Friday, July 2, 2010

Some Planetary Exploration Challenges

Some of the most interesting places in the solar system unfortunately present some daunting challenges to explore.  The intense radiation fields at Europa are probably the quintessential example of this problem.  With a decade of technology development to push radiation hardened engineering and a $3B+ budget, the proposed Jupiter Europa Orbiter will survive for an estimated 9 months or so after it enters orbit.  With that short of a lifetime and a whole world to explore, a fairly small percentage will be imaged at high resolution.  Eventual lander missions (if the findings of the orbiter warrant what would likely be a Flagship class mission) will be equally time pressed.

The hellishly hot surface of Venus provides its own challenge.  Landers and probes have lasted on the surface for an hour or so in the past.  The next generations of landers under consideration by NASA are targeting landed or near surface lifetimes of two-five hours.  These landers extend their life by including phase change materials within the landers.  Much as the ice in your ice chest does, these materials absorb heat as they change from solid to liquid.  Once the phase change is complete, however, the interior of the lander's temperature will rise as heat soaks through the shell.  The lead author of the Venus Mobile Explorer, Lori Glaze at the Goddard Spaceflight Center, explained to me in an e-mail the challenges faced by designers of Venus landers.  "The real challenge here is that if you want to keep the lander 'cool' you have to provide more phase change material, which has mass...At some point, you just can’t get this massive system off the ground at Earth."  As a result, Venus landers seemed destined to have just a few hours to perform their studies.  Long term studies can be done from balloons and orbiters, but long-term surface missions are beyond our capabilities.  (In theory, refrigeration units powered by plutonium 238 could resolve this problem.  However, these systems operate based on the difference in heat between the  plutonium and a thermocouple.  It's hard to "dump" the heat of the Pu-238 into an already hellish hot atmosphere to maintain the temperature difference.)

Titan has neither radiation fields nor heat to deal with.  It's atmosphere and surface are hellishly cold, but nuclear power systems do operate well in cold environments (easy to maintain that heat differnce).  This is a world that has a surface that in many ways may be most Earth-like of any body in the solar system with river valleys, seas and lakes, mountains, and a host of other interesting terrains.  Titan deserves the high resolution imagery that has advanced our understanding of the other Earth-like surface, Mars.  Unfortunately, the atmosphere of Titan and the dim light so far from the sun makes high resolution imaging from orbit almost impossible.  At Mars, orbiters can operate at 150 km altitude, while the atmosphere of Titan requires an altitude of 1500 km.  To see through the haze in a spectral window, a camera has to operate in near infrared bands.  The sun is dimmer at these wavelengths than at visible wavelengths and Saturn is far from the sun.  So to collect enough light to illuminate each pixel sufficiently for a clean image, a large mirror would be needed for high resolution imaging.  As a result of these limitations, the proposed Titan Flagship orbiter would have imaged the surface at 50 m per pixel where Mars is imaged at 30 cm per pixel.  (Radar instruments would have their resolution degraded by the increased altitude compared to what could be done at Venus or Mars.)  In addition, either optical or radar imaging system generate lots of data, which from the distance of Saturn require high powered communications systems.  High power communications systems require large power systems (and likely lots of Pu-238) leading to a large spacecraft.  Net result is that global, moderate resolution (~50 m) mapping of Titan may require a small Flagship-class (($1.5-2.0B?) missions.  In the meantime, in situ probes like the proposed TIME lake lander and the AVIATR plane offer ways to increase our knowledge of Titan at much more moderate costs.

1 comment:

  1. Excellent post! You could write a whole book on the subject.

    Most of the bodies in the outer solar system (by number, not by mass) have solid surfaces, no atmosphere, and are very cold. I have always wondered how materials would behave in such conditions. Is water ice slippery? Do plastics and ordinary materials break like potato chips? If you are walking on the surface, in a space suit, and fall down, greatly increasing your surface contact area, do you freeze within seconds? I have not been able to find any information on this.