Another interesting article from AviationNow

The long-simmering concept of automated satellite servicing is stuck short of implementation by questions about technical feasibility, economics and who will risk billions of dollars to make the first step. To move the concept along, the Orbital Express demonstrator, set for launch in October, will try to show that at least the technology is ready to refuel, repair and otherwise service spacecraft.

The Orbital Express program is run by the Defense Advanced Research Projects Agency (Darpa), with Boeing Advanced Systems as the prime integrator and builder of the Astro servicing, or "chase," satellite, and Ball Aerospace building the client target satellite NextSat (next-generation serviceable satellite). They will be joined together for launch on an Atlas 5 rocket, part of the Defense Dept. Space Test Program-1 mission.

Proponents of spacecraft servicing believe it will enable new types of missions and enhance existing ones. A prime military mission would be to refuel reconnaissance satellites so they can maneuver to improve coverage, increase surprise and be more survivable. But commercial operators especially will want to see a strong economic case, and there are fear and inertia to be overcome.

Just a year ago, NASA's Demonstration of Autonomous Rendezvous Technology (DART) satellite collided with its target, aborting the mission (AW&ST May 22, p. 37). And spacemen's lives were put in peril when the robotic docking of a Progress supply ship at the Mir space station in June 1997 resulted in a collision that punctured the pressure vessel. Economic models show that even a fairly small chance of collision can ruin the payback for orbital servicing.

Orbital Express is to conduct the first autonomous component exchange and first U.S. refueling, as well as autonomous rendezvous and docking. And just as important, "it will take the technical excuse off the table for on-orbit servicing," says USAF Lt. Col. (select) Fred G. Kennedy, the Darpa Orbital Express program manager.

In many cases, orbital refueling probably won't make economic sense because other components are often pretty well degraded by the time propellant runs low. However, for reconnaissance satellites, the ability to refuel means they can be maneuvered much more agressively and stay on orbit longer, making them potentially much more useful and cost effective.