Neptune Orbiter, Another proposed mission Options

[PhilHorzempa]

[quote name='algorimancer' date='Mar 27 2007, 11:52 AM' post='86914']
Personally I think that unmanned exploration really ought to be getting at least fifty percent of the Nasa budget, and that the science budget ought to be immune from scavenging to fund the manned exploration program.


Here, here! I totally agree with those comments. Space Science should be at 50% of
NASA's budget, not the 30% that it gets now. Griffin acts as if that 30% level is written
in legislation. In fact, in recent comments by him, he bemoans the fact that manned spaceflight
gets ONLY 60% of the budget, double that of Space Science.
I don't see any reason why Manned flight MUST gobble up that much of NASA's budget.

With a 50-50 split, the astronauts would learn to live with fewer missions, while we supporters
of UMSF would begin to see some of our dreams come true -

Mars Sample Return, Saturn Ring Observer, Europa Orbiters and Landers and Submersibles,
Planet-Detecting Space Telescopes,
Comet Sample Return, Venus Balloons and Landers, High-Resolution Venus Radar Mapper,
Neptune Orbiter, Triton Lander, Enceladus Lander, Titan Orbiter and Rover, Venus Sample Return,
Mercury Sample Return, Main Belt Asteroid Sample Return.

Thiose are all missions that have been studied/proposed by NASA.
We have a whole Solar System (and new ones) to explore.
What are we waiting for?


Another Phil

[djellison]

Ahem.

Politics people - stop dancing with the rules.

[JRehling]

BAD QUOTING - STOP IT - Doug

I think the turn of topic serves to signify that a Neptune mission is as far out in the future as it is in kilometers. The only advantage Neptune has over the other flagship mission candidates of the outer solar system is that the approach that is required is less mysterious. While the nature of the orbit, and the possibility of entry probes for Neptune or a sounder of some kind would be up in the air, a Neptune mission will obviously be a kind of mini-Cassini. Exploration strategies for Europa, Titan, or Enceladus are much harder to pin down, and will be a kind of Twenty Questions game. Triton may get to that point eventually, but it is obvious that whatever mission follows up the NEXT mission to Neptune will be very far out in the future.

Budget and top-level priorities come up in discussion of Neptune because Neptune's as low as a priority could be and still be on the list. It hasn't been said yet in X vs. Y terms, but Enceladus (by matching the highest expectations of how interesting a place it seems to be) will probably knock Neptune out of contention for a very long time to come. Picture the competition for outer solar system flagship missions as a horserace. Whatever place Neptune was in before, it has seen Titan surge to rival Europa for the lead, seen Enceladus move out of a pack to almost challenge the others for the lead, and such places as Io, Ganymede, and Uranus aren't quite completely dead. In comparison with any of these, Neptune fares terribly in terms of distance from Earth -- even Saturn is over three times closer! And that's only accounting for outer solar system competition for funding, to say nothing of a (prioritywise) colossus like Mars or dollar-gobbling one-off possibilities like a Venus Sample Return.

Pondering Neptune missions is therefore probably a mere delight for our imaginations. I strongly believe that only an unforeseen mission of opportunity (like a physics / solar magnetosphere mission that flies by Neptune just for what-the-heck, or an entry probe that sails off of a jovian gravity assist) could possibly get there before 2040. I don't think it's very likely a Neptune Orbiter will fly before the ground rules of outer solar system exploration change in some unforeseeable way (other nations taking a role; new kinds of propulsion, etc.)

I'd be overwhelmed if Europa, Titan, and Enceladus each receives a followup before 2040.

[Greg Hullender]

Here's a question: why is the distance such a big deal? The energy difference between getting to Saturn and getting to Neptune shouldn't be that great. I may have computed this wrong, but it looks to me like the Saturn-to-Neptune potential energy difference is just 1/12 of the Earth-to-Saturn difference.

Clearly it takes a lot longer to get there, but how much of the cost of a mission is the cruise time? I'd expect that to be pretty small compared to the up-front probe cost and the data analysis cost at the end.

