[X] My Assistant

[Bjorn Jonsson]

I have been wondering, even though it is a slight chance, if we are seeing any hazes or clouds at Pluto.

I was reading about Triton's photometric parameters a few days ago and came across something interesting: Triton's atmosphere contributes to Triton's limb darkening (at least at at low phase angles), i.e. Triton would be less limb darkened if it was a completely airless body. So the amount of limb darkening exhibited by Pluto might be affected by Pluto's atmosphere too - but I don't know the amount of limb darkening that Pluto should exhibit if it was airless. Also at high phase angles, Triton's brightness at the limb is affected by its atmosphere.

[gallen_53]

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.

[TheAnt]

...... 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.

Aha, I suspected that at someone at least had taken a look at the problem of getting a lander to Pluto together with NH. Turned out to be 2 studies done even. =)
Even without knowing any details I suspected it would be a near impossible feat, cotton candy subjected to 33g sounds pretty much like that.
Thank you for the info that that suggest it might be possible to send a low mass lander some time in the future.

[gallen_53]

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.

[djellison]

the Dawn spacecraft is very exciting because it has fully demonstrated the capability of solar electric propulsion

This was done 15 years before by Deep Space 1.