A European company, however, is developing a spacecraft to help satellites have longer, more productive mission lifetimes.

"This is an opportunity to open an untapped market in satellites," explained Phil Braden, CEO of the London-based aerospace firm Orbital Recovery, Ltd., which is developing the new spacecraft. "There is an economic pressure on [telecommunications] satellite providers to make the most of their satellite assets."

Orbital Recovery's vehicle, dubbed ConeXpress Orbital Life Extension Vehicle, is an ion-propelled spacecraft designed to fit into what until now has been empty space aboard an Ariane 5 rocket. The space tug, which is expected to conduct its first service call in 2007, is aimed at the telecommunication satellite industry, which depends on spacecraft in geosynchronous orbits to provide global television, telephone and online services among others.

Braden told SPACE.com that Orbital Recovery has identified about 83 telecommunications satellites orbiting Earth through 2011 whose missions could be extended by ConeXpress. Those high-end satellites, he added, were launched along with many others in the late 1980s and early 1990s and are approaching the end of their planned mission lifetimes.

ConeXpress adopts a slow and steady approach to satellite service calls by using an ion engine to reach ailing spacecraft. The engine uses electricity from tug's solar panels to charge xenon gas, then spits ionized particles through a nozzle to provide thrust.

Once the space tug arrives at a satellite it uses a docking probe to attach itself to the kick motor of the target craft, then it fulfills all the spacecraft's navigation and propulsion needs. Altogether, ConeXpress should be able to increase a single satellite's operational lifetime by up to a decade.

But one the primary challenges facing Orbital Recovery is the need to convince satellite providers that an orbital lift by ConeXpress is more useful and cost effective than launching new spacecraft.

The project is currently in a blueprint and design study phase with Dutch Space, a Netherlands-based company that will build the final spacecraft. While commercialization of the space tug may be a challenge, the spacecraft uses already proven technology for ion engines, as well as docking hardware and software, Braden said.

The space tug program itself is an evolution of Orbital Recovery's Geosynch Satellite Life Extension System (SLES) and Dutch Space's ConeXpress efforts to develop an efficient auxiliary craft for the Ariane 5 launch vehicle.

Monica Morgan, a spokeswoman for the Princeton, New Jersey-based telecommunications company SES AMERICOM, which has launched a fleet of satellites, said more satellite providers would be likely to use ConeXpress' services after a final design is available or the spacecraft has proven itself in flight.

Orbital Recovery, she added, would also have to compete with technological advances in satellite systems that could make spacecraft life extension missions unwarranted if more advanced versions are available for launch.

Morgan also said that a space tug could be useful in emergency situations where satellites have had to burn more fuel than planned trying to reach their geosynchronous orbits.

"We've been very fortunate that our launches have not required [unplanned burns]," Morgan said.

SES AMERICOM's latest launch, the AMC-11 spacecraft to provide high-definition cable service to the U.S., flew earlier this month without a hitch.

The ConeXpress plays another role in space access in addition to its space tug mission, that of space-saving passenger aboard an Ariane 5 rocket.

The spacecraft is designed to serve as a payload adapter connecting the fuel-carrying portion of an Ariane 5 to its payload compartment. Currently, the payload adapter aboard all Ariane rockets has been a void in an otherwise efficient launch vehicle. On the Ariane 5, the payload adapter measures almost three feet (nearly one meter) tall and is 3.6 feet (1.1 meter) wide at the top and 8.5 feet (2.6 meters) wide at the bottom.

"It's a pretty wide piece of equipment," said Arianespace's Jean-Michel Desobeau of the payload adapter. Desobeau is a program manager with the France-based Arianespace, which builds the Ariane 5 rocket. "You could easily fit yourself with some friends inside," he added.

Desobeau told SPACE.com that Arianespace engineers have been working for years to find a use for the empty space inside payload adapters in its Ariane launcher family, but the space has been too small for spacecraft that relied on chemical propulsion.

Since the ConeXpress space tug uses an ion engine, it takes up less space and can fit in the adapter compartment. Because of its position on the rocket, the overall spacecraft does not impact any other satellites aboard the Ariane 5, making it ideal as an auxiliary payload.

"I think the real advantage to this is it gives customers more flexibility in choosing their launch dates," Desobeau said, adding that a space tug could merely service a satellite until its replacement arrived.

Currently, ConeXpress is designed to be a tugboat for spacecraft that lack only the fuel to keep them going. A satellite provider, for example, would order a space tug lift before their craft ran out of propellant, and the orbital tow truck could latch on in time to keep its target craft running smoothly.

While pre-ordered ConeXpress flights are expected to be the norm, the space tugs could also be lofted into space on a more regular basis and serve as stand-by spacecraft ready to tow a satellite in distress at short notice.