Ion Rocket to Map Moon with X-Rays

jralls writes "The Guardian is reporting that a European ion-rocket has taken the last year to reach the moon and is about to enter lunar orbit. Once it slows and gets into a very low orbit, it will probe the surface with x-rays in an effort to solve the long standing puzzle of the moon's origin."

Comments

Great title

[Chairboy]

I expected the story to read "But when Flash Gordon approaches, will the moon people fight back with their electro-guns? Watch next week to find out!"

Moon mining?

[FiReaNGeL]

From the article :

"The sun emits X-rays and these are reflected back into space by atoms on the Moon's surface. A magnesium atom will reflect an X-ray in a different way from an iron atom, and Grande's detector can detect these differences.

Flying over the lunar poles, so that it covers the entire Moon as it revolves below, Smart will create strip maps of the surface - and eventually a global map of its composition."

Look like useful data to me if we were in the 'mine the moon' business... maybe in a not so distant future?

Re:Where do you think the Moon came from?

[MrWim]

It was a spaceship!

AKA "Thats no moon"

Re:A year?!

[Garion Maki]

unlike in the 60s and 70s, they are using ion engines for this mission, which can run of solar power.
they give less trust/second, but they can keep burning for allot longer, since the sun gives a constant supply of fuel (in the form of electricity from solar panels).
so you've got a smaller probe, which means easier to get into orbit from where it can fly on it's own power, so even tho it takes longer to get where you want, it will be cheaper to get it into orbit.

btw, they are planning on bigger engines in the future, so hopefully they will go faster someday.

Re:A year to reach the moon?

[SpryGuy]

The ultimate speed of ion propulsion is higher than that of chemical propulsion.

But the mass being expelled at high speeds (the ions) is so low, that accelleration is VERY slow. So it takes a long time to get up to speed, but the maximum speed you can theoretically reach is much greater than that of chemical rockets.

Possible Resolution to US moon landing hoax theory

[jeoin]

Will these guys be able to snag some good shots of the trash we left on the moon? Exluding the flag of course, which can't be trash cuz its on a stick.

Re:Visibile from Earth?

[NetKraft]

Ion rockets can't generate very high accelerations. They can, however, keep going for a long time.

wow

[flacco]

sobering thought that that headline sounds exactly like something you might hear in a pulp sci-fi movie from the 50's...

Re:This begs the question:

[marsonist]

... the beagle2 is laughing at your post all the way from mars

In all seriousness it's nice to see some other serious large-scale attempts being made by countries other than the US and Russia. As with all things scientific, the more head working together the more we all learn.

Re:A year?!

[GileadGreene]

Ion engines still use propellant (typically Xenon, but I haven't looked at what this particular mission is doing), they simply accelerate the propellant using electrostatic fields (in the case of ion engines) instead of chemical combustion. The key to ion engines isn't so much the solar power, as the fact that they have a much greater specific impulse (a rocket engineering term that relates to the efficiency with which propellant is used). Where a chemical engine may top out at ~400 s of specific impulse, ion engines have hit around 4000 s, or an order of magnitude greater propellant efficiency. That translates into a much smaller amount of propellant to do the same mission.

The tradeoff is betwen the extra time it takes to get to the destination (due to the low thrust of an ion engine), and the reduced cost created by being able to launch a much smaller amount of mass into space in order to do the mission.

btw, they are planning on bigger engines in the future, so hopefully they will go faster someday.

The issue with ion enginer thrust is not so much size, as it is power. The thrust you get is directly proportional to the amount of power you can generate. If you're using solar arrays, then you're limited to something between 15-20 kW (the Boeing 702 has solar arrays that produce ~15 kW at end-of-life).

Re:A year to reach the moon?

[GileadGreene]

Actually, the maximum speed that you could theoretically reach is the same in both cases: c

The difference is that it will take a chemical rocket much more propellant to get there, because it is far less efficient in its use of propellant mass (i.e. it has a lower specific impulse).

Re:Visibile from Earth?

[wertarbyte]

If we used some sort of higher powered rocket to generate the velocity, I wonder if ion rockets could hold that velocity for a long time.

