Excerpts from a new press release from the Messenger Team:
Also from the release...
"A 3.3-minute firing of its bi-propellant engine provided nearly all of the probe’s 177 meter per second (396 mile per hour) increase in its speed relative to the Sun."
Quite a little kick -- it moves the periapsis of Messenger's orbit very close to Mercury's. Somehow, capture at periapsis is most energy-favorable, but it is not casually obvious why. (Maybe someone will be kind enough to post an explanation...)
Oh, that's easy - it's called the http://en.wikipedia.org/wiki/Oberth_effect. The deeper in the gravity well you are, the more energy gained from a burn.
I think the Oberth effect is only an oblique influence in the positioning of the capture point of Messenger in Mercury's orbit. There may be some advantages from the positioning of the 3-body problem as far as Messenger's entry into Mercury's gravity well, but there were not rocket firings in any of the fly-by's, and the capture rocket firing will be for losing energy relative to Mercury (admittedly centered at Mercury closest approach, so that aspect of the capture rocket firing is Oberth-enhanced). What is not obvious is why this near-Mercury capture is more efficient at Mercury's perihelion, rather than elsewhere in its orbit.
Perhaps the net trajectory deflection by Mercury's gravitational field can be viewed in some sort of approximation as a delta-v impulse, and that is why the fly-by's and capture are grouped near Mercury's perihelion, and that is how the Oberth effect is realized.
Is there a bigger/better communication window at perihelion? If there needs to be a flurry of important transmissions, might as well schedule the crucial points for such a position?
I know someone I can ask this question -- let me see if I can get a response.
That was quick! This is from Jim McAdams, MESSENGER Mission Design Lead Engineer. I can't say I understand it all -- the physics of trajectories is not one of my strengths.
"is required to place the spacecraft in the science-defined initial orbit about MESSENGER." How does the spacecraft called Messenger orbit around it'self called Messenger. Surly he meant Mercury ?
The explanation is way beyond what Joe Public would understand.
So basically it's overall safer to go in fast and at the low point, is that what he's trying to say ?
Well I can't speak for anyone else but posts like that are _precisely_ why I come to UMSF.
One of the (many) joys of UMSF (and, it has to be said, Emily's excellent blog for The Planetary Society, for which she gets nowhere NEAR enough credit) is that we can enjoy info and input here that simply isn't available anywhere else. Joe or Josephine Public can come here and see gorgeous pics and read soundbites, others with more knowledge about the more scientific side of things can enjoy the more in-depth info.
Thank the Universe UMSF is here - a little Babylon 5 of sanity in the insane internet cosmos; we'd all be stuffed without it.
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
A quick read tells me the perihelion option would save propellant during the initial insertion, however the inclination wouldn't have been too favorable, and more flybys would have been needed, so they chose another option. By the way I like the term "periherm".
That was a great reply and discussion about capture at perihelion. After thinking about it a bit, an easier-to-understand visualization came to mind...
When Mercury is at perihelion, the sun's gravity is stronger relative to Mercury's at a given distance from Mercury (compared to Aphelion), thus reducing the size of Mercury's "gravity well", and thus requiring less delta-v to stay in it.
So Messenger can get closer to Mercury before the dynamics become Mercury-dominated, rather than Solar-dominated. (Remember, this is a visualization, not a precision calculation...)
Just a few weeks over one year away now, Messenger just passed the four-billion-mile mark.
http://messenger.jhuapl.edu/news_room/details.php?id=142
This time next year, things should be hopping!
--Greg
A new post from the Messenger team, noting that we are exactly one year away from MOI!
http://messenger.jhuapl.edu/news_room/details.php?id=144
--Greg
And a new map of some of the newly named craters too:
http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=377
Over the next few months, Mercury will appear to go back-and-forth relative to the sun as seen from Messenger, as the planet and probe travel their elliptical orbits, with Mercury slowly lapping Messenger. The back-and-forth-relative-to-the-sun motion marks the dynamics of passing through the halfway period of the "lapping", with Messenger doing 5 orbits to Mercury's 6, between the third flyby and orbit insertion. Just a chance for some Keplerian visualization cheap thrills...
I'd love it if someone could locate (or create) an animation or video of Messenger's orbit insertion process since launch so I could wrap my brain around it all. So far I haven't had any luck tracking one down on my own.
http://messenger.jhuapl.edu/the_mission/movies/encounters/od131cruiseorbitsandtimeline.mov
From
http://messenger.jhuapl.edu/the_mission/ani.html
Mercury is about to pass behind the Sun in its pursuit of Messenger at the halfway point between the 3rd flyby and orbit insertion. Mercury is at periapsis and Messenger at apoapsis. Go Messenger!
