To The Moon, Alice

One of the nice things about having a blog is that you can self publish. This is the original piece that I submitted to Popular Mechanics, which inspired them to ask for a "revision" which they then edited to what was actually published. I thought that readers here might appreciate it.

Location, location, location.

Those are the proverbial three rules of real estate. They aren't restricted to terrestrial transactions--location matters a lot, sometimes a lot more, in space.

Recently, Michael Benson, a guest columnist at the Washington Post, proposed that the problem with the International Space Station is that it is in the wrong place. He proposes that it be refitted as an interplanetary spaceship.

It's a novel proposal, and he's in good company--a lot of people are thinking about what to do with the ISS after 2015, for which there is currently no official US policy. The foreign partners and other stakeholders recently met to discuss the issue, though if this particular option was discussed, there is no mention of it in the reporting, or the joint statement they provided after the meeting. There's probably a good reason for that.

Mr. Benson is clearly earnest, but the concept is not as well thought out as he seems to think. The ISS is designed for operations in low earth orbit (LEO), but that is a unique environment, and had trips beyond that been its intended use, both the requirements and the design would have looked very different.

What does NASA think?

I called Mike Curie, in the NASA Public Affairs Office for the ISS, to get the official agency response. It was predictable, concise, and (in my opinion) correct: "We welcome and share Mr. Benson's enthusiasm for the space station program, but the proposal is not feasible."

He suggested that I talk to Tom Jones, four-time Shuttle astronaut (and Pop Mechanics space consultant) for further elaboration, so I did.

The idea has several problems," he told me. "If you do it with chemical propellant, the structure won't be able to take those high thrust levels, particularly the fragile solar panels that were designed for zero gee. Also, the Station isn't designed to operate for long periods of time without resupply of things like food, water, and spare parts for maintenance. You'd have to develop a duplicate interplanetary system just to deliver the supplies and rotate the crew."

"Once out in deep space, the ISS doesn't have the radiation shielding it would need for either lunar operations, or even traversing through the Van Allen belts, particularly if you did it slowly with a low-thrust system, as he suggests."

"The Station is also overdesigned for an interplanetary mission in some ways. It's a laboratory facility designed to rely on frequent resupply and contact with Earth. This is not an operational space vehicle. It's more of a technology test bed, to learn how to do things in space, and take advantage of the near-Earth space environment. It's really better and more cost effective to keep it here and use it for what it was designed."

In fairness, Mr. Benson attempted to anticipate these objections:

It's easy to predict what skeptics both inside and outside NASA will say to this idea. They'll point out that the new Constellation program is already supposed to have at least the beginnings of interplanetary ability. They'll say that the ISS needs to be resupplied too frequently for long missions. They'll worry about the amount of propellant needed to push the ISS's 1,040,000 pounds anywhere -- not to mention bringing them all back.

There are good answers to all these objections.

Well, he has answers, but they don't seem to be very good ones. One wonders if he actually ran any numbers.

How much propellant would it take? Well, to leave LEO and go almost anywhere else, you need to have escape velocity. In orbit, that means adding about forty percent to your current speed of twenty-five thousand ft/sec, or about ten thousand ft/sec. The station weighs on the order of a million pounds. Assuming that you could provide the necessary thrust without snapping off the solar arrays, using liquid oxygen/hydrogen (the most efficient practical propellant combination we have today at a generous specific impulse (I_sp) of 480 seconds (not far from theoretical), it would take almost as much propellant as the payload (over 900,000 lbs).

Now that's not necessarily a lot--it would be a couple dozen launches of, say, a Delta IV, which might cost a few billion dollars. But the problem is that all that does is get the ISS out of earth orbit. It doesn't have any way to park in orbit when it gets to the moon or Mars, or even an asteroid encounter. To do that it needs (in Mr. Benson's words) a "drive system and steerage module" (whatever that means) which he hand waves off as "technicalities."

You also need propellant. A lot of it.

That means that we not only have to accelerate the ISS itself out of LEO, but also all of the propellant that it will need at its destination as well, which would likely be many hundreds of thousands of more pounds. So we have to recalculate our escape, and now we need, say, a million pounds of propellant to send with the station to its destination, and another two million to blast the whole lot out of earth orbit. So now we're up to many billions of dollars for the propellant delivery to LEO, even ignoring the "technicalities."

Ah, you say, but he suggested using low-thrust high-I_sp ion-propulsion systems, which will require much less propellant.

So he did, but he didn't consider the radiation problem, as Tom Jones noted. You'd fry the crew and the electronics, including solar panels, in short order, even if you're lucky enough not to be hit by a solar flare in all that time.

Considering all the other factors he explained, clearly, the ISS is built for LEO, and it should stay in LEO.

But that raises another question. Is it in the right LEO?

The ISS is in a 52-degree inclination orbit. This location was chosen in 1991, when it was decided to bring the Russians into the program, using some of their modules as the core of the station. At the time (and now) their primary launch site was Baikonur, and that was the lowest inclination to which they could launch from that location. The Shuttle pays a high payload penalty to reach that orbit (the original space station plan was to have it at 28.5 degrees, the same as the Cape's latitude, so they could get there with a due-east launch and maximum payload). In fact, every vehicle that goes to the ISS would deliver more payload if it were in a lower inclination. With Russian plans to start launching Soyuz out of the Arianespace launch site in Kourou, near the equator, they will have the capability to get to almost any inclination, so the old Baikonur constraint will be gone.

It might be worth doing a trade study to see if its inclination could be lowered, using ion propulsion, over a period of months or years, as I suggested several years ago. This would avoid the radiation problems of sending it out of LEO by this technique, because the whole trip would remain in LEO, and in fact the radiation reduces with the inclination. This would not only save money on resupply costs (or rather, provide more payload for the same amount of money, because the cost of the flights is fixed, while their payload can vary), but also perhaps put it in a more desirable location to serve as a way station to beyond LEO. It would also put it to use as the test bed that Tom Jones pointed out that it truly is, proving out long-duration ion thrusters that might allow future vehicles to operate more effectively.

So it might be time to consider a move to a better neighborhood--just not one quite as out of this world as Mr. Benson suggests.