Since we are using this space, I wonder what we would need the ion rocket for to hold that velocity. In space, there is not much that could slow you down.

From the article -- galactic bowling physics?

[jdkane]

Smart's map should provide that data and show if scientists are right in believing that the Moon coalesced from a vast ring of debris generated when an ancient planet the size of Mars destroyed itself after crashing into Earth. Understanding the origins of the Moon will therefore give insights into the nature of our planet.

Doesn't this mean earth should have some huge dent in it, and not be so round? Look at the sizes of Mars and Earth. Are you surprised earth is still here after a crash of that magnitude? I am. Maybe earth was a lot bigger before a Mars-like planet destroyed itself crashing into earth, but then I go back to my question about the roundness of the earth.

Maybe someone more knowledgeable wants to talk about that. The article doesn't go into any great detail on that, which causes a lot of questions to be raised.

Re:From the article -- galactic bowling physics?

[The Mgt]

The earth wasn't a solid mass 4 billion years ago.
It's still not a solid mass now.

Re:From the article -- galactic bowling physics?

[barakn]

Take a look at the moon. Those dark spots are the sites of enormous ancient impacts. They may have been holes briefly, but they then filled up with lakes of lava. As far as the Earth goes, the impact was so devastating that the outer layers of the Earth had to reform by falling back down.

The following contains some links to mostly non-technical explanations of planetary roundness. I'd like to point out that part of this explanation, by "Derek Sears, professor of cosmochemistry at the University of Arkansas and editor of the journal Meteoritics and Planetary Science," is wrong. He says "Planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it." But this is a circular argument (pardon the pun). Generally a non-spherically symmetric distribution of matter doesn't have a gravitational field that acts as if it originates from the center of the body (the "center" being the center of mass). Spherically symmetric mass distributions do have this special property, so what Sears really implied is that planets that are already round will have gravitational fields that point towards the object's center of mass. This does absolutely nothing to address cases of objects that deviate from perfect roundness, i.e. all celestial bodies. This explanation by Dr. Sten Odenwald suffers from the same argument, and there's even a hint of it here. Nonetheless, these explanations are approximately true, and require bizarre shapes to break them.

For example, imagine a homogenous, perfectly shaped doughnut (a torus with a circular cross section). At the center of the doughnut hole we'd feel no gravitational field at all (a perfectly balanced tug-of-war). But deviate from the exact center just a tiny amount, and the closer side of the doughnut becomes more attractive than the other. One suddenly experiences a gravitational field that points away from the center of mass.

Re:A year to reach the moon?

[WindBourne]

There was no extra weight margins on the launch vehicle to do so. However, if you have a more powerful booster (i.e. lots more money), you can then get higher intial speed.

This mission was to prove to EU that their ion engine worked. So they wanted it to keep running for quit some time. NASA did this be creating deep space one, which ran around picking up steam via its ion engine.

Down the road, you can bet that EU will launch a number of deep space probes based on ion engines with high initial speeds.

In addition, their will be a real push for micro sats with ion engines to control them. Makes a lot of sense to send these to other planets. think of 100 small satillites going though out jupitor or saturn planets. Or better yet, small micro sats around Mars providing surface to space communication, pictures, glp, etc. Send about 100 of these to orbit mars and we would have a very through pic of mars, moon, etc.

Biq == Round

[DumbSwede]

Beyond a certain size, gravity pulls things into a spherical shape. The immense pressure makes the insides molten and irregular structures eventually sink down in. Mars has Mons Olympus, the tallest volcano in the solar system, this is because Mars is smaller and has less gravity than Earth. The larger the planet the more regular it has to be. Asteroids can be highly irregular because they haven't the size and gravity to collapse them into spheres.

The mountains on Earth may appear huge to us insects on the surface, but from a distance the earth appears as smooth as a billiard ball.

Ironically this event was so big, that unlike latter smaller hits, all evidence in the way of dents will be gone as the entire globe virtually liquefied and coalesced again. Though I wouldn't rule out some exotic mass distributions that might lend evidence of it.

MOD DOWN PARENT

[Anonymous Coward]

At each pole, there are such deep caverans that sun never reaches it. That includes X-rays and normal sun light. That is why it is possible for ice to be there. And yes, it is considered part of the surface.

