Meanwhile, Space science continues its course

[Spiffor]

Originally posted by DanS
To design, build and fly a 100 kilowatt-class fission reactor.

They have a program attached to it (Jupiter Icy Moons or JIMO), but the fission reactor is really the star of the show.

[Dinner]

Dan: I've always read that Ion engines would be used for the main part of the spacecraft's trip will solid or liquid fuel engines would be used to intially accelerate and to decelerate when they arrive at their destination.

[DanS]

If the spacecraft already has an ion engine on board, then simplicity would seem to dictate that you use that. It's the most fuel efficient, in any event.

[Urban Ranger]

It makes sense for NASA to use ion engine for the deep space probe, but using it for SMART-I is kinda strange. I reckon the ESA is using this chance as a live trial for the technology.

[Whaleboy]

Agreed. Presumably Nasa would use a rocket motor to get it up to a decent speed, let it cut out and use the Ion engine to accelerate further.

[Spiffor]

Originally posted by Urban Ranger
It makes sense for NASA to use ion engine for the deep space probe, but using it for SMART-I is kinda strange. I reckon the ESA is using this chance as a live trial for the technology.

That's a likely explanation, considering that two probes are in the making, that will go much farther than SMART-1 (Sun and Mercury). I think SMART-1 was mostly a test on the technologies used for future probe missions (not only the ion Engine), especially future SMART missions. If anything, SMART-1 has proven that the ESA can pack a serious payload on very light interplanetary craft, and thus it seriously reduces the launching costs.

Edit: By re-reading the ESA's website, it really seems the ion Engine was the star of the show - four instruments in the payload were designed to measure the ion engien's behaviour alone. Two other technologies were tested in this mission (laser communicationand automated piloting). Less than half the payload was designed to study the Moon.

[DanS]

As stated, it takes almost as much energy to send spacecraft to the moon as it does to send spacecraft to other locations in the solar system. Indeed, if you want to land on the moon, as a trip it takes more energy than landing on Mars. That's why Heinlein said: "Once you get to Earth orbit, you're halfway to anywhere in the solar system." ESA proved the system for their purposes (although the concept was developed and proven by others before ESA).

It really only matters how much patience you have and how much on-board energy you have as to whether ion engines are good for your application. ESA could afford to be patient and was staying near Earth, where solar energy is relatively abundant.

[Urban Ranger]

Yes, getting out of earth's gravitational well is a big part of launching space vehicles.

[DanS]

I've been noodling things and doing some online research and it looks like Hall thrusters have much more potential for the stuff we want to do than ion engines. This is a shame, since Hall thrusters are mostly Soviet technology built on the original American invention while ion engines are mostly American technology build on the orginal American invention.

Hall thrusters are similar to ion engines in that they are both electric propulsion, but the engines are smaller, they scale up well, and have higher specific power (thrust). Currently, the Hall thruster specific impulse (fuel efficiency) is a little more than half the ion engine specific impulse, but NASA is working on improving efficiency. In any event, both methods are light years ahead of chemical propulsion with regard to efficiency (if they hit their targets, Hall thrusters will have 10x the fuel efficiency of chemical rockets). Hall thrusters are used frequently in commercial satellites.

NASA has a couple of test 50 kilowatt hall thrusters built under Project Prometheus. I guess they'll scale it up to 100 kilowatts, once they hit their efficiency, operational, and reliability targets.

http://www.grc.nasa.gov/WWW/hall/