IPB

Welcome Guest ( Log In | Register )

3 Pages V  < 1 2 3 >  
Reply to this topicStart new topic
Mercury Science
Mariner9
post Sep 29 2007, 12:19 AM
Post #16


Member
***

Group: Members
Posts: 216
Joined: 13-October 05
Member No.: 528



My understanding has always been that it was pure luck that the planned mission trajectory was such that it was possible to modify it into a multiple encounter mission.

Remember that this encounter trajectory was not purely a function of the flyby altitude, but also of when and how you flew past Venus.

I really doubt they could have just as easily switched gears and get it into a '3X' orbit. Such an orbit would have had a substantially greater Solar apopasis (I'm probably using the wrong term here), and hence a lot of delta-vee would have had to somehow been introduced into the mission to get there. Since they didn't have much fuel on board Mariner 10, we might have been looking at multiple Venus flybys, or even Venus-Earth-Venus-Mercury scenarios.

Rmember that this was the very first multiple planet flyby. It was hard enough for them to figure out how to get the Venus-Mercury-Mercury-Mercury thing to work. The billiard-ball style of mission planning (such as Venus-Venus-Earth-Jupiter-Saturn) didn't spring into existance over night.
Go to the top of the page
 
+Quote Post
tasp
post Sep 29 2007, 02:49 AM
Post #17


Member
***

Group: Members
Posts: 883
Joined: 30-January 05
Member No.: 162



A couple of points:

* The 2x period was desirable because with the needed higher altitude sunward second pass, almost the entire sunlit hemisphere could be observed. This allowed the inbound and outbound flyby 1 mosaics to be stitched together.

* IIRC, the post encounter orbit wasn't realized to be near 2 X 88 days until pretty late in the mission planning, and Mariner 10 was just about the cheapest planetary mission ever flown. No big changes would have been possible. Bruce Murray had overseen a slew of changes in the mission up to that point (the 1500 mm telephoto lens, advanced computer control, high data rate telemetry, lunar flyby on leaving earth vicinity, etc.) and the mission managers most likely would not have gone along with another change.

* A 3 X 88 day orbit will have perihelion beyond earth's orbit. Modifications to Mariner 10 for this would be a problem (IIRC, the space craft antenna would not have been able to slew properly earthward there) Additionally, there might be a problem with a gravitational deflection at Venus changing the eccentricity of the resultant orbit that much, (I am not smart enough to do the math on that one) there might not be a flyby distance above the solid Venusian surface to realize the math for that trajectory change.
Go to the top of the page
 
+Quote Post
tasp
post Sep 29 2007, 02:54 AM
Post #18


Member
***

Group: Members
Posts: 883
Joined: 30-January 05
Member No.: 162



{Anticipating a bad joke}

Yeah, we have discussed litho-braking here before.


Now I am bringing up litho-deflection.


blink.gif
Go to the top of the page
 
+Quote Post
tasp
post Jan 31 2011, 03:36 PM
Post #19


Member
***

Group: Members
Posts: 883
Joined: 30-January 05
Member No.: 162



It is interesting, now that we are on the verge of Messenger orbiting Mercury, to contemplate the Mariner 10 mission in a hypothetical 3X88 orbit.


Let's keep the geometry of the first flyby the same. One big difference would be the encounter speed. To come back after 264 days, instead of 176, Mariner 10 would need to go faster to climb further away from the sun. Mercury is well lit, so spacecraft motion won't blur the pictures, but with a faster flyby, we have less time to take all the pictures. This would probably cut the # of close range pictures, and probably reduce the extent of the mosaics.

The 2nd flyby, if at a similar 50,000 km sunward direction, would have imaged the opposite hemisphere. Foreshortening along the terminator in the pictures would have made it harder to stitch the mosaics together, however, the Caloris basin would have been seen in it's entirety (in 2 strips) , and the outer ring would have helped align them. We might have had some coverage of 75 to 80 % of Mercury, but with lower resolution along the edges of the covered areas of the 2 flybys.

If a second flyby could have had a geometry similar to the actual first or 3rd Mariner 10 flyby, we would have seen the west adjacent area of Caloris on approach, and on departure, we would have seen the eastern adjacent area of the first flybys approach hemisphere. We would have 4 large 'stripes' of coverage of Mercury. Stitching the 4 bands together would be tricky, all the adjoining areas on the limb would be foreshortened, and all the terminator seams would have illumination from 180 degrees around.

