Isn't there a probe on its way to one of those planets?

This is great news and congrats to the team! Maybe if WISE has captured Tyche we could get Neptune and Uranus on the way there.

Nope.

Aside from the Voyager 2 fly-by in 1986 (Uranus) and 1989 (Neptune) we haven't visited.

There were proposals for an outer planets mission that would orbit one or t'other of the smaller gas giants, but I'm not aware of anything getting funded yet other than design studies, and it'll take at least five years to get there even if they go for something expensive like a nuclear-powered VASIMR mission.

There is the New Horizons probe, which is en route to Pluto-Charon right now (and maybe another KBO afterwards), but that's strictly a fly-by mission too.

One of my favorite factoids about the MESSENGER mission is its highly-elliptical orbit. Apparently it gets heated by both bodies when passing between Mercury and the Sun and thus requires a long cooling off period between perigees.

Bravo, NASA!

Woot! Go APL!

There is a good (podcast) talk on New Horizons at the Silicon Valley Astronomy Lectures site. pity the associated slides aren't available.

If over the next several years VASIMR engines were developed into a viable and practical technology would we really still be waiting years and years? I would have thought that the advance in specific impulse and thrust would allow probes to cross journeys like that in months rather than a number of years.

I suppose even if it still took the advantage of the VASIMR could be that once the probe has spent a number of years around one planet it could boost to look at the moons as well (im thinking one of the gas giants here). Hell after a substantial number of years it could boost onto another planet if practicality permits

*even if it still took months,

If over the next several years VASIMR engines were developed into a viable and practical technology would we really still be waiting years and years?

Yes, unfortunately.

While VASIMR looks like a great leap forward relative to, say, ion propulsion, there's a devil lurking in the details: how to deal with waste heat. The motor NASA have commissioned (which was originally going to be used to reboost the ISS) is highly efficient; if I remember correctly it produces 7 Newtons of thrust. Unfortunately it also sucks 50-80kW of juice to do that.

Getting 50-80kW of juice inside the orbit of Jupiter is do-able with photovoltaics, but for missions beyond Jupiter orbit it's going to take a full-on nuclear reactor (not an RTG). And once you've got the power, you then have to dump it off your spaceship -- which, remember, is in vacuum (a very good insulator) which means you're stuck with radiative cooling (convection and conduction into a heat sink being unavailable).

My conclusion is that VASIMR will hopefully work wonders for missions as far out as Jupiter, but outer planet missions are a whole lot harder (working backwards from: getting permission to strap a critical mass of HEU on top of a rocket in the wake of last week's events in Japan).

I don't imagine we're too likely to get space probes into orbit around Uranus or Neptune until we actually have the infrastructure in Earth orbit to let us build them there, with resources mined from the moon or a near-Earth asteroid.

However, once we do reach that point, I imagine costs per mission would take a quantum leap downwards, and would enable the sort of mission we can only dream about right now - we could build probes that were far too big to launch from Earth directly, allowing them to carry a lot more equipment, or we could send much more specialised probes, or select destinations that at the moment just aren't considered worthwhile.

That will be a very long time indeed.

You're talking about us getting the infrastructure in Earth orbit to build something at least as complex as the space shuttle that's just in the process of being retired, using materials from off-earth. Which implies the existence of a mining and materials processing supply chain. Even if you allow for shipping up the most complex assemblies (probably electronics and biologicals) plus possibly fissionable fuels from Earth, and even if you compromise on materials tech so that you can build the structure of such probes out of readily available metals without exotic/rare earth alloys, you're talking about the equivalent of having an aluminium processing plant and a petrochemical refinery in microgravity.

That's very non-trivial, because historically toys like that have been the climax products of an industrial economy, not the first tentative steps. (Traditionally we've tended to send dirt farmers and the odd miner in advance -- not an option in our solar system, as there's a shortage of land suitable for "forty acres and a mule" colonization initiatives -- "forty robots and a reactor" is a whole lot pricier as a start-up value proposition.) And there are a whole bunch of h-u-u-u-u-g-e technical obstacles to overcome before we can operate such large material processing plants. Never mind use them to build spacecraft-grade components and assemble them into spaceships in on-orbit assembly yards.

