MESSENGER: Three days out from Mercury

By Phil Plait | September 27, 2009 7:37 am

The spacecraft MESSENGER is just three days away from its third encounter with Mercury. The past two have been nothing short of frakkin’ amazing, so I’m really looking forward to this final pass. Even though it was 1.3 million kilometers away from the tiny planet on the 25th, it snapped this serene shot:

messenger_sep2509

I love moody astronomical pictures. Here, a crescent Mercury sits in the inky black as MESSENGER screams down on it at 3.3 km/sec (2 miles per second). ON this final pass, the spacecraft will slow enough that it will be able to enter orbit around Mercury in March 2011. The nominal lifespan of the mission will be for one solid (Earth) year of observations, and will map the planet with 18 meter resolution. That’s the size of a house.

That is so going to rock. But that’s 1.5 years away, and we’re still waiting on this third pass. I hope to be able to post pix when they come in, but I’m traveling to the UK for TAM London not long after, so we’ll see. But you can always keep an eye on the MESSENGER website, as well as Emily Lakdawalla’s blog. They’ll be up-to-date… especially, if I know her, Emily, who will be spending the flyby looming over her keyboard and hitting "refresh" every 30 seconds or so.

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Mercury, MESSENGER

Comments (26)

Links to this Post

  1. Fischblog | September 28, 2009
  1. Sili

    I seriously hope that it’ll prove to live longer than a year – given how it’s taken to get to Mercury in the first place.

    And I hope that some random genius figures out a way to deëccentrice the orbit so they can get high resolution of the Southern hemisphere too.

  2. mus

    but but but… where are the stars?

    FAKE!!! it’s all a hoax!

  3. Gary

    http://www.astrologycom.com/mercret.html

    “Retrograde periods, although often problematic for us earthlings, are not particularly uncommon. Each planet retrogrades, except the Sun and Moon. Although a powerful astrological influence, Mercury is quite a small planet that travels at a relatively fast speed through the zodiac. Despite being the closest planet in our solar system to the Sun, he is not always in the same sign as the Sun. This time, Mercury turns retrograde in Libra (relationships; tastes; harmony) while the Sun is in Virgo (analysis; critical ability), but retrogrades into Virgo at the Virgo New Moon on Sep. 18 (while the Sun of course is still in Virgo). The Sun moves into Libra at the Equinox on Sep. 22 while Merc is still in Virgo, but Mercury soon heads back to join the Sun in Libra on Oct. 10.”

    So I wonder if the Messenger probe being near a retrograde Mercury will effect our “relationships,tastes, harmony or critical ability? Of course we all know that followers of such woo woo lack “critical ability” in the first place.

  4. Not John Price (aka APL Minion)

    “will be spending the flyby looming over her keyboard and hitting “refresh” every 30 seconds or so. ”

    And working my web servers to death, but we do *so* love the PR. It means I get to keep working. :)

    Thanks for posting!

  5. So if I’m understanding this correctly, it’s using Mercury’s gravity to slow down, the opposite of how the Voyagers and others used Jupiter and Saturn’s gravity to accelerate to the outer planets. Do you do that by crossing in front of the planet’s orbital path instead of behind it?

    Edit: Just watched the video. MESSENGER passes behind the planet. Now I’m confused – wouldn’t that speed the spacecraft up instead of slowing it down?

  6. KC

    Woot! Go Messenger!

  7. Wow! We’re gonna be able to see houses on Mercury!

  8. Torbjörn Larsson, OM

    toasterhead, orbital mechanics isn’t my forte. But obviously Mercury must be the fastest planet in the system (relative to a Sun observer) as it orbit closest to the sun, way down the gravity well.

    So Messenger has to speed up relative to Sun to catch Mercury, i.e. to slow down relative to it. According to the APL Messenger site, Messenger starts off traveling at 29.8 km/s relative to Sun at Earth orbit (coincidentally, that is Earth orbital speed, see Wikipedia) and ends up traveling at 47.9 km/s at Mercury arrival (which is Mercury orbital speed, see Wikipedia again).

    Gravity assist changes speed relative to the sun. [See Wikipedia for the principle.] That might explain if the gravity assist is as you described.

  9. MNPundit

    So as I have often begun to, I wonder, how will this keep our species from exterminating each other?

  10. NelC

    MNPundit, it keeps wonder alive in the human soul. Without wonder we have no hope and then we might as well kill each other as not.

  11. Loaf Of Bread

    MNPundit, it won’t keep us from exterminating each other.

    Those who want to kill each other will go on killing each other, regardless of what we discover in the universe around us.

    And those of us who enjoy what we discover in the universe will go on enjoying what we discover.

  12. Wayne

    #8 Torbjörn Larsson,

    “A” for effort, but orbital mechanics is weirder than that. Imagine that from Earth orbit I loose enough velocity to go into an elliptical orbit with Earth orbit at the high point (aphelion) and Mercury orbit at the low point (perihelion). When at Mercury’s orbital distance, I’m GOING FASTER than Mercury, or else I wouldn’t swing back out towards Earth. So, you have to loose energy to get your perihelion down to Mercury’s orbit, and then LOOSE MORE energy in order to stay there.

    As Robert Heinlein says in “The Cat who Walks through Walls”, “The most cock-eyed, contrary to all common sense, difficult aspect of ballistics around a planet is this: To speed up, you slow down; to slow down, you speed up.” Of course, what is true of orbiting the Moon is true of orbiting the Sun as well, plus it’s my favorite Heinlein quote.

  13. Troy

    I’m trying to imagine 2 miles per second wow!
    I do know that Mercury is hard to catch, the trajectory so far has been quite elaborate with many assists. A lot of the difficulty is the fact that Mercury it is so far out of the ecliptic. Even if the probe were to suddenly die now the mission would be an astonishing success. The entire globe of Mercury has now been imaged with modern equipment.