This would have been an interesting flyby. We would have perhaps double the areal coverage, but with less of an overall global context. I would be cautiously optimistic that perhaps Mariner 10 might have still accomplished 3 flybys. Due to the loss of attitude gas, Mariner 10 used the solar panels to help control the spacecraft attitude between the flybys. This technique would still work in the 264 day orbit, so maybe, just maybe, they could have squeezed a third pass out of the old girl. Another problem however, was the thermal effects on the craft. In the 3X orbit, Mariner 10 would have been a lot further out at perihelion. The antenna feed had a temperature sensitivity problem, and this might have exacerbated that severely. This problem has the potential to drastically cut the data rate of the antenna system. Also, the on board tape recorder would still have been lost, and with the high flyby speed, this would seriously impact the amount of data returned.

All in all, the mission as flown was most excellent. Given the twitchiness of the craft, I bet the controllers and flight personnel were relieved they did not have the 3X orbit, but I bet they could have still done a magnificent mission if they had.
Go to the top of the page
 
+Quote Post
tasp
post Jan 31 2011, 07:18 PM
Post #20


Member
***

Group: Members
Posts: 883
Joined: 30-January 05
Member No.: 162



Maybe someone with a bit more ken of orbital mechanics can address this; but is the obstacle to the 3X orbit in getting the perihelion of the spacecraft orbit beyond earth's orbit? It would seem a deflection at Venus into a 264 day orbit would be 'easier' than a deflection into a 176 day orbit, but achieving the eccentricity needed for the former implies a much faster approach speed to Venus than you would get than from simply 'falling' from earth's distance from the sun towards Venus.

(even with my limited understanding of the math, I love this trajectory stuff)



Now that we already have pretty good coverage of Mercury from the Messenger flybys, if someone would like to 'markup' some global Mercury pictures appropriately, we could get a pretty good idea of what Mariner 10 might have pictured in the hypothetical 3X orbit. Just transpose the second flyby coverage 180 degrees for the distant flyby option, or shift the first flyby coverage 180 degrees for the close option 2 Mercury 'days' later.

(I might even be able to do this with scissors and paste, LOL!)

Seems you are having a good chat with yourself, tasp.
Go to the top of the page
 
+Quote Post
Hungry4info
post Jan 31 2011, 09:33 PM
Post #21


Member
***

Group: Members
Posts: 998
Joined: 26-July 08
Member No.: 4270



I don't understand why a spacecraft with an orbital period three times that of Mercury has to have a perihelion outside Earth's orbit. 264 d < 365 d. This requires a smaller semi-major axis (and consequently, a lower perihelion).


--------------------
-- Hungry4info (Sirius_Alpha)
Go to the top of the page
 
+Quote Post
tasp
post Jan 31 2011, 10:28 PM
Post #22


Member
***

Group: Members
Posts: 883
Joined: 30-January 05
Member No.: 162



I was extrapolating from the 176 day orbit having an aphelion between Venus and earth, and accepting a perihelion at Mercury. Enlarging that orbit would seem [[based on pure assumption] to get you uphill of earth's orbit. Sadly, I can't do the math to get a precise dimensions.

And sorry about mixing up perihelion and aphelion. I am home sick today with a nasty head cold. Snowed in too. Beautiful day here for Titan, not so good for midwest USA.
Go to the top of the page
 
+Quote Post
Greg Hullender
post Feb 2 2011, 03:31 PM
Post #23


Senior Member
****

Group: Members
Posts: 1009
Joined: 29-November 05
From: Seattle, WA, USA
Member No.: 590



From Kepler's laws, I figure an orbit with a period of 264 days to have a semi-major axis of 0.8 AU. If that orbit's perihelion was at Mercury's aphelion (0.467 AU) then it's aphelion should be at 1.14 AU.

Anyone want to double-check that?

--Greg
Go to the top of the page
 
+Quote Post
Littlebit
post Feb 2 2011, 07:07 PM
Post #24


Member
***

Group: Members
Posts: 153
Joined: 14-August 06
Member No.: 1041



I get -1.137, so I probably have a sign error somewhere:)
Go to the top of the page
 
+Quote Post
Paolo
post Feb 2 2011, 08:16 PM
Post #25


Senior Member
****

Group: Members
Posts: 1307
Joined: 3-August 06
From: 43 35' 53" N 1 26' 35" E
Member No.: 1004



QUOTE (Greg Hullender @ Feb 2 2011, 04:31 PM) *
Anyone want to double-check that?


checked. That's correct.
264 days = 0.72 years = T
a = T^(2/3) = 0.80 AU (from Kepler's law)
q = 0.467 AU

from q = a*(1-e)
e = 1-q/a = 0.42 (orbital eccentricity)
hence Q = a*(1+e) = 1.14 AU


--------------------
I'm one of the most durable and fervent advocates of space exploration, but my take is that we could do it robotically at far less cost and far greater quantity and quality of results.

