Commands were transmitted from Earth to the Mars-bound Mariner 7 today to complete the star acquisition sequence begun shortly after the spacecraft was launched on March 27.
For the first five days of flight, Mariner 7's star tracker has been "locked onto" the star Vega. One of the com- mands, generated at the Jet Propulsion Laboratory, Pasadena, Calif., caused the spacecraft to drop lock on Vega and continue its roll search until it acquired and locked onto the star Canopus.
The commands were transmitted from the NASA-JPL Deep Space Network station at Woomera, Australia, across a distance of about 1,120,000 miles.
Canopus, one of the brightest stars in the galaxy, provides a second celestial reference--along with the sun--upon which to base the midcourse maneuver and the pointing of the science instruments at Mars.
Mariner 7 was allowed to remain locked onto Vega for several days while engineers analyzed some unexpected behavior observed in the Mariner telemetry during the launch phase of the mission.
Most of the anomalies are now understood and the spacecraft is operating correctly.
During the powered phase of flight, the telemetry data indicated a decrease in the radio transmitter power lasting for about eight minutes. However, the decrease has not been verified by ground receiver measurements and the performance at present is correct. Although the telemetry indication has not recurred, this inconsistency remains under investigation.
An unexpected change in the power profile was observed when sunlight fell on one of the spacecraft sensors. This occurrence was normal, but was not recognized as such at the time.
The Central Computer and Sequencer memory also has been checked out and found to be correctly loaded. It is believed that a spurious electrical signal at the time Mariner separated from the Centaur caused some unintended switch settings in the CC&S. A command from the ground today reset the switches and all are now in the normal position.
Preliminary analysis of the Mariner 7 trajectory indicates that the launch accuracy was as good or better than the near-perfect launch of Mariner 6 a month earlier. Without a midcourse maneuver, Mariner 7 would fly over the south polar cap of Mars at an altitude of about 16,000 miles on the night of August 4. A maneuver, however, is planned for next week to change the closest approach altitude to about 2000 miles.
At 12 noon PST today, Mariner 7 was 1,120,000 miles from Earth and traveling at a velocity of 9311 miles per hour relative to Earth. Distance to Mars was 76 million miles. Mariner 7 will clock 195 million miles in its 130-day voyage. Of this distance, the spacecraft has completed 8.5 million miles.
Mariner 6, now in its 36th day of flight, is 6.2 million miles from Earth and 71 million miles from Mars. Its flight path is 226 million miles long and duration of flight from Earth to Mars is 156 days. Mariner 6 has already flown 64 million miles.
Mariner 7, one of two automatic scientific spacecraft destined to rendezvous with Mars this summer, today executed a midcourse maneuver adjusting its flight path for a 2000-mile- altitude swing by the planet.
Now in its twelfth day of flight and more than 2 1/2 million miles from Earth, Mariner 7 acted upon commands from the ground to turn the proper position then fire its rocket engine for 7.7 seconds for a velocity change of about 10 miles per hour.
Mariner Project officials at the Jet Propulsion Labora- tory in Pasadena, California, said the maneuver shortens the Earth-to-Mars flight time by 15 minutes and brings the spacecraft closer to the planet. On its pre-maneuver trajectory, Mariner 7 would have passed Mars at a distance of 16,500 miles at 10:20 p.m. PDT on August 4, 1969. The new arrival time is 10:05 p.m. at a distance of about 2000 miles.
Maneuver commands, prepared at the Space Flight Operations Facility at JPL, were transmitted from the Deep Space Network station at Woomera, Australia. The DSN station at Johannesburg, South Africa, also had the spacecraft in view and both stations monitored the performance of the maneuver.
Mariner 7 was launched March 27 from Cape Kennedy. It carries an array of science instruments to obtain high resolution television pictures of Mars and measure surface temperatures and atmospheric composition and density. Mariner 7 will conduct its experiments in the southern hemisphere of the planet, including a portion of the south polar cap.
Mariner 6, an identical twin, is now in its 43rd day of flight and is more than seven million miles from Earth. Mariner 6 will arrive at Mars at 10:18 p.m. PDT on July 30, with instruments conducting similar experiments near the Martian equator.
A series of radio commands has been transmitted to Mariner 6--one of two U.S. spacecraft on the way to Mars--to alter the method of spacecraft stabilization as Mariner flies past the planet this summer.
Transmitted as a precautionary measure because of a possible problem with the spacecraft receiver and a known problem with the Canopus sensor, the commands programmed the on-board computer to turn on the gyroscopes automatically for the science- taking phase of the mission.
