Science: Command Correction

As the U.S. Venus probe Mariner II raced through space last week, it slipped past a significant milestone: it effectively escaped from the earth’s gravitation and became a satellite of the sun. But far out as it reached, it did not escape from the earth’s control. Back on the fast-receding planet, 1,500,000 miles away, the scientists who built the spacecraft could still hear its radio voice, still send it orders.

Agonizing Wait. At Goldstone control center in California’s Mojave Desert, the scientists had been composing their message for more than a week. They knew with precision what maneuvers Mariner II must perform if it was to pass within a useful distance of Venus. The question was: Would the faraway spacecraft accept the orders, store them in its electronic memory and execute them properly at the proper time?

The first command—”Roll minus 9.33 degrees”—sped across space from the great dish antenna at Goldstone. After an agonizing wait for radio travel time, Mariner II acknowledged the command and repeated it accurately. For more than an hour the long-distance conversation continued, carried on in a language of carefully spaced pulses of radio energy. At Goldstone these pulses appeared as mere dots on a slowly moving tape. But each combi nation of dots represented numbers in the two-digit binary code that computers understand best. Finally, Goldstone sent “Signal RTC-6,” which told Mariner II to execute all the commands one hour later.

The spacecraft’s electronic nervous sys tem took over and issued commands of its own, starting a one-hour warmup period. It turned off instruments and turned on guidance gyros. It swung the directional radio antenna aside to get it out of the blast of the mid-course rocket motor. At the end of the warmup, Mariner II was ready for the crucial maneuver of its long voyage. Replaying the commands from earth, it rolled 9.33 degrees and pitched its nose around for 139.83 degrees. This turned its mid-course rocket motor forward, putting it in position to slightly reduce the spacecraft’s speed. The motor fired for 29 seconds, as ordered. Then the spacecraft switched off its gyros, switched on its instruments, turned its sensors toward the sun, and pointed its directional antenna toward the earth.

Lonely Cruise. All these actions were duly reported by radio and received at Goldstone. When the maneuvers were finished, and Mariner II had resumed its lonely cruise, Project Director Jack James analyzed the data. Sounding almost as if he did not believe it himself, he announced: “Everything worked exactly as planned.” According to calculations based on radio measurements of the spacecraft’s decreased speed, Mariner II had corrected its course neatly; instead of missing Venus by 233,000 miles, it would pass it on Dec. 14 at the ideal observation distance of 9,000 miles. Some peril of hostile space may yet put Mariner II out of action before it reaches its goal, but already its voyage is a triumph of U.S. technology.

As Mariner II moved on toward Venus, the National Aeronautics and Space Administration announced that the Soviets have made six attempts to send probes to Mars or Venus. One probe, launched on Feb. 12, 1961, passed somewhere near Venus, but with its radio long dead. The other five tries were unqualified flops.