Pluto/Triton Lander Deceleration, using thin atmospheres for EDL Options

[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.

[djellison]

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.

[Y Bar Ranch]

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.

Whippin' out my Casio...based on the Pluto radius and a 14 km/s approach speed, you'd have to pull 17 Gs at the maximum to stay in the atmosphere. With a swag on density rho of 2 x 10^-5 kg/m^3, and a mass of entry probe of 1 KG and C_L_max of 1.5, you'd need a....really, really big wing.