The board first laid out ground rules for drafting recommendations for each of the remaining Gemini missions. Benefits for Apollo and for science and technology were weighed against risks to crew safety. Mission planning policies were examined - was too much being programmed or too little?73 With Gemini X scheduled for 18 July, planning for that flight was nearly firm. The board did measure mission objectives against the new ground rules, but there was neither time nor opportunity for more than minor changes.74
Gemini X, like VIII and IX, was a complex flight with multiple objectives.  Among these was a dual rendezvous involving two Agenas - one launched for the mission, the other a passive target left over from Gemini VIII. Using the target's main engine to propel the docked Agena/spacecraft combination to high altitudes had been hotly debated on two previous missions. When the Atlas dropped into the Atlantic Ocean on 17 May 1966, the time for discussion was past. Since neither Gemini VIII nor IX-A had provided the hoped-for experience of firing the Agena's main engine while it was docked to a spacecraft, a decision had to be made promptly. There were only three flights left in the program. Nor would there be any preliminary, low-level practice first. The next day, Mathews told his staff that Gemini X would dock with Agena 10 and together they would climb to Agena 8.75
On 24 January 1966, John Young and Michael Collins were named to fly Gemini X.** When Young first heard about the dual rendezvous plan, he thought, "they must be out of their minds." The astronaut had two worries. Could he slow down the linked vehicles and stop them in time to keep from crashing into the second Agena? VIII's Agena, having run out of electrical power, was dead, with no radar transponder or other apparatus to help in the search. Could he even find the old Agena, using only optical equipment? Young recalled, "We hadn't worked on any of these procedures. The problem with an optical rendezvous is that you can't tell how far away you are from the target. With the kind of velocities we were talking about, you couldn't really tell at certain ranges whether you were opening or closing."76
Young also remarked, "We didn't have an EVA program," but that soon changed. Collins would do experiments, retrieve packages from both the spacecraft and the passive target, test a zip gun, and visit an unstabilized vehicle. The backpack was dropped for missions X and XI and replaced by a 15-meter umbilical to supply oxygen and electrical support.77
Deciding what to do was only the beginning; how to do it was the bigger challenge. The second part of the double rendezvous (with the passive Agena) was particularly tricky. Agena 8, like all Earth-orbital vehicles, had been precessing above and below the equator on its orbital path. With no help from the dead target possible, the Gemini X Agena and spacecraft would have to be launched at very precise times. Suppose circumstances delayed the launches? It had happened before - more often than not! The mission planners would have to come up with a new set of numbers in a hurry. With events so closely related,  delay or failure at any point threatened all aims of the flight.
While shaping the Gemini X mission for the dual rendezvous, the planners decided to give the crew some helpful experience in onboard navigation using optical equipment, charts, and the spacecraft computer. The crew would join its first target in the fourth orbit. Mission sequence was the next consideration. When should the dual rendezvous take place - the second day or the third day? Mission planners eventually decided that the second day should be devoted to experiments, the third to chasing the passive target. This, in itself, appeared to create a conflict of aims; although Agena 10 was needed to carry the spacecraft to the second target, many of the planned experiments could not be performed while the vehicles were docked.
About 50 people kicked this problem around at a trajectories and orbits meeting on 28 April 1966. Obviously, the launch dates would have to be jockeyed to get the best phase relationship between the spacecraft and target for both the dual rendezvous and the experiments.78
Even assuming that both launches went as planned, shaping the second rendezvous was an exacting task. The North American Air Defense (NORAD) Command, at Colorado Springs, had kept track of Agena 8's whereabouts ever since it ran out of electrical power. To begin the rendezvous, the docked Gemini X/Agena 10 combination should first go into a large elliptical orbit, 298 kilometers at perigee and 752 kilometers at apogee. After six revolutions to judge phase relationships, Agena 10 would then maneuver down to an approximately 398-kilometer circular orbit near Agena 8's space lane, as reported by NORAD.
