Crossing the equigravisphere

Between the Moon and Earth, there came a point where the gravity of the approaching body became stronger than that of the receding body. When this point of gravitational equality was reached, it was customary for mission control, and especially those concerned with flight dynamics, to switch their frame of reference from one world to another. However, because the Moon itself was in motion around Earth, the numbers representing the spacecraft's speed and position appeared to jump. Journalists found it difficult to make sense of this change in the velocity figures being fed to them by the NASA public affairs people, and some got the impression that a 'barrier' was being crossed that would surely be felt by the crew.

Mike Collins later related how Phil Shaffer, one of the flight dynamics controllers in the MOCR struggled to explain the truth to reporters: ''Never has the gulf between the non-technical journalist and the non-journalistic technician been more apparent. The harder Phil tried to dispel the notion, the more he convinced some of the reporters that the spacecraft actually would jiggle or jump as it passed into the lunar sphere. The rest of us smirked and tittered as poor Phil puffed and laboured, and thereafter we tried to discuss the lunar sphere of influence with Phil as often as we could, especially when outsiders were present.''

As a homeward-bound Apollo 11 crossed the imaginary line between the gravitational spheres of influence of the two worlds, Capcom Bruce McCandless called the spacecraft to inform the crew: ''Apollo 11, this is Houston. Stand by for a 'mark' leaving the lunar sphere of influence.'' He then indicated the moment's passing, ''Mark. You're leaving the lunar sphere of influence. Over.''

Collins saw a chance for some mischief. ''Roger. Is Phil Shaffer down there?'' The FIDO console was being manned by Dave Reed rather than Shaffer. ''Negative,'' said McCandless, ''but we've got a highly qualified team on in his stead.''

''Roger. I wanted to hear him explain it again to the press conference,'' teased Collins. ''Tell him the spacecraft gave a little jump as it went through the [equigravisphere].''

''Okay. I'll pass it on to him. Thanks a lot,'' said McCandless, ''and Dave Reed is sort of burying his head in his arms right now.''

Crews continued to play with this confusion throughout the programme. As Endeavour headed home, Capcom Joe Allen let Apollo 15's crew know they had entered Earth's sphere of influence. ''Be advised at my mark, you are leaving the sphere of lunar influence; and it's downhill from here on in... Mark!'' ''Roger, Thank you, Joe,'' replied Scott. ''That's nice to know.'' ''Did you notice anything there, Dave? Discontinuity in velocity or anything like that?'' teased Allen.

''Well, Joe,'' returned Scott. ''That's one of the mysteries that we'll probably have to keep to ourselves.''

''I was afraid of that,'' replied Allen.

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