This apparently paradoxical condition occurred because, in order to intercept Venus, Mariner had been launched in a direction opposite to the Earth’s course around the Sun. The midcourse maneuver turned the spacecraft around and slowed its travel away from the Earth while allowing it to increase its speed around the Sun in the direction of the Earth’s orbit. Gradually, then, the spacecraft would begin to fall in toward the Sun while moving in the same direction as the Earth, catching and passing the Earth on the 65th day and intersecting Venus’ orbit on the 109th day.
At the time of the midcourse maneuver, the spacecraft was travelling slightly inside the Earth’s orbit by 70,000 miles, and was behind the Earth by 1,492,500 miles.
THE LONG CRUISE
After its completion of the midcourse maneuver, Mariner reoriented itself on the Sun in 7 minutes and on the Earth in about 30 minutes. During the midcourse maneuver, the omnidirectional antenna was used; now, with the maneuver completed, the directional antenna was switched back in for the duration of the mission.
Ever since the spacecraft had left the parking orbit near the Earth and been injected into the Venus trajectory, the Space Flight Operations Center back in Pasadena had been the nerve center of the mission. Telemetered data had been coming in from the DSIF stations on a 24-hour schedule. During the cruise phase, from September 5 to December 7, a total of 16 orbit computations were made to perfect the planet encounter prediction. On December 7, the first noticeable Venus-caused effects on Mariner’s trajectory were observed, causing a definite deviation of the spacecraft’s flight path.
On September 8, at 12:50 p.m., EST, the spacecraft lost its attitude control, which caused the power serving the scientific instruments to switch off and the gyroscopes to switch on automatically for approximately three minutes, after which normal operation was resumed. The cause was not apparent but the chances of a strike by some small space object seemed good.
As a result of this event, a significant difference in the apparent brightness reading of the Earth sensor was noted. This sensor had been causing concern for some time because its readings had decreased to almost zero. Further decrease, if actually caused by the instrument and not by the telemetry sensing elements, could result in loss of Earth lock and the failure of radio contact.
After the incident of September 8, the Earth sensor brightness reading increased from 6 to 63, a normal indication for that day. Thereafter, this measurement decreased in an expected manner as the spacecraft increased its distance from the Earth.
Mariner II was now embarked on the long cruise. On September 12, the distance from the Earth was 2,678,960 miles and the spacecraft speed relative to the Earth was 6,497 miles per hour. Mariner was accelerating its speed as the Sun’s gravity began to exert a stronger pull than the Earth’s. On October 3, Mariner was nearly 6 million miles out and moving at 6,823 miles per hour relative to the Earth. A total of 55,600,000 miles had been covered to that point.
Considerable anxiety had developed at JPL when Mariner’s Earth sensor reading had fallen off so markedly. This situation was relieved by the unexplained return to normal on September 8, although the day-to-day change in the brightness number was watched closely. The apparent ability of the spacecraft to recover its former performance after the loss of attitude control on September 8 and again on September 29 was an encouraging sign.