Today marks the 50th anniversary of the launch of NASA’s Mariner IV spacecraft (November 28, 1964). In total, the mission gave us 21 complete images of Mars, including this, our first close view of the planet–courtesy of data transmitted by the interplanetary probe and earth-bound scientists wielding pastels (below).
How did the image come to be? As Robert B. Leighton wrote in The Photographs from Mariner IV (Scientific American, April, 1966):
“Experience had shown that the best way to send a weak radio signal through space in the presence of background noise is to use a signaling method known as pulse-code modulation. In this signal-coding method the output of an electrical device, whether it be a thermometer or a television camera, is coded into a sequence of “bits,” or binary digits made up of 0′s and 1′s, that represent a particular level of intensity. Accordingly the output of the Mariner IV television camera was translated into a six bit code that identified the brightness of each picture element on a scale that had 64 steps from full black to full white. The 64 steps of the sequence ran from 0 to 63. A sequence of six 1′s represented full black, or no light at all; a sequence of six 0′s represented full white, or maximum light. To encode the information contained in 40,000 picture elements therefore required 240,000 binary digits.
As the signals arrived they were recorded on magnetic tape to provide a permanent record, and they were also typed out simultaneously…on 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 crayon, 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 closeup picture of Mars emerged line by line in the form of a hand-colored mosaic. Within the day it was framed and presented to William H. Pickering, director of the Jet Propulsion Laboratory.”
How has space-imaging technology changed in the last 50 years? “Instead of the brief glimpse of Mars provided by Mariner, current probes often hang around and orbit (or even land on) their targets. We don’t just take pictures of solar system bodies; we map mineral composition, explore planetary atmospheres in three dimensions and monitor their complex magnetic fields,” says Robert Simmon (a data visualizer partly responsible for the NASA Earth Observatory’s blue marble image, currently with satellite startup Planet Labs). Storage capacity has increased too: ESA’s Rosetta spacecraft can hold 25 gigabits of data between radio transmissions to Earth–nearly 4,771 times the capacity of Mariner IV.
Thanks to Robert Simmon for first bringing this image to my attention in his post on encoding quantitative data with color.
For more details and images from the mission, don’t miss the classic three-part Scientific American series: The Voyage of Mariner IV (March,1966), The Photographs from Mariner IV (April,1966), and The Scientific Experiments of Mariner IV (May, 1966).