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Greg Hullender
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

MarkG
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...)
maschnitz
Oh, that's easy - it's called the Oberth effect. The deeper in the gravity well you are, the more energy gained from a burn.
MarkG
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.
brellis
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?
elakdawalla
I know someone I can ask this question -- let me see if I can get a response.
elakdawalla
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.
QUOTE (Jim McAdams)
The lowest arrival velocity for a spacecraft on a ballistic trajectory approaching Mercury is achieved when Mercury is at perihelion. For much of the interplanetary cruise phase, this option served as a contingency back-up for the MESSENGER prime trajectory. As much as 200 m/s delta-V savings were possible with this option. This option required an additional two Mercury flybys (one a few days before the current MOI and another 88 days later) and an increase in launch-to-MOI time of 128-130 days – slightly less than 1.5 orbits of Mercury around the Sun.

Now here is why this trajectory option was not chosen as the baseline for MESSENGER.

The timing of MESSENGER propulsive maneuvers is based on selecting a spacecraft orientation that positions the sunshade between the Sun and sensitive components of the spacecraft. At close range to the Sun, permanent damage or spacecraft failure can occur in as little as an hour without sunshade protection. A peer-reviewed complex orbit insertion sequence of 6-7 maneuvers (vs. 1 or 2 maneuvers for the nominal mission plan) is required to place the spacecraft in the science-defined initial orbit about MESSENGER. This process takes over 6 weeks, adding significant risk and further delay. The primary reason that the sequence is so complex is that the Mercury-relative arrival geometry leads to an initial orbit inclination of 89-90 degrees – not the 80-83 degrees desired for initial orbit inclination. To achieve a low-cost orbit inclination change, the initial orbit must have a much larger orbit period. This subjects the orbit to large perturbations from solar gravity. Solar gravity alone can be used to make most or all of the orbit inclination change from 89-90 degrees to 80-83 degrees, but other changes to the orbit introduce the need for multiple corrective maneuvers after initial orbit insertion. With arrival near Mercury’s aphelion and achieving the required spacecraft-Sun-relative orbit orientation for thermal stability, the solar gravity perturbations have the OPPOSITE effect that they do for the primary mission with orbit insertion near Mercury’s perihelion. That is, periherm altitude decreases, leading rapidly to impact with Mercury’s surface in the absence of corrective maneuvers. So the orbit periherm altitude no longer drifts from 200 to 450-500 km followed by periherm-lowering delta-V, but drifts from 450-500 km down to 200 km with periherm-raising delta-Vs. After onboard propellant runs out, the potential for extended mission options would be very minimal. Another complicating factor is that the 5th Mercury flyby must occur at relatively low altitude with the spacecraft flying between the Sun and Mercury when Mercury is near its perihelion – subjecting the spacecraft to heat from solar radiation off Mercury’s surface. This altitude can be kept sufficiently high, but there still is a substantial increase of thermal input to the sensitive portions of the spacecraft.
MahFL
"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 huh.gif ?
elakdawalla
QUOTE (MahFL @ Dec 4 2009, 10:35 AM) *
The explanation is way beyond what Joe Public would understand.
Well, yes. Isn't that what you guys come to UMSF for?
helvick
Well I can't speak for anyone else but posts like that are _precisely_ why I come to UMSF.
Stu
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. smile.gif
remcook
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
scalbers
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".
MarkG
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...)
Greg Hullender
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
Greg Hullender
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
Explorer1
And a new map of some of the newly named craters too:
http://messenger.jhuapl.edu/gallery/scienc...mp;image_id=377
MarkG
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...
infocat13
QUOTE (scalbers @ Dec 5 2009, 08:40 PM) *
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".




burn less fuel = extended mission! smile.gifsmile.gif
ElkGroveDan
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.
MarkG
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!
ElkGroveDan
If it is the planet pursuing the craft, shouldn't it be "Go Mercury" ?
MarkG
QUOTE (ElkGroveDan @ Jun 25 2010, 08:49 AM) *
If it is the planet pursuing the craft, shouldn't it be "Go Mercury" ?

...well, my odds of successfully urging a spacecraft are far better than successfully urging an entire planet...
Greg Hullender
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
stevesliva
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.
MarkG
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?
peter59
100 days before orbit insertion !
Click to view attachment
I hope that it will be without such problems as case of Akatsuki.
peter59
One Year until Mercury Orbit Insertion !
Relax, Mercurian year.
tasp
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.
MarkG
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....)
Phil Stooke
35 million km away... I wonder if there will be any good approach imaging?

Phil
MarkG
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?
jasedm
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 here

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.


stevesliva
Come on, Juno, you're late. wink.gif
jabe
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
B Bernatchez
QUOTE (jabe @ Feb 23 2011, 04:16 PM) *
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


According to this website: http://messenger.jhuapl.edu/orbit_insertio...tionkeeping.htm, "MESSENGER's thrusters must slow the spacecraft by just over 0.86 kilometers (0.53 miles) per second."
climber
QUOTE (stevesliva @ Feb 22 2011, 05:33 PM) *
Come on, Juno, you're late. wink.gif

And Voyager's?...probably out of this world rolleyes.gif
machi
QUOTE (climber @ Feb 24 2011, 01:28 AM) *
And Voyager's?...probably out of this world rolleyes.gif


Maybe this way:

Functioning spacecraft in orbit (VIII.2011) at:

Mercury (1)
Venus (1)
Earth (legions)
Moon (2)
Mars (3) +2? on the surface
Vesta (1)
Saturn (1)
Sun (several (>7))
Milky Way's central BH (3)
ElkGroveDan
QUOTE (machi @ Feb 24 2011, 03:31 AM) *
Functioning spacecraft in orbit (VIII.2011) at:
...
Earth (dozens)
...

I believe the number is in the thousands, somewhere around 3,000 functioning satellites in orbit.
machi
Updated smile.gif
jabe
QUOTE (B Bernatchez @ Feb 23 2011, 11:07 PM) *
According to this website: http://messenger.jhuapl.edu/orbit_insertio...tionkeeping.htm, "MESSENGER's thrusters must slow the spacecraft by just over 0.86 kilometers (0.53 miles) per second."


ok..I was blind...no idea how i missed that.... thanks for the link
jb
SolarSystemRubble
Quiet here in this thread, less that 2 weeks from orbital insertion!!

Wayne
Astro0
Everyone's just cruisin' I guess! smile.gif
nprev
smile.gif

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.)
stevesliva
I thought we had established that Dione is totally boring.
Bjorn Jonsson
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.
JohnVV
QUOTE
I thought we had established that Dione is totally boring.

well the voyager teem thought that the Jovian moons would be "the moon" boring and boy were they wrong
volcanopele
Nope Rhea is boring. But it's monotonous surface does make for nice desktop backgrounds. So it's good for something.
Astro0
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. smile.gif

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