In fact, it seems that if we really did want to study all the outer planets, it'd be most cost-effective to make 4 of the same probe and send them off to all four gas giants more or less at once, then process the results over a period of years as they arrive.

But even if (as I know) we can't do that, I'm still puzzled why a Neptune probe would be more than (say) 20% more than an equivalent Saturn probe. Error in my math? Or some other factor I'm missing?

--Greg

[JRehling]

BAD QUOTING - STOP IT - Doug

For a flyby like NH, the distance is not as much of a factor as it is for a putative orbiter, simply because the orbiter has to stop. The minimum-energy path that would make that easy would be prohibitively slow for Neptune (ie, it would be similar to the trajectory of Halley's Comet). Faster trajectories mean more resources (aero- or propulsive) to brake on the other end. Cassini's flyby velocity at Phoebe (before a lot of Saturn's gravity well had come into play) was 6.4 km/s, and a delta-v of only one tenth that was needed for SOI. A cruise to Neptune like New Horizons is taking to Pluto would mean almost triple Cassini's arrival velocity. Even though Neptune's gravity well is not as deep as Saturn's, and we could tolerate a more elliptical orbit (more time spent far from Triton, etc.), that's an expensive engineering constraint, and could lead to a sacrifice in terms of instrumentation, etc.

Someone is surely more qualified to speak to the operational costs in terms of ground crew (NH being a good model?) and the quantitative engineering costs of the aforementioned problem.

[algorimancer]

What I'd like to see is a paradigm shift over to taking advantage of microelectronics and microfluidics to develop really small (light) yet capable spacecraft. Heck, my cell phone has a more powerful computer than my desktop had less than ten (five?) years ago. Combine that with microfluidics & ion drive, and the only real remaining challenges are a compact nuclear power supply and communications (improve the DSN's sensitivity to allow lower power comm?). I can envision a pretty competent little spacecraft massing on the order of a kilogram or several. We could send swarms of these things all over the solar system and beyond for the cost of a single mission today, a Neptune orbiter would be trivial.

[JRehling]

BAD QUOTING..STOP IT - THREE BY YOU IN ONE PAGE - Doug

Rodney Brooks (robotics/AI, MIT) proposes things like this.

One problem is that while many critical subsystems have been miniturized into invisibility (or to massive capacity) others can't be. A good telescope still has to have a big mirror or lens. A radio transmitter still has to crank out some power for reception 30 AU away. A nuclear power source can't be tiny. Firing a thruster to get orbital insertion -- you just can't do that with a pocket-sized craft.

I think there's some future to this idea, though. Maybe centralizing what needs to be centralized on one comm-craft with lots of little drones in other locales. I could especially imagine the space weather around the Sun and Jupiter being explored by a large number of little orbiters spread out in different orbits. That would be tremendously valuable. Exploring a world in situ would be the other great application: Imagine lots of little Surveyor rovers in hundreds of locations around Mars. The asteroid belt or the outer jovian/saturnian satellites might benefit from this approach.

An orbiter devoted to surveying static phenomena would seem to be immune to this approach, though. Why not just have one big orbiter? Lots of little spaced-out (no pun intended) orbiterlets don't seem to offer much as an alternative. Eventually, the one orbiter will get all of the spatial perspectives anyway.

[helvick]

Danger Will Robinson - Serious arm waving ahead.

Anyway, assuming we can just linearly scale things down to this size which is almost certainly untrue but ..

A ~250g Plutonium RTG would yield around 1 watts sustained for many years. If we were to shrink Deep Space 1's Ion drive\Fuel load by a factor of 500 we would have a drive that required ~164 grams of fuel which combined with a reliable 1 watt power supply could provide about 3600 m/sec delta V to a 1 kg probe. I seem to recall from somewhere that we'd need about 15km/sec delta-V to get to neptune so we're looking at slightly more than four times that fuel load - let's say 650g.

