The Arch Mission Foundation sent its first archive to space in 2018 in the glove compartment of Elon Musk’s Tesla, which is now in a 30-million-year orbit around the sun. That archive contains Isaac Asimov’s Foundation trilogy, which is inscribed in a quartz disc using an experimental 5D optical technology developed by physicists at the University of Southampton. But that storage medium has limitations. Digital technologies and encoding standards are