James Van Allen
Go to the top of the page
 
+Quote Post
tasp
post Feb 2 2011, 08:26 PM
Post #26


Member
***

Group: Members
Posts: 883
Joined: 30-January 05
Member No.: 162



As always, the folks with the math gene are very much appreciated!

Interesting orbit, compare to Phaeton that we were discussing on the Hyabusa thread.

Bruce Murray had a nice scaled drawing of the Mariner 10 176 day orbit in his book Journey Into Space, and also a veritable treasure trove of insider information on the amazing Mariner 10 mission.

Bruce met Beppe Columbo! That's how they all found out about the resonant orbit they had accidentally set up. Murray had tremendous respect for Beppe. So too his colleagues setting up their own Mercury mission to be named after him.
Go to the top of the page
 
+Quote Post
Paolo
post Feb 2 2011, 08:47 PM
Post #27


Senior Member
****

Group: Members
Posts: 1307
Joined: 3-August 06
From: 43 35' 53" N 1 26' 35" E
Member No.: 1004



QUOTE (tasp @ Feb 2 2011, 09:26 PM) *
Bruce met Beppe Columbo!


ColOmbo, not ColUmbo


--------------------
I'm one of the most durable and fervent advocates of space exploration, but my take is that we could do it robotically at far less cost and far greater quantity and quality of results.

James Van Allen
Go to the top of the page
 
+Quote Post
tasp
post Feb 3 2011, 03:56 AM
Post #28


Member
***

Group: Members
Posts: 883
Joined: 30-January 05
Member No.: 162



Thanx for the correction!

Ebay has a copy of Murray's book Journey Into Space for $4.99 if any one is interested.

The Mercury reminisces are worth that, and he also writes about exploring Mars, the Venera series, and Voyager. The story behind the 1500 mm Mariner 10 camera system and the radio upgrades that made the transmission of those great pictures is especially interesting.
Go to the top of the page
 
+Quote Post
ElkGroveDan
post Feb 3 2011, 05:23 AM
Post #29


Senior Member
****

Group: Admin
Posts: 4636
Joined: 15-March 05
From: Sloughhouse, CA
Member No.: 197



QUOTE (Littlebit @ Feb 2 2011, 11:07 AM) *
I get -1.137, so I probably have a sign error somewhere:)

I keep getting 1.8675309 over and over again. I should probably turn off the radio while I'm working on orbital calculations.


--------------------
If Occam had heard my theory, things would be very different now.
Go to the top of the page
 
+Quote Post
tasp
post Feb 3 2011, 05:47 AM
Post #30


Member
***

Group: Members
Posts: 883
Joined: 30-January 05
Member No.: 162



Googling "Mariner 10 orbit" and checking the images, (it's the first one tonight) will show the 176 day orbit diagram. It has aphelion further out from the sun than Venus. Intuitively, just glancing at that diagram, suggests an aphelion for the 264 day orbit that is going to be a nice bump further out than that. (And I realize that doesn't help with the calculations)

Like I said, sorry I can't do the math, but I really appreciate those that can.

Does the perihelion velocity for the 176 and the 264 day orbits 'pop out' of the calculations easily? (It would give an ~ idea of the Mercury encounter speed of the craft)
Go to the top of the page
 
+Quote Post

3 Pages V  < 1 2 3 >
Reply to this topicStart new topic

 



RSS Lo-Fi Version Time is now: 23rd April 2014 - 07:53 PM
RULES AND GUIDELINES
Please read the Forum Rules and Guidelines before posting.

IMAGE COPYRIGHT
Images posted on UnmannedSpaceflight.com may be copyrighted. Do not reproduce without permission. Read here for further information on space images and copyright.

OPINIONS AND MODERATION
Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators.
SUPPORT THE FORUM
Unmannedspaceflight.com is a project of the Planetary Society and is funded by donations from visitors and members. Help keep this forum up and running by contributing here.