The Great Big Disco Balls in the Sky

The simplest satellites were some of the coolest!

Inthis blog post, I want to talk about some of the most awesome satellites out there (in my opinion). They’re actually the simplest artificial satellites that we’ve put out there. They’re passive satellites, they contain no electronics, no propulsion systems, no power supply, no sensors, no communication equipments… They’re as the title says, great big disco balls in the sky.

I want to talk about two sorts of satellites here. Some of them being big great balls and some other being disco balls. The former being passive communications and geodetic satellites and the latter being laser ranging satellites. Both of them however have provided great value to humanity despite being metal balls floating mostly undisturbed. They enabled us to measure the shape of the Earth with unprecedented accuracy. Plus, those that were inflatable gave us a pretty cool word, “satelloon”!

Project Echo

Echo 2, inflated to undergo tensile stress test. 1964 (Source)

Project Echo is just so cool to me, NASA wanted to put a ball in space to bounce radio waves off of it in order to communicate between distant points on Earth. You could already do a similar thing by bouncing radio waves off of the ionosphere, but it was not so reliable, because the ionization of that layer of the atmosphere depends on space weather. So what NASA came with is a dedicated metallic ball in the sky. As long as both points that intend to communicate have a line of sight to the satellite, then it should work out. Given that the satellite was in a polar orbit, that condition should be met a good portion of the time.

The satellite wasn’t launched in the shape that you see in the picture. It was actually a mylar sheet all neatly folded up in a rocket and inflated once in space to reach a diameter of 30–40 meters! there were two Echo satellites, Echo I launched in 1960 and Echo II in 1964.

A worker inspects Echo II for leaks during a test inflation. How cool is this pic really!? I could only find it in one place on the internet and they thankfully provided their source so that it doesn’t get lost forever. I extracted the highest res version possible. (Source)

Echo I was successful at evaluating space-based communication. It was successfully used to transmit the first satellite two-way telephone conversation in 1960 and a television program in 1964. How mind-blowing is this? Imagine the people communicating by bouncing signals off of a 30-meter ball in the sky in 1960 no less! You can learn more about it in this video from 1960.

Now because the communications experiment was a success, it allowed scientists to use the satellites as a triangulation device. To me, that’s where the real interesting bit begins. Scientists were able to precisely locate the position of two ground stations by bouncing a signal off of the satellite between them. A few years before, earlier satellites such as Sputnik were used for that purpose, but those were limited to determining the rough shape of the Earth and measuring that the Earth is not perfectly round.


PAGEOS was built to succeed Project Echo. It was specifically designed for geodesy, to compute the shape of the Earth and provide reference points everywhere on Earth that would be part of the same reference system. Previously, there were separate systems of measurements and reference points which were combined together in [WSG 60]

PAGEOS was part of the Worldwide Satellite Triangulation Network, consisting in 46 ground stations that were spaced 3,000–5,000 km apart and covered the whole Earth. This satellite enabled scientists to measure the position of these stations to within 3–5 meters, an incredibly leap of 20x in precision versus previous methodologies. Imagine that! Before the 60’s, to the best of our knowledge, we could only ever locate any given point on Earth to a precision of 60–100 meters!! If you’re interested in how we used to measure the Earth before satellites came about, you can read about such expeditions as the French Geodesic Mission to the Equator, an expedition charged with measuring a whole degree of latitude on the ground, by hand. For the sole purpose of making a new definition for the meter.


The giant soccer ball we sent to Aliens as an offering.

An honorable mention for OV1–8, which just rolls off the tongue. Earlier passive satellites had a problem in that they were susceptible to the pressure of the solar wind. Being low density, shiny balls, they get pushed along by solar radiation and the solar wind. This disturbs their orbital elements and means that imprecisions are introduced in their location. It also means that eventually, the satellite decays and returns to Earth.

To work around this constraint, the US Air Force created a satellite that had an aluminium mesh embedded in a plastic film. Once inflated in space, the plastic film would disintegrate in the UV rays of the sun and leave behind only the aluminium shell that would be minimally disturbed by the sun. That’s a really neat trick. And it did work for 11 years despite the satellite being put in an orbit that had much more drag in order to study the effect of the upper atmosphere on the satellite’s motion.


This Disco ball was launched into space at the peak of the Disco Era!

Ok, now forget everything we said so far, because it’s time for LASERS! Yep, that’s right LASERS! Everybody likes lasers and you bet NASA does too. That’s why they decided to build The Great Big Disco Ball in the Sky!

I guess by now you understand pretty clearly what the concept is here, but I’ll explain it regardless. This satellite was used for laser ranging. This is the technique that allows you to know the precise distance between a laser source and a reflector. You may have guessed what these mirrors are on the satellite. They are retroreflectors. What these do is very simple, they reflect light from the exact direction it came from. It’s a pretty neat little trick that only requires you to create a corner of mirrors.

So if you know the position of the satellite, you can know your position extremely precisely by measuring it a bunch of times by sending tiny pulses of light to the satellite and measuring when they come back. And they made this satellite purposely super dense such that it would not be disturbed as much by forces present in Earth orbit. This means that the orbit of this satellite is super well defined and almost constant. Its orbit was actually so constant that it was used to measure the Earth’s gravity. Seeing how the orbit gets deflected indicates a gravitational anomaly.

This specific satellite allowed us to measure the Earth‘s shape to an unprecedented precision, less than 2 cm anywhere on Earth. It also allowed us to measure continental drift. That’s insane! Those only move by a few centimeters every year, measured over a distance of several thousand kilometers.


I hope I was able to transmit my enthusiasm for passive satellites. I hope you learned a lot and will keep learning about space, the Earth and satellites. I’ve compiled a few more examples of interesting satellites at the end of this blogpost. Honorable mention to the Lincoln Calibration Sphere 1, which is the oldest satellite still in use after 50 years!

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