You could probably combine photovoltaic VASIMR with conventional rocket thruster. You know, different stages? use VASIMR up to Jupiter, then dump it and use hydrozine or solid fuel stage.

The most efficient option is still nuclear VASIMR, by a long way -- even with the added weight of radiators. Seriously. Storable hypergolics or solid fuels are two to three orders of magnitude less efficient -- we're talking first generation steam locomotives circa 1810 compared to a modern airliner's high bypass turbofans, and then some.

The problem with building an industrial complex in orbit is figuring out how to make it cheaper than simply making whatever it is on Earth and assembling it in orbit. To me it's one of those classic bottlenecks with space (how to lift megatonnes of redesigned-to-work-in-space factory equipment out of the well) that isn't likely to be over come until we really start getting good at fablab type technology. Until a small conglomeration of cargo-freight sized payloads can mine and refine whatever they need to build from their environment (moon/asteroid/desert) we're probably not going to see many shipyards in orbit

I know, but one have to start somewhere? We could test VASIMR without complicating the design with a nuclear reactor.

That's happening already.

Oh, I agree - barring some very serious developments, I think I will be very lucky indeed to see it happen in my lifetime. The state of the space industry has remained a crushing disappointment for me since childhood.

Me too -- and I'm in my mid-40s.

"Getting 50-80kW of juice inside the orbit of Jupiter is do-able with photovoltaics, but for missions beyond Jupiter orbit it's going to take a full-on nuclear reactor"

The assumption that photovoltaics must use un-concentrated sunlight seems false to me. This assumption is often invoked, although I do not know the original [NASA] source. While perhaps a little unwieldy, I see no fundamental reason why the weaker sunlight at Jupiter could not be concentrated with lightweight mirrors or fresnel lenses onto the PV arrays. O'Neill was suggesting using this approach to have his colonies orbit out to 3.7 LIGHT DAYS and still have earth orbit intensity sunlight.

Technology development surely favors solar in terms of rapid development of lighter structures, which will push the nuclear vs solar trade off orbital radius ever further outwards.

Or am I missing something fundamental?

Wikipedia suggests that there are absolutely trillions of dollars(at todays prices) in many average asteroids. If there is one group I trust to succeed at getting money from difficult places, it is the resource exploitation cartels.

Also, has some one already written the book where New Horizons post-Pluto/Charon Kuiper Belt exploration discovers ....something!.... and there are several crash program to get people to the ....something!...?

While I'm all in favour of space exploration, the resources argument always rang hollow. Surely earth based mining will always be cheaper, and we have a lot of terrain yet to cover.

Submarine mining operations, ever deeper mining operations will being to approach the difficulty of space mining, but even when it becomes objectively more difficult to make and Even Deeper Hole than to fly out into space path dependence and sunk costs and accrued experience will probably keep us heading downwards.

Adding concentrators, whether mirrors or fresnel lenses means more mass needs to be accelerated for the same mission plan within the same energy budget. The 7-newton VASIMR motor being tested for the ISS reboost project weighs over 500kg as it is. 50kW of solar panels (about what the current ISS solar array generates) plus concentrators to keep the power flowing in Jupiter orbit will add several tonnes of mass and that's before you include propellant, the system bus, comunications antennas, science instruments etc. A 20-tonne vehicle is not out of the question whereas something like the Cassini-Huygens spacecraft weighed 2.5 tonnes all-up at launch.

I wonder if it would be feasible to use a ground-based laser system to "pump" a solar cell array at that sort of distance, to deliver a few hundred watts per square metre of usable energy to make up for the increasing distance from the sun?

Purely by coincidence, I guess (*), over the past few days I've been contemplating a design reference mission where the design is intended to highlight the limitations of our current abilities to get around the solar system.

DRM: No more than three years after Earth departure(**), place a 2,500 kg probe in a 40,000 km circular polar orbit around Neptune.

Relaxed DRM: Five years and 1,000 kg.

What would we need to do that?

(*) Or synchronicity or morphic resonance.

(**) For generous but not overly contrived values of "Earth departure." Say it means "the first time the Neptune probe is more than 1,000,000 km from the barycenter of the Earth-moon system."

Getting a fast flyby of Neptune by a 2.5 ton probe within three years? We can probably do that, but we might have to hulk up an existing booster.

