PDA

View Full Version : Orbital question regarding a hypothetical mission (hubble)



Moose
2004-Dec-07, 02:02 PM
Okay, I understand that under "current" safety regulations, all shuttle missions must at least rendez-vous with the ISS, and that the big issue with a Hubble repair mission is the change in inclination necessary to reach the ISS from Hubble's orbit.

But if you were to rendez-vous with the ISS first, and then try for Hubble. Is the change in inclination just as much of an obstacle, or would it be easier to smooth out and lower one's orbit?

ngc3314
2004-Dec-07, 02:07 PM
Okay, I understand that under "current" safety regulations, all shuttle missions must at least rendez-vous with the ISS, and that the big issue with a Hubble repair mission is the change in inclination necessary to reach the ISS from Hubble's orbit.

But if you were to rendez-vous with the ISS first, and then try for Hubble. Is the change in inclination just as much of an obstacle, or would it be easier to smooth out and lower one's orbit?

Just as big a problem - the change in inclination takes so much energy that it would be nearly as easy to de-orbit Hubble and relaunch (entering a complete fantasy world...). If we still had a Saturn V or Energiya booster available, it might just work. But then in that parallel world, we probably wouldn't be dealing with this particular problem at all.

Moose
2004-Dec-07, 02:21 PM
Okay, I think I see.

As far as deorbiting it goes, I take it that Hubble definitely would impact the surface rather than burn up on re-entry, hence the emphasis on de-orbiting it?

Would it have been prudent (or possible) to have built in thrusters to de-orbit it when the time came, rather than have to send something up to do the job?

Kaptain K
2004-Dec-07, 02:36 PM
Okay, I think I see.

As far as deorbiting it goes, I take it that Hubble definitely would impact the surface rather than burn up on re-entry, hence the emphasis on de-orbiting it?

Would it have been prudent (or possible) to have built in thrusters to de-orbit it when the time came, rather than have to send something up to do the job?
I doubt it. Space is a hostile place. There would be no gaurantee that a booster would still work as planned after years or decades in space.

I feel that if we are going to go to the trouble of sending a booster to Hubble, we should boost it into a higher orbit, so that when we come to our senses, we can repair it and get more use from it!

ToSeek
2004-Dec-07, 02:58 PM
Okay, I think I see.

As far as deorbiting it goes, I take it that Hubble definitely would impact the surface rather than burn up on re-entry, hence the emphasis on de-orbiting it?

Would it have been prudent (or possible) to have built in thrusters to de-orbit it when the time came, rather than have to send something up to do the job?

The original thought was to have a shuttle go up and bring Hubble down for eventual installation in the Smithsonian. With the present budget restrictions and paranoia about shuttle safety, that's not going to happen.

The current plan is to have a module with thrusters go up and dock with the Hubble, then de-orbit it in a controlled fashion when the time comes. All the talk of a robotic servicing mission is really just a look at enhancing this module to provide servicing capabilities as well as deorbit/maneuvering capabilities.

ngc3314
2004-Dec-07, 03:07 PM
Okay, I think I see.

As far as deorbiting it goes, I take it that Hubble definitely would impact the surface rather than burn up on re-entry, hence the emphasis on de-orbiting it?

Would it have been prudent (or possible) to have built in thrusters to de-orbit it when the time came, rather than have to send something up to do the job?

HST has big pieces that would almost certainly make it to the ground. NASA became very worried about such things after the Skylab fiasco - there are now (I believe) legal standards for impact probability which require new spacecraft to have the on-board capabilityfor controlled deorbit. (I seem to recall a debris footprint of no more than 8 square meters, derived from the probability of hitting anyone being less than 1/250,000).

HST was part and parcel of the early Shuttle program, which everyone in and out of the project was told would be flying hundreds of missions, so servicing was an exciting opportunity. Furthermore, orbital reboost with onboard thrusters was not very desirable, since there are not only te usual weight and control issues, but optical contamination problems. They worled very hard to get an unprecedented UV efficiency for the mirrors, somethiing easily compromised by even traces of - well, a great many things. Even plastic bakeout does this within one of the cameras. On top of that, use of on-board thrusters for any maneuvering would cause stability problems in pointing as the telescope waitied to damp out residual vibrations. Given that background, and the fact that the Skylab uncontrolled re-entry had not yet occurred when the key design decisions were frozen, not putting re-entry engines on HST seemed like a no-brainer (aside from the reliability issues mentioned by the previous poster).

The contamination and stability issues didn't apply to Compton, which was perfectly capable of dropping itself into the Pacific. Chandra and Spitzer are both beyond geosynchronous orbit, Chandra so high that tidal coupling will move its orbit outward and Spitzer in heliocentric orbit, so the issue doesn't arise there.

To get a real headache, start contempating what will need to happen when ISS reaches its internationally-agreed end of mission. The Russian modules do have engines, but you need a very large and pretty quick impulse for a precision drop into that magic empty piece of the Pacific.

Kaptain K
2004-Dec-07, 03:24 PM
To get a real headache, start contempating what will need to happen when ISS reaches its internationally-agreed end of mission. The Russian modules do have engines, but you need a very large and pretty quick impulse for a precision drop into that magic empty piece of the Pacific.
The ISS will probably have to be de-orbited in sections.