Interstellar Probe

Tim · 2004-02-19 18:19:36

http://www.nasa.gov/missions/deepspace/ … probe.html

I saw this last week and was pretty suprised, I hadn't realized that NASA had done enough R&D on solar sails to make this a possibility any time soon. Since they seem to making good progress with nuclear electric propulsion you would think that they would just stick with that.

An artist's depiction of a spacecraft under the power of solar sail. NASA is exploring the use of such alternatives to traditional rocket propulsion for cleaner, more efficient methods of reaching deep space.

Stephen · 2004-02-20 08:49:29

Define "any time soon". It's unlikely to launch for at least another decade (albeit I notice one page suggesting it could set out as early as 2010).

For a whole lot more info, check out:

[http://interstellar.jpl.nasa.gov]http://interstellar.jpl.nasa.gov

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Stephen

~Eternal~ · 2004-02-20 10:28:15

I've always seen Photon Sails as pointless in interstellar travel, sure they're fast, but are they fast enough?
Of course they are but I wouldn't want to be inside a ship that was propelled by Photons towards a distance star.
The gravity exerted on your body would be immense!
And the time difference would be really upsetting if you left loved ones behind.
Though we're speaking of probes...
But I really only see Rodenberry Warp Drive as the only practical interstellar travel method.

Mad Grad Student · 2004-02-20 18:52:53

Of course they are but I wouldn't want to be inside a ship that was propelled by Photons towards a distance star.
The gravity exerted on your body would be immense!
And the time difference would be really upsetting if you left loved ones behind.

No I don't think the force would be very intense. Something like the 200 meter sail proposed on interstellar probe would exert only a miniscule amount of acceleration, like that of ion-electric drive. Additionally, it gets weaker the farther you are from the sun, the main disadvantage of solar sails. The one big advantage they have over other types of drives, though, is that sails are are totally propellantless, probably the reason that NASA wants to pursue this in interstellar probe.

Another drive type they might want to consider is M2P2. This also generates acceleratoin by solar wind, but recieves its propulsion by suspending a huge helium plasma "baloon" that weighs practically nothing and can be over 20 miles wide. Something that big would consume about two pounds of helium a day, though, so assuming we have 500 pounds of fuel it could only be used for 250 days. However, when you think about it, you could get to Saturn in about that time if you were constantly accelerating, as soon as M2P2 is considered reliable it should be used IMHO.

Then again, there's always the old standby of interstellar drive systems first thought of in the 1950s, Orion. For those not familiar, Orion drive uses small nuclear bombs exploding in quick succession behind a shock plate, transfering kenitic energy into forward thrust. This was the original plan for a probe called Nomad that could hopefullly reach 10% of the speed of light and then study some nearby star systems like Alpha Centauri. If you're familiar with Star Trek, in one episode Nomad was supposedly launched in 2002 (Sorry, still waiting for the Botany Bay's 1996 launch ) on a mission to find life in other solar systems. It ended up at the buisness end of a Klingon probe meant to extinguish all life it meant (Or something thereof), and when the two collided, well, things didn't go as well as planned.

This is a cool idea for a probe, I'm glad to see that NASA seems to be creative. Actually, never mind, they've always been among the most creative guys out there. What would be really nice is if, within 30-40 years, they were able to make a probe capible of reaching Alpha Centauri within 40 years, then decelerate to an acceptable speed and make a flyby of any planets in the system. It would be the first ever mission to a planet in another solar system! Now that would be something I would like to see.

atomoid · 2004-02-21 05:32:55

that snew to me, I had thought the only real work being done on a solar sail, was by the Planetary Society. The ~100 foot diameter test prototype "Cosmos 1" is set to launch as soon as the cheap converted russian surplus missile is ready to go... [http://planetary.org/solarsail/index2.html]http://planetary.org/solarsail/index2.html

czech it out --> [http://www.solarsail.org/]http://www.solarsail.org/

Euler · 2004-03-31 23:52:54

The main problem with the plans for the solar sail interstellar probe is that it requires a solar sail about an order of magnitude thinner than is currently available. It is unlikely that the required aerial densities can be achieved in the near future, so such a vehicle is probably at least several decades away.