The high altitude aspect of the flight raised its usual qualms. Although the Gemini Program Office no longer resisted the use of the big Agena engine while the vehicles were docked, McDonnell did not like the idea of the vehicles passing through so many high orbits, which might affect a safe emergency reentry if the retrorockets did not perform as needed. There was also the South Atlantic radiation zone to be considered. In a trajectories and orbits meeting at the end of June 1966, the maximum acceptable altitude for the dual rendezvous was set at 298 by 1,065 kilometers, based on radiation constraints and actual radiation levels measured in 1964. But the decision to use Agena for docked maneuvers had already been made, and any misgivings had to be laid aside. After careful study, the planners concluded that an emergency reentry from an elliptical orbit with a perigee of 298 kilometers could be made even if only three out of the four retrorockets fired. Finally, they plotted the spacecraft's orbital track with great care, to avoid the heavy radiation patches.79
With the memory of past flights still fresh - when no one had been sure what target, if any, would be waiting - they made alternate and  contingency plans for Gemini X. If the target vehicle for this flight did not reach orbit, the mission would be renamed X-A, and the spacecraft would be launched into a 162- by 385-kilometer orbit to rendezvous with the Agena 8 on the 16th revolution. The alternate plans also covered experiments, extravehicular activity, and systems tests.80
* Members were Edgar M. Cortright (NASA Deputy Associate Administrator for Science and Applications), Major General Vincent G. Huston (Commander, Air Force Eastern Test Range), and MSC GPO Manager Mathews.
** Lovell and Aldrin were selected as backup command pilot and pilot, respectively. On 21 March 1966, after the deaths of See and Bassett, they were moved into the backup positions for Gemini XI-A. Bean and Clifton Williams then became the alternate crews for the first Apollo manned flight: Gus Grissom, Edward White, and Roger Chaffee (prime); James McDivitt, David Scott, and Russell Schweickart (backup).
71 Memo, Seamans for Mueller, "Gemini IX Review," 9 June 1966; Meyer, notes on GPO staff meeting, 14 June 1966, p. 1.
72 Memo, Mueller to Dep. Adm., "Review of Gemini Missions X, XI, and XII," 23 June 1966, annotated: "Action approved. Robert C. Seamans, Jr. June 24, 1966"; letter, Mueller to Gilruth, 23 June 1966, with enclosure, "Gemini Mission Review Board," 22 June 1966; letter, Seamans to Harold Brown, 21 June 1966; letter, Brown to Seamans, 29 June 1966; letter, Gilruth to NASA Hq., Attn: Mueller, "Gemini Mission Review Board," GP-62275, 30 June 1966.
73 James C. Elms, telephone interview, 29 Oct. 1969; [Elms], "Committee Action on Gemini X Mission," 5 July 1966.
74 Elms interview; Elms, "Interim Report Gemini Mission Review Board," 15 July 1966,
75 "Gemini Program/Mission Directive," Appendix A, Sec. 10; Meyer notes, 18 May 1966,
76 Jack Amerine, "Young, Collins Named Crew of Gemini-10," Houston Chronicle, 25 Jan. 1966; Jim Maloney, "Young and Collins Gemini 10 Crew," The Houston Post, 25 Jan. 1966; TWX, Mathews to McDonnell, Attn: Burke, "Contract NAS 9-170, Gemini, Astronaut Fit Check," GP-7513, 5 April 1966; "Gemini and Apollo Crews Selected;" John W. Young, interview, Houston, 8 May 1967.
