Saturday, October 10, 2009
All of a sudden it seems like there's water everywhere. During the press conference announcing the presence of water on the Moon the scientists noted that if the water was created due to an interaction with the solar wind, it could be created on other similar areas as well - bodies without an atmosphere and the requisite oxygen contained in the minerals making up the planet or asteroid in question. It seems that asteroid 24 Themis also has ice water on its surface.
Whenever the presence of water on a body is indicated the next question always is: does this make it a good destination for human colonization some day? The next thing to do is to compare it to Ceres, another body that we are quite sure has a huge amount of water.
24 Themis though is a ways farther out than Ceres (3.13 AU compared to 2.7663 for Ceres), although still well within Jupiter's orbit.
A single orbit for 24 Themis takes 5.54 years compared to 4.6 years for Ceres. Since manned exploration of any destination out this far (including Mars) simply isn't possible without a new form of propulsion such as VASIMR, a longer orbital period is probably a plus since it means that launch windows are that much more frequent, while the extra time required to travel isn't all that different. Here the two are about equal.
Gravity: Ceres has about 3% the gravity of Earth, and 24 Themis about half that. The effect on low gravity human physiology won't be known until we have spent some time on the Moon, though don't forget we have spent up to 437 days in space at one time with no gravity at all with no severely negative effects. A lower escape velocity for 24 Themis is a bit of a plus though, with 313 kph for 24 Themis compared to 1836 kph for Ceres, meaning that the return rocket could be much less massive.
Length of day: a single day on Ceres is 9 hours, on 24 Themis it's 8.3. A shorter day means less night to get through which is a plus when sending battery-powered probes, but the difference is pretty small and is balanced out by Themis being farther from the Sun.
Inclination: Themis wins big here. Inclination for Ceres is 10.585° whereas that for Themis is 0.760°, meaning that Themis rotates the Sun at almost the same plane that the rest of the planets do. Ceres is a ways off though, requiring extra fuel to be spent to reach compared to Themis where a probe will only need to go straight forward, in the same way probes are sent to other planets that orbit along mostly the same orbital plane as ours.
Conclusion: keep 24 Themis in mind for later as it could be an interesting location once we are able to travel to it with ease. It has a diameter of 198 km which would give it a surface area roughly equivalent to that of Greece. Note though that the recent discovery of how water might be formed could mean that ice could be present on many more asteroids besides 24 Themis.