![]() The mission comprises two science orbiters which will be delivered into complementary orbits around the planet by the Mercury Transfer Module in 2025. It will swoop by the planet at an altitude of about 200 km, capturing imagery and science data that will give scientists a tantalizing first taste of what's to come in the main mission. As stated on the main Mercury page, depending on latitude an observer would see the Sun do some very strange things.Hot on the heels of its last Venus flyby in August, the spacecraft's next exciting encounter is with Mercury at 23:34 UTC on 1 October (01:34 CEST 2 October). Such is the case with a highly elliptical orbit and an orbital-rotational resonance ratio other than 1:1. It is much quicker around aphelion (position 4) which is 43.4 million miles from the Sun. Observe how slow Mercury's precession is for our observer around the Sun close to perihelion. You may have noticed something about Mercury's revolution around the Sun. 175.8 Earth days has elapsed for our observer. Notice how we are finally back to our noontime position where we started, directly at perihelion. After another 58.6 Earth days, location 15 shows the third rotation of Mercury completed and the planet having just revolved around the Sun twice. Our person of interest has now just endured a long night of temperatures falling to between -300 and -350 degrees F. It has now taken 117.2 Earth days to get from our observers noontime placement on position 1, to just before sunrise at number 10. ![]() Position 10 shows the second rotation complete, and our observer is likely starting to see some "morning" twilight just before sunrise. Location 7 shows the time of day is now midnight, and Mercury has completed one orbit around the Sun. He/she gets their wish as a long night settles in. After baking in temperatures near 800 degrees F., our observer would probably like to cool off. It took 58.6 Earth days to get from position 1 to 5, with the Sun being visible to our observer right up until just before position number 5. Two-thirds of the way around the orbit, location 5 shows the first rotation complete and nightfall beginning. Our "observer" is denoted by the small dots black for the first rotation, green for the second, and purple for the third rotation. ![]() Starting at the top of the image in location number 1 at perihelion (28.6 million miles from the Sun), we can consider this "noon" for the observer. In this example, our fictitious observer is shown as the small circle "attached" to Mercury standing on or near the equator. The image above shows how Mercury completes three rotations around the Sun for every two orbits. Mercury does not tilt on its axis, therefore the Sun would constantly be situated on the horizon at the poles). (This is assuming an observer is not at one of the poles. However due to an orbital-rotational resonance ratio of 3:2, a fictitious observer on Mercury would see that a solar day from noon to noon would take about 176 Earth days to complete. One rotation takes nearly 59 Earth days to complete.
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