Using the gyros to provide a stable platform for pointing the science instruments at Mars and the high-gain antenna toward Earth is an alternate attitude stabilization procedure. Normally three-axis stabilization is provided by optical sensors that lock onto the sun and the star Canopus.
According to the Mariner Project manager, H. M. (Bud) Schurmeier, of the Jet Propulsion Laboratory, the Canopus sensor was "distracted" by tiny bright dust particles apparently shaken off the spacecraft both at the time of the mid-course maneuver and when Mariner 6's scan platform was unlatched. Both events correlate with the firing of explosive devices.
When the platform was unlatched, the spacecraft was locked onto Canopus. The particles caused Mariner to go into a roll search for the star and Canopus was reacquired properly. At the time of the maneuver, the spacecraft was on autopilot-gyro control and the particles were seen but ignored.
Another squib-firing event is associated with the Mars encounter sequence and could cause Mariner 6 to begin a star search at the last moment if particles are again in view of the sensor while it is locked onto a star.
Mariner 7, which will reach Mars five days after Mariner 6, also experienced the dust particle problem when it executed a mid-course maneuver. Mariner 7, however, is still programmed for the normal sun-Canopus attitude control scheme, but will be reprogrammed for gyroscopes at a later date.
The commands to Mariner 6 were sent early in the mission because of a recent change in the telemetry reading from the radio receiver indicating the possibility that at a later date the spacecraft might not accept commands from the ground. The problem, however, has not recurred.
The Canopus sensor on Mariner 6 also has failed to accept a "look angle" update command, one of a series issued periodically by the spacecraft to keep the star within the field of view of the sensor. Backup commands from the ground indicated the sensor can be switched to only two of its five view positions.
An analysis of other stars that can be used as a roll reference when Canopus is out of view is underway at JPL. Because it is now planned to complete the Mars passage on inertial con- trol, with the gyros, a star reference is necessary only to establish the initial roll position for the gyros.
Mariner 6 was launched from Cape Kennedy on February 24 and will arrive at Mars at about 10:18 p.m. PDT on July 30. Mariner 7 was launched on March 27 and makes its closest approach to the planet at about 10 p.m. PDT on August 4. Bh Mariners will pass Mars at a lowest altitude of about 2000 miles. ow in its 59th day of flight, Mariner 6 is more than 10 million miles from Earth and about 50 million miles from Mars. Mariner 7 is 6.7 million miles from Earth and 53.8 million miles from Mars.
Two Mariner spacecraft enroute to Mars logged a combined total of 272,363,358 miles of space travel at noon today with 167 million miles--and 8 weeks--to go to Mars encounter.
Mariner VI is 21,731,091 miles from Earth and will fly past Mars--at its point of closest approach--at 10:18 p.m., PDT, July 30, after traveling 241,838,160 miles since launch last February 24.
Mariner VII is 19,526,893 miles from Earth and will fly past Mars at 10:00 p.m., PDT, August 4. Its total travel distance will be 197,137,830 miles. At noon today it had covered 110,808,438 miles since launch on March 27.
The distance from Mars to Earth at encounter will be approximately 60 million miles.
Each of the 850 pound spacecraft are identically equip- ped with two television cameras to photograph Mars at medium and high resolution and with instruments to measure the temperature of the surface and to determine the composition of the upper and lower atmosphere. Atmospheric density will also be measured by transmitting the spacecraft radio signal through the atmosphere of Mars as the spacecraft curves behind the planet.
The first data on Mars will be transmitted to Earth on the night of July 29 in the form of 33 pictures of the full disc of the planet taken by Mariner VI during its approach to Mars.
The pictures will be stored on a tape recorder in the spacecraft and played back when Mariner VI is over the 210-foot diameter communications antenna at the Goldstone station of the Deep Space Net in the California Mojave desert. (Additional pictures are then recorded on the tape.) Use of the sensitive 210-foot antenna will allow transmission of one picture each five minutes at a rate of 16,200 bits per second. By comparison it required 8 1/2 hours to transmit each picture from Mars in 1965.
The two spacecraft are programmed to return a total of 141 pictures of the disc of Mars in five transmission sessions as they approach Mars. When the spacecraft fly past Mars the tape recorders will then store 24 surface pictures for each spacecraft and science data from the other instruments. These will be transmitted to Earth when the spacecraft have gone beyond Mars.
The 1965 pictures contained 240,000 bits each compared with 3,900,000 bits for the pictures expected from the current missions.
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