The Partnership: A History of the Apollo-Soyuz Test Project|
22-23 July - ExperimentsSome minor experiment hardware problems developed during the final days of the mission, but for the most part the crewmembers worked through their flight plan - which included 23 independent experiments - with few difficulties.  CSM 111 was truly the best - as well as being the last - Apollo to fly. After a relatively quiet day of work on the 22nd, the major part of the next day was devoted to preparing for and conducting the doppler tracking experiment (MA-089). Paired with the geodynamics experiment (MA-128), these investigations were designed to verify which of two techniques would be best suited for studying plate tectonics (movements of the earth's substrata) from earth orbit. Where the geodynamics experiment utilized Apollo and ATS 6 in an attempt to measure these movements (the so-called low-high approach), the doppler tracking experiment involved the use of two satellites in low earth orbit (the low-low approach) to measure the existence of "mass anomalies" greater than 200 kilometers in size. When the jettisoned docking module and the CSM were separated by 300 kilometers, they would theoretically have their orbits affected by the greater gravitational forces exerted by these mass anomalies. As their orbits were perturbed, the radio signals transmitted from one to another would correspondingly be affected.
Prior to releasing the docking module on its separate journey, the crew had participated in a second press conference from space. During that 32-minute session, the crewmen were asked to philosophize about the future of manned space flight in general and upon such diverse topics as trips to Mars and their own participation in the Shuttle program. Their answers were filled with optimism and good humor. Deke Slayton's statement that he had done nothing in space that his 91-year old aunt could not have done sent reporters scrambling to find out her name (Mrs. Sadie Link) so they could meet their deadlines. Following the press period, CapCom Crippen told the crew, "you guys did a great job there. Professional as always." He also gave them the news that Leonov had been promoted from colonel to major general.
With congratulations over, Stafford told the ground, "Now, back to work." After donning their space suits, the crew vented the command module tunnel and at 2:41 jettisoned the docking module. Filled with all their trash and used equipment that need not be returned, the DM tumbled into space at exactly the proper rate. Stafford and his team then executed their separation maneuver so that they could take the necessary doppler measurements. The docking module would continue on its way until it re-entered the earth's atmosphere and burned up in August 1975.49
49. Program Operations Office, "ASTP Technical Air-to-Ground Voice Transcription," pp. 925-940 and 989-990; and "Astronauts Look Forward to the Space Shuttle Era," Washington Star, 23 July 1975.