The Surveyor II spacecraft, which was to soft-land on the moon tonight, lost communications with Earth tracking stations at 2:35 a.m. PDT today--45 hours and three minutes after it was launched from Cape Kennedy.
The mission ended during the firing of Surveyor's 10,000 pound thrust retro-rocket, one of a number of engineering experiments performed when it became apparent that a soft landing would not be possible.
For more than 24 hours, engineers at the Jet Propulsion Laboratory attempted to correct an out-of-control tumbling condition which began during the midcourse trajectory correction.
Surveyor Project officials said the 60-rpm tumble was caused by the failure of one of three vernier (throttleable) rocket engines to ignite during the thrust phase of the midcourse maneuver at 10:00 p.m. Tuesday.
Flight control engineers at JPL first tried to stabilize the spacecraft with small nitrogen gas jets, part of Surveyor's attitude control system, but abandoned the attempt because the rapid tumble was beyond the correction capability of the jets.
In order to stabilize the spacecraft, it was necessary that all three vernier rocket engines fire simultaneously. The same three-engine firing is required to perform a controlled landing.
A total of 38 attempts to fire the engines at various thrust levels and durations were conducted beginning at 12:28 a.m. yesterday. In each case, only two engines ignited.
The final attempt, at 1:05 a.m. today, was commanded from the Canberra, Australia, tracking station of the Deep Space Network and called for a full-thrust motor burn for 20 seconds. While a proper three-engine firing might have corrected the 92rpm tumble, the partial, off-center thrust increased the tumble rate to 136 rpm.
Because the tumble prevented the spacecraft's solar panel from locking onto the sun, Surveyor was operating on battery power alone. Concern that the battery life was nearing an end prompted project officials to conduct experiments in an effort to obtain as much engineering data as possible before the mission ended.
Early today it also was apparent that the major objectives of the mission could not be met.
Commands were transmitted from the Canberra station to vent helium gas used to pressurize the vernier engines, erect the solar panel, turn on the radar altimeter and doppler velocity sensor normally used during the final descent phase of Surveyor flight and fire the main retro engine.
Activation of the radar system gave engineers important data on the capability of a weakened spacecraft battery to provide electricity to a system with large power requirements.
Helium venting provided information on the reliability of a possibly faulty pressure sensor.
Because of centrifugal force of the tumbling, the solar panel bounced as it was commanded to move in small increments. The attempt to point the panel toward the sun was abandoned when the bouncing continued with each series of commands. In addition to the possibility of obtaining some solar energy, it was hoped that movement of the panel would provide mechanical data.
The main retro motor was ignited at 29 seconds after 2:34 a.m. Planned as the last event in the mission of Surveyor II, it was expected that firing of the 10,000-pound-thrust engine would terminate communications with the spacecraft.
The Canberra station was able to track Surveyor during 30 seconds of the 40-second-duration firing. Loss of radio lock occurred at 2:35 a.m.
Helium tank depressurization increased the tumble rate from 136 rpm to 146 rpm. At the time of communications loss during retro fire, the tumble slowed to 116 rpm.
Whether or not the spacecraft was damaged structurally by the large stresses from retro firing was not known.
Surveyor's trajectory remains an encounter course with the moon where it will impact about 8:30 p.m. PDT today at a velocity of about 6,000 miles per hours.
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