If it is the planet pursuing the craft, shouldn't it be "Go Mercury" ?
Yesterday was the sixth anniversary of Messenger's launch, but the Messenger web site didn't comment on it. I suppose they're far more interested in getting ready for their big day in March.
On the trivia front, I'm figuring Messenger will be two orbits away from MOI on August 10 (next week) and it'll be just one orbit away on November 23, which is roughly American Thanksgiving (Nov 25th this year to be exact.)
Disclaimer: My method is apt to have a few days of error at this point, so don't use these figures to pilot your own spacecraft!
--Greg
Still looking for Vulcanoids. From Twitter:
@MESSENGER2011: 4-part vulcanoid survey will be conducted from 8/14-17. Long-exposure images from MDIS in search of obj existing w/i the orbit of Mercury.
Messenger is pulling away from Mercury for the last time. As Messenger approaches perihelion, Mercury is just past aphelion, so it will pull father away for a little while. But soon after Messenger passes perihelion and begins its last solar orbit before insertion, Mercury will close the gap, and the following perihelion will be orbit insertion around Mercury.
Any results in the Vulcanoid search? Will there be another round of searching during this last independent perihelion?
100 days before orbit insertion !
One Year until Mercury Orbit Insertion !
Relax, Mercurian year.
It's just amazing they racked up over 4 billion(!) miles on the odometer so quickly. New Horizons seems screaming fast, but Messenger could blow it's doors off!
(Yeah, I know we are just seeing the effects of the sun's gravitational well, but still)
Even more amazing when you consider all the flybys of Venus and Mercury were to slow the craft. I recall the Mercury and Gemini astronauts were trained in orbital mechanics by having them drive in circles of differing sizes at differing speeds to impress upon them how it all works.
Messenger is at its last Aphelion before capture into Mercury orbit. Cool (well, as cool as it will ever again get, unless it goes into Mercury's shadow....)
35 million km away... I wonder if there will be any good approach imaging?
Phil
At Messenger's current distance from Mercury, a sharp-eyed human observer would be able to make out the tiny crescent shape. It won't be long now. Will any navigation images be released, I wonder?
A little over three weeks to go! - have been genning-up on the upcoming orbit insertion and the mission generally - very much looking forward to results from the mission (especially the data on any volatiles at the poles)
The orbit insertion is unusual I believe; in that the burn and turns will be 'line of sight' from earth throughout, so no nail-biting, peanut-eating occultation phase whilst we wait for re-acquisition of signal to confirm whether insertion went as planned. Four DSN stations will be tracking, with a fifth as backup during OI. Full information at the official site http://messenger.jhuapl.edu/orbit_insertion/stationkeeping.htm
Astonishing that the spacecraft has travelled 4.8 billion miles already - enough to take it well past Pluto's orbit had it been heading outwards from the sun.
We've been spoiled by having unfettered access to all the raw images from the MER's and Cassini as soon as they're available, but I understand this won't be the case with Messenger, so we'll have to be patient with image releases as this mission unfolds...
Incidentally, if all goes to plan, by August this year there will be amazingly, functioning spacecraft in orbit at :
Mercury (1)
Venus (1)
Earth (dozens)
Moon (2)
Mars (3) +2 on the surface
Vesta (1)
Saturn (1)
Very lucky to be living through this era.
Come on, Juno, you're late.
Been trying to find what the Delta-V is for the burn? Going to be neat to see the burn in "real time" watching the progress of the burn for a change.
jb
Updated
Quiet here in this thread, less that 2 weeks from orbital insertion!!
Wayne
Everyone's just cruisin' I guess!
No news is good news now. I expect the future Messenger main mission thread(s) to get very busy indeed, though...Mercury is not as simple as it seems.
(Disclaimer: no inside knowledge claimed, just a very confident prediction. We've consistently found that upon close enough examination no Solar System body is merely a nondescript rock...in itself, one of the most profound revelations of UMSF.)
I thought we had established that Dione is totally boring.
There's not even a single planet/satellite/asteroid/comet that is boring in my opinion (and BTW I suspect you are confusing Dione with Rhea).
There are already some hints from the Messenger flyby data that Mercury is more interesting/complex than previously expected.
Nope Rhea is boring. But it's monotonous surface does make for nice desktop backgrounds. So it's good for something.
I've been rummaging through some boxes and found a bunch of old NASA/JPL newsletters.
Thought that the covers of these might help wet your appetites for MESSENGER's pending MOI.