So you are left with ~ 100g with which to build the micro-probe's chassis, fuel tank, sundry scaffolding and all the comms / sensor equipment. Now that would be tricky but basically that's more than enough mass to build a cell phone with a camera today and the DSN can certainly pick out a signal broadcast at cell-phone range power (~1 watt) from Jupiter type distances although the data rate would probably be pretty poor. OK space rating all this is a monstrously untrivial task but we're not talking about magic here just some steady technical progress.

Plausible within the realms of mad arm waving at least.

[tty]

Being an aerospace person I feel I have to do some counter arm-waving. It takes a looong time for new technology to become mature enough and robust enough for flight-rating, much less space-rating. Components that doesn't even exist yet probably won't be flying until the 2020's at the earliest.

Also not everything can be miniaturized. Maybe DSN can handle a 1 watt signal from Jupiter - but not a non-directional one or communications with Galileo would not have been a problem. So You need a directional antenna, and good directivity means that the antenna has to be large compared to the wavelength of the signal.

Ion engines are just dandy for the long haul but they are utterly useless for orbit insertion, so the flight profile would either mean having to decelerate for nearly half the distance to avoid arriving with too high speed or adding a conventional rocket for the insertion burn. In either case much of the savings in weight/time is lost.

[djellison]

Basically - you're not going to the outer solarsystem with a cube sat

Doug

[Greg Hullender]

I wonder if someone can double-check my figures here. I figure a minimum-energy orbit for Neptune from Earth needs a delta-V of 11.65 KPS and takes 30.6 years. (Technically that's from Earth's orbit to Neptune's orbit -- I'm ignoring the gravity of both objects for the moment.)

I agree -- 30 years is a long time to wait.

However -- and this is what surprises me -- if you use 12 kps instead of 11.65, it cuts the time to 15.68 years. That's a big improvement for one third of one kps, I think. Add another third, and it's just under 13. (And your orbit is parabolic). At 13 kps of delta-v, you get 10.38 years.

I stipulate that we could wait ten years. I know extra delta-v costs a lot because of the exponential in the rocket equation, but it doesn't seem like this should be enough to sink the boat.

Is there some other factor? Or (again) do I have the math wrong somehow?

--Greg

[helvick]

Greg - I think your numbers are correct but I'm certainly no expert. My understanding was that 15km/sec delta-V gives the flexibility for a fast (10-12 year) mission including orbital insertion.

TTY - As far as oribital insertion is concerned I understand where you are coming from but as I understand it a DS1 type system with a DS1 type Ion drive engine is able to hit 3.6km/sec delta-V eventually. DS-1 consumed 100g per day under minimum thrust so a full load of of 82Kg of fuel would require 820days to expend in getting that 3.6km/sec delta-V. Extrapolating that out to my 4x DS-1 model we need to spend four and a half years accelerationing initially followed by a year (or three) of cruise and then a four and a half year deceleration into Neptune capture orbit. If ~15 km/sec delta-V is the magic number what's wrong with that approach?

With regard to Antenna size. The Messenger phased array Ka band antenna assembly weighs <3kg and is good for 3-4kbps comms over 100million km at ~11watts. Yes there are significant constraints in reducing that by (slightly more than) an order of magnitude in terms of both mass and power consumption that I can't solve but it seems to be in the ball park and it doesn't feel totally impossible at first glance.

While it is true that high res optics such as carried by MRO are simply physically impossible on a platform like this what you choose to carry surely depends on what you are attempting to achieve. You can certainly build a very poor camera for less than 20g, I have a 1.2 megapixel example in my phone in front of me. I reckon you could put some effective sensors in there one way or another although they might not be cameras.

Worst case - scale the whole thing up by a factor of 10 and you really should be able to design and build it today. Look at the Beagle 2 build specification and what they managed to cram into that, the idea of a midget deep space probe is not so mad.

The point being that if you _could_ build such a thing within a 1kg (or 10kg) mass budget then you could potentially build and launch a large number of them at once, as Algorimancer suggested. And you would definitely get some efficiences of scale if you were to build tens or hundreds and scatter them off a launch similar to New Horizons (ie with oodles of delat-V up front from those nasty chemicals).

