Adam asked the following:

>Vernor Vinge is known for getting his
>science right. In his story "Long Shot"
>[anthologized in "Explorers"] he
>presents a scenario in which an
>unmanned vehicle deccelerates via an
>aerobrake from ~ 110 km/s in the
>atmosphere of a terrestrial planet. Is
>such a thing feasible?

The Galileo Probe hit Jupiter's atmosphere at a relative velocity of 47.4054 km/sec at 450 km altitude. Specific energy is called C3 and is defined as 0.5 * V^2. C3 is a measure of how intense the thermal environment will be. The C3 for the Galileo Probe was 1.124e9 Joules/kg. For Ving's "Long Shot" the C3 would be 6.05e3 Joules/kg. This is 5.4 times worse than the Galileo probe. The peak stagnation point heat flux experienced by the Galileo Probe was 16200 watts/cm^2 with a peak shock layer temperature of 16500 deg.Kelvin (hotter than the Sun's photosphere). In contrast the peak stagnation point heat flux experienced by the Mars Pathfinder aeroshell was 106 watt/cm^2. The Galileo Probe was over a hundred times hotter than Mars Pathfinder.

This is all preamble to the punch line: Given the ridiculous thermal environment, the Galileo Probe was feasible only because Jupiter has a hydrogen-helium atmosphere (0.1359 mole fraction for helium). Hydrogen and helium have a very high heat capacity and thus sucked away most of the Probe's entry heat. The old NASA guys who designed the Galileo Probe tell people that if the Probe had entered the Earth's mainly nitrogen atmosphere at 47.4 km/sec then it would have vaporized in a flash. If Vinge's "Long Shot" entered a mainly nitrogen atmosphere then it would probably vaporize. Of course Vinge could use a statis field or make the heat shield out of unobtainium (the Galileo Probe used carbon phenolic).

Gary