During day five of the flight, the crews concentrated on docking exercises and experiments that involved the two ships in the undocked  mode. During the interval between the first undocking and the second docking, the Apollo crew placed its craft between Soyuz and the sun so that the diameter of the service module formed a disk which blocked out the sun. This artificial solar eclipse, as viewed from Soyuz, permitted Leonov and Kubasov to photograph the solar corona. Ground-based observations were conducted simultaneously, so that the Soviet astronomer G. M. Nikolsky could compare views of the solar phenomena with and without the interference of the earth's atmosphere. Skylab had provided a long term look at the corona, and the ASTP data would give scientists an opportunity to compare findings made a year and a half later. This "artificial solar eclipse" (MA-148) experiment would be the last American chance for such information gathering until the Shuttle era.
Another major experiment, "ultraviolet absorption" (MA-059), was an effort to more precisely determine the quantities of atomic oxygen and atomic nitrogen existing at such altitudes as the one in which Apollo and Soyuz were orbiting. Again this information could not readily be obtained from ground-based observations because of the intervening layers of atmosphere. Apollo, flying out of plane around Soyuz, first at 150 meters, then at 500 meters, and finally in plane at 1,000 meters, projected monochromatic laserlike beams of light to retroreflectors mounted on Soyuz. When the beams were reflected back to Apollo, they were received by a spectrometer, which recorded the wavelength of the light. Subsequent analysis of these data would yield information on the quantities of oxygen and nitrogen. Some very precise flying was called for in these experiments.
After being docked for nearly 44 hours, Apollo and Soyuz had parted for the first time at 7:12 a.m. while out of contact with the ground. Slayton advised Bobko after radio contact was re-established that they had undocked without incident and were stationkeeping at a range of 50 meters. Meanwhile, Soyuz had extended the guide ring on its docking system in order to test the Soviet mechanism in the active configuration. Once they completed the solar eclipse experiment, with Slayton at the controls, Apollo moved towards Soyuz for the second docking. As he did, Stafford called out to the ground, "Okay, Houston, Deke's having the same problem with the COAS washout that I had." As Slayton explained it, he could see Soyuz and the target initially when they were against the dark sky, but at "about 100 meters or so, it went against the earth background and zap. Man, I didn't have anything." Although worried that he might run over Soyuz, he pressed on with the docking "by the seat of the pants and I guess I got a little closer than they or the ground anticipated."38 There was too much light flowing into the optical alignment sight for Slayton to get a good view of the docking target. Contact with Soyuz came at 7:33:39, and Leonov advised  the Americans that he was beginning to retract his side of the docking assembly.
As viewed via Apollo television, this docking looked as if it had been harder than the first, and the two ships continued to sway after capture had been completed. Slayton, speaking in a debriefing, later said:
The docking was normal, you guys gave me contact as usual and then I gave it thrusting. The only thing that happened then was they seemed to torque off. I was surprised at the angle they banged off there after we had contact.39
Despite this oscillation, the Soyuz system aligned the two craft and a proper retraction was completed. Subsequently, there was some discussion of this docking, and the Soviet docking specialist Syromyatnikov was at first worried that an unnecessary strain might have been placed on the Soyuz gear. Bob White said that analysis of the telemetry data indicated that Slayton had inadvertently fired the roll thrusters for approximately 3 seconds after contact, and that this sideways force caused the craft to oscillate after the docking systems were locked and rigid.