Getting a 2.5 ton probe into orbit around Neptune in 3 years is going to be hard because of the slowing-down problem. How about aerobraking? Crank up to speed using a solar-powered VASIMR, then send a pilot probe or two to dash on ahead and feed real time meteorological data back to the main payload so that it can make last-minute adjustments to its entry course?

Don't forget electromagnetic tether braking. You can accelerate a probe up to good speed in the inner solar system using solar ion/solar/vasmir drive.

At Jupiter, the probe would deploy a conductive wire some 10 km long. Cutting the planet's magnetic field lines would generate braking through back EMF. The process is very effective in Jupiter's case; the main problems being dealing with the current loads and radiating away the heat you generate to dump the current.

If the probe is in an orbit around Jupiter that is longer than Jupiter's rotational period (10hrs. approx), then the field lines are moving faster than the probe is, so they impart momentum, which can be used to boost the probes orbital period or generate electricity for the ion/vasmir drive or do some combination of both.

While the other outer planets have weaker magnetic fields than Jupiter's, the same principle could be applied to some extent.

Crap. Having a hard time posting here. Wasted a whole long post here :(

Well, the gist was, what would be better: improving propulsion technology (the better to send out probes and whatnot)?

Or improving imaging technology (why send a probe to Neptune if you can see it just fine from here?)?

Sure, there'd be more to it than that (physical exploration and all), but do to the difficulty of putting objects in orbit around distant objects, would not just improving imaging techniques not be more cost effective (in the short term - say, until some 'event' brings costs down)?

There are few rules of thumb that really work, but this one does perfectly. If you have a telescope or camera and want to see something ten times bigger, you have two options: 1) Get ten times closer. 2) Make your telescope ten times bigger.

Telescopes on a space probe are on the order of 24cm in diameter. The Hubble Space telescope has 240cm in diameter.

Propulsion it is, then.

For any given payload, we only have to compare the energy source masses. 80KW of solar panels are approaching the 10 Kg/KW range, suggesting < 1 tonne of mass for the panels and supporting structure. Inflatable plastic foil mirrors or fresnel lenses are much less massive, but let's say that a 25x concentrator for earth level insolation at Jupiter's orbit masses the same again. We are talking perhaps a few tonnes of power supply compared to how many tonnes an equivalent nuclear generator would mass? Plus, no possibility of politically unpopular reactor launches or a launch fail that could sink the nuclear approach, permanently.

I'd certainly like to see some serious studies of using lasers to power PV arrays at astronomical distances. Obviously easier than hypothetical interstellar solar sails, but probably still a difficult engineering feat.

There are an awful lot of phenomena you cannot examine by using remote means. Just look at any science reports from any space probe you can think of and ask yourself how would the same data be collectible using telescopes (of any wavelength) or other detectors, irrespective of size.

Charlie @ 18 Crushing dissapointment? Think what it's like if you are 65, like me, then!

Don't forget the extending/folding frames required to carry the panels inside a launch vehicle fairing and the control servos needed to keep the panels aligned with the sun for optimal performance when considering the weight of the solar arrays needed to power such a VASIMR motor. There is also the inertial load to worry about when rotating the entire spacecraft in tight manoeuvering timescales as it enters orbit, necessitating powerful reaction control gyros and very strong solar panel frames to take the inertial loads. If you want to hang concentrators on the booms to boost the panel output that's just going to add to the mass and complexity.

Frankly, buying an ex-Soviet RORSAT reactor and sticking that on the probe is a lot easier solution as long as you wear earplugs because of all the screaming from folks downwind of the launch site.

Greg, I'm [just] old enough to remember watching Neal Armstrong climb down that ladder on TV as it happened.

I hope to live long enough to see someone else do the same thing.

Getting a fast flyby of Neptune by a 2.5 ton probe within three years? We can probably do that, but we might have to hulk up an existing booster.

I'm not sure we could do even that without some considerable amount of development. Neptune is ~ 30 AU away, 4.5e9 km. Three years is ~ 9.5e7 seconds, so we're taking about 47 km/sec average speed, plus climbing out of the sun's gravity well from the orbit of Earth. Voyager 1, the fastest probe escaping the solar system, is currently going at 17 km/sec according to Heavens Above and that after a couple of gravity assists.

