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The past and the present
It is interesting how our perspective has changed since the last Hubble repair mission in 1999. The previous missions to Hubble were conducted with the same launch system that lead to the destruction of Columbia and were launched assuming an acceptable level of risk. Today we are launching shuttles with a system that has been redesigned to be safer. Therefore, this mission should be even less risky than all of the previous Hubble missions. Yet we balk at the level of risk we have to contend with. But maybe this concern is justified given that Discovery’s extremely successful mission (STS-124) to deliver the KIBO module to the International Space Station (ISS) was marred by foam falling off the main fuel tank. This mission utilized upgrades to the fuel tank to prevent this problem. Fortunately, there was no damage from the falling pieces of foam.
To counter the higher level of risk, Atlantis will carry a tile inspection system to check all the critical areas of concern and a repair kit to repair damaged tiles and reinforced carbon-carbon (RCC) panels should they find any. If the damage is too great, the only alternative is to launch the shuttle Endeavour, which will be on standby for the very purpose of a rescue mission (STS-400). Atlantis will be able to support her crew for a maximum of about 23 days  giving NASA a window in which to launch Endeavour that would take several days to prepare. All of this concern is minimized when a shuttle launches to the ISS. The station becomes a safe haven in the event of an accident that prevents the shuttle’s return to earth. This gives NASA an extended period of time to ready a rescue launch.
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What about the International Space Station?
This point of the ISS being a safe haven brings many to ask why the shuttle servicing the space telescope can’t simply fly over and dock with the station if a problem arises. The laws of physics, which determine the orbital mechanics for the shuttle and space station, coupled with our technical limitations—how much fuel the shuttle can carry—all conspire to produce a hard truth: the shuttle cannot change its orbital path to rendezvous with the station. Hubble lies in an orbit which is inclined about 28 degrees from the equator, while the space station’s orbit is inclined almost 52 degrees from the equator. (See figure 1) It would take an amount of fuel almost as massive as the orbiter itself (111,000 kilograms) for Atlantis to change its orbit! It doesn’t carry that much fuel to begin with and we cannot launch a fuel tank to supply the craft. We are too conditioned by Hollywood and TV’s interpretation of space travel showing our favorite space heroes rocketing off to some distant planet, or bounding about a solar system without regard to how much energy it would take to do this. Unfortunately, we don’t have their technology at out disposal—yet. At this time we have to live with the reality that in a worst-case scenario, the Shuttle Endeavour will launch to rescue the crew of Atlantis. (But one must realize that when Endeavour launches, it will be using the same hardware system that Atlantis launched with just days before and may itself now be in need of rescue.)
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The risks do not end with the launch of the rescue shuttle. The orbital rendezvous and transfer of astronauts present new and untested procedures for NASA. Here is a link that details the docking and astronaut transfer to Endeavour. Atlantis, depending on its condition may or may not be remotely piloted to land, most likely at Edwards Air Force Base. If it cannot land then it will probably be deorbited over the Pacific Ocean to minimize the risk to people on the ground.
I look forward to this mission and hope for a flawless, successful flight that brings Atlantis and her crew safely home and breathes new life into Hubble so we can be even more amazed with its images of the universe. Godspeed, Atlantis!
(For the latest information about the successful final Hubble repair mission, read The Fifth and Final Hubble Repair Mission.)
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 NASA Evaluates Rescue Options for Hubble Mission
Figure 1: “To Fix Hubble, an Orbital Complication", New York Times