  14. Asimov Fan

    Looking forward to the fly by tomorrow. Great image thanks BA. :-)

    Would the resolution of this crescent Mercury be about the same as the unaided eye view of our Moon from Earth?

    @ 13 Troy:

    The entire globe of Mercury has now been imaged with modern equipment.

    Has it? My understanding was that a few thin strips & spots on Mercury have yet to be mapped – maybe less than 5% or so but still something left to discover now & when MESSENGER goes into orbit in 2011.

    ***

    Hmm .. Having trouble posting here for some reason on this particular thread. Tried my other email address which seems to have worked but made a fair number of attempted posts that never appeared here even after getting a ‘duplicate message detected’ message. Is something funny going on here? Anyone else experiencing trouble posting here? :-(

  15. Asimov Fan

    Great image thanks BA.

    Looking forward to hearing about the fly-by tomorrow. :-)

  16. Dr Cy Coe in NL

    Speaking of moody astronomy: The camera seems to love Titan as there are quite a few of these crescent shots of Titan (http://www.nasaimages.org/luna/servlet/view/search?q=titan+AND+crescent&search=Search)

    My fav: http://photojournal.jpl.nasa.gov/catalog/PIA11567

  17. #3 Gary:
    A challenge to astrologers… They say Mercury will “enter Libra” on 10 October. So…
    I will personally give one million dollars to any astrologer who can photograph Mercury, on 11 October, among the stars of the constellation Libra.
    Of course, I don’t have a million dollars, but I don’t think I need to worry!

  18. toasterhead

    12. Wayne Says:
    September 28th, 2009 at 12:52 am

    As Robert Heinlein says in “The Cat who Walks through Walls”, “The most cock-eyed, contrary to all common sense, difficult aspect of ballistics around a planet is this: To speed up, you slow down; to slow down, you speed up.”
    _________

    Ok, I think I got it. So you’re using the relatively slower speeds of the inner planets to bleed off your initial Earth-orbit velocity.

    It is cock-eyed, but beautiful. Interplanetary ballet.

  19. BigBob

    3rd pass already!? Doesn’t the time fly. I drooled all over my keyboard during the last pass and this one will be no different. On the astrology front, from Messenger’s point of view, Mercury will fly through all constellations several times a day when Messenger settles into a more circular orbit.
    Loaf at #11, great lines, well put. They’ll go on doing what they do, and we’ll go right on floating in Astronomy goodness.
    Bob(Big)

  20. NelC

    But, Mus@2, everybody knows there aren’t any stars that close to the Sun.

    ;p

  21. Torbjörn Larsson, OM

    #12 Wayne,

    Thanks for the kind words, but truth is I messed up big time as I glossed over the potential energy loss before acquiring the orbit. Luckily toasterhead listened to saner advice.

    I like your argument, because then I don’t need to analyze orbital motion for its effect on energy balance.

    Yet another way to see that a round orbit is the least energy state is to realize that this is the reason for hydrostatic spherical shape of planets. [More generally, this condition must mean stability of this type of field, or, say, planets would split and sunder.] So the probe going in another trajectory at the moment it hits Mercury orbit (acquires the same potential energy) must have higher kinetic energy in the balance, and needs to bleed off speed in the gravity assist.

  22. Bryan Feir

    Wayne @#12:

    Or, as one of my physics profs used to say, in orbital mechanics you have to go forward to go up, go up to go back, go back to go down, and go down to go forward.

    (Forward to go up: forward acceleration shifts you into a higher orbit. Up to go back: going straight up causes you to lose angular velocity because you have greater distance per orbit. Back to go down: reverse acceleration causes your orbit to decay. And down to go forward, dropping down increases your angular velocity because the orbit is smaller but your linear velocity stays similar.)

    Real-time orbital corrections can be highly non-intuitive.

  23. Troy

    Asimov fan, yes there are probably small parts not imaged.

  24. Gary Ansorge

    23. Bryan Feir

    That’s actually a much better description of orbital ballistics than Heinliens, since the closer you are to the sun, the faster you have to move in order to remain in orbit but it’s also true that to drop your orbit closer to the sun you have to slow down from current velocity. The thing is, as your orbit decays(gets closer to the sun) your velocity WILL increase. It’s much more about ones potential energy(height above the gravity source times mass times the acceleration) being converted to kinetic energy(velocity^2 times mass/2). We start at a high potential energy with some kinetic energy,ie,earths velocity is about 30 km/sec and we’re about 150 million km above the sun, so we have both potential and kinetic energy. That’s what makes it so darned counter intuitive to figure out how to go where we want to go. To get closer to the sun, we reduce our orbital velocity, drop down and our Velocity then increases while potential energy decreases. If you twirl a weight around on a string, pull it in closer to you(pretend you’re the sun) it will increase the objects velocity. If you reduce the velocity, it’s distance from you will decrease. Not the greatest model, because it’s also affected by earths gravity but maybe it will give some insight.
    Jus an addendum: Mercurys velocity is about 48 km/sec.

    GAry 7

  25. Asimov Fan

    @ 24. Troy – Thanks. :-)

    @ 17. Neil Haggath Says:

    A challenge to astrologers… They say Mercury will “enter Libra” on 10 October. So… I will personally give one million dollars to any astrologer who can photograph Mercury, on 11 October, among the stars of the constellation Libra. Of course, I don’t have a million dollars, but I don’t think I need to worry!

    Be careful there – there’s always photoshop! ;-)

    You’d be well advised to stress a genuine, unfaked and confirmed photo. Then agian I wonder how many astrologers could even find Libra in the sky or name any of its stars and their spectral types. ;-)

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