Today we take for granted that we can instantly access images of almost any part of the Earth taken from space using an ever growing collection of satellites. These images, in addition to supplying information for numerous practical applications ranging from weather to intelligence gathering to the assessment of Earth resources, also provide the general public with inspiring views our home planet from a unique perspective. But such images did not exist before the advent of space travel. The first photographs of the Earth from space were taken by high altitude rockets starting in 1946 (see “First Pictures: The View of Earth from Space – October 24, 1946”). But these early efforts, with the camera payloads following suborbital ballistic paths, could only acquire a handful of photographs before plummeting back to Earth. The ideal vantage point for observing our home planet from space for extended periods of time is from an Earth orbiting satellite.
A view of the Explorer 6 satellite with its solar panels extended. (NASA)
The first successful attempt to image the Earth from orbit came on August 14, 1959 – almost two years into the Space Age. The satellite responsible for this feat was NASA’s Explorer 6 built by STL (Space Technology Laboratory – a division of what would become TRW). Designated S-2 by NASA before its successful launch using a Thor-Able rocket on August 7, Explorer 6 was a spheroid 66 centimeters in diameter and 74 centimeters tall that relied on its spin rate of 168 rpm to maintain stability. The satellite was fitted with a set of four deployable solar panels 46 centimeters on a side, which earned it the nickname as a “paddlewheel probe”, that provided power for the spacecraft systems and experiments for its nominal 60-day mission in an extended Earth orbit.
This diagram shows the arrangement of internal systems inside Explorer 6. Click on image to enlarge. (NASA)
The 64.4-kilogram Explorer 6 carried an impressive suite of instruments to study space radiation, magnetic fields, micrometeorites, and Earth’s ionosphere from orbit. Also included on the satellite was a 1.1-kilogram scanning imager built by STL. Based on an earlier version which flew on NASA’s unsuccessful Pioneer 2 mission (see “The First Race to the Moon: Getting Off the Ground”), this primitive imager (which was charitably referred to as a “television camera”) consisted of a 5-centimeter mirror that focused an image onto a photocell whose measured brightness was converted into one of eight possible values and transmitted in real time back to Earth. With the device canted at a 45° angle to the satellite’s spin axis, the imager relied on the spin of the satellite to scan a line across the Earth and the satellite’s forward motion to build up an image one scan line at a time. With each rotation of the satellite, a single brightness value along the scan line was measured and transmitted. This process was repeated 64 times for each line of the image before the scanning of the next 64-pixel line started. In this way an image with a scale of about 8 kilometers per line could be created.
Here is the launch of Explorer 6 on August 7, 1959 from LC-17A at Cape Canaveral. (NASA)
With its successful launch on August 7, 1959, Explorer 6 had been placed into a 237 by 41,900 kilometer orbit with an inclination of 47.0° and an orbital period of 12.6 hours. The apogee of Explorer 6 was initially placed at 21:00 local solar time with the spin axis pointing at right ascension 14 hours, 28 minutes and declination 23° in the constellation of Boötes (The Herdsman). The one major malfunction upon reaching orbit was the failure of one of the satellite’s four solar panels to deploy properly cutting the total power output of the arrays to just 63% of the expected value. Slow changes of the satellite’s orientation with respect to the Sun as well as degradation of the solar panels’ performance resulting from exposure to the space environment caused the power level to slowly decrease over time.
This diagram details the orbit of Explorer 6 on August 16, 1959. Click on image to enlarge. (NASA)
With the requirements of the altitude and position of the Earth with respect to the satellite being met while in range of a tracking station, the first imaging attempt by Explorer 6 started at 01:05 GMT on August 14, 1959 with the spacecraft crossing over Mexico at an altitude of 27,000 kilometers. The imager’s data were transmitted in real time to the 18-meter tracking antenna in South Point, Hawaii where it was 3:05 PM local time on August 13. The 40-minute communication session ended at 01:45 GMT with the equivalent of 110 scan lines returned. The magnetic tapes with the recorded raw data were flown to Los Angeles where scientists at STL began the long process of decoding the data and creating an image.
During the course of studying the data, a number of problems were encountered. The images were built up by hand using individual strips of paper for each scan line with their exact placement judged by eye and a number of assumptions about the actual pointing. It was also found that the camera’s logic circuits had malfunctioned resulting in only every fourth measurement along each scan line being usable. Also affecting the image quality was a wobble in satellite’s rotation resulting from the single solar panel which had failed to deploy. Data dropouts further complicated creating a coherent image. After six weeks of effort, NASA was ready to unveil the first of its kind image during a press conference on September 28, 1959.
On the right is the publicly released image of the Earth taken by Explorer 6 on August 14, 1959. The globe on the left provides context for the image. Click on image to enlarge. (NASA)
While an important achievement for the US in these early days of the Space Age, the quality of the image shared by NASA was, at best, disappointing and, at worst, prompted claims by some in the press that it was faked (which, of course, it was not). Clearly discernable was part of the crescent Earth, despite a data dropout which caused a black bar in the upper right corner. The sunlit portion of the Earth at this time was the central Pacific viewed at a slant range of about 32,000 kilometers with North America in darkness. Comparisons between this image and rough cloud maps (created by hand using scattered ground and air observations) seemed to show cloud banks in the upper and lower parts of the crescent. A bright area near the equator was interpreted as being a possible specular reflection of the Sun from the ocean or clouds in the area. Despite the crude nature of the image, especially compared to photographs taken by ballistic missiles of the era (see “First Pictures: Atlas 71D Color Photos of Earth from Space – October 13, 1960”), it did show the feasibility of imaging the Earth from orbit using electronic means.
In the meantime, several additional attempts were made by the Explorer 6 team to image the Earth through August 25. Due to the poor quality of the images, none of these data were publicly released. Explorer 6 was last heard from on October 6 when the satellite’s power levels fell below the minimum required to operate. The pioneering spacecraft, which transmitted a total of 827 hours of analog and 23 hours of digital data, finally reentered Earth’s atmosphere on July 1, 1961.
Related Video
Here is a brief period newsreel about Explorer 6.
Related Reading
“First Pictures: The View of Earth from Space – October 24, 1946”, Drew Ex Machina, October 24, 2021 [Post]
“First Pictures: Atlas 71D Color Photos of Earth from Space – October 13, 1960”, Drew Ex Machina, October 13, 2020 [Post]
General References
Craig Lewis, “Explorer VI Reveals New Radiation Data”, Aviation Week, Vol. 71, No. 14, pp. 29-31, October 5, 1959
Gideon Marcus, “Earthbound Pioneer (Explorer 6)”, Quest, Vol. 19, No.1, pp. 38-49, 2012
“Paddlewheel Satellite Probes Radiation”, Aviation Week, Vol. 71, No. 7, pp. 30-32, August 17, 1959
“Satellite Relays Earth Image”, NASA Press Release No. 59-226, September 28, 1959