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Space

Spacecraft Buzzes By Mercury 62

Posted by CmdrTaco
from the it's-time-to-buzz-the-planet dept.
Riding with Robots writes "The robotic spacecraft MESSENGER is making its second fly-by of the first planet today, skimming just 200 kilometers above the surface. The fly-by will reveal portions of the planet that have never been seen before, but the main purpose of the maneuver is to prepare for an orbital insertion in 2011. The mission site offers extensive information, along with the first pictures that are already arriving on Earth, with many more expected in the coming hours and days."
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Spacecraft Buzzes By Mercury

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  • Re:The coldest place (Score:4, Informative)

    by Anonymous Coward on Monday October 06, 2008 @09:20AM (#25272237)

    Some areas in the polar regions of Mercury (deep crater floors) may be permanently shadowed and hence very cold. Similarly to some areas on the Moon poles. This is due to the very low obliquity of the planet. This was discovered by radar studies done from Arecibo, which had anomalously high signal return in some restricted polar regions. This will answered most definitely by MESSENGER itself when it gets into orbit in a few years.

  • by mbone (558574) on Monday October 06, 2008 @10:10AM (#25272793)

    Because it is energetically tough to get to Mercury they are trying to get into with as little fuel expenditure as possible, to send as much payload as possible. Since there is no atmosphere, aerobraking is not possible, and thus they are using gravity assists to help reduce the orbital insertion delta-v to a manageable number. Each flyby speeds up the spacecraft a little, to better match Mercury's orbital velocity, and they decided on 3 of these to get the performance they wanted. There is a synodic period (the orbital beat period) between each such opportunity, so it takes a while to complete three flyby gravity assists.

    The mission FAQ [jhuapl.edu] has more information on this.

  • by mbone (558574) on Monday October 06, 2008 @10:13AM (#25272827)

    sorry - "speeds up" should be "slows down," above.

  • by Anonymous Coward on Monday October 06, 2008 @10:15AM (#25272843)

    The flybys and the ridiculously indirect route are not to speed up the craft, its actually to slow it down. Mercury is a very small planet, a little bigger than our moon, so the flybys are meant to slow down the craft enough so that it can be "caught" in the very low energy level orbit of Mercury.

  • Re:delta-v (Score:5, Informative)

    by Migraineman (632203) on Monday October 06, 2008 @11:02AM (#25273375)
    Moving around in space is all about changing your velocity. There are a number of ways to effect that change - gravitational slingshot, aerobraking, big sails, thrusters ... Each has advantages and disadvantages. For example, direct thrust may provide the most direct path to your objective, but the fuel requirement may be impractical. The mission designers have chosen a method of getting MESSENGER (about 1000kg of payload) to it's objective with enough fuel on-board to perform it's mission. Many variables have been considered - launch vehicle requirements, time to arrival, duration of mission, required consumables, etc. It's a horribly complex optimization.

    The most efficient time/location to make orbital adjustments is apogee or perigee. If you enter into a highly eliptical orbit and wish to circularize at a much lower altitude using only a fractional-Newton thruster, yeah, it'll take a while. MESSENGER has a 650N main thruster, but only about 600kg of propellant. That equates to "not a lot" of thruster time. The main engine has a Specific Impulse (Isp) [wikipedia.org] of 318 seconds. [spaceref.com] On Earth, you'd get about 318 seconds (5+ minutes) of operation. That gravitational element doesn't really apply out in space, so the available thrust-time will be longer. The NASA PDF [nasa.gov] indicates that the final orbital insertion burn will consume 30% of the propellant, and will last about 14 minutes. Extrapolating, that indicates that MESSENGER has about 42 minutes of propellant on board.

    There's also a nice explanation of the orbital maneuvers on the JHUAPL website, [jhuapl.edu] and also a nice PDF showing the orbital insertion cost plots. [nasa.gov]
  • Re:delta-v (Score:4, Informative)

    by p3d0 (42270) on Monday October 06, 2008 @02:04PM (#25275561)

    The main engine has a Specific Impulse (Isp) of 318 seconds. [spaceref.com] On Earth, you'd get about 318 seconds (5+ minutes) of operation.

    No. Specific impulse, despite being measured in seconds, has nothing to do with how long the rocket can fire. That obviously depends on how much propellant you carry.

    Take another look at that Wikipedia article you linked on specific impulse [wikipedia.org].

  • Re:delta-v (Score:3, Informative)

    by Migraineman (632203) on Monday October 06, 2008 @07:07PM (#25278753)
    Crap ... typing too fast ... not enough sleep ...

    Fthrust = Isp * (mass flow rate) * (gravity on Earth), which allows us to solve for the mass flow rate:
    650N = 318s * MFR * 9.8m/s^2
    MFR = 0.209 kg/s

    With 600kg of propellant on board, you'd be able to fire the engine for 600kg / 0.209kg/s = 2871 seconds on the Earth's surface ... a little over 47 minutes. At least that's consistent with the other derived number. Sorry about that, Chief.

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