Re:A year?!

[Mr2cents]

SMART-1 has been making bigger and bigger orbits around the earth, because of the smaller thrust explained above. It goes faster and faster, and because the craft gets further from the earth, it becomes easier to get into a bigger orbit. So at first the orbits became larger very slowly, but the last months it has grown faster than ever before.

The last months the orbit was also synchronized with the moon. The highest part of SMART-1's orbit coincided with the lowest point of the moon's orbit. This helps the craft to get an extra boost every month. Take a look at a graph of the orbit here.

Oh, and they do have normal propellant onboard, there's some 70kg left iirc. I think it was installed in case the ion engine failed, but I'm not sure of that. It could also be to correct the initial orbit if the launcher would have placed it in a wrong one. Anyway I *hope* it will be used to attempt a soft landing after the mission is over.

Re:A year to reach the moon?

[Brett Buck]

> The ultimate speed of ion propulsion is higher than that of
> chemical propulsion.

Depending of course on the fixed mass of the spacecraft, vs it's propellant mass, of course. You get more momentum change from given amount of propellant, but if you only had a teaspoon full of propellant, or the spacecraft was exceptionally massive, you wouldn't get more velocity.

> But the mass being expelled at high speeds (the ions) is so
> low, that accelleration is VERY slow. So it takes a long
> time to get up to speed, but the maximum speed you can
> theoretically reach is much greater than that of chemical
> rockets.

To expand, the measure of efficiency of a rocket engine is the specific impulse or ISP. It's how much momentum change you get per unit of propellant mass, and the usual unit is seconds (lb-sec/lb). The highest actually-achievable ISP from a chemical rocket is somewhere in the 475 seconds. The Saturn 5 first stage was more like about 350, and monopropellant thrusters used for many satellite propulsion systems is more like 150-180! That means that if you want to change the velocity a lot, you need a whole lot of propellant.

I'm not sure which engine this particular program uses, but the ISP of the typical Xenon ion thruster is something like 1800. So you have to carry fantastically less propellant for a given velocity change, meaning it can weight less at liftoff, meaning you can use a weaker/cheaper booster.

The downside is that you don't get something for nothing. It takes, not surprisingly, a whole lot of electrical power to make it go. So you put in 4000-5000 watts of power, and it only generates .04 lb of thrust - .64 of an ounce, pushing a spacecraft weighing thousands of pounds on the ground. So the acceleration is very small, meaning takes a long time to get going. The other downside is that the Xenon ions, although chemically pretty neutral, shoot out at such high speeds that anything that gets in the exhaust gets eaten away. This may or may not be an issue depending on there you put it relative to the rest of the spacecraft.

Brett

The ion drive is the real story

[SimURL]

Ion drive technology allows you to explore space in ways that chemical rockets simply can't.

Quoting from the article,
"We have shown that even a small ion engine like Smart's can get us across space. Now we are planning to build space telescopes and robot probes to planets such as Mercury, using bigger and more powerful ion engines. These will take years off space-travel times. Instead of decades-long missions, we will take only a couple of years to cross space for future projects."

But,
"Ion engines need electricity and only solar panels can provide enough at present. So ion engine missions will be restricted to planets and moons near the Sun."

So the solution to deep space exploration is nuclear-powered ion-drives and NASA is working on it.

Basic rocket physics makes it a bit clearer

[Evil Pete]

Look up any reasonable book on mechanics and you will find a formula for the final velocity of rockets that have a empty mass M, mass of fuel m, and have an exhaust velocity v. The final velocity of the vehicle is ...

V = v . ln( (M + m)/M )

In other words ion rockets will beat chemical rockets because they eject their exhaust at a reasonable fraction of c, whereas chemical rockets have exhaust velocities more like velocities we see on earth (e.g. bullets). So chemical rockets need lots of mass, but that's ok because they throw out lots of mass. Trouble getting to space is expensive ... each kilo of fuel you put in orbit better be wisely used ... so in space ion rockets make sense (apart from the fact you can't use them on Earth anyway ... wouldn't be able to lift off even).

Hope this makes things a bit clearer.