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European Moon Mission Ready for Launch
merryprankster writes "Europe's first mission to the Moon is set for blast off from Kourou in French Guiana just after midnight, local time, on Sunday. SMART
1 will study the composition of lunar rock through X-ray observations. The probe uses a new solar electric propulsion system which converts solar energy its panels into motion via the expulsion of ions. Details at the ESA mission site."
I hate to burst everyone's bubble, but NASA used ion propulsion on the Deep Space 1 mission several years ago. Yes, cool technology but like most stuff it's been researched for years and used before.
And the payload isn't really greater at all.
The payload in the Apollo program was launched from a Saturn V, which can put nearly 120,000 kg in low orbit and so had enough oomph to put it's payload immediately into a lunar trajectory. The payload for this mission will be launched from an Ariane V, which can only put ~16,000 kg in low orbit, but at (IIRC) a tenth the price.
Hopefully they can perfect the ion drive, however through this to increase the speed and payload capacity.
The drive itself is pretty near perfect; the problem is that if you're going to kick out exhaust at such insane velocity that you don't need a whole lot reaction mass to get good deltaV, then you need a whole lot of energy to get good deltaV instead. And these guys are getting that energy from solar panels, which takes a while. If we had that anti-matter reactor you want (or cold fusion, or anything else providing lots of energy from little mass) we might eventually want something fancy like VASIMR, but in the short run we'd probably just plug the reactor in to bigger or more ion drives.
Sure the Ion drive is a really neat addition, but it's soooo slooooow. It's going to take them 15 MONTHS to get there!
Actually, Ion propulsion is faster. It just has much less tork, if I may use the analogy.
Unlike rocket propulsion, Ion propulsion has a constant push. Although is exerts a smaller force, it can accelerate for as long as there is fuel, and it uses very little of it.
That's how they'll be able to send people on mars in a matter of months (last figures I had was 9 months) instead of more than a year (it was reported to be somewhere around 16-18 months using rocket propulsion).
For a moon mission, though, speed isn't necessarily important, so they can afford the longer trip time to same money and augment the payload (instead of carrying all that fuel).
Solar panels work great when you're this far into the solar system. From Mars and beyond the solar intensity is much lower and solar panels would need to be prohibitively large and heavy to provide the same amount of power as a 45 pound radioisotope thermal generator.
You have to understand that there is more than one reason for using ion engines. Some include, reduced cost, reduced complexity, proving the improved technology really works and extending the mission life. The final one it important, since what usually ends a probe's mission is component failure or more often running out of fuel. As long as there is a star in our Solar system, then SMART 1's mission can last a good while. The only thing that could extend the mission even more is an xenon collector and an extended mission budget.
Jumpstart the tartan drive.
From yesterday's article article about Smart 1 at the BBC:
Just another wannabe fantasy novelist...
"It takes longer to get any large loads going" -No, not really.
/dt
Yes. Really. quoth the article: "Ion propulsion systems are less powerful than conventional chemical rockets..."
Power = Energy/Time = Mass * dV^2
If average power is less, velocity increases at a slower rate. Therefore, the probe will take longer to get to a specific destination than a conventional rocket. MUCH longer.
It may be slightly more economical..." just slightly? considering that the available fuel is practically infinite with no fuel from earth being used (except for leaving earth)
Again, quoth the article: "...but can run for ten times as long using the same mass of propellant."
So even the ion drives have a finite amount of "fuel" which must be loaded before liftoff. So much for that.
Combine these two, and you have an engine that is very poorly suited for transporting humans.
It is, however, very well suited for probes and the like, since the engine is smaller and lighter than chemical booster. That can decrease cost of lift and size of craft by a good margin... and since most probes don't starve to death you can afford to take 15 months to get there.
The moral of this story is, ion drives as they currently stand are not the end-all solution for space transportation. I agree it's well suited for this particular use, though.
=Smidge=
Solar electric propulsion is hardly new. It's been used for getting communications satellites out to their final geosynchronous orbits for a number of years now, and NASA demonstrated using solar-powered ion engines for interplanetary primary propulsion on Deep Space 1 back in '98.
What ESA is claiming is new about this mission is that they'll be combining ion propulsion with gravity assist maneuvers. AFAIK that hasn't really been done yet (although I know some guys at JPL who're working on it), and given how difficult it can be to work out low-thrust trajectories in the first place I would imagine that successfully throwing gravity assists into the mix would be a significant acheivement.
What exactly is innovative about this mission? It is the same mission as flown by Clementine years ago. Solar electric propulsion is commonplace. Here are some spacecraft that have flow them to date:
I don't think this story is slashdot worthy.
an ill wind that blows no good
If average power is less, velocity increases at a slower rate. Therefore, the probe will take longer to get to a specific destination than a conventional rocket. MUCH longer.
You forgot about time. Your chemical engine accelerates your spacecraft at 10 m/s^2 for ten minutes, and then runs out of fuel and has to coast the rest of the way. Total delta-V: 6 km/s. Your ion drive accelerates at 0.1 m/s^2 for a year. Total delta-V: 50 km/s. At the end of the year, the ion drive vehicle is going a hell of a lot faster.
And in case you think that a year of continuous thrust is infeasible, Deep Space I's ion drive ran (on and off) for about 600 days.