If over the next several years VASIMR engines were developed into a viable and practical technology would we really still be waiting years and years? I would have thought that the advance in specific impulse and thrust would allow probes to cross journeys like that in months rather than a number of years.

First of all, there's another dimension to the tradeoff, and one that I am told is generally more plausible for these sorts of missions: You can increase the size of the payload instead of completing the mission faster.

This makes a lot of sense when you think about it; since no humans are harmed in the making of this movie, it really doesn't matter how long the voyage takes. But if you get up close and personal with your target and all of a sudden find out that you really need that high-resolution magnetometer you put off "for another mission" because it just massed too much for the respective science return, well, you're out of luck.

Additionally - and this has been hashed through many, many times on all the old space groups, it turns out that if you want to optimize for velocity, you want the specific impulse of your rocket to be about 2/3 of your delta-vee. That wanting to cut it off rather than go higher may seem counter-intuitive, but here's a nice little analysis. Read through the whole thing - it may be a trifle earnest, but the math is sound.

Greg .. Shame on You !

I'm not all that far behind you on AGE and I do remember watching the Moon Landing far into the early morning in the COLD light of dawn in my mothers Grimly un-comfortable council house in the North East of England as the coal fire died and the monochrome TV relayed pictures from the Moon ...no home insulation or efficient heating systems way back then ! ..and I do remember my sleep lacked Sense of Sheer WONDER that WE ..the Human Race .. had DONEit! Granted that all of my family had got tired of the whole damn thing and gone to bed but .. I WAS THERE!

Of course if the general sense of optimism and ignorance of the precarious, cold war fueled, financial balance of the enterprise had held up then we would have a base on Mars, and Shiny tm Space drives to Infinity and Beyond by now, but we have what we have and that isn't all that bad.

Given our extended life spans we Two ... Million ?? .. or so whose parents had the genetic fortune to live to 87, as my Mother and Grandmother did ..may well live to see the discovery of Earth Human inhabitable planets in the Wonderland of Space.

Space Colonization is an entirely different thing and on that I do strive for Optimism but, well .. I cant find the casually interesting newspaper report that featured 100 plus people in the UK, but ...

" “The ‘oldest old’ people, those who are aged over 85, have more than doubled in number since 1984 from 660,000 to 1.4million,” informed the ONS report.

Numbers are projected to grow The rate of people, who crossed the 100 years mark was less than 2 percent before 1940, but it surged past the World war era."

http://www.themedguru.com/20101003/newsfeature/better-lifestyle-increasing-life-expectancy-more-100-plus-people-86140700.html

As I recall the people in that newspapers, human interest, report - that I cant find just yet - were reassuringly normal; some gloomy some cheerful, all reduced in physical and mental capability, but all still here apart from the woman who had just died leaving her partner of just below her 109 years behind.

I am 62 and I plan on really Pissing Off the current UK Tory government by living to be 202 and beyond and claiming State Pension all the while ..whilst considering the advantages of migrating to the nearest in-habitable world to our own.

Wot The Hell ..I can't think of anything better to do to pass the time.... other than reading The Laundry Series Book 152 by our, by then, Ever So Ancient Host.

I'm sure I'll find something more fun to do than extrude more Laundry novels by the time I'm 188 years old.

Grump.

(Ignore me, I'm just getting my teeth stuck into the death march to the end of "The Apocalypse Codex", a stage in the process which always feels like kicking a dead whale up a beach. But seriously: writing more than half a million words on a single topic gets old fast.)

Ditto, and I hope Pournelle's observation will allow that to be possible. :-/

"I always knew that I would see the first man on the Moon. I never dreamed that I would see the last."     -Dr. Jerry Pournelle, in Rainbow Mars (by Larry Niven)

And with any luck he will live - "Jerry Pournelle at the 2005 NASFiC. Born, August 7, 1933 (1933-08-07) (age 77) " - to see the next human landing on the moon ..though that landing is likely to be made by a Chinese crew.And frankly that is a prospect that delights me as being one that extends the scope of endeavor by the whole human race.

You CAN do it! Push That Whale! Strike an Heroic Posture, Stiffen the Sinews ..and persuade your publishers to Release "The Apocalypse Codex" before next year. Damn It, I want to know what Happens Next.

