THE PHOTOGRAPHS ARE MADE

The story of the launching and flight of Mariner IV, after the structural failure of the shroud of Mariner III, was told in last month’s issue of Scientific American by
J.N. James. To recapitulate briefly,] Mariner IV was launched on November 28, 1964. On the 78th day of flight a command was given to remove the lens cover from the television camera (to avoid the possibility that an attempt to remove it just before encounter, as originally planned, might cause last-minute problems that could not quickly be corrected. (One fear was that moving the lens cover might jar loose dust particles that would gleam like stars in the sunlight and disorient the Canopus-sensor.)]

At the same time that the lens cover was removed the scanning platform that carried the camera was tested and left in a position that was correctly, aimed at Mars, [on the basis of the computed flight path. The concern here was that the platform bearings might "freeze" in the course of seven-and-a-half-month flight through high vacuum of space and not move on command when sensing devices responded to light reflected from Mars.

On July 14, 1965] on the 228^th day of flight, when Mariner IV was about 20.000 miles from Mars, a command from the earth switched on the Mars-acquisition system that was linked to the scanning platform. [The platform responded.] Simultaneously the television system was switched on and began warming up preparatory to the actual picture-taking. The shutter began operating and the [Vidicon's] electron beam scanned the blank "pictures" of space, but according to plan none of these pictures was recorded.

The actual recording of pictures could have been initiat­ed in any of three ways: by the narrow-angle planet-sensor, which responded to sunlight reflected from Mars; by a sufficient brightening of the television images, indicating a bright object in the picture, or by direct command from the earth. [In case both of the built-in systems failed, a precisely timed command sent from the earth 12 minutes earlier would order the picture-taking and recording to begin when Mariner IV came to within 10,00Л miles of Mars.] In actu­ality the narrow-angle planet-sensor [is believed to have] triggered the sequence. [The direct command from the earth arrived about two minutes later.] Because the spacecraft was then about 130 million miles from the earth, the signal telling us that the first picture had been taken did not reach Goldstone until 12 minutes later. By then the 10th picture had already been taken. Had anything appeared abnormal at that time, a corrective command from the earth, even if made immediately, would barely have reached Mariner fV before the planet had passed from view. We finally received word that 22 pictures had been taken in a 26-minute period, but we still had no information about their content—or about whether any images had been recorded at all.

[The project staff was somewhat disturbed by a signal indicating that the end of the first track of the 330-foot loop of tape-had been reached after the fifth picture had been taken and that the tape had then begun recording on its second (and last) track. This seemed to imply a serious malfunction—for example, that only five pictures were recorded altogether, or that five pictures were recorded on the first track and that 10 or 11 more were recorded on the second track.]

Eighteen minutes after the end of the picture-taking Mariner IV passed within 6,118 miles of the surface of the planet. [An hour and 18 minutes later it went behind Mars, where its radio was blacked out for 54 minutes, and three hours and 20 minutes later, as the earth turned, Маriner IV's radio beam no longer reached the Goldstone station. We would have to waif until Mars—and Mariner – rose next morning over the Johannesburg tracking station in South Africa to receive the playback of the first picture, and to learn whether our mission had been a success or a failure.]

Ten hours and 59 minutes after the last picture had been taken the slow playback began. [As the signal was received at Johannesburg it was relayed, bit by bit, to the Jet Propulsion Laboratory in Pasadena. At first all numbers were 63’s, because all pictures were black for a few lines along the top edge and down the left side. After an hour or so it was noted that the numbers were no longer 63's and, more important, that they were different from the numbers that had been left on the tape after the final tests at the launch Site. So we knew at last that some information from Mars itself had been received.

Even then, however, we remained puzzled for several minutes, because all the six-bit sequences were very much the same and indicated a tight intensity about a fourth of the maximum possible value. If the camera had been looking at the sky next to the limb, or edge, of the planet, the light intensity (we thought) should have been low or close to zero. If the camera had been looking at the planet, we had hoped to see more variation in the numbers. Finally, how­ever, the signal jumped suddenly nearly to maximum intensity and we felt sure we were recording the sunbathed planet itself.]

As the signals arrived they were recorded on magnetic tape [to provide a permanent record, and they were also typed out simultaneously as a sequence of O's and l’s on a paper tape that resembled adding-machine tape. Many people were clustered around the machines producing these tapes. It was an exciting experience to realize that we were actually receiving knowledge from a man-made machine almost 150 million miles away. Of course we were seeing only a sequence of bare numbers. What would the picture look like? Eight hours seemed an eternity to wait.

Then someone conceived the idea of cutting the tape from one of the printers into short lengths, each containing a series of 200 numbers representing the light intensity of one line of the picture. These sections of tape could be stapled together, one next to the other, to build up a two-dimensional picture of the numbers. To make the picture “readable”, each element was filled in with one of five different colors of depending on the light level indicated by its numerical code. Each color of crayon was applied by a different person. In this way the first close-up picture of Mars emerged line by line in the form of a hand-colored mosaic.

Date: 2015-04-20; view: 1018