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How Precise Are Orbits Today?

Posted: Thu Aug 22, 2013 7:12 am
by AF1624
Hey all,

I've taken to playing a little game called "Kerbal Space Program", it's a neat game which is both entertaining and quite informative, which allows you to build rockets and space ships, and launch them in a virtual universe that admittedly resembles ours quite a lot.

I started by creating simple rockets to put satellites onto Orbit, but realised quickly that my orbits are never perfectly circular. Which brings me to my question: just how precise are Orbits today? Specifically, what kind of precision do we expect when putting an object on a circular orbit? How different is the periapsis from the apoapsis?

Thanks!

RE: How Precise Are Orbits Today?

Posted: Thu Aug 22, 2013 10:36 am
by connies4ever
Because the Earth is not a perfect sphere, because there are mass discrepancies between hemispheres, principally. Satellites in geosynchronous orbit have to adjust their locations from time to time to correct drift. Depending on the number and size of solar panels, there would a minute effect due to the solar wind.

Frequently the Earth's shape is called an oblate spheroid, kind of like a grapefruit. But even that is a little misleading, as the northern hemisphere, IIRC, is just a tinch bigger than the southern. I may have that backwards, but it's 0630 here.

A fairly straightforward explanation:

http://en.wikipedia.org/wiki/Figure_of_the_Earth

RE: How Precise Are Orbits Today?

Posted: Thu Aug 22, 2013 11:44 am
by autothrust
As far as i know, the Apollo 11 achieved almost a perfect orbit of 111km around the moon. That means almost none eccentricity.

Even more suprising was the fact the Apollo Computer computed position of the spacecraft matched very precisely the calculations made on earth and measured by Radars.

To come back your question, depending on the object you want to orbit, it's today possible with help of computers, Magnetometers, RCS, Magnettorquers, Control moment gyros, star trackers to achieve a very precise orbit.

RE: How Precise Are Orbits Today?

Posted: Thu Aug 22, 2013 12:47 pm
by connies4ever
Quoting autothrust (Reply 2):
As far as i know, the Apollo 11 achieved almost a perfect orbit of 111km around the moon. That means almost none eccentricity.

Which kind of conflicts with these:

http://phys.org/news/2013-05-team-moon-mascons-mystery.html

http://www.britannica.com/EBchecked/topic/367793/mascon

It was noted 1966-67 that the Lunar Orbiter missions, to survey possible Apollo landing sites, had their initially near circular orbits affected by local changes in the lunar gravitational field.

RE: How Precise Are Orbits Today?

Posted: Thu Aug 22, 2013 1:56 pm
by autothrust
Quoting connies4ever (Reply 3):
Which kind of conflicts with these:

You sure are right, i didnt want to imply it was such a good orbit the whole time. I meant more the arrival at the moon.

Have this information from the documentary Moon Machines Apollo Guidance Computer.

RE: How Precise Are Orbits Today?

Posted: Thu Aug 22, 2013 3:00 pm
by connies4ever
Quoting autothrust (Reply 4):
You sure are right, i didnt want to imply it was such a good orbit the whole time. I meant more the arrival at the moon.

Agreed the initial few orbits would be essentially circular. Over time (they were on site 3 days I think) the orbit would start to alter. Not even an ellipse, actually. But an orbit where the CSM would rise and fall slightly based on there the mascons were located.

Those were the days, man !

RE: How Precise Are Orbits Today?

Posted: Thu Aug 22, 2013 3:36 pm
by autothrust
Quoting connies4ever (Reply 5):
Those were the days, man !

Indeed, shame we(humans) weren't there again, and even worse how many stupid people deny this incredible achievement.

     

RE: How Precise Are Orbits Today?

Posted: Thu Aug 22, 2013 3:53 pm
by connies4ever
Quoting autothrust (Reply 6):
Indeed, shame we(humans) weren't there again, and even worse how many stupid people deny this incredible achievement.

In my heart I agree, shame we're not there again on a mostly permanent basis (lunar base, maybe 6 people). But the cost would be stupendous, make ISS look cheap.

As for stupid people, there will always be Flat Earthers.

One thing people need to bear in mind is that Apollo, while it did some decent science, was not a science-driven program. It was a Cold War program to demonstrate to the other side that "we are ahead of you, and we will stay that way, in fact get further ahead of you". The flight crews knew that, knew there was a good risk they wouldn't be coming back, but accepted the missions. Apollo was a program driven by the politics and competition of that era.

RE: How Precise Are Orbits Today?

Posted: Fri Aug 23, 2013 7:17 am
by AF1624
Quoting connies4ever (Reply 7):
Apollo was a program driven by the politics and competition of that era.

It's history repeating itself.

Scientists with lack of funding and research have always used wars and political tensions as an excuse to make progress, if you will. That is a good thing even though it doesn't sound like it.

Think of it like an ascending thermal if you're flying a glider. You can do nothing about it, these thermals are there in the first place, but will you pass an opportunity to make good progress on science that is dear to you (sorry, lift that is vital to you) or will you ignore it? I don't know if you get the metaphor...

Anyway, yes, these were the times. And as the world changes the technology and the research changes with it. Look at what's funded today: security projects, IT security, counter-digital-terrorism measures, etc. Space exploration? Meh, not so much. It's just not today's priority anymore, which is a shame.

Thanks for all the replies  

RE: How Precise Are Orbits Today?

Posted: Fri Aug 23, 2013 9:43 am
by autothrust
Quoting connies4ever (Reply 7):
In my heart I agree, shame we're not there again on a mostly permanent basis (lunar base, maybe 6 people). But the cost would be stupendous, make ISS look cheap.

Agreed, however the with International effort it is doable, besides this the expenditures on Nasa have decreased massively.

On long term, one have to include the stimulation of new technologys needed for such a program and the gaining retrieved from them.

(The Apollo program did bring a great leap in Technology in many areas)

Quoting connies4ever (Reply 7):
One thing people need to bear in mind is that Apollo, while it did some decent science, was not a science-driven program. It was a Cold War program to demonstrate to the other side that "we are ahead of you, and we will stay that way, in fact get further ahead of you". The flight crews knew that, knew there was a good risk they wouldn't be coming back, but accepted the missions

So true(sadly that it needs a "Cold"-War), i recall Armstrong saying the chance for a successfull lunar landing was like 60%.