Can you use dynamite on the whale? Or is that too messy?

The Mercury probe is pretty cool, shame NASA is still wasting $BNs on the ISS instead of using the money on 10x more robot planetary probes.

History has fairly conclusively shown that dynamite and whales are a very, very bad combination.

Well, if you're lucky, this is using the cetacea definition of whale; some porpoises and dolphins are around 30 to 40 kg, e.g. this one:

http://en.wikipedia.org/wiki/Finless_Porpoise

SCNR.

if you want to optimize for velocity, you want the specific impulse of your rocket to be about 2/3 of your delta-vee.

OK, if we say that we want to have a delta-vee in the 50 to 100 km/sec (5e4 to 1e5 m/sec) range, that means the Ve is 3.3e4 to 6.6e4 m/sec (Isp 3300 to 6600), no?

I was thinking about aerobraking on Neptune, but I suspect we don't know enough about the atmosphere to pull off that stunt properly (and you have to dodge the ring going in and coming out, too). We may have to wait for a magsail or something similar. Thinking about it, I wonder if one of those M2P2 minimags could somehow be reconfigured to serve as a brake in the magnetosphere of Uranus or Neptune.

Or we might want to send some sort of two stage bolo shot. One stage gets slung through a flyby on Uranus/Neptune to shoot towards Pluto (or wherever), and the other stage gets slowed down enough to settle into orbit. Probably the math doesn't work for this, but I'm trying to figure out a nice way to shed a lot of delta-V in a hurry without just wasting it as spent fuel. Somehow pitching the delta-V into a spacecraft continuing on would useful

Ignore me, I'm just getting my teeth stuck into the death march to the end of "The Apocalypse Codex", a stage in the process which always feels like kicking a dead whale up a beach.

The best time to do a Uranus or Neptune orbiter would probably have been the late 1970s or early 1980s, because of the wonderful variety of gravity-assist opportunities available then. Nowadays, you're probably limited to Jupiter, maybe a VEEGA-type preliminary. At least, without electric propulsion, which would more or less require nuclear at those distances, which means it is at least 10-20 years away from launch, even assuming Obama and Congress woke up tomorrow and said, "Gee, we really need to develop nuclear space propulsion". VASIMIR is not necessarily the best choice, either, just the same way a gas turbine is great for airplanes and ships, okay for tanks, and pretty lousy for cars.

Anyways, NASA has discussed the idea before; there's a wealth of information on the NASA Technical Reports Server, NTRS, if you particularly care about the details. If it weren't for costing $20 billion or so to develop and interfering with Constellation, the Prometheus/JIMO project might have led to a nuclear electric Neptune or Uranus orbiter at some point. Currently, the National Academy of Sciences is looking at Neptune and Uranus orbiters for the Planetary Science Decadal Survey. Their concept is a solar-electric vehicle that has a 13-year cruise time to Uranus (yes, I know I said that it would probably need nuclear above). Well, a direct Hohmann transfer would take 16 years, so that is an improvement. I predict that, like in 1971, outer planet science will be decidedly down on the priority list, though. Instead, they're likely to go for Mars. Maybe Venus. The big outer planet mission will be yet another Jupiter mission. Or perhaps a Saturn mission, although Jupiter looks decidedly more likely.

I would also take a certain amount of issue with the idea that we need an orbital probe to have the beginnings of an idea about a planet. Telescopes nowadays are very, very good. Very good. We know a fair bit about Neptune and Uranus, even without orbital probes. And especially in outer planet science, a lot of the focus nowadays is on the moons, not the planets, and only Neptune (which is obviously rather hard to get to) has a big interesting moon, Triton. Uranus doesn't have any additional pulls like that, so everyone sticks with comparatively easy-to-reach Jupiter and Saturn, with their big moons.

@Heteromeles: Theoretically, you probably could do that, by shooting the "go-ahead" probe out of something like a giant cannon and relying on conservation of momentum to slow the "stay-behind" probe. But the cannon would be so much extra dead-weight that you'd be better off replacing it and the "go-ahead" probe with more fuel, instead.

To see what NASA might actually be up to with planetary missions in the 2013-2022 time frame, please see

http://solarsystem.nasa.gov/docs/Vision_and_Voyages-FINAL.pdf

Always assuming the current masters of the House of Representatives don't get their way and cut the non-defense part of the US Federal budget to zero.

