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Rocketdoc Notes – Week of September 6, 2020

Interesting Exploration Opportunities - Titan

I attended a webinar last week for NASA’s Dragonfly mission to Saturn’s moon Titan. It is a fascinating, well-thought-out mission but is going to push the limits on artificial intelligence (AI) to be successful. The problem is that the Dragonfly moves around by flying from one location to the next using rotors, and the roundtrip for a control command from Earth is 166 minutes (2 hours 46 minutes), and that is less than the flight time. So, Dragonfly has to autonomously fly around, find locations of interest, find a safe landing spot in or near that location of interest, and land safely; all without help from Earth.

The NASA team had a well-thought-plan for this. In essence Dragonfly always flies a roundtrip scouting mission to find the nearest point of interest within it’s flight radius and records detailed surface characteristics over the flight. The ground crew on Earth downloads the data, determines if there was a site worth visiting during the flight and uses the recorded surface characteristics data to select the landing site, which is the goal for the following flight. Looks like a successful Con-Ops (Concept of Operations) plan to me.

Dragonfly can fly on Titan with eight relatively small rotors because the surface pressure on Titan is 150% of that on Earth and the temperature on Titan is -296 degrees F (94 K) so Titan’s atmosphere is very dense (7.3 times Earth at Sea Level). Since Saturn is 10 AU from the sun solar power is negligible so a nuclear-powered Radio Isotope Powered Generator (RTG) rides on the back to charge the batteries for flight (see figure 1 below).

Figure 1 RTG-Powered Dragonfly on Titan

In addition to cameras and neutron spectrometers to record and measure Titans features Dragonfly has drill units on the landing legs to sample the surface minerals and look for precursors of life. Titan is very similar to very early possible conditions on earth before the sun heated up, so life precursors are possible. Scientists expect Dragonfly to travel about 200 miles during its baseline 32-month mission on Titan and it does this with a 5 miles flight range with fully charged batteries. Launch is scheduled for April 2026 with arrival on Titan in 2034. The first landing will be a test of the onboard terrain recognition AI and will undoubtable have a high pucker factor. Total cost for Dragonfly is estimated at one billion dollars. This is an exciting exploration mission and I wish them the best of luck.

Interesting Exploration Opportunities - Venus

Another aerodynamic related space exploration mission would be a long-term probe of Venus using either a balloon or a solar-powered airplane. I was involved in scoring NIAC proposals last year and one was a solar-powered airplane/glider that charged its batteries high in the Venusian atmosphere using solar cells and then glided down to lower altitudes below the cloud layer where it scanned the ground. It then used its charged batteries to climb back above the cloud layers so it could charge its batteries again. The Venus airplane was a very original concept, but if I were in charge of the Venus’ atmospheric probe, I think I would stick with a more proven balloon concept for my first attempt.

If the balloon(s) are at 35 to 40 miles altitude where the temperatures are Earth-like, the high-speed winds would carry the probe(s) around the planet in about four days. If the mission is designed for a limited mission duration, batteries could power the atmospheric sensors and a detailed synthetic aperture radar surface-scanner. If the mission is designed to be a long-term mission then an RTG could power the sensors and allow the design to include the altitude changing features of a hot-air balloon. Note that the sulfuric acid cloud layer on Venus ranges from 29 to 43 miles altitude so our balloons and payloads need to be weatherproofed.

There have been numerous balloon exploration missions to Venus proposed, but as yet none have been funded. Venus has a harsh atmosphere but there is much we could learn about planetary science with a low altitude synthetic aperture radar so I would support a mission. The long-term knowledge applied to the evolution of earth-like planets around other stars should be worth the expense.

Interesting Exploration Opportunities – Jupiter

It turns out that Jupiter’s atmosphere also has a sweet spot where both temperature and density are within the capability of current balloon technology. The main problem is how to survive the tremendous entry speeds involved (~ 43 km/sec) so the probe can be deployed. The structure of the upper Jovian atmosphere is shown in figure 2 below.

Figure 2 Structure of the Upper Jovian Atmosphere

As you can see in Figure 2 there is a point in Jupiter’s atmosphere, just below the clouds, where both pressure and temperature are near Earth normal. Of course, the atmosphere is mostly hydrogen so we’re going to need a hot-air balloon filled with hydrogen and the gravity is 2.4 times that of Earth, so everything has to be lightweight. Numerous proposals have looked at hot-air type balloons for Jupiter, but as yet nothing has been funded. This would be much harder than a balloon probe on Venus so maybe we should let them go first.

Thanks for your attention.

Dana Andrews

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Contemplating flight in foreign environments is always a fascinating exercise. I went through the numbers and determined that my old Globe Swift would be incapable of flight on Mars even if the powerplant could function there. It would hit exceed its critical Mach prior to liftoff, even accounting for the 3/8 gravity. Ballooning in Jupiter would certainly be complicated by the low MW H2 atmosphere, but I think the increased gravitational acceleration washes out in the equations for buoyant systems. The change in density as a function of altitude due to gravity would precisely cancel the increased buoyancy required to resist the gravity. Flying on Titan sort of splits the difference between flight and swimming, which brings up the thought of adding…

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