NASA's 12-foot-wide satellite lost its radio signal in space; scientists still got the data they needed

Explorer 9 was nothing like the metal spacecraft people usually imagine from the early space race, according to the Smithsonian National Air and Space Museum. It was a light sphere, about 12 feet wide, built from thin plastic film and aluminium.

NASA sent it up on February 16, 1961. The idea was to watch how the sphere travelled along its orbit and use that to gauge the density of the upper atmosphere. Then the radio beacon stopped working.

The experiment survived anyway. According to NASA’s historical records, observers on the ground tracked the sphere optically, and those sightings still produced useful data.


That mattered because of where the sphere was flying. The upper atmosphere is too thin for ordinary weather balloons to study in the same way, yet dense enough to act on any satellite passing through it.

Why send a balloon into orbit?

Explorer 9 belonged to an early push to measure the upper atmospheric density. Air up there is extremely sparse, but it is not absent. The few molecules that remain push back on anything moving through them.

Over time, though, it changes a satellite’s motion, and by studying that change researchers can estimate how much air is present at different heights.

NASA’s records describe Explorer 9 as a 3.66-meter inflatable sphere meant to determine atmospheric density through air drag. A large surface and very little mass made it sensitive to the faint forces scientists were after. It was also reflective enough to be seen from the ground, which counted for a lot. The method needed its position fixed again and again so that changes in its orbit could be worked out.


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The idea took years to reach orbit

According to NASA’s historical data, the idea came from William J. O’Sullivan at NASA’s Langley Research Center. NASA’s data traces its work back to the 1950s. When researchers were actually looking for practical ways through which they can measure air at higher altitudes.

The inflatable sphere offered various combinations, such as a large surface, low mass density and a shape that could be tracked easily through eyes. Nothing complicated had to sit inside it. The way it moved was the actual measurement.

Getting one into orbit took more than a single try. NASA’s records describe earlier launches of inflatable spheres that failed before Explorer 9 finally reached orbit in February 1961. The successful flight, then, came from an idea that had already absorbed several setbacks, not from a design that worked the first time.

When the radio beacon failed

Once in orbit, Explorer 9’s radio beacon did not work as intended. One way of following the satellite was gone. The sphere itself was still up there to be seen.

NASA’s historical data say it was tracked optically, which let scientists obtain useful measurements despite the failure.

Explorer 9 was never meant to photograph the atmosphere or sample its air directly. Scientists inferred density from how the satellite’s motion changed, and the optical observations supplied the positions for those calculations. The sphere’s shape, mass, and movement were the method, so a dead beacon did not end the experiment.


What the changing orbit revealed


A record published through NASA’s Technical Reports Server documents air-density measurements from Explorer IX. Later NASA technical work describes how Explorer 9 and related satellites helped researchers rework their picture of the upper atmosphere.

Data published through 1973 NASA’s Technical Reports Server states air density measurement from Explorer 9. Later it describes how an observation helped researchers understand the upper atmosphere.

The program included Explorer 9, Explorer 24, and Explorer 39. Their measurements helped scientists to examine various variations in air density and its composition at different heights.

These observations helped researchers and scientists much beyond the original experiments done. The upper atmosphere changes with factors such as solar activity, and thus that can affect the motion and life of satellites.


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A small satellite with a practical purpose


Explorer 9 carried no astronauts and relayed no calls. Its contribution was quieter. It helped measure the environment that other spacecraft would fly through.

For engineers, air density matters when predicting how an orbit will change. Even slight drag adds up over time, shifting a satellite’s path and affecting how long it stays up.

The failed beacon adds its own lesson. Researchers still had ground observations from which to recover useful information. The workaround did not erase the equipment failure, but it kept the mission’s remaining evidence valuable.

Explorer 9 was a simple sphere compared with the complex spacecraft that followed it. Yet its measurements contributed to a better understanding of the thin atmosphere surrounding Earth, a region where the air is sparse, but its effects on objects in orbit are real.