Everybody is rightfully concerned about sending the shuttle into an orbit where it can't reach the safe haven of the Station Alpha (ISS). And if you think about it, it wouldn't make sense to launch the untested OSP to Hubble unless it had the capability of reaching the ISS, too.

Finally, Soyuz can't reach the HST unless launched from somewhere further south and it might require two launches. And even then, the Soyuz couldn't reach the ISS from there, either.

Whatever transportation system is used, it looks like the Hubble will meet its demise after the last servicing, since it seems to have been decided that making a special trip to bring it down in the shuttle cargo bay is an unnecessary expense and danger.

All these problems, except for the as yet non-existence of the OSP, come about for the same reason. The HST and the ISS are in different orbits. The ISS orbits at about 400 kilometers versus the HST's 600 km.

If it were just for that, it wouldn't really be a problem. We know the shuttle put the thing up there, so it can reach the HST and then later lower its orbit by a mere 200 kilometers. In fact, I figured that part out with the remnants of an unused and twenty five year old Astronomy degree. That, and this really great link from the Danish Space Research Institute

One only needs to change the Shuttle's velocity (in two steps) by a total of about 100 meters per second to accomplish this. No, the problem is that the Earth wears the orbit of the Hubble like a crooked waistband and the ISS orbit is more like Tarzan's over-the-shoulder style suspender. To be a bit more technical, the ISS orbit has an inclination of 51.9 degrees and the HST's is 28.5 degrees.

Now, why, you may ask, have they done such a silly thing when part of the reason for having a space station was to have a base of operations in space? Simply put, its politics. Having the Russians as partners in the ISS means that they need to be able to launch to the station.

The Soyuz cannot reach a low inclination orbit because they launch from the Baikonur Cosmodrome, which is at the same latitude as St Paul, Minnesota. We launch from Florida, around 28 degrees latitude (that number should sound familiar). We can make the ISS orbit, but not that and the HST on the same mission, apparently.

Now way back when we first thought up the idea of a space station, there were three vital components to be built, the Station, the Shuttle and an Orbital Transfer Vehicle. Well, we could only afford to build one and since we could not build a station without a reusable orbiter (we erroneously thought), we built a Shuttle.

Finally we get a chance to build another component. The Orbital Transfer Vehicle this time? Well, without a station we have nowhere to transfer to or from, now do we? So we build the station. We wanted the Russians in, and that meant a high inclination orbit, and NOW we have an orbital transfer problem.

OK, well how much of a problem is it? Now those calculations, unless I am very much mistaken (a possibility not to be taken lightly), are actually a bit easier to accomplish - just some vector geometry. This is one of those cases where velocity is distinguished from speed. We shall not change speed, but only direction. In order to go from HST's orbital inclination to that of the ISS, a delta V (change in velocity) of some 3000 meters per second is required.

For a point of reference, it took us about seven thousand meters per second of delta V to get to orbit in the first place. Coming back from orbit is much easier because we only need to make the orbit intercept the atmosphere (a few 100 m/s delta V) and air resistance does the rest.

So, that much delta V is just not practical if you want to take a whole Space Shuttle Orbiter along. What you need is an...(all together, now) Orbital Transfer Vehicle. Yes an OTV needs to go out and latch itself onto the HST and bring it to the ISS.

Now, hands up all you who think that will take less than a decade and less than 10 billion to produce. Isn't there anyone? Heck, we don't really need anything new, just maybe a solid rocket motor with (excuse me while I calculate)...GUACAMOLE! Its EIGHT METRIC TONS of propellant.

Actually, it's not that big a deal, the shuttle carries like 70 tons or more. But, we weren't going to send the shuttle to HST, remember? And we can't really use a solid because changing orbits is a multi step procedure (solids don't turn off) and I don't want my telescope being pushed around so roughly anyway.

A Progress might do it but there is that same problem with St Paul - I mean Kazakstan launch site and the orbital inclination. Besides, the whole point in having an OTV in orbit is that you can use it again. Nope, what we need is a lightweight high efficiency low-acceleration propulsion source with guidance system. One that could navigate up to the HST and then gently push it to another orbit.

Well, students, such a craft does exist. We can't use the original because its WAY out yonder now, having completed its test of a high efficiency low-acceleration propulsion source and autonomous navigation system. It's (ta-da) Deep Space One. DS One uses an ion propulsion system powered by solar panels, that spews out Xenon atoms at tremendous speed. This has the effect of being extremely fuel-efficient.

Now when I calculate the amount of fuel needed to move the HST it comes out as about one metric ton of Xenon gas. I couldn't find a mass for the whole Deep Space probe but the ion motor with all the accessories (power lines, etc) weighs a whopping 48 kilograms. There are solar arrays and power converters and sunshades and stuff but it's still a tiny package. Call the whole thing 1000 kilograms total. With just 88 kilograms of fuel, DS One propelled itself to a delta V of 4,500 meters per second.

We'll have to add a grapple fixture, but with low acceleration, it need not be too beefy. The Hubble can point itself quite accurately so the SDSO doesn't need oversized maneuvering jets or anything like that. So, attach it to Hubble (11000 kg) fuel it with another ton and a half of Xenon and you have more than enough delta V to get the job done. The extra half ton is to push the DS One Clone (maybe we could call it Spirit of Deep Space One (SDSO)) from the station (where the shuttle just delivered a load of supplies and, oh yes, SDSO.

So then we just tell it, "Go and fetch me the HST, please!" (It's autonomous, remember.)

"Your whim is my mandate, oh Carbon Unit!", it might reply, "It will, however, take 80 thousand hours."

That's the one fly in the ointment. Actually, DS One has higher thrust modes so it won't take 10 years. ...Oh, all right! Maybe we need some more solar panels and an extra engine or two. But it can be done, you understand. Solar panels are light as a feather these days and, as I understand it, exhaust speed (and fuel efficiency!) Goes up with voltage. There is the idea that the atmospheric drag, while tiny, might be too much for the ion thruster.

Well, most of the thrusting will be at right angles to the direction of motion, so I'll bet it'll still work. Also, when your aim is to lower an orbit, drag is your ally. The scope can't observe while under thrust so it might take a few years of intermittent maneuvering, but we have a few years before its ready to be repaired.

So, what do we do with it when we're through fixing it up? Can it operate at the same orbit as the ISS? If not, the Spirit can take it up to 600 km again but this time on the same inclination as the ISS! Again, that's the easy part. And when the time comes to dump the poor old thing in the ocean like some kid's broken toy spyglass from the dollar store?

Well, for the same delta V (less, actually) we could bring it back to the station and just park the thing there. Just tie it on a truss somewhere with a bungee cord and duct tape. Its mass is only about three percent of the station itself. It might cost a bit more in orbit maintenance, but not much. Maybe someone will decide to do something with it later. Like looking at Mercury, which we haven't done yet because the scope might catch a glimpse of the sun and fry its eye out.

So there you have it, trekkers. The Spirit of Deep Space One - existing, tested technology, efficient, cheap and lightweight. Bring the Hubble over to the Station's neighborhood and we have solved the problem.

• Bibliography
• Can A Soyuz Service Hubble And Save A Bundle by Bruce Moomaw
• Delta-V Calculation for Lowering Perigee of Cubesat by Danish Space Research Institute
• DS 1 drawings:
• Deep Space One Technology Validation Report