A NASA satellite launched in 1976 carries a Carl Sagan–designed plaque sealed inside its core, mapping Earth's continents 268 million years ago, at launch, and 8.4 million years from now — and that last date is no accident, because it's roughly when the satellite is expected to fall back to Earth and finally be opened.

On 4 May 1976, NASA launched a satellite called LAGEOS-1 from Vandenberg Air Force Base in California. It is one of the simplest objects ever put into orbit. It is a sphere about 60 centimetres across, weighing roughly 400 kilograms, with no electronics, no sensors, no power source, and no moving parts. It is a solid mass of brass and aluminium studded with 426 reflectors.

The name stands for Laser Geodynamic Satellite. Its job is to be a target. Ground stations fire laser pulses at it and time how long the light takes to return. Doing this from many stations over many years measures the slow movement of Earth’s tectonic plates, the wobble in the planet’s rotation, and small shifts in its centre of mass. LAGEOS-1 has no instruments because it does not need any. The instruments are on the ground. The satellite only has to sit there and reflect.

That passivity is also why it will last. A satellite with electronics fails when the electronics fail. A polished metal sphere in a stable medium-Earth orbit, about 5,900 kilometres up, has almost nothing to go wrong. The original estimate was that LAGEOS-1 could remain in orbit for about 8.4 million years before atmospheric drag finally brings it down.

What it carries

Because the satellite will outlast not just its mission but most of recorded geological change, NASA chose to put a message inside it.

NASA turned to Carl Sagan to design a plaque. What Sagan produced, in the same period he was working on the Voyager Golden Record, is a small stainless steel plate, about 10 by 18 centimetres. According to NASA’s description of the plaque, two identical copies were placed inside the satellite. To reach one, a finder would have to break the satellite open.

The plaque is not a text. It is a diagram, designed to be read by someone with no shared language and no shared moment in history. The top of it establishes a vocabulary: the numbers one to ten in binary, and a small drawing of Earth orbiting the Sun, with the binary number one marking the length of one orbit, one year. That sets up the unit the rest of the plaque depends on.

Three maps

The lower part of the plaque carries three maps of Earth’s surface, and they are the heart of the message.

The first map shows the continents about 268 million years in the past, gathered into the single landmass of Pangaea. The middle map shows the continents as they were at launch, in 1976. The third map shows the predicted arrangement of the continents about 8.4 million years in the future. NASA cautions that the maps were not meant as exact reconstructions, but as a dramatic way to show continental drift across deep time.

That third date is not arbitrary. It is, very roughly, the satellite’s estimated re-entry. The plaque’s future map is dated to around when the satellite is expected to come down. The message and the object are timed to arrive together.

The logic of the three maps is a clock. Continents drift at a rate of roughly a few centimetres a year, slow but steady and, across millions of years, large. A finder who recovers LAGEOS-1 can compare the arrangement of continents on the plaque with the arrangement of continents they see around them, and read off, roughly, how much time has passed. The plaque dates itself by the geography of the planet it fell back to. NASA’s account notes that this is the same physical process, continental drift, that the satellite was built to measure. The method on the plaque and the purpose of the mission are the same idea.

What the message assumes, and what it does not

It is worth being clear about what the LAGEOS-1 plaque is and is not.

It is not addressed to aliens, in the way the Voyager and Pioneer plaques are often described. The Voyager records are leaving the solar system. LAGEOS-1 is not going anywhere. It is in orbit around Earth and it will come back to Earth. The plaque is a message to whoever is on the planet when it returns, and the honest description of that audience is that it is unknown. It may be a later human civilisation. It may be something else. It may be no one.

The plaque does not assume a finder will exist. It assumes only that if one does, the most useful thing to hand them is a way to know when the object they are holding was made. That is the same modest logic behind the uranium clock on the Voyager record. The designers did not predict a recipient. They prepared for the possibility of one.

The timescale is the part worth sitting with. LAGEOS-1 is expected to return roughly 8.4 million years from now. That is a span longer than the existence of the human species to date. Whoever, if anyone, eventually opens the satellite and finds the plaque will be separated from 1976 by more time than separates 1976 from the earliest members of our genus. The plaque was designed in full knowledge of that, which is why it carries no words, and why it dates itself by the one clock that will still be running: the slow rearrangement of the continents.