Why can't the shuttle reach the ISS from Hubble?

I am sure there is a very simple answer for this but I cannot understand why most (if not all) articles state that the shuttle would be unable to reach the ISS from Hubble space telescope in the event of a problem with the orbiter.

Surely they are both on orbital paths which must come close together at some point. Is the issue time rather than manoveuring/distance.


Chris aka BoobBoo

3 Answers

  • 1 decade ago
    Best Answer

    This is an excellent question, and I think the answer has to do with fuel.

    In space, you don't require constant thrust to keep moving (that's something that many sci-fi movies get wrong). Once you get into orbit, you can let gravity do the driving. Oh sure, you'll need to make minor corrections every so often as your orbit is perturbed by the influence of the Moon and the miniscule drag of the extreme upper atmosphere, but you're pretty much on easy street once you've obtained a stable orbit (figuratively speaking, of course...I don't REALLY think an astronaut's job is easy!)

    Moving from one orbit to another orbit, however, is NOT easy. Even if you're staying within the same orbital plane, you have to use fuel (technically, propellant) to move to a higher or lower orbit. If you want to CHANGE your orbital plane, I hope you brought along extra juice. An object as big as a space shuttle moving at orbital speeds has a good deal of momentum. In order to change your orbit, you have to change the direction of that momentum (and perhaps the magnitude as well). Changing momentum is a matter of applying force over time, and providing force requires fuel. The bigger the change in momentum, the more fuel you're going to need.

    So once the space shuttle reaches a certain orbit, it's pretty much stuck there - except for a few minor corrections - until the astronauts are ready to come home. A jet plane can change direction on Earth without using a lot of extra fuel because the air provides a lot of force, but the space shuttle doesn't have that luxury.

    I hope that helps. Good luck!

  • 1 decade ago

    I'd like to know why you think the orbital paths must come close together at some point.

    The ISS orbits at an altitude of 51.6 degrees and about 350km altitude. The HST orbits at an inclination of 28.5 degrees and an altitude of 565km. Even when they pass each other at the point their orbital planes intersect they are still more than 200km apart and travelling in different directions. It is not enough just to get to the right height, at orbital speeds you have to be travelling in the same direction too if you want to rendezvous with something in any way that doesn't involve a high-speed crash and the loss of both craft. Changing orbital altitude takes a lot of fuel. Changing orbital *inclination* takes even more. The shuttle just doesn't carry enough fuel to effect a change of 200km in altitude *and* 23 degrees of inclination, hence it cannot move from the HST to the ISS.

  • Anonymous
    1 decade ago

    The problem is velocity. While both orbits sure cross at two points (where their planes intersect), you have to match velocities for rendezvous, so both station and shuttle travel in the same direction afterwards.

    The difference in inclination is minimal 23.2°. At a typical orbital velocity of 7700 m/s, this is equal to a required maneuver of 3100 m/s for turning the direction of travel of the Shuttle from the HST orbit to the ISS orbit. (This can be calculated simply by trigonometry, it is just the difference between two vectors)

    The Shuttle has only enough fuel for a maneuver budget of about 600 m/s, after reaching orbit.

    Also, for reaching Hubble and returning to Earth, the space shuttle will use about 75% of it's whole fuel. only 25% will be left for reentry and station keeping.

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