Can't.

My publisher is not in the business of publishing my books as fast as I write them; they're in the business of publishing roughly 150 books a year, on a conveyor belt, three a week. I'm a small supplier throwing raw product at a production line, in other words.

There are occasions on which the production line gets cleared to manufacture a single bespoke product at high speed ("The Pope wants a purple metallic SUV with custom wheel rims, stat!") but this tends to cause chaos so it doesn't happen unless million-plus sales are at hand. The Laundry is not a million-plus seller.

(If you want an example of this happening in genre fiction: George Martin has nearly finished writing "A Dance with Dragons", just in time for the Game of Thrones to hit TV; there's a publication date in June which coincides with the TV series airing: earlier books in the series are already New York Times top 10 bestsellers: this one is going to go nuclear if the lumps of dead trees hit the stores while the program is on-air. However, you need to be (a) a NYTimes bestseller, (b) overdue, and (c) with a unique sales opportunity (major tie-in TV series or movie) before they'll disrupt the publishing schedule for 90-odd other books to squeeze it out in less than the regulation 12 months.)

I would also take a certain amount of issue with the idea that we need an orbital probe to have the beginnings of an idea about a planet. Telescopes nowadays are very, very good

Indeed, telescopes are very good, but they can only provide a subset of what a space probe in orbit can do. Firstly, there is, as already mentioned, the advantage that a probe gets in being several orders of magnitude closer with a camera only one order of magnitude smaller than Hubble.

And secondly, the probe gets a better geometry. Whenever we look at an outer planet from here, we're seeing it from the direction of the sun. We don't get to see the shadows, because we're pretty well in the light source. The probe is picturing it from all angles, and can thus see elevations much better.

That's just the optical.

What probes do very well is things that you can't do at all from range - such as radar mapping the surface, or examining the magnetic field, or looking at the content of space around the body. You can't even start to do those from long range.

That's a very good point. Replacing ion drives with VASIMR engines could result in us seeing very large probes. Individual payloads could be boosted into orbit where they connect together. Strap on the power supply and a VASIMR and in the same time it takes an ordinary probe to boost to another planet the VASIMR could deliver several different ones all packed together. Or even one large "probe" comparable to the ISS

Friend of mine managed a Boeing proposal for the Neptune mission. Added a probe dropped into the atmosphere. Consulted with top expert on bathscaphes.

if you want to optimize for velocity, you want the specific impulse of your rocket to be about 2/3 of your delta-vee. OK, if we say that we want to have a delta-vee in the 50 to 100 km/sec (5e4 to 1e5 m/sec) range, that means the Ve is 3.3e4 to 6.6e4 m/sec (Isp 3300 to 6600), no?

Yes, that's right. Did you read the whole paper? At the end the author optimizes for payload, which might be a better way to go. Hmm . . . this is, er, one man's interested opinion to say the least, but I'll give a quote to provide an explanation as to why optimizing just for velocity might not be a good idea:

Robert Terry, a physicist who worked for over two decades in the plasma physics division of the Naval Research Laboratory, expressed concern about “leaks and losses” in the VASIMR design that could reduce its effectiveness. He also pointed to a recent analysis that looked at how much usable payload could be sent to Mars as a function of both the specific impulse of the propulsion system and the mass fraction of the aeroshell that would capture the spacecraft in Mars orbit. As it turned out, improvements to the aeroshell’s mass fraction had a much bigger effect than increasing the specific impulse, even to the very high levels (10,000 to 30,000 seconds) of a system like VASIMR. “Even if VASIMR works,” he said, “there may not be a use for it in the context of Mars.”

