An unpiloted Soyuz spacecraft carrying supplies and an experimental robot attempted to dock with the International Space Station early Saturday, but the spacecraft, unable to lock onto guidance signals, began slewing widely about its long axis, prompting Russian flight controllers to order an abort.

In the tense moments leading up to the abort, cosmonauts Alexander Skvortsov and station commander Alexey Ovchinin, monitoring the approach from inside the station's aft Zvezda module, lost sight of the gyrating Soyuz in their television monitors.

At that point, the spacecraft was less than 300 feet away from the target docking port atop the Russian Poisk module. "We're making every attempt we can think of to try to track it down visually right now," one of the cosmonauts radioed Russian flight controllers.

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NASA controllers at the Johnson Space Center in Houston then radioed the station's other four crew members to make sure they were up and aware of the apparent malfunction with the Soyuz KURS rendezvous system. "We just want to let you know the KURS is having difficulty locking onto the target and we wanted to make sure you're awake," mission control called. "And you have a go to open windows for situational awareness as needed."

A few moments later, the cosmonauts again reported they had lost visual contact with the Soyuz and Vladimir Solovyov, the chief flight director in the Russian mission control center, decided he'd had enough.

"Station, Soyuz is aborting," U.S. flight controllers confirmed for NASA astronauts Christina Koch, Drew Morgan, Nick Hague and European Space Agency astronaut Luca Parmitano.

Dutifully executing the abort procedure, the Soyuz headed for an orbit that would permit a second rendezvous attempt within 24 hours, but Russian engineers quickly ruled that out.

Immediately after abort commands were issued, the Soyuz began backing away from the space station. NASA officials said the station crew was never in any danger. NASA

Based on telemetry and the performance of the Soyuz, engineers believe the culprit may have been a faulty amplifier in the KURS rendezvous equipment inside the space station. The amplifier is part of a system that sends signals to an approaching Soyuz that are used by the ship's computer to help calculate the proper approach trajectory.

Several hours after the missed approach, the Russian space agency Roscosmos posted a statement saying the proposed plan forward calls for Skvortsov, Morgan and Parmitano to strap into the Soyuz MS-13/59S spacecraft that carried them into orbit last month, undock from the Zvezda module's aft port and manually re-dock at the Poisk module.

All Soyuz pilots are trained to fly their ships manually and the suspect KURS system used in the aborted automated approach to the Poisk port will not be utilized.

Assuming the relocation goes well, the Soyuz MS-14/60S spacecraft would be directed to attempt an automated docking at the vacant Zvezda port. The proposal requires the approval of all the station's international partners.

"On August 27 (Moscow time), the second automatic docking attempt will take place, with the Soyuz MS-14 docking to the Zvezda module," Roscosmos said in the web posting. "Tomorrow, Alexander Skvortsov will take additional training according to the normal procedure using the onboard simulator to redock the Soyuz MS-13.

"The situation is complicated, however is under control."

The Soyuz MS-14/60S spacecraft was launched Wednesday night U.S. time from the Baikonur Cosmodrome in Kazakhstan. Instead of a crew, the three-seat ferry ship carried 1,450 pounds of cargo and an experimental robot known as the Skybot F-850.

The primary goal of the flight was to verify the suitability of the upgraded Soyuz-2.1a booster to launch astronauts and cosmonauts to the station and to check out a variety of Soyuz upgrades. The booster performed flawlessly and the Russians are expected to start using it for crewed missions starting early next year.

The two-day rendezvous also went smoothly until the final minutes of the approach. After carrying out additional tests, the Soyuz slewed about its long axis before briefly lining up on the Poisk module docking port. But instead of staying locked on, the nose drifted off in large arcs, almost as if the spacecraft was searching for a signal to home in on.

Ovchinin and Skvortsov were monitoring from the Zvezda module at the back end of the space station. Unlike unpiloted Progress supply ships, which have equipment on board that allows cosmonauts to take over manual control if problems develop, the normally crewed Soyuz relies on the vehicle commander if trouble develops.

"It's such a shame we were not able to use TORU," one of the cosmonauts said, referring to the backup control system. "We would grab it and bring it in."

While the Skybot robot strapped into the commander's seat of the Soyuz MS-14 spacecraft is being developed to carry out a variety of relatively complex tasks, piloting a Soyuz is far beyond its capabilities.