However, there are many interesting scientific questions that could be answered by such mission, so I thinking about whether or not a mission with similar objectives could be launched using current or near term technology. The main alternative to solar sails is nuclear ion propulsion, so I decided to do some calculation on the performance of an NEP probe.

Assuming an isp of 10,000 and 50% propellant mass, the NEP probe would match the solar sail probe's maximum velocity of 14 au/year. An isp of 10,000 is a little above that used in any current thruster but there are not any technical problems preventing higher isp values- shorter ranged missions use lower isp values because high isp comes at the expense of thrust. Since efficiency increases with isp for ion engines, the overall engine efficiency should be over 85%. If the reactor and thruster have a 10 kg/kW specific mass and are 25% of the vehicle's mass, the total burn time would be about 3.5 years. This is about the lifetime of current ion engine thrusters. 10 kg/kW is achievable for current or near future high power nuclear power plants. Nuclear power is not nearly as efficient on small scales though, so this would have to be a fairly large probe. The remaining 25% should be sufficient for fuel tanks, structural mass, and a formidable array of scientific instruments.

Note that none of these numbers have been optimized. Better performance could probably be achieved with a larger % of the mass being used for power instead of fuel and with an even higher isp. If the thrusters and power plant can operate for longer than 3.5 years, the isp can be raised still further with a longer burn time.

While the NEP version of the probe would be somewhat larger and more expensive than the projected solar sail version, the propulsive technology needed to make it work is available now. With sufficient funding, it should be possible to launch the probe within 10 years.

SBird · 2004-04-01 13:05:00

One way to really boost the performance of a solar sail is to launch it closer to the sun. Since you benefit from the inverse square law, you get a lot more delta V that way. Then your primary difficulty is makinga sail that can survive the heat. It doesn't even have to be terribly lightweight - the increased thrust as you approach the sun more than makes up for the increased spacecraft mass. Assuming a spacecraft that desn't melt from the heat, I once calculated that a solar sail that departed from about one solar radius woud hit something like 3% c. Over 90% of that acceleration was finished before it even crossed Mercury's orbit.

Euler · 2004-04-01 18:15:50

Yes, if you can get closer to the sun, the terminal velocity will be higher. However, the terminal velocity only increases with the inverse square root of the distance, while solar radiation increases with the inverse square of distance. The interstellar probe estimate of .25 au is probably already at the limit of current technology, and most people working to improve sail tech are focusing on thickness rather than temperature tolerance.

Recently, I have also begun investigation options for a "real" interstellar probe to Alpha Centauri. This is of course much more difficult than the 200-400 au probe because Alpha Centauri is at about 300,000 au. With regards to solar sails I found an interesting [http://www.pioneerastro.com/USSIT/Ultra … al_Rpt.doc]article by Robert Zubrin. The conclusion is that a solar sail could reach Alpha Centauri within a century, but only if it uses some sort of exotic carbon nanotube mesh for its sail.

dicktice · 2004-04-01 19:34:04

MadGrad: If i'm not mistaken, you mean hydrogen plasma balloon (not "helium") and--if you agree--how about correcting your post, since a lot of newcomers don't appear to have known about the possibility of Solar Wind propulsion. Exciting prospect, already checked out as feasible in a space chamber, ripe for scaled microgravity testing in orbit, along with light (photon) sails.

SBird · 2004-04-01 19:51:57

I was working with the assumption of a dusty M2P2 sail that was about 90 km in diameter and a 1000 kg spacecraft. Of course, the ability for an M2P2 sail to retain structural integrity at those kinds of light pressures is highly suspect. Nonetheless, the performance was quite striking once you got to within a solar radius or two.

~Eternal~ · 2004-04-01 20:20:08

How about,
Scramjetting to as close as-close-as-you-can-get orbit around the Sun, unleashing a massive solar sail and jetting away at 3 to 5 percent the speed of light.

You could definately get to Alpha Centauri in 60 - 70 years, though don't trust my math :b.

902 km/s -> 10 AU in 19 Days -> 300,000 / 10 * 19 = Days to Alpha Centauri.

Euler · 2004-04-01 21:08:36

though don't trust my math

903 km/s is .003c, which would get you to Alpha Centauri in about 1500 years.

I looked at magnetic sails as a form of propulsion... unfortunately they are limited by the velocity of the solar wind which is less than .01c. They might be useful for slowing down near the destination star system though.

I looked at electric propulsion, and the power requirements are way beyond any conceivable fission power plant. Using multiple stages helps some, making so that a super advanced fission plant might be able to power the ship. This would require a huge ship though, and even with many stages it is well beyond current technology. Fusion or antimatter power might help some, though it seems quite possible that electric propulsion will never be practical for interstellar travel.

Well, if interstellar travel were easy, aliens would already have colonized Earth

atomoid · 2004-04-02 03:10:08

Wasn't there an approach of beaming an extremely powerful laser at the sail from the Earth or Moon or from the Space Platform, so you wouldnt have to accelerate all that heavy propulsion equipment and fuel? just a couple hundred kilograms for a science payload and radio transmitter... if the laser doesnt lose much coherency over distance, you could keep accelerating it as long as you can budget, although if it gets going too fast thered be no way to slow it so it would fly by its destination too fast... better pack a 'light' parachute!

SBird · 2004-04-02 10:45:22

Euler - A dusty M2P2 sail derives almost all of its thrust from reflected sunlight. Basically, you use magnetic, highly reflective dust trapped in the sail to basically act as a super-light solar sail. The performance is a compromise between the heavier nanotech sails Zubrin talk about and the fact that they are ~100% opacity/albedo and the ~1% of the dust cloud. After you get past Venus or so, you'd shut off the sail. At that point, I'd advocate using a nuclear salt water reactor to boost for a few minutes and add another significant fraction of c so that we cover ground quickly. The remaining few kgs of probe would use a nuclear photonic drive to veeery slowly continue accelerating. The nuclear photonic drive is useful because of the high levels of light coming out the back. Properly aimed, it could be used as a low bandwidth transmission unit back to Earth - scientific data sent by Morse code!

atomoid - the problem is that lasers are notoriously inefficient. The new diode pumped solid state ones can push into the lows tens of percent for effocoency but the high powered ones are usually something like 0.001% efficient. I once saw an estimate for the power and material requirements for such a laser and it ended up being something like 100 times the current world gross product to construct.

Euler · 2004-04-02 20:11:18

A dusty m2p2 may be workable as a first stage, but I have my doubts about the salt water reactor and the photonic drive. The sources that I found all listed the salt water reactor's isp at 10,000, which is completely insufficient for interstellar purposes. A rocket that was 90% fuel would need an isp of 1.3 million to reach .1c. I also don’t think that a nuclear photonic drive achieve the thrust or even the impulse required by relying on fission and photons alone. Light just takes too much energy to produce for the impulse it provides.

Beamed energy/momentum is actually one of the better options available to us in the near future. Rather than using lasers, current plans would use either microwaves or a particle momentum beam. Microwaves could be produced much more efficiently than laser light. The momentum beam would accelerate ions to high velocities where they would push against a mini-magnetosphere, providing thrust. The using particles produces thrust more efficiently than light, but the particle beam would disperse at a shorter range. I have read a proposal for accelerating a small probe to .1c that needed "only" 100 GW to power the microwave/particle beam. Of course 100 GW is still more than the total power consumption of most states.

SBird · 2004-04-02 23:38:30

Sorry, I meant to put orion drive down, I was thinking of something else. eg: M2P2, orion, nuclear photonic.

The nuclear photonic has minimal thrust - a quick number crunch I did a couple weeks ago showed that a few millinewtons were all you could expect. The aceleration is minimal but if you run your reactor for several decades, you get a significant increase in velocity. The advantage is that it's a very small, self-contained drive. Also, it's the most efficient use of the matter-energy conversion you're going to get out of fission - your exhaust velocity is c.

Using beamed energy is fine with this scenario, a particle beam canno works particularly well with M2P2 although I question how much thrust you can get off of it. You'd just fire up the Orion drive after you are out of range of solar/beamed propulsion. At the distances we're talking about, anything you can do to eek a little more velocity is critical, especially if we're talking about distances of tens of light years.

Euler · 2004-04-03 00:11:11

Also, it's the most efficient use of the matter-energy conversion you're going to get out of fission - your exhaust velocity is c.

Actually, it is not the most efficient use of energy. In this case the spacecraft is energy limited rather than propellant limited. Methods of propulsion with lower exhaust velocities create more impulse with the same amount of energy. The most efficient engine would maximize the ratio of impulse/(propellant mass + fission fuel mass). This is maximized when propellant is equal to fission fuel mass. Really, the best thing to do is to use the fission fuel as propellant. This principle is used in Orion, which is why it is one of the few fission technologies that might be practical for interstellar travel.

Euler · 2004-04-03 18:04:48

The more I look at interstellar travel, the more difficult it seems. I read a paper on muclear photonic drive today where the fuel:dry mass ratio was calculated for varius speeds... to get up to .1c, the photonic drive would need a fuel:dry mass ratio of 10^44.

GCNRevenger · 2004-04-03 20:27:29

To build an interstellar probe, you either have to go really light or really heavy i'd think, so a sail + NSWR salt water rocket isn't such a good idea.

The ability to push a vehicle with a solar or other sail with an external beam sounds like the best idea, but i'd like to know how well it could focus at such a distance.

As far as the "heavy" rockets go, options are limited for a reasonable vehicle mass to an antimatter rocket or the ORION drive, which has an Isp that increases with size.

SBird · 2004-04-04 23:04:50

Euler - I've seen 5*10^6 listed for the Isp from a nuclear photonic drive. For long duration thrust, Isp and efficiency are really the only things that you should look at. If I'm missing something here, I'd like to know what.

I agree that nuclear photonic produces laughably low amounts of thrust. However, basically all of the energy released by the nuclear fission goes towards generating thrust. An Orion drive already throws away half of the thrust in radiation aimed away from the pusher plate. Furthermore, much of the energy is thrown into particles like electrons and nuclei which will have a much lower 'exhaust velocity'.

An Orion drive is great for gettin a lot of initial thrust and is the clear winner for anything under about 30-50 LY. However, if one posits a low thermal output nuclear photonic drive and a very low mass craft, the constant, low acceleration eventually wins out. Of course, it takes a freakish long time to actually get to any decent velocity with a photonic drive - I haven't run updated numbers for the lower thrust photonic drive but I suspect that it doesn't become competetive for anything under 1000 LY. Hence the staged version where I propose where an initial high delta V comes from an Orion drive to be followed by long term thrust from a photonic drive.

GCNR - The addition of a dusty M2P2 1st stage is dependent upon how well M2P2 will work for driving large masses and large sail areas. Provided that a large area, high reflectivity magnetic sail can be made, there's no reason to use it. A close sun pass launch can impart a significant fraction of C to a spacecraft for what is essentially free. There's no reason to not take advantage of that if it's possible.

Euler · 2004-04-05 01:55:24

I have also seen the isp of photonic drive listed as 5*10^6, and that is what wikipedia lists it at. I am not sure how that number was arrived at. It could be argued that since the exhaust velocity is c, the isp should be 3*10^7s. However, Uranium-based fission only has a mass to energy conversion ratio of about .00075. This means that 99.925% of fuel has an exhaust velocity of 0, while 0.075% has a velocity of c. Therefore the average exhaust velocity is .00075c= 22,500s isp. The impulse/energy ratio is inversely proportional to exhaust velocity, which means that if we used the same amount energy to accelerate all of the fuel to a moderate speed, it would produce more impulse. My (unchecked) calculations show that fission has an "ideal" isp of about 820,000s. In practice, it would be difficult to approach this value, and the highest isp estimates I have seen for Orion/Medusa drives is 100,000s.

SBird · 2004-04-05 10:31:30

The problem is that with an Orion drive, your average exhaust velocity is no better. The total system energy is the same - the stored energy in the unstable nuclear 'bonds' in your fission pile. You've got a lot more mass to toss around but it's accelerated by that input energy. The total momentum cannot be any greater than what you get from the original radiation pressure. Plus, with an Orion drive, geometry dictates that only half of the blast energy hits your pusher plate, lowering your energy efficiency by half.

I might be missing someting esoteric about the coupling efficiency of radiation pressure that makes it less effective at moving a spacecraft but as far as I can see, it represents the most efficient way of harnessing that power. If you've got a nuclear pile, it wants to emit its energy as X-rays and neutrons. With a radiative jacket that absorbs most of that radiation, you down convert that energy into IR radiation which can be effectively reflected. As long as your core supports are smal and poor heat conductors, virtually all of the fission energy goes towards thrust.

Nuclear photonic gives almost laughable amounts of thrust - millinewtons at the best. However, there's no reason why you can't run a small, lightweight engine for decades, occasionally tossing in a new fission pile as you exhaust the old one. Assuming a mechanism where small fission piles can be swapped out of the drive as they expire, a 100 kg 'dry' drive mass, 150 kg of fillible material, and a 50 kg probe, I calculate that you're going 6% c at fuel burnout. (this number is a lowball since I'm assuming the spacecraft mass remains constant over the 450 years of drive thrust - in actuality, the loss of mass from ejected fission cores will make that value considerably better.

If you conminbe this 300 kg final stage with a small Orion drive to get a nice, fast initial speed boost, you can cover a lot of ground. With an Orion stage that hits 3% c, your top speed now goes up to 10% c. Plus, from a mission planning standpoint, you get to 'close' targets much faster because of the rapid initial acceleration. Such a staged probe would hit Alpha Centauri in under 100 years. Furthermore, it's still usable for further exploration since the photonic stage still has 350 years of acceleration left in it.

GCNR - If you recall our earlier discussion about these drives, I downrated the engine to have an internal energy flux of about 10 MW/m^2 which should be ammenable to realistic engineering solutions.

Euler · 2004-04-05 15:46:48

Momentum is proportional to velocity. Kinetic energy is proportional to the square of the velocity. Therefore it is possible to get more momentum with the same amount of energy if the exhaust velocity is lower.

atomoid · 2004-04-06 01:35:19

I remember reading some old article years ago about a probe that, if they had started building it at the time, would be arriving at Alpha Centauri around 2020. The probe was to be powered by microwaves or laser beamed from earths vicinity. The amount of power needed to do this essentialy relegated the idea to a pipe dream.

I found [http://www.space.com/businesstechnology … 217-1.html]this intersting article:

...before the beginning of the 22nd century, both nuclear and solar drives should be approaching their interstellar potential. If a craft was launched then using such propulsion, it could reach Centauri within about 1,000 years, he said.
-----
...the best way to do interstellar propulsion seems to be to use beamed momentum - use the pressure of a laser beam or microwave beam to accelerate a reflective "sail" up to high velocity. In Kare's SailBeam design, a stream of small sails carries the momentum from the laser accelerator to a vehicle. At the far end, a magnetic drag brake can be used to slow down.
"It's still a huge project," Kare admitted. Launching a one-ton probe to another star with SailBeam would take many gigawatts of laser power for several years.
"But it doesn't take any new physics, and the technologies required are only modest extrapolations from what we can do now. My very rough guess is we could start building a SailBeam launcher in 20-30 years, and be launching probes by 2050," he said...
-----
...a small instrument payload could be sent to 250 AU [Kuiper belt] in 10 years using 30 milligrams of anti-hydrogen.
"This amount of antimatter is clearly within the product on potential of the U.S. within the next 40 years using currently accepted accelerator technologies, Howe said. Preliminary calculations also indicate that a similar probe could be sent to the next star, Alpha Centauri, actually a triple star system, in 40 years using grams of antimatter, he explained.[Antimatter Driven Sail]

SpaceNut · 2004-11-10 06:44:45

Thou no specific probe or mission is stated at the start of this thread it however has a beautiful solar sail.

Cosmos 1 is closing in on that magical date to show that a sail can be pushed by photons, costing under 4 million. So how cheap is that russian sub rocket?

Planetary Society’s Cosmos 1 Solar Sail Ready for Flight

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#1 2004-02-19 18:19:36

Re: Interstellar Probe

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