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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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  • The coldest place (Score:5, Interesting)

    by CRCulver (715279) <crculver@christopherculver.com> on Monday October 06, 2008 @08:01AM (#25272043) Homepage

    Larry Niven's first published short story was titled "The Coldest Place" (collected in 3 Books of Known Space [amazon.com] ), based on the idea that the regions of Mercury not hit by the sun would be the coldest place in the solar system. The story was infamous out of date by the time it hit print, as some studies of Mercury had shown that it never got that cold. Nonetheless, reading the story as a child awoke a certain interest of that planet which never gets as much attention as the sexier Mars or Venus or the gas giants. I look forward to following this mission.

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

      by Anonymous Coward on Monday October 06, 2008 @08:20AM (#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.

      • Some areas in the polar regions of Mercury (deep crater floors) may be permanently shadowed and hence very cold.

        That doesn't make a lot of sense to me. Okay, the not being in the sunlight hence cold makes sense, but wouldn't heat be conducted through the planet? That would make it warmer than just being out of the sun, say at Mercury's L2 (assuming that that's in the umbra).
        • Re: (Score:3, Insightful)

          by CheshireCatCO (185193)

          Sure there will be some conduction, but rock doesn't conduct heat terribly well. So while I'd expect Mercury's polar craters to be warmer than the Moon's polar craters, I would expect them to still be really cold. (Not necessarily the coldest place in the solar system, though. In fact, you can guess that it may be a polar region of a Jovian moon since Jupiter's obliquity is only 3 degrees. Or a Neptunian moon; tilt is a lot higher, but 1/r^2 comes in big time there as does the high albedo of the bodies.)

          • L2 wouldn't necessarily be that cold for Mercury, by the way. The planet is still a warm surface radiating toward L2.

            Good point. I hadn't considered that. The same would probably hold true for the gas giants. Although for Coldest-Place-In-The-Solar-System(tm), I'd still be looking more toward Pluto and Sedna, than anywhere inward of the asteroid belt.
            • Actually, Pluto (at least on the average) is warmer than Triton. Albedo wins in that case. For actual coldest place, the rapidly expanding frontier is making that a moving target, I'm sure.

          • by dougmc (70836)

            Rock does conduct heat very well. Well, some do. Others not so well, but none of them are what we would call good insulators.

            But hundreds or thousands of miles of *anything* will make a very effective insulator. :)

            • It's not a good conductor compared to, say, metals, which is my point. Coupled with the point I didn't make explicit, but you did (thanks!), that LOTS of rock is just not a good conductor.

            • "ERROR 589: Your argument is out of relation."

              A huge sun right in front of you for billions of years makes a *very* effective heating device too. :)

              • by dougmc (70836)

                Well, which is going to win? Billions of years of sun, or hundreds or thousands of miles of rock?

                I suspect the sun will. The core of the Earth is hot too, but the surface isn't. But if you go down just a few miles, the temperature increases so much that humans can't survive without special cooling. The Earth has had a few billion years to find an equilibrium too.

                In any event, my only point was that rock isn't a particularly good insulator, that it only seems that way because there's so much of

    • Re: (Score:3, Interesting)

      by DerekLyons (302214)

      Mercury doesn't get as much interest as Venus because it is very, very hard to get to and has an extremely hostile orbital environment once you get there. Venus gets less attention than Mars because it very hard to get there, has a hostile orbital environment and very difficult to learn anything once you do because of the cloud cover.
       
      Mars, by comparison, is merely hard to get to, has a relatively benign orbital environment, and has a transparent atmosphere.

  • by Anonymous Coward
    ... but the main purpose of the maneuver is to prepare for an orbital insertion in 2011

    I thought this kinda thing wasn't happening when I read the No Space Porn [slashdot.org] article?

  • Anybody read the headline and get really excited for a second? Must be because I'm reading through Hitchhikers Guide to the Galaxy (again).
    • No, but as soon as I read it, I thought I was going to be reading about some new, mercury-based propulsion system - or about a spacecraft that had suffered a mercury containment failure and this had caused some weird kind of resonance/electrical short.

      Too much Star Trek, I guess.

  • Could someone please explain why according to the web site the orbit insertion is going to take another pass and another 3 years. Does it really take that long to slow the spacecraft down?

    • by mbone (558574) on Monday October 06, 2008 @09:10AM (#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.

    • Re:delta-v (Score:5, Informative)

      by Migraineman (632203) on Monday October 06, 2008 @10:02AM (#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 @01:04PM (#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: (Score:3, Informative)

          by Migraineman (632203)
          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.
          • by p3d0 (42270)

            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.

            True, though your calculation has nothing to do with Earth's surface. (When Isp is measured in seconds, they multiply by the gravity on Earth's surface just for fun.)

    • If it was easy, they would have done it before now. We managed flybys of Merc 30 years ago, but those are much easier since you don't have to slow down.
  • by rotenberry (3487) on Monday October 06, 2008 @11:15AM (#25274245)

    Note that MESSENGER used solar sailing to correct its trajectory for this flyby:

    http://www.planetary.org/blog/article/00001674/ [planetary.org]

  • This is what we should be doing, instead of quibbling over small things like creationism and Paris Hilton. We should launch several satellites orbiting each planet and few satellites for some of the more interesting moons(Europa, io etc)
    • Re: (Score:3, Insightful)

      by tnk1 (899206)

      AFAIK, neither creationism nor Paris Hilton has had any appreciable effect on Space exploration.

      In any event, most Intelligent Design folks don't deny the usefulness of space science, they just believe that someone started created the Universe. The position that God (or some imaginary man with a long white beard) created the universe does not preclude one from exploring said created universe.

      The actual cause is that too few people are interested in a project which will only become economically significant

    • by mbstone (457308) on Monday October 06, 2008 @12:05PM (#25274821)

      When they eventually build a hotel on Mercury, I want an ice machine that works and doesn't keep running out of ice. So how big would such an ice machine have to be on Mercury? Would they have to charge $3 for a soda? I hate those tacky signs that say "No Filling Ice Chests."

Possessions increase to fill the space available for their storage. -- Ryan

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