-237 is produced in very small amounts in a uranium reactor, but as it's the only Neptunium isotope produced in any significant quantity, there's no need to do an isotope separation on it. It can be chemically separated from the other elements in the spent fuel, which is much easier, although certainly still not easy.

In any case, with the end of the Cold War there are indeed fewer facilities willing & able to produce Pu-238 in kilogram quantities. In the US, spent fuel from commercial reactors currently cannot be re-processed at all, due to proliferation concerns.

Oh. Thanks for the detailed explanation and correction about the enrichment process. Do you think that once we have asteroid mining we'll be able to get Uranium (and consequently Plutonium) in higher quantities so that the economic costs will be viable again? That is assuming we can overcome the legal/security concerns by then.

That's a good question - AFAIK nobody has any real idea how much Uranium is present within nearby asteroids. Hopefully they will create some more RTGs & send some probes to find out!

It actually makes me hopeful that New Horizons will have a chance to look further on things in the Kuiper Belt. It's a very interesting area that doesn't get much attention.

I was upset about not orbiting Pluto initially but the objects they've identified in the Kuiper Belt are dissimilar so that information will be just as valuable IMO.

We just need to start setting up bases!

ALL YOUR PLUTO BELONG TO US!!!

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