Mercury Orbit Insertion, Events and Discussion leading up to MOI |
Mercury Orbit Insertion, Events and Discussion leading up to MOI |
Nov 25 2009, 05:30 AM
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Senior Member Group: Members Posts: 1018 Joined: 29-November 05 From: Seattle, WA, USA Member No.: 590 |
Excerpts from a new press release from the Messenger Team:
QUOTE Deep-Space Maneuver Positions MESSENGER for Mercury Orbit Insertion The Mercury-bound MESSENGER spacecraft completed its fifth and final deep-space maneuver of the mission today, providing the expected velocity change needed to place the spacecraft on course to enter into orbit about Mercury in March 2011. . . . today's maneuver began at 4:45 p.m. EST. Mission controllers at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., verified the start of the maneuver about 12 minutes, 49 seconds later, when the first signals indicating spacecraft thruster activity reached NASA's Deep Space Network tracking station outside Goldstone, Calif. "The team was well-prepared for the maneuver," said MESSENGER Mission Systems Engineer Eric Finnegan, of APL. "Initial data analysis indicates an extremely accurate maneuver execution. After sifting through all the post-burn data I expect we will find ourselves right on target." --Greg |
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Dec 5 2009, 11:07 AM
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Rover Driver Group: Members Posts: 1015 Joined: 4-March 04 Member No.: 47 |
I think it's more efficient at Mercury''s perhelion simply because the relative velocities between messenger and Mercury are smaller. MEssenger is moving closer to the Sun as it goes from Earth to Mercury. So, it will be in an orbit where it meets Merury close to its perihelion. At this point it will be moving faster than Mercury (it has a larger orbit for the same position, so the velocity at that place must be larger). At Mercury's perihelion it will be moving fastest, so the velocity difference between Messenger and Mercury will be smaller. It is slightly closer to the Sun, so also Messenger will be moving a bit faster, but relatively less so than the increase in Mercury's speed, because (sorry for the maths):
Velocity in elliptical orbit: V= sqrt( GM * (2/r - 1/a)) r= radius from sun a= semi-major axis Position r will be the same for spacecraft and mercury. For the spacecraft a will be bigger. Qualitative example with simple numbers: r=9/10 at percientre r=1 for somewhere higher than pericenter a=1 for mercury a=2 for spacecraft GM = 1 (2/r - 1/a) = 1 for mercury, away from pericenter = 1.5 for spacecraft " " Difference = sqrt(1.5) -sqrt(1) = 0.225 = 1.222 for mercury at pericenter = 1.7222 Difference = sqrt (1.7222) - sqrt (1.222) = 1.31 - 1.105 = 0.205 is less than 0.225 |
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