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Author Topic: STS-125 question
MinutiaeMan
Living the Geeky Dream
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I'm putting this question in Starships and Technology because it's a ship-related question pertaining to technology. I don't care if it's real-life, so there! [Wink]

Everyone keeps talking about how the space shuttle mission can't dock at the ISS if there's a problem because of the "orbital parameters" or some such. Obviously there are people much smarter than me who've calculated everything, so I don't doubt that it's true... but it doesn't make sense to me.

Has anyone found a more detailed explanation other than "they just can't"? It seems to me that, considering how the shuttle and the station zip around the planet with orbits every 90 minutes or so, slowing down a bit for one or the other to catch up should be simple. And being in a higher/lower orbit wouldn't be a problem, would it?

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Guardian 2000
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If they were on the same plane (e.g. orbiting around the equator, for instance), slowing down to go to the ISS would be easy.

But Hubble and the ISS orbit on different planes. The ISS is intended to have occasional overview of as much of the planet as possible, so its orbit has a high inclination (i.e. hellacious tilt compared to the equator).

Hubble is not in such an orbit.

You would basically need a starship impulse engine to make the necessary orbital changes. The shuttle *might* be able to do it within some long period of time provided it had a fully-fueled aux tank, but they rather stupidly burn the fuel out of it and drop it off every time they go up. [Wink]

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OnToMars
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In orbit, when you change altitude, you change your speed. And vice versa.

And when your path is a combination of circles and ellipses, it makes things kind of complicated.

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Joshua Bell
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One way to think about it: imagine that the shuttle is orbiting above the equator, while Hubble is orbiting pole to pole. (Which is completely inaccurate, but bear with me.) Even ignore the difference in orbital altitude, and pretend the orbits are perfectly circular.

When they cross paths, they're traveling at around 10 km/sec relative to each other. That velocity came from launch, and was a significant amount of the energy used - you can think of achieving orbit less about going "up" and more about going "over" quickly enough that you "fall sideways and miss". The "up" bit just gets you above the atmosphere, and you can do that with a balloon.

To intercept, it's not enough to cross paths - you also need to match velocities. (Otherwise, you call that a "collision".)

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Omega
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To match velocities, you have to change your velocity, which is called acceleration, which requires fuel. It's a question of how much fuel. In this case, the answer is "way too much".
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The Ginger Beacon
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It's closer to 14 km/second closing speed for Hubble and the ISS, but they are still really far away from each other.

STS-125's altitude will be about 570km at rendez vous with Hubble, whereas the ISS is at about 350km. The inclination of Hubble's orbit (the angle at which it crosses the equator) is about 28.5 degrees, and the ISS is at 51.6 degrees (mostly due to their launch sites - Florida for Hubble via the Space Shuttle, and from Baikonur in Russia for the 1st part of the ISS).

As a result, the difference between the two oribts is pretty vast, and the fuel required to reduce the orbiters velocity in order to enter the ISS's orbital altitude from the telescope is more than the shuttle can carry.

Aother point is that, supposing you had enough fuel etc. reduce your altitude the required 200 or so km, is that you'd have to re-orientate your orbital track, placing yourself into a track that will intercept the ISS, for which even more fuel is required, along with alot of complicated maths.

Finaly, the orbits would also need to carry the two satelites close enough to each other to allow such a course change.

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Jason Abbadon
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quote:
Originally posted by Guardian 2000:
The shuttle *might* be able to do it within some long period of time provided it had a fully-fueled aux tank, but they rather stupidly burn the fuel out of it and drop it off every time they go up. [Wink]

I think they do that so they do stupidly explode on the way down. [Wink]

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-Aeschylus, Agamemnon

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Jason Abbadon
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er..so they DONT stupidly explode, that it is.
I'm dark, but not that dark of spirit.

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Justice inclines her scales so that wisdom comes at the price of suffering.
-Aeschylus, Agamemnon

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Mark Nguyen
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ON a related subject, amongst the tings they're installing on Hubble this time is a docking mechanism so that a future spacecraft can safely de-orbit the telescope when the time comes. It only leaves me to wonder what they were thinking they could do in the first place? Let it drop uncontrolled on the Soviets?

Mark

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MinutiaeMan
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I should'a figured that fuel would be the reason. That makes more sense, when combined with the orbital inclination thing.

quote:
Originally posted by The Ginger Beacon:
The inclination of Hubble's orbit (the angle at which it crosses the equator) is about 28.5 degrees, and the ISS is at 51.6 degrees (mostly due to their launch sites - Florida for Hubble via the Space Shuttle, and from Baikonur in Russia for the 1st part of the ISS).

But space shuttles visit the ISS all the time, so why would the launch site affect the orbital inclination? Wouldn't it be the orbital insertion angle or something?

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“Those people who think they know everything are a great annoyance to those of us who do.” — Isaac Asimov
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The Ginger Beacon
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Yes, the inclination is as a result of the insertion. All right smartypants!

I don't know why the angles are so different off the top of my head, but I remember being told many years ago in a physics class. I think it's something to do with the spin of the earth and the sites lattitudes make those insertion angles the most obtainable using the least fuel.

To change inclination mid flight you need to alter velocity, which takes fuel. Much easier to do it from launch, and insert into an orbit of the desired inclination.

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