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Mercury closeups
Jan. 16th, 2008 10:36 pmThe MESSENGER team has released three close-up images from the flyby, in the image gallery. Emily Lakdawalla has, as usual, interesting things to say about one of them, the closeup of crater Vivaldi.
This is an area that Mariner 10 actually got pictures of in September 1974, but the sun angle was totally different. This is the opposite of the situation with Caloris Basin—Vivaldi was right on the border between day and night when MESSENGER flew past, so the new pictures pick out the topography much more starkly, but for Mariner 10 the sun was shining on it at a high angle and the old picture mostly shows brightness variations.
MESSENGER's eventual mapping orbit in 2011-2012 is interesting in this regard. Earth's rotational period and its solar day are almost the same—they only differ by about four minutes—because Earth takes about 365 of its days to go around the Sun. But Mercury's solar year is only one and a half times as long as its rotational period, so Mercury's motion around the Sun is a major contributor to the Sun's apparent motion from Mercury's surface. It works out so that, while the rotational period (or sidereal day) is two-thirds of a Mercurian year, Mercury's solar day is two Mercurian years long, or about 176 Earth days. So the whole of Mercury's surface will go through two full days over the course of MESSENGER's primary science mission. I guess that allows MESSENGER to see the whole surface under varying lighting conditions—except that the spacecraft's own orbit will be highly elliptical, and the portion of the planet where it will get the closest pictures will vary only slowly over the course of Mercury's rotational period. What that all adds up to is...I don't know, you work it out.
This is an area that Mariner 10 actually got pictures of in September 1974, but the sun angle was totally different. This is the opposite of the situation with Caloris Basin—Vivaldi was right on the border between day and night when MESSENGER flew past, so the new pictures pick out the topography much more starkly, but for Mariner 10 the sun was shining on it at a high angle and the old picture mostly shows brightness variations.
MESSENGER's eventual mapping orbit in 2011-2012 is interesting in this regard. Earth's rotational period and its solar day are almost the same—they only differ by about four minutes—because Earth takes about 365 of its days to go around the Sun. But Mercury's solar year is only one and a half times as long as its rotational period, so Mercury's motion around the Sun is a major contributor to the Sun's apparent motion from Mercury's surface. It works out so that, while the rotational period (or sidereal day) is two-thirds of a Mercurian year, Mercury's solar day is two Mercurian years long, or about 176 Earth days. So the whole of Mercury's surface will go through two full days over the course of MESSENGER's primary science mission. I guess that allows MESSENGER to see the whole surface under varying lighting conditions—except that the spacecraft's own orbit will be highly elliptical, and the portion of the planet where it will get the closest pictures will vary only slowly over the course of Mercury's rotational period. What that all adds up to is...I don't know, you work it out.