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Temperatures at Mercury Orbit
sorry if this is a repeat post...
How hot would a nearly ideal black body object get in Mercury orbit? I've read that Mercury's daytime temperatures can spike to 800F, but my calculation for average temperatures at Mercury orbit was 440K/328F. Mike Miller, Materials Engineer |
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"Henry Spencer" wrote in message ... In article .com, wrote: How hot would a nearly ideal black body object get in Mercury orbit? Mercury's average distance from the Sun is about 0.39 Earth's, so by the inverse-square law, a black body gets about 6.6x as much solar energy. However, energy radiated as heat is proportional to the fourth power of temperature, so temperature only needs to rise to 1.6x to match this. However however, that's *absolute* temperature, so a black body's temperature goes from around 300K at Earth to around 480K at Mercury, i.e. from around room temperature to around 200degC. Caution, I oversimplify in several ways. Inverse square is not exactly right for a source of finite size and this effect is not negligible for Mercury; Mercury's orbit is noticeably elliptical and the semimajor axis is not really the average distance; and most important, Mercury is not an ideal black body in several ways, and that can make a big difference. I've read that Mercury's daytime temperatures can spike to 800F, but my calculation for average temperatures at Mercury orbit was 440K/328F. See above. Your calculation's certainly in the right ballpark and is probably correct, but it's for an oversimplified model. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | What would be the temperature underground on mercury say from 20 to 100 feet down. |
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Henry Spencer wrote:
Mercury is not an ideal black body in several ways, and that can make a big difference. Thank for the answer and explanation. I was actually thinking of a black sunshield for a Mercury orbiting object, but the other issues are noted. Two follow-up questions: *If the sunshield is uninsulated, would the IR radiation from the sunshield warm the objects behind the sunshield as much as if they were directly exposed to the sun? *If the sunshield is "perfectly" insulated on its back (leaving it with half the area to radiate heat), would its absolute temperature climb by the fourth root of 2 (1.189) to about 570K? Mike Miller, Materials Engineer |
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" writes:
sorry if this is a repeat post... How hot would a nearly ideal black body object get in Mercury orbit? I've read that Mercury's daytime temperatures can spike to 800F, but my calculation for average temperatures at Mercury orbit was 440K/328F. Mike Miller, Materials Engineer Your calculations are about right for a spherical black body at Mercury's distance from the sun. A flat plate at the same distance, front face painted black and pointed at the sun, back face coated with an ideal insulator, will reach 640K or 700F. Same heat input, less surface area for radiant heat output. As Mercury doesn't have an atmosphere or an ocean to complicate things, we might reasonably conclude that the *average* temperature on Mercury would be somewhere around 440K and the *maximum* temperature would be more like 640K. On an equatorial plain of loose, dark regolith at local noon. -- *John Schilling * "Anything worth doing, * *Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" * *Chief Scientist & General Partner * -13th Rule of Acquisition * *White Elephant Research, LLC * "There is no substitute * * for success" * *661-718-0955 or 661-275-6795 * -58th Rule of Acquisition * |
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