Onufrienko served as a pilot in the Soviet (later Russian) Air Force, where he logged over 800 flight hours.

Starting March 1994, he entered flight training to be the commander of the stand-by crew of the Mir-18 expedition aboard the Soyuz TM-21 spacecraft and the Mir space station as part of the Shuttle-Mir program.

The Soyuz TM-23 spacecraft carrying Onufrienko with cosmonaut Yury Usachov lifted off from the Baikonur Cosmodrome on February 21, 1996, at 12:34:05 UTC.

After two days of autonomous flight, the Soyuz spacecraft docked with the Mir space station's forward-facing port on February 23 at 14:20 UTC.

Onufrienko and Yuri Usachov were joined by French astronaut Claudie André-Deshays after the departure of Shannon Lucid.

On September 2, 1996, Onufrienko, Usachev and Claudie André-Deshays returned to Earth on board the Soyuz TM-23 capsule.

During a 6+1⁄2-month stay aboard the ISS, the 3 member Expedition 4 crew (Onufrienko and NASA astronauts Daniel W. Bursch and Carl E. Walz) performed flight tests of the station hardware, conducted internal and external maintenance tasks, and developed the capability of the station to support the addition of science experiments.

[6] The Expedition 4 crew returned to Earth aboard STS-111, with Endeavour landing at Edwards Air Force Base, California, on June 19, 2002.

The two spacewalkers also inflated an aluminum and nylon pup-up model of a Pepsi Cola can, which they then filmed against the backdrop of Earth.

They installed the modular optoelectrical multispectral scanner (MOMS) outside Priroda and handrails on the Kvant-2 module to facilitate moving around outside the station during future extravehicular activities.

During the spacewalk, Onufrienko and Usachov installed the Rapana truss structure (an experiment mounting point) to the Kvant-1 module.

Onufrienko and Usachev also manually deployed the saddle-shaped traverse synthetic aperture radar antenna on Priroda.

Onufrienko and NASA astronaut Carl Walz relocated the cargo boom for the Russian Strela crane.

Onufrienko performed his eighth career spacewalk on January 25, 2002, when he and NASA astronaut Daniel Bursch ventured out into space from the Pirs airlock.

[8] During the spacewalk, Onufrienko and Bursch installed six deflector shields for the Zvezda Service Module's jet thrusters.

This article incorporates public domain material from websites or documents of the National Aeronautics and Space Administration.