Soyuz 13 launched 1973

Soyuz 13, launched December 18, 1973, was modified to carry the Orion 2 Space Observatory, becoming the first manned space observatory and the Soviet Union's first mission dedicated to science.  Soyuz 13's crew consisted of Commander  Pyotr Klimuk and Flight Engineering Valentin Lebedev, both rookies.

They observed spectral lines of aluminum and titanium in the spectrogram of a planetary nebula.  They also studied Comet Kohoutek.

Klimuk and Lebedev returned to Earth December 26.

Mariner 10 flies by Mercury, 1974

Mariner 10, launched November 3, 1973, became the first spacecraft to encounter Mercury, the closest planet to the Sun. It made ultraviolet observations of Comet Kohoutek in January 1974 and passed Venus February 5. It made its first flyby of Mercury March 29, 1974.  The second was September 21 and the third (and last) March 16, 1975.

It was decommissioned about a week later.

Skylab 4 launched 1973

Carr, Gibson and Pogue

The third manned Skylab mission, the third and final crew to the first space station, lifted off November 16, 1973.  The crew, Commander Gerald P. Carr, Science Pilot Edward G. Gibson and Pilot William R. Pogue, spent 84 days in space, the longest period at that time.

During the mission, the crew complained they were being pushed too hard, being made to work long hours without sufficient rest.  NASA thought they weren't working hard enough or efficient enough.  On New Year's Day 1974, the crew, fed up with their work schedule, turned off their communications and enjoyed the view of Earth for a day.  NASA did not find that even remotely funny,  but their work schedule was modified and the crew completed more work than scheduled.

Part of their mission was to make observations of Comet Kohoutek.