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    Having lived in the southern hemisphere for 2 years, the 'man in the moon' from my UK childhood is becoming a distant memory. In fact I can't find any reference to the face I was seeing. Would the moon surface be different in appearance in Australia and New Zealand than in Britain?

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    Where can we find some actual (normal visible light) photographs of the sun taken from outside the near-earth environment? There seem to be no photos of the sun available, taken, for example, from a point half-way between the earth and the sun, or taken from the vicinity of Saturn, Jupiter, Uranus, Neptune or Pluto. Voyager 1 and 2, Pioneer 10 and 11, and numerous other deep-space spacecraft, from the USA as well as from Russia and from the ESA, surely must have taken some photos of the sun. Where can we find them? Why has NASA never released them?

Thank you very much.

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    What makes the Sun hot? Energy is released in the core of the Sun by nuclear processes, which combine atoms of hydrogen into atoms of helium. Four hydrogen nuclei (aka protons) combine to form one of helium, with some energy left over, and that energy provides the heat.

    Many chemical reactions require a higher temperature (boiling an egg, for instance), and so do nuclear reactions. The temperature required for the solar energy release is enormous, and creates enormous pressure. Only near the center of the Sun can these conditions exist, with the weight of the higher layers of the Sun keeping the energy-release region confined. If that confinement did not exist and it had the chance to expand, it would cool down and all nuclear energy release would end.

1. What concept of physics causes the space shuttle to get so hot that it almost burns up when returning to earth? 2
2. What one word describes this concept?
3. What kind of energy is present during reentry?

Also how are these problems fixed if nessecary?

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    Many of these problems were addressed on the space station "Skylab". Astronauts tend to lose weight, muscle tone and bone--in a way which somewhat resembles problems of patients who are bedridden for a long time. Exercises help prevent these from occurring. I am not sure about more subtle effects--you will have to search on the web.

    Unfortunately, I'm getting hung up on that lifting gas. I suspect that Helium will be in short supply on Mars. On earth, it is a byproduct of natural gas, but on Mars, there should be little, or no natural gas, and helium would have to be mined by itself.

    My idea however, is that since the atmosphere is mostly CO2, blimps with much heavier gasses could be used. I would expect hydrogen, oxygen, and nitrogen blimps to be common, since these three gasses are highly useful commodities in their own right on a world with little water, and no breathable atmosphere.

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    About the lifting power--it equals to the weight of the displaced atmosphere, minus the weight of the gas inside the balloon, required to keep it inflated. Whether that is hydrogen or helium should make little difference, because either gas weighs much less than the air (or on Mars, carbon dioxide) which is displaces. Hydrogen should be available on Mars, if only a little water can be extracted there, which seems likely. Hydrogen was of course used in all the Zeppelins in WW-I and with proper caution is quite usable.

    Oxygen and nitrogen are rather heavy, cutting the lifting power by a factor of about 4: considering how marginal any balloon is on Mars, they are clearly unsuitable..

[Note: The above turned out to be a false rumor, which had quite wide circulation. The reply below was written before the new media reported on it.]

    Beats me. The average Mars-Earth separation is 0.5237 AU, and with 1 AU equal approximately to 150,000,000 km or 93,000,000 miles, this comes to 48.8 million miles. The actual closest approach may be larger or smaller, because of the ellipticity of the orbits (mainly the one of Mars),

    In any case, Mars seen by the unaided eye is never as big as the Moon. It is true, though, that viewed through a telescope, Mars appears to be about as big as the Moon seen by the unaided eye (although, unless seeing conditions are very good, the image may shimmer and distort).

    If they do continue to show interest more than a few days in the telescope project, then I hope to help them build (instead of buy) a set of telescope drive controls. So far their enthusiasm has not waned, and they are asking a lot of questions about how to find objects in the sky and point the scope. My experience doing some controls engineering will help, but I want to let them do some of the programming to make the telescope follow the right target. My challenge will be to keep their challenges at the right level.

    Your pages provide a nice balance. Appropriate examples, and reasonable approximations that make some of the fundamental calculations of motion of celestial objects trackable. I think if I can learn that art of finding and sharing simple examples, then the telescope project will generate several fun and useful lessons. Otherwise, it will become just another project.

Thanks for your contributions in making physics, astronomy and earth sciences fun for non-scientists.

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    Also, what are the leading "hot" topics on Solar Physics now?

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    The solar wind starts not from the Sun's surface, but from the corona, and is accelerated somewhat gradually. Obviously, it has to overcome solar gravity, which I suspect is one of the conditions needed for accelerating the solar wind--maybe like a lid on a pressure cooker, holding down the hot corona until it can just barely escape.