77 John Young interview; Meyer, notes on GPO staff meeting, 1 Feb. 1966, p. 2; "Gemini Program/Mission Directive," Change No. 2, 15 Feb. 1966, p. D-10; memo, Mathews to Asst. Dir., Flight Crew Ops., Attn: Chief, Flight Crew Support Div., "Fifty-foot umbilical for Spacecraft 10," GS-64116-A, 25 Feb. 1966; memo, James V. Correale to GPO, "Gemini extra-vehicular life support system," 17 Sept. 1963; "General Requirements for an Engineering Study and Preliminary Design of a One-Man Propulsion Device for the Gemini Program," Exhibit A, "Statement of Work," 19 Sept. 1963; memo, Richard S. Johnston to Mgr., GPO, "Gemini Extravehicular Life Support System Development," 25 March 1964; TWX, Mathews to McDonnell, Attn: Burke, "Contract NAS 9-170, Gemini Configuration Control Board Meeting Number 94, 3-12-66," GV-12376, 14 March 1966; TWX, Mathews to McDonnell, Attn: Burke, "Contract NAS 9-170, Gemini - Action Items of NASA/McDonnell Management Meeting,3-23-66," GP-7510, 1 April 1966; Burns et al., "Gemini Extravehicular Activities," pp. 3-12, -13; Bell et al., "Life Support Systems for Extravehicular Activity," pp, 4-88, -89; Johnson, Schultz, and Huber, "Maneuvering Equipment," D. 6-4.
78 Mathews memo, GV-66208, 1 Oct. 1965; "Abstract of Meeting on Trajectories and Orbits, February 16, 1966," 3 March 1966; TWX, Mathews to McDonnell, Attn: Burke, "Contract NAS 9-170, Gemini, Meeting on Gemini X Onboard Rendezvous Procedures," GS-10104, 22 March 1966; TWX, Mathews to McDonnell, Attn: Burke, "Contract NAS 9-170, Gemini, Onboard Procedures for Gemini X Primary Rendezvous," GS-10107, 7 April 1966; memo, Mathews to Asst. Dir., Flight Ops., "Gemini X Mission Plan," GV-66389, 7 April 1966; "Abstract of Meeting on Trajectories and Orbits, April 28, 1966," 31 May 1966; TWX, Mathews to McDonnell, Attn: Burke, "Contract NAS 9-170, Gemini Configuration Control Board Meeting Number 102, May 9, 1966," 10 May 1966; Meyer notes, 14 June 1966.
79 "Trajectories and Orbits Meeting, February 16, 1966"; "Trajectories and Orbits Meeting, April 28, 1966"; Howard W. Tindall, Jr., telephone interview, 5 Nov. 1969; TWX, Networks Ops. to W. H. Wood, "GT-8 Agena," 25 March 1966; Mathews memo, GV-66447, 26 May 1966; memo, Mathews to Asst. Dirs., Flight Ops. and Flight Crew Ops., "Gemini X Mission Changes and Priorities," GV-66454, 10 June 1966; Elvin B. Pippert, Jr., J. V. Rivers, and Tommy W. Holloway, "Gemini X Flight Plan," Final, 22 June 1966; TWX, Mathews to SSD, Attn: Gardner and Hull, "Revisions to the Gemini Program Mission Directive for Gemini X," GV-12456, 27 June 1966; memo, Mathews to NASA Hq., Attn: Schneider, "Gemini X Mission Changes and Priorities," GV-66458, 27 June 1966; "Abstract of Meeting on Trajectories and Orbits, June 29 and July 8, 1966," 14 July 1966; Larry D. Davis, telephone interview, 12 Nov. 1969.
80 Elms, "Interim Report," pp. 5-6; "Abstract of Meeting on Gemini IX Agena Real Time Mission Evaluation Support, April 27, 1966," 6 May 1966; letter, Mathews to NASA Hq., Attn: Schneider, "Contingency Missions for Gemini X, XI, and XII," GV-66461, 23 June 1966; TWX, Mathews to SSD, Attn: Hull, and McDonnell, Attn: Burke, "Pertinent Gemini X Information," GV-12455, 24 June 1966; letter, Schneider to Kraft, 7 July 1966; TWX, Mathews to SSD, Attn: Hull, and McDonnell, Attn: Burke, "Gemini X Alternate Mission," GV-12461, 8 July 1966; Gemini 10 News Center Release No. 10, "Alternate Gemini 10 Plans," 16 July 1966.