A small capsule the size of a beach ball was stranded in Earth orbit early Tuesday after an attempt to return the craft from space via a revolutionary technique using a nearly 20-mile-long tether.

The 12-pound Fotino re-entry capsule was to be released from the tether to parachute to a landing in Kazakhstan shortly before 0800 GMT (4:00 a.m. EDT) Tuesday, but the deployment procedure apparently hit a snag, according to the European Space Agency, a co-sponsor of the mission.

The Young Engineers' Satellite 2 test mission was sponsored by ESA as part of its experiment package on the Russian Foton M3 microgravity research mission. Delta-Utec, a Dutch contractor specializing in tether systems, provided the technology and solicited the help of about 450 students from across Europe.

"We are very proud of the students' work, although we didn't reach the full 30-kilometer (18.6-mile) deployment," said Roger Walker, YES2 project manager for ESA's Education Office. "The hard work of the YES2 team over the past five years has paid off with this largely successful demonstration.

Everything went as planned until the mission encountered deployment problems, officials said.

"We are currently assessing the orbit of Fotino to understand when and where the capsule will return to Earth on its parachute," wrote Fabio de Pascale, integration manager for YES2, on Delta-Utec's Web site.

Fotino carries a radio beacon to broadcast its location and science data after landing to receivers up to 200 miles away. The spherical capsule also carries an impact-proof beacon to communicate with the satellite-based ARGOS system to assist with recovery.

Early estimates predict the capsule will re-enter the atmosphere between four and 11 days from now, according to Michiel Kruijff, Delta-Utec technical director and lead engineer for the YES2 mission.

Batteries aboard Fotino should be able to last until re-entry, giving officials hope of successfully recovering the capsule after it lands. The batteries control the radio beacons and parachute system, and on-board electronics are designed to handle cases like this, Kruijff said.

A GPS unit was removed from the mission at the last minute due to an unexplained problem, Kruijff told Spaceflight Now.

Kruijff said the deployed tether and attached sensors will continue to fly in orbit for several days. The objects could be visible from the ground in the early morning and evening hours.

"Many of YES2's mission objectives have been achieved," Kruijff said. "More than half the deployment objectives, the technology development and qualification of tether and capsule, the satellite design, its construction and its [successful] operation by 400 students, and possibly the Fotino landing objective will still be confirmed later this week."

Telemetry relayed from the system's Foton mothership indicated the tether unreeled to a length of about eight-and-a-half kilometers, or about 5.3 miles. The tether deployed about one-quarter of its intended length.

Automated timers aboard the spacecraft commanded the Fotino capsule's separation from an expendable telemetry package called MASS about a half-hour before the scheduled touchdown time.

Officials with Delta-Utec said the deployment snafu was due to problems during the second phase of the process. The tether was designed to briefly pause at a span of three-and-a-half kilometers before continuing out to its full length.

"The [three-and-a-half kilometer] first stage is intended to increase landing precision and is the hardest part of YES2," Kruijff said in an update on the mission's Web site. "Its success is therefore also a big success for YES2."

The preprogrammed release sequence was designed to begin about 50 minutes after the start of the second stage of deployment, but the deployment was slower than expected, meaning that the tether could not unfurl to its full length in the constrained time period, according to Delta-Utec.

"Somehow the friction level was significantly too high and could no longer be controlled," Kruijff said. "We still have to find out the details."

The deployment velocity was expected to reach more than 25 miles per hour during the last few minutes of the sequence.

Earlier mission activities proceeded as planned early Tuesday, beginning with powering up the experiment at about 0200 GMT (10:00 p.m. Monday). Engineers stationed at the Russian control center near Moscow confirmed the health of the payload a few hours later.

Fotino and MASS were spring-ejected from a spool on the forward end of the Foton capsule at 0447 GMT (12:47 a.m. EDT).

It was not immediately clear what caused the slower-than-planned deployment, but officials were busy analyzing recorded data from the experiment on Tuesday. Engineers were not able to follow the events in real time because of limited ground station coverage.

Fotino was to have dropped from orbit after a pendulum-like swing at the end of an 18.6-mile (30-kilometer) tether, which was to be the longest ever deployed in space.

The backward swing was designed to reduce the capsule's orbital velocity enough to drop it into the atmosphere.

The test was a critical milestone in the development of a "Space Mail" concept that could return small payload packages from spacecraft in orbit, including the international space station.

Once tested, tether concepts could be used in a variety of applications, according to Kruijff.

"The door is wide open for other applications [such as] electrodynamic propulsion, artificial gravity [and] tethered launch assist," Kruijff said.

"A success for a 30-kilometer tether would mean that the path we follow is a consistent and secure path to follow towards real applications," he said.

Kruijff said the Space Mail return system faces several safety issues before it is cleared for use on the space station. Similar issues forced the cancellation of a NASA mission called ProSEDS, which was designed to demonstrate tether-generated thrust that could change a craft's orbit.

"Currently, tethers are going through a hard time," he said. "After a lot of interest in the early 90s, [there were] some cancellations and failures. Both Europe and [the] U.S. all but stopped the tether effort."

Small-scale tether research continued, but the experiments were limited in scope, Kruijff said.

YES2 was the second mission designed specifically for tests of tethered propulsion for re-entry. A precursor mission had similar objectives, but the experiment did not unreel the tether due to unsafe orbit conditions.

Other tether demos, including those flown aboard space shuttle flights more than ten years ago, were focused on deployment methods and survivability in space.

The Foton spacecraft, with a menagerie of international experiments stuffed inside its return capsule, is scheduled to leave orbit Wednesday morning for a landing in Kazakhstan.