If a time capsule is supposed to stay closed for 250 years, making the contents last is only the first problem.

The Library of Congress has taken a particularly ambitious run at that problem. For America's semiquincentennial, it supplied a tiny metal container filled with synthetic DNA encoded with digital copies of collection material. The larger time capsule was buried in Philadelphia on July 4 and is meant to be opened on July 4, 2276.

The files include Thomas Jefferson's rough draft of the Declaration of Independence, Francis Scott Key's handwritten lyrics for “The Star-Spangled Banner,” the 1791 L'Enfant plan of Washington, D.C., Passamaquoddy audio recorded in 1890, a family tree containing more than 1,500 names, and early congressional records.

The storage medium is impressive. The Library says synthetic DNA can hold about nine terabytes in a cubic millimeter and remain stable for thousands of years with little maintenance or power. The DNA strands in this project fit inside a metal vial about the length of a pencil eraser.

Alongside them is a small chip containing decoding instructions.

That may be the most interesting thing in the container.

The people in 2276 have to know what they are looking at

A paper letter brings much of its reading machinery with it. If the paper survives and the language remains intelligible, the marks are there to inspect. Digital information is more demanding. Preserving the physical carrier does not automatically preserve the route from that carrier back to a document, recording, map, or image.

A future reader of the Library's DNA has to know how the information was encoded and how to reconstruct the sequence as digital data. After that come the more familiar questions about file formats and software. The Library cannot know what computers in 2276 will look like, but it can reduce the amount of lost context those readers will have to reconstruct from scratch.

This is an exaggerated version of a problem that appears much sooner than 250 years. A disk can still contain data after the software needed to open it has disappeared. An old research folder can be full of usable files whose abbreviations and version names no longer make sense. A family archive can keep every photograph and still lose the names of the people in them.

The failure in each case is slightly different, but none is solved by storage alone.

A tiny medium still needs context

The extraordinary density of synthetic DNA matters to a library. Physical storage has costs: space, environmental control, media replacement, copying, power, and maintenance. If molecular storage eventually becomes practical at large scale, its density and durability could change some of those calculations.

The time capsule experiment is interesting precisely because the Library has not mistaken density for completeness. It is sending a way of interpreting the storage along with the storage itself.

There is no guarantee that this solves the handoff. The chip is an artifact too. People in 2276 will need to read its instructions, understand the assumptions behind them, and translate those instructions into whatever technical environment exists then. A 250-year gap leaves plenty of room for conventions to disappear.

Libraries and archives have dealt with smaller versions of that gap for a long time. Finding aids explain collections. Editors explain manuscript conventions. Researchers leave READMEs beside datasets. Names get written on the backs of photographs. Documentation is part of what allows an object to remain useful after the person who created or organized it is gone.

When the capsule is opened, its synthetic DNA may still be physically intact. The little decoder chip is an attempt to make sure the contents are more than intact: that somebody can still get from the thing in the box to Jefferson's draft, the Passamaquoddy recording, the map, and the rest of the collection the Library meant to send forward.

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