But even with the extra thrusting, the second docking was within the limits of safety established for the docking system. Slayton's docking took place at a forward velocity of 0.18 meter per second versus 0.25 meter per second for Stafford's docking, but the difference lay in the inadvertent thrusting. Momentarily an issue, the extra motion of Slayton's try was not a serious concern after all the data had been evaluated. Even Syromyatnikov had to concede that "the mechanism functioned well under unfavorable conditions." It was a case of things looking worse than they really were. In the end, the incident only demonstrated the reliability and hardiness of the new docking system.40
It was 10:27 when Apollo and Soyuz undocked for the second and final time. This 4-minute exercise was conducted by Leonov, since it was a Soyuz active undocking. Slayton then moved his ship to a stationkeeping distance, about 40 meters away. As he did, Leonov opened the retroreflector covers so that the ultraviolet absorption (UVA) experiment could be performed. A difficult series of maneuvers were called for in this test. As Soyuz continued its circular orbit, Slayton took Apollo out of plane with Soyuz and oriented his craft so that its nose was pointed at the reflector on the side of the other ship. Orbiting sideways in this configuration, Slayton flew Apollo in a small arc from the front of Soyuz to the rear of that ship while the spectrometer gathered the reflected beams. On the 150-meter phase of the experiment, light from a Soyuz port led to a misalignment of the spectrometer, but on the 500-meter pass excellent data were received; on the 1,000-meter pass satisfactory results were also obtained.
After nearly 3 hours of tough flying, Bobko congratulated the crew. "You people flew it fine." Slayton responded:
Okay. Great, Bo. And you can thank ol' Roger Burke, Steve Grega, and Bob Anderson, down there, that everything came off right. 'Cause they sure did all the work to make it go.41
The three men Slayton mentioned had spent hours in the simulators working out the procedures to fly this complicated maneuver. Burke, who had worked with developing flight procedures for years, felt that this was one of the hardest experiments a crew had ever been called on to do, especially since the flight plan for it had continued to evolve until a couple of days before launch.42 Slayton later noted that it had taken all three Apollo crewmen to complete the ultraviolet absorption experiment. "I was doing the flying, Vance was running the computer and we had Tom down in the equipment bay opening and closing doors, turning on sensors and so forth. So, it was a busy time for all of us." He indicated that the maneuvers were difficult because orbital mechanics came into play as they tried to fly around Soyuz. When the Apollo crew changed the velocity of their craft, they also affected its orbit. They would have no difficulties if they had had unlimited fuel resources, but being out of plane and playing orbital mechanics with "a very limited fuel budget . . . made it a great challenge."43 Stafford added  that the thruster firings had to be timed because the onboard accelerometers could not measure the changes in velocity.44
Apollo performed a separation maneuver at 1:42 to prevent re-contact with Soyuz, placing the American craft in a 217- by 219-kilometer orbit. With all the joint flight activities completed, the ships were going their separate ways. Soyuz was below and moving ahead of Apollo at a rate of 6 to 8 kilometers per orbit. Leonov and Kubasov prepared to go to sleep, but the American crew had several hours of work scheduled in their crowded flight plan after their mid-afternoon meal before they could settle down for a rest period. The fifth day of ASTP - the second of joint activities - had been a success, and everyone in the Moscow and Houston control centers was pleased that all had gone so well.
38. Crew Training and Procedures Division Training Office. "ASTP Technical Crew Debriefing," p. 4-27.
39. Program Operations Office, "ASTP Technical Air-to-Ground Voice Transcription," pp. 494-497; and Crew Training and Procedures Division Training Office, "ASTP Technical Crew Debriefing," p. 4-27.
40. Interview, White-Ezell, 19 Aug. 1975; Roscoe Lee to Task ASTP E-101 File, TRW memo, "Control System Post-flight Analysis for the Initial Mission Report," 30 July 1975; James O'Kackson, "Russ Bare Soyuz Strain in Linkup," Chicago Tribune, 21 July 1975; and Robert C. Toth, "Russians Upset by Hard Space Docking," Los Angeles Times, 21 July 1975.
41. Program Operations Office, "ASTP Technical Air-to-Ground Voice Transcription," pp. 539-540.
42. Interview, Roger A. Burke-Ezell, 14 Aug. 1975.
43. Interview, Donald K. Slayton-Ezell, 2 Mar. 1976.
Stafford-Ezell, 6 Apr. 1976.