I'm mentioning the application to human-crewed flights because, while it seems fairly clear for unmanned probes that travel times don't matter all that much[1], sometimes exotic space propulsion and very high speeds are justified on the grounds that this is "better" for the meatbags. To say that this is still being debated is to put it rather mildly :-)

Riffing off an earlier thread, one of the problems with propulsion schemes like vasimr is that these guys are real energy hogs because of the physics involved: Doubling the exhaust velocity (or specific impulse for us USians :-) requires four times the energy, all other things being equal. But for the same total delta-vee, this cuts the propellant requirements to (again from the physics) 1/Sqrt[2]≈0.7 of the original mass ratio. So just to put some blue-sky numbers in, if the original mass ratio was 10 - about right for a Mars mission[1] - then for the same total delta-vee, the amount of propellant could be reduced by a third. Why is this relevant? Because the energy has to come from somewhere; a higher exhaust velocity means more power-plant, which means it's extra mass budget comes at the expense of some other component. So to get the same overall performance, if the original mission called for 1200 tons of propellant, you'd have at most 400 extra tons to play with. That may seem like a lot, but even with the best state of the art, you're looking at about 20 kg of power-plant per kW, so 400 tons buys you at most 20 MW . . . which again seems like a lot to play with, but did I mention that these alternative propulsion guys are energy hogs? The very best chemical rockets (Isp≈450) are burning up about 10 MJ/kg, or about 10 MW if you assume a mass flow of 1 kg/sec. Assuming the same thrust profile, you only need a mass flow of 0.5 kg/sec if you double the specific impulse; but that factor of four mentioned above means that you're using all your generating capacity! So for any mission that requires more than 1 kg/sec consumption of propellant, something like vasimr is a loser, odd as that may sound at first. Would something like a manned Mars mission require the consumption of more than 1 kg/sec of propellant at any stage? Well, you design the mission and see what you can come up with :-)

Incidentally, this sort of analysis is - surprise! - nothing new, in fact has also been rehashed endlessly. I mention this again despite repeating myself too much because this one is a classic battle between the giants going back to the mid-50's:

Stuhlinger envisioned an ion engine powered by a nuclear reactor. A sodium-potassium fluid would take reactor heat and drive a turbine to produce electrical power. The heat would be dissipated in an enormous rotating flat disc of coolant tubes - a parasol. Crew quarters would be arranged in a 'donut' shape within the parasol, providing artificial gravity to the crew. The reactor would be at the other end of the parasol, where an umbrella's handle would be. At the center of gravity on the shaft in between, the cesium propellant and the Mars lander would be located. The very low thrust-to-weight ratio of the spacecraft made for a longer mission time than the classic Hohmann profile used by von Braun for his chemical rocket powered expedition concept. The mission profile was as follows: <snip> The trip was slower than that envisioned by Von Braun, but per crewmember the mass needed in low earth orbit was over eight times less, due to the much higher efficiency of the ion engines. Stuhlinger would advocate the use of ion engines again several times in the 1960's, but was overruled by those that favored use of nuclear thermal rockets, which were less efficient but allowed shorter mission times.

Wow. The more things change . . .

[1]Ever notice how these sorts of missions look an awful lot like rolling up numbers for a D & D campaign :-)

("The Pope wants a purple metallic SUV with custom wheel rims, stat!"

Heh. The new Council Chairman in the District of Columbia (there's a mayor above him) asked for a black Explorer with moonroof, entertainment system, and black in and out. They could only get him one with tan inside which he wouldn't take and they got him one with gray inside. The WashPost made this public, plus the fact that he's not supposed to drive a city car and that each of them is costing almost $2K/month for the year. He's now willing to give them up, but so far, the rental companies won't let the money go.

Only the mayor is supposed to have that kind of a car because he has security.

The latest NASA Decadal Survey recommended doing an Uranus orbiter and Probe mission as a flagship mission: http://sites.nationalacademies.org/SSB/SSB_059323 (All the other looked at mission concepts can be found on this site: http://sites.nationalacademies.org/SSB/SSB_059331 ). It seems an Uranus orbiter is perfectly doable with current state of the art technology. The proposed mission would be accelerating with an solar electric propulsion stage for 5 years in the inner system (including one earth flyby) and then go directly to Uranus (without the need of a Jupiter gravity assist), discarding the SEP stage beyond Jupiter. Flightime would be 13 years. But for a Neptune orbiter aerocapture technology would be necessary for any useful payload and/or flightime.

When do they start turning it into nanocomputers?

That's what I was thinking of - New Horizons. I couldn't remember which planet it was going to.

Plug a tsunami into NIGHTMARE GREEN and claim your bespoke publishing crown. (As befits an antipope.)

APL engineers? Excellent!

I had no idea ...