I wonder what happens if you dial that number at the bottom...
I used to do that! For Voyager, not Mariner 10, but it was the same thing. Don Bane from the Public Affairs Office would record a phone message, updated every week during slow times, and daily or more often for busy times like a flyby. It was the equivalent of checking into UMSF every morning.
I'm at LPSC this week... I'll post a few pics when I get back... but regarding Mercury, I asked Sean Solomon if his spacecraft was going to impact on Mercury at the end of the mission so I could have a point to plot on a map. I think he preferred not to think about that just yet... but yes, it will. Still not clear to me if it will be a controlled impact, or just left to strike at an unknown location.
Phil
I'd hardly call that a "morbid" interest, kwp. Phil has been diligently mapping space hardware/man-made crash sites on the Moon, and high-speed impacts are obviously of scientific interest since they frequently expose fresh subsurface material. Knowing Messenger's final impact region would greatly facilitate spotting the nice fresh crater it will make from some future Mercury orbiter (Bepi-Columbo?)
The Pioneer Venus craft experienced large perturbations from it's proximity to the sun at Venus distance. That affect will be worse at Mercury, and with the Mercurian orbital eccentricity, the effect will be variable over time, too.
(I am deliberately neglecting Messenger altitude and eccentricity, it is still early here)
She's getting closer
Ingenious idea but impossible! Remember, we didn't have a rocket big enough to get Cassini to Saturn without several gravity assists to help it, so the reverse is bound to be impossible with a little bit of residual fuel. And Cassini's fate is decided - burn up in Saturn's atmosphere.
Phil
If I understand things correctly (and I probably don't), it might actually be easier in terms of delta-V to crash into Mercury from the outer Solar System. Saturn's heliocentric orbital velocity is much lower than that of the Earth, so presumably that means less thrust would be required to negate it & 'fall' into the inner system.
However, we're still probably talking about a change in velocity of several (if not tens) of km/sec, plus escaping from Saturn orbit. I doubt that Cassini could have done this even if it was fully fueled at the beginning of the maneuver.
The sun-grazer and long-period comets are an extreme example of this right? A tiny nudge in the Oort cloud is more than enough to send them in a more or less straight line sunward.
Yeah, basically. The stuff way out there is barely moving in comparison to the planets; probably doesn't take much to negate their orbital motion at all (e.g., gravitational nudges from passing stars over long periods of time, perhaps occasional outgassing from the cometary bodies themselves?)
If I have calculated correctly, you need a delta-V of 5.4 km/s to go from Saturn's orbit into a Hohmann transfer orbit intersecting Mercury. Dawn could have done it (ignoring distance from Sun issue), but I doubt if Cassini ever had that much of delta-V.
At the present time (this Sunday Evening), Mercury would appear from Messenger to be about the same size as the Moon from Earth, with the Sun looming 3 times that diameter.
I think Messenger must halve its current distance to Mercury to enter the Hill Sphere... Soon!
The cheapest (in terms of energy) way to get a spacecraft very close to, or into, the Sun is indeed a Jupiter flyby. Scientifically not very interesting because the spacecraft would not spend much time near the Sun during periapsis. See the Solar Probe Plus trajectory options at
http://solarprobe.jhuapl.edu/mission/docs/2015-2018missions.pdf
pages 3 and 4. The Jupiter flyby option requires by far the least C3.
Not far now!
The orientation of the day side is to the right vs. to the left in http://messenger.jhuapl.edu/whereis/index.php. I had wondered about the latter since Messenger is ahead of Mercury in its orbit waiting for the planet to "catch up" to it. EOTSS appears to have the view in accord with the convention of North pointing up.
A minor milestone just occurred to me: Not only will Messenger become the first spacecraft to orbit Mercury, but for the first time we will have orbited every terrestrial planet in the Solar System...in fact, we will have active spacecraft orbiting every major body in the inner Solar System.
Maybe that's not such a minor milestone, actually... ...wow!
nprev,
The milestone is that every planet known to the ancients, all the classical planets, will now have been the host of an orbiter from Earth. I think this is a very significant milestone. I find it interesting that Mercury was the last of the classical planets to be explored by an orbiter.
We are nearing the end of the initial reconnaissance of the Solar System. What a time to be alive!
Ron
And apologies for those who have seen this before, but Ron reminded me just how significant Messenger's impending detailed of exploration of Mercury really is in the Big Picture; here's a column I did once upon a time for Rui Borges' spacEurope blog:
http://spaceurope.blogspot.com/2008/05/maps-of-history-by-nicholas-previsich.html
nprev:
The galaxy beckons.
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