[tasp]

Imagine, if you will, an ion engined probe, already in Neptune orbit. Further, in view of Neptune's large Hill sphere, imagine the probe to be in an extremely elliptical orbit with a period of ~ 1 year. At apogee, even relatively minute adjustments to the orbit, made with the ion thruster perhaps over a period of a couple of months, will result in large controllable variations in the perigee portion of the orbit, ~6 months later. At perigee, orbit adjustments can be made via gravitational interactions with Triton. IIRC, large changes in period and inclination can be made easily at the very fringe of the Hill sphere. The long orbital period about Neptune is also conducive for using a relatively low power, slow data rate from Neptune to send back the perigee data.

Ignoring the challenge of the initial Neptune orbit insertion for the moment, it seems Neptune might be rather hospitable to a probe with an ion engine, as long as the probe was taking advantage of the large Neptune Hill sphere, and we would be content with a series of Neptune encounters at roughly 1 year intervals, for as long as the probe could remain functional. Considering the success with the Voyagers, we might anticipate a probe duration at Neptune of 10 years, possibly substantially more.

[algorimancer]

Let me address a few issues related to the micro-probe concept.

Communications. Need a light-weight directional radio antenna? How about a half-silvered mylar balloon, inflated with a minimal pressure slightly above vacuum. I'm sure this could be improved upon, so that the balloon material hardened over time and the internal pressure was no longer needed, allowing for degradation of the material over time (inevitable). Better yet, use a diode laser like that in a laser pointer - I've seen cheap kilowatt versions of these lately.

Ion Drive. These things have progressed a ways since DS1, and are a fair bit more efficient these days, meaning less propellant needed or more available delta-v from the same propellant.

Telescope. While I know there are inherent weight limits behind telescope optics (though see above under Communications for a light-weight solution), there is an old fashioned solution to the lack of zoom-lens problem... move closer to the target and take more pictures.

And don't forget that aero-capture still works for something at this scale.

As to space-rating, heck at the price of these things you can afford to waste a few dozen working-out the bugs. Start with low earth orbit, do the moon, asteroids, mars, then head for Neptune. If someone in the field got serious about it, these things could be flying within a few years. You could easily piggy-back a few at a time on conventional launches, especially those to geostationary orbit. Opt to use Windows Mobile as an OS and you might even be able to get Microsoft to fund the development for the publicity.

[tty]

Imagine, if you will, an ion engined probe, already in Neptune orbit. Further, in view of Neptune's large Hill sphere, imagine the probe to be in an extremely elliptical orbit with a period of ~ 1 year. At apogee, even relatively minute adjustments to the orbit, made with the ion thruster perhaps over a period of a couple of months, will result in large controllable variations in the perigee portion of the orbit, ~6 months later.

I fully agree. The problem I wanted to point out is that orbit insertion means essentially to convert an orbit with apogeum at infinity to one with a finite apogeum. This has to be done at the perigeum where orbital speed is necessarily quite high and You have only a very short time to do it in - ergo a chemical rocket is needed. Theoretically You could do it by aerobraking, but doing it by a single short aerobraking means going deep into the atmosphere which means very high temperatures and probably some kind of an ablative heat shield.
Some kind of a balute might work, but it would weigh a fair amount too. Also I strongly doubt that we know enough about the Neptunian atmosphere to bring this off on a first try.

As to space-rating, heck at the price of these things you can afford to waste a few dozen working-out the bugs. Start with low earth orbit, do the moon, asteroids, mars, then head for Neptune. If someone in the field got serious about it, these things could be flying within a few years.

The trouble with that method is that You only get to know that you have bugs, not what they are, so you can't fix them. Or do you plan to include advanced diagnostics and telemetry in those dead-cheap cubesats?

Believe me, nobody wants to spend years and millions flight-rating and space-rating equipment, but (expensive) experience shows that it's necessary.

Better yet, use a diode laser like that in a laser pointer - I've seen cheap kilowatt versions of these lately.

Laser communication over such distances will require quite exact pointing of the laser. Not easy to do cheaply and "weightlessly". You would need either a very precise attitude control system for the whole probe or a very good pointing system for the laser.