Pluto has a very thin atmosphere. I believe it was originally measured using stellar occultation from ground based telescopes. Many years ago, I was asked to do a quick pre-Phase-A study for a Pluto atmospheric probe to be carried by the New Horizons spacecraft. At the 11th hour, someone raised the reasonable point that if we were going to travel all the way to Pluto then maybe we should drop a probe into Pluto's atmosphere. Unfortunately, the rules defining the study were impossible to satisfy. The entry speed in the inertial frame at 700 km altitude was 15 km./sec. The maximum allowed entry mass was 15 kilograms. To get the vehicle to slow down, I had to assume a very large base radius to yield a tiny ballistic coefficient. The base radius that I was forced to assume was 2 meters. That assumption yielded a ballistic coefficient of 0.62 kg/m^2 at peak dynamic pressure occurring at 36.5 km altitude. The assumed free stream density at that altitude was about 5.5e-6 kg/m^3. For purposes of comparison, the Stardust probe had a ballistic coefficient of 60 kg/m^2 at peak dynamic pressure (two orders-of-magnitude greater). The density of air at the Earth's surface is 1.225 kg/m^3. Under the study requirements, the proposed Pluto probe was effectively made out of "cotton candy" but still had to shield against a significant peak heat flux of 36 watts/cm^2. That sort of heat flux meant the "cotton candy" had to be some sort of carbon fluff (how do I deploy it with a total mass constraint of 15 kg?). However the peak g-load was 33g. Under that sort of g-load, the carbon fluff would have crushed with the aerodynamics becoming unstable. The design refused to close so we knew it was game-over and walked away from the problem. That was an unfortunate conclusion. It would have been very cool to have obtained images from the surface of Pluto like what the Huygens probe acquired for Titan along with an atmospheric model based upon in-situ data.
Wasn't there a balloon decelerator studied for a Pluto Lander? I guess that doesn't fit your mass-budget, but it does make the prospect of a lander-mission carried by a fly-by bus at least conceivable. That'd be a perfect fit for an E-Sail propelled bus.
The design that I earlier described assumed a spherical section geometry. Given the vehicle's low density, that was effectively a balloon. I wanted to use an umbrella design because that could have withstood the peak deceleration and have a straight forward deployment scheme. However the 15 kg constraint almost immediately removed an umbrella from consideration. People are always suggesting some sort of inflatable concept for entry vehicles but inflatables usually have issues with dynamic instability, i.e. the surface vibrates until it tears itself apart. Dynamic pressure scales as velocity squared. Imagine the force your hand feels when you stick it out your car's window while driving down the freeway. Now scale that dynamic pressure up by three or four orders of magnitude.
Did you consider any sort of trap to have a cloud of ions or ionized dust as your drag surface? I've long pondered that for solar sail purposes - trapping such as:
* Superconducting dust in a magnetic field
* Ions in a magnetic field
* Ionized dust in a magnetic field
* Ions in a RF trap
* Ionized dust in a RF trap
... and so forth. Tiny particles means rapid radiative cooling, and suspending them magnetically or electrically means easy deployment and no issues of structural strength, while having a surface area to weight ratio that no macroscopic sheet could hope to meet. Then again, any such method would certainly introduce new challenges... It's kind of tempting to try to model such a system to see what sort of drag to weight ratio one could realistically get. Maybe I should pull out Geant4 and play around with trap geometries...
How much do we actually know about Pluto's atmosphere? I've seen some diagrams before (which I can't seem to locate any more) showing that despite its extremely low pressure, its change in pressure with altitude is very small (aka it extends very high off the surface).
New topic established for this discussion. Please remember rule 1.9 re 'SF engineering'; gotta keep things in the realm of the feasible. Have fun!
Here's the paper I was thinking of (has been a while since I read it too)...
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/17006/1/99-0422.pdf
There's been some work on using plasma drag devices for decelerating/aerobraking. Have you looked at them?
I've always had a lot of respect for your re-entry opinions, ever since you said (years ago) that "Beagle 2" would dig a hole, due to the modeling being done using the wrong atmospheric composition - N2 instead of CO2 dominated atmosphere.
I'd really like to see a low-mass decelerator option for Pluto and Triton lander missions. Both destinations need up-close study, but orbiters would take too long to arrive. A flyby bus for a lander payload would open up the option space IMO.
Now this is funny. Gary and I have had this conversation before, as quoted on NASA Spaceflight's Forum...
http://forum.nasaspaceflight.com/index.php?topic=22381.msg633293#msg633293
...originally on http://www.centauri-dreams.org/?p=11271#comment-78828 about 5 years ago.
Now it's somewhat more relevant. I guess we'll have better models of the Plutonian atmosphere shortly.
Ah yes, the Internet never forgets.... Along this topic, last year I repeated that same line of speculation about Nereid, Triton, Pluto and Charon coming from a common body while I was on a business trip at the Applied Physics Lab (APL). I was there to present some results concerning a Uranus probe study at the Workshop on the Study of the Ice Giant Planets. Many of the guys at the workshop were high level planetary scientists. One of the planetary scientists made the obvious response that there was no reason to assume that Triton and Pluto came from the same body since objects were raining down from the Kuiper belt all the time. Since many different objects are coming in all the time, it makes more sense that Triton and Pluto should have originated from separate objects rather than assume the unlikely event that they came from a single object after going through complicated orbital mechanics.
The Ice Giant Planets workshop was held in Building 200 at APL where apparently they do unclassified work like the New Horizons spacecraft. The entry room for Building 200 has full scale models of their interplanetary spacecraft hanging from the ceiling including New Horizons and Messenger. I was struck by how small a spacecraft New Horizons was. If you go to the museum at JPL, they have a full scale model of the Cassini spacecraft on display. Cassini is huge and many times larger than New Horizons. While I was looking at the New Horizons spacecraft, it occurred to me that there was no way I could have stuck an entry probe on it. New Horizons really is just a minimalist heliocentric flyby spacecraft.
Pluto is interesting mainly because it is a Kuiper belt object. Neptune's moon Triton is also a Kuiper belt object. I suspect that Triton and Pluto are very similar. Triton also has a very tenuous atmosphere. I don't know if Triton's atmosphere is dense enough to significantly slow down an entry vehicle. The idea has been kicked around to aerocapture a spacecraft into the Neptune system using Neptune's atmosphere. The problem with aerocapture is one must do a periapsis raising manuever at apoapsis otherwise the vehicle reenters the atmosphere on the first return periapsis. An idea that I like is to use Triton's atmosphere for the periapsis raising maneuver. One could theoretically get a spacecraft into a Triton orbit with no fuel consumption by doing clever aerocapture maneuvers off of Neptune's and Triton's atmospheres. The problem with the concept is the spacecraft needs to spend a long time in Neptune's atmosphere to aerocapture to Triton (about 10 minutes). Supposedly the heat load is so great from doing that maneuver that conventional thermal protection materials like carbon-phenolic will not work, i.e. the heat soak eventually convects through the carbon-phenolic and fries the payload. Also, the planetary scientists have told me that Triton may not be all that interesting as a Kuiper belt object because it got cooked during the capture process that put it in orbit around Neptune. The hard truth is that any world past Saturn is too far away and requires too much travel time. That situation might change if they could get solar electric propulsion sorted out (the Dawn spacecraft is very exciting because it has fully demonstrated the capability of solar electric propulsion). We found in our Uranus probe study that solar electric propulsion was an enabling technology that not only got us to Uranus quicker but also freed us from the tyranny of planetary assist orbital mechanics. My suspicion is that if we want to study Kuiper belt objects then we should probably focus on Saturn's moon Phoebe. Saturn is a fantastic system for "one stop shopping". Not only is Saturn interesting in its own right but there is also Titan, Saturn's rings, Phoebe and Enceladus. This fixation over Jupiter's moon Europa is something that I have never understood. If one wants to do a "water world" then Enceladus is the obvious choice. Enceladus has water geysers and does not have the problem of Jupiter's extreme radiation environment. If I was king, I'd send a flagship class spacecraft to Saturn. While entering the Saturn system, I'd drop a lander on Titan that could float on one of Titan's ethane lakes, split off a separate lander to Phoebe, do the Saturn orbit insertion burn and then send the main spacecraft to Enceladus to look for sea monsters.
The caption of http://astronomy.com/sitefiles/resources/image.aspx?item={39DA8E59-C65A-4463-B942-5BB09F98D10E} in http://astronomy.com/year-of-pluto/2015/07/artist-renderings-of-pluto-through-the-ages suggests there may have been at least one more study of a Pluto lander (unless it refers to one of the ones already mentioned? Or maybe never more than a wild idea? I've done 0 research).
Now we need to develop propulsion systems that use practically no propellant. Only recently, the idea was regarded as controversial, but the RF resonant cavity thruster is starting to look promising. (or at least not impossible)
http://ntrs.nasa.gov/search.jsp?R=20140006052
While a new thruster technology would be a welcome addition to the collective toolkit, for the purposes of this discussion we should limit it to proven and near-term available thrusters and technologies.
To drop from 14km/sec to 1.2km/sec ( which is what you basically have to do ) in, say, an 1800km quarter circumference of Pluto would require an average deceleration of 5G. In your example - you would have to generate even more acceleration than that! It's just not going to happen.
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