Salyut 4
The Soviet Union launches its fourth orbital space station, Salyut 4. Much like Skylab, Salyut 4 is fitted with a solar telescope and X-ray astronomy equipment, which is trained on the X-ray source (and potential black hole) Cygnus X-1 during its flight. Three crews will go on to occupy Salyut 4 before it is deorbited in 1977; it is the first Soviet space station to successfully be occupied by multiple crews.