Decades before satellite TV or GPS, NASA used a huge silver balloon to prove global communications via satellite possible. This was Echo, the U.S.’s first communication satellite, which paved the way for many current communication technologies.
Technologically, Echo was quite simple. One hundred feet in diameter and made of Mylar and aluminum, the balloon functioned as an enormous mirror in orbit. But it was not originally designed to be a communications satellite.
Aeronautical engineer William J. O’Sullivan first conceived of the satellite as an air density experiment. He represented the National Advisory Committee for Aeronautics (NACA), the precursor to NASA, at the Upper Atmosphere Rocket Research Panel. This panel developed experiments to advance knowledge in the burgeoning field of space science. At a meeting in January 1956, O’Sullivan found the methods the panel discussed to measure air density in the upper atmosphere insufficient. He set about designing a better craft for the job. By December 1956, O’Sullivan was head of a new group tasked with developing it.
O’Sullivan needed to consider many design aspects for the new satellite. When a craft moves through the air, it experiences something called “drag.” The craft pushing on the air causes the air to push back in the opposite direction. Think of this like trying to walk through waist-deep water. The water resists your movement, making it harder to move forward. The amount of drag an aircraft experiences is proportional to the density of the atmosphere it travels through. Therefore, researchers can determine atmospheric density by measuring how drag alters the satellite’s orbit. This was O’Sullivan’s first approach.
Echo’s initial concept showed promise, but there were many more problems to solve before the design was complete. The force exerted on a craft while in the thin atmosphere of low-Earth orbit is relatively small. The satellite’s construction material needed to be sensitive enough that even a small amount of drag would alter its orbit. However, during its launch, the force exerted on the satellite would be quite large. A sensitive material would be too fragile to survive, but a strong material would be too rigid to be affected enough by drag.
However, if Echo was a spherical balloon, it could be packed into a strong container for launch, which would open once in orbit. Without the atmospheric pressures closer to the ground, a small amount of inert gas could inflate the balloon.
O’Sullivan’s group met with a new challenge. Echo needed to withstand extreme temperatures from -80°F in Earth’s shadow, to 300°F in direct sunlight. There were already plastics which could handle the cold temperatures, but not the heat. O’Sullivan’s group determined if a reflective metal was added to the plastic, it would reflect light from the sun, lowering the heat sufficiently.
One challenge made it clear that Echo could have applications beyond measuring air density. A typical satellite tracking system of the time would add mass to the balloon, reducing its sensitivity to drag. The team determined that the only way to track it at all times would be through radar. If you could bounce radar off the satellite for tracking, you could also bounce TV, radio, or telephone signals off it. This would make global communications much faster than conventional methods.
Soon, an opportunity to test this concept presented itself. In October 1957, Russia launched Sputnik 1, the first human-built satellite to orbit Earth. People around the world gained a new interest in space. Amateur astronomers and professionals alike looked to the skies for a glimpse of this new technological achievement.
Seeing the attention Russia was getting, the U.S. wanted a satellite just as visible as Sputnik to show off our own achievements to the world. NACA’s director of research convinced Congress that, rather than develop a completely new craft, Echo could fulfil this need. What’s more, the light reflected by its mirrored surface made the craft visible to the naked eye. Project Echo was no longer an air density experiment, but instead a communications relay.
A large radio antenna was built in Holmdel, New Jersey, specifically to support Project Echo. After Echo’s successful launch in August 1960, scientists in Holmdel communicated by phone with the Jet Propulsion Laboratory in Pasadena, California, using the satellite rather than landlines. This proved that global communication via satellite was possible.
Project Echo continued for years, and gave rise to numerous new technologies and techniques. That first balloon spawned other “satelloons,” as they were called (a combination of “satellite” and “balloon”). Echo 2 fulfilled Echo’s original purpose to measure air density, as well as solar radiation. Another satelloon demonstrated a triangulation technique, which better calculated the distance between two points on Earth, making maps more precise. In addition, Echo’s metallized plastic helped researchers evaluate the long-term durability of similar materials for other spacecraft.
By the mid-1960s, “active” communication satellites were determined to be a more effective solution than a “passive” system like Echo. Using a passive system, the transmitted signal needs to be strong enough to make the trip all the way into orbit and back. With an active system, the transmitted signal only needs to be strong enough to get to the satellite. The satellite provides power for the return trip.
Thanks to Echo, we now have advanced communications satellites such as the Tracking and Data Relay Satellites (TDRS), operated by NASA’s Space Network. The ten TDRS in operation receive and transmit signals through radio frequency antennas, recharge their batteries with large solar arrays, and have thrusters for maneuvering. These spacecraft allow communications coverage from anywhere in Earth’s orbit. They provide hundreds of hours of service every day for a wide range of scientific and commercial missions. This space communications network supports critical missions like the Hubble Space Telescope and the International Space Station.
By Matthew D. Peters
NASA’s Goddard Space Flight Center, Greenbelt, Md.