The James Webb Space Telescope will deploy (unfold mechanically) while on the way to L2. Couldn't it do so in LEO, where it is potentially serviceable? Starliner CST-100 and Dragon are planned to soon allow affordable crewed missions to LEO, and Soyuz could maybe already do it. At least Orion is designed to allow for an EVA, and it is a $bn 8.8 telescope. Even if JWST is not designed to be upgraded, a moving part gotten stuck might be moved manually during an EVA. The upper stage would then bring it to L2. I do think some other spacecrafts have orbited Earth before having set off to their interplanetary destinations. Why won't JWST deploy in LEO where it is potentially serviceable? (like Viking?).
JWST is being launched on an Ariane V with a cryogenic upper stage. That upper stage has to be used immediately to launch it on a trajectory to the Sun-Earth L2. The stage operates on batteries, and the cryogenic fuel is boiling off. So there would be no time to do anything even if you deployed the telescope before departure.
Furthermore, the deployed mirror and sunshade would have to be designed to take the loads from the thrust of the upper stage, which would increase their mass and complexity. Pretty much needlessly increasing the mass for the entire lifetime of the telescope for a 20-minute event.
The only way to do this would be to have an electric propulsion upper stage. Then you could have it in a low-Earth orbit for a short time, short enough to mitigate the debris, and still accessible to Orion or Dragon. You could use the electric propulsion to raise it above most of the debris while waiting for servicing (more than 2000 km), and lower it for the servicing mission. A few dings in the mirrors is better than no telescope at all. However you'll have the van Allen belts to consider in the waiting orbit. The electric propulsion would be very low thrust, so the deployed telescope would hardly notice, and it could wait as long as desired before departing. The telescope would also have to be designed to be serviceable, which it currently isn't.
What would be really nice is if it had an electric propulsion system with enough propellant to take to L2, and bring it back to LEO. Then you could service it, refill the Xenon tank, and send it back off.
Alas, all of that would just increase the cost of an already rather costly telescope, in order to mitigate a risk that can and has been mitigated through design and test.
The James Webb Space Telescope will not be deployed in Low Earth Orbit because there is too great a risk of the optics being damaged by debris.
[T]he environment around the ISS is not suitable for the exposed optics that JWST has and would have had the possibility to damage or contaminate the optics. The deployment of JWST happens far above Low Earth Orbit and the debris that resides there.
Location, location, location.
Earth is too hot for a telescope that needs such low temperatures to operate. They are building a 5 layered sunshield to protect the JWST from the heat of the Sun. By 'hovering' in Sun-Earth L2, a million miles from Earth, they can avoid essentially all of the radiated heat from Earth.
The goal is to keep it as cold as possible passively to minimize the active cooling needed, to allow it to work longer.
There are so many reasons why an L2 orbit was chosen for the JWST. Though the specific advantages over LEO include;
- It can use Infrared instruments since the heat from the Earth and Sun can be radiated away from its field of view.
- It will have a constant, uninterrupted view of deep space.
- It will require less orbital corrections throughout its lifetime.
- Obviously, the risk of space debris in LEO is a growing problem.
- L2 has been a favoured orbit for space observations by astronomers for a long time.
Due to the delicate nature of the telescope's deployment, yes, it would be great if we could get people there in case something goes wrong. However, this is also the reason why it cannot be deployed in LEO and then sent to L2 as it simply isn't structurally designed to withstand that sort of journey and would have to pass through altitudes of even greater levels of debris.
This website talks about the advantages of the L2 orbit for the JWST: http://www.nasa.gov/topics/universe/features/webb-l2.html