Kosmos 1686 was a heavily modified TKS spacecraft

TKS spacecraft

TKS spacecraft was a Soviet spacecraft design in the late 1960s intended to supply the military Almaz space station. The spacecraft was designed for manned or autonomous cargo resupply use...

 which docked unmanned to the Soviet space station

Space station

A space station is a spacecraft capable of supporting a crew which is designed to remain in space for an extended period of time, and to which other spacecraft can dock. A space station is distinguished from other spacecraft used for human spaceflight by its lack of major propulsion or landing...

 Salyut 7

Salyut 7

Salyut 7 was a space station in low Earth orbit from April 1982 to February 1991. It was first manned in May 1982 with two crew via Soyuz T-5, and last visited in June 1986, by Soyuz T-15. Various crew and modules were used over its lifetime, including a total of 12 manned and 15 unmanned launches...

 as part of tests to attach scientific expansion modules to stations in Earth orbit. The module which docked to the station was the FGB component of a TKS vehicle launched on 1985-09-27, and was designed to test systems planned for use on the Mir Core Module

Mir Core Module

Mir , DOS-7, was the first module of the Soviet/Russian Mir space station complex, in low Earth orbit from 1986 to 2001. Generally referred to as either the core module or base block, the module was launched on 20 February 1986 on a Proton-K rocket from LC-200/39 at the Baikonur Cosmodrome...

. The spacecraft docked with Salyut 7 on 1985-10-02, during the long-duration stay of the cosmonauts of its fifth principal expedition, which arrived on Soyuz T-14

Soyuz T-14

-Backup crew:-Mission parameters:* Mass: * Perigee: * Apogee: * Inclination: 51.6°* Period: 88.7 minutes-Mission highlights:...

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Notable features

• The spacecraft's Merkur capsule was greatly modified to carry instruments - the retrorocket and parachute packages were replaced by scientific equipment, including an infrared telescope and the Ozon spectrometer.
• The combined Salyut 7-Kosmos 1686 complex massed 43 tons, with Kosmos 1686 delivering 4500 kg of cargo to Salyut 7 and nearly doubling the amount of habitable volume available to the station's crew.
• On August 19-August 22, 1986, ground controllers boosted the vacant Salyut 7-Kosmos 1686 complex to a 474 km by 492 km orbit using engines on Kosmos 1686. This reduced the propellant supply of the complex to 70 kg (about 500 kg were required for controlled deorbit). In addition, Kosmos 1686 and Salyut 7 each suffered major systems breakdowns soon after they were abandoned, making the complex impossible to control.
• All previous space stations over which the Soviets maintained control were intentionally deorbited after their last cosmonaut crew departed. The Soviets estimated that the reboost gave the complex an 8-yr lifetime in orbit. They considered recovering the station using the Buran shuttle. However, following delays to and the eventual cancellation of the Buran program, Kosmos 1686 underwent uncontrolled reentry with Salyut 7 on February 7, 1991, reentering over Argentina, scattering much of its debris over the town of Capitan Bermudez
  Capitán Bermúdez
  Capitán Bermúdez is a city in the province of Santa Fe, Argentina, located within the metropolitan area of Greater Rosario, , on the western shore of the Paraná River. It has a population of about 27,000 inhabitants per the .The town was founded in 1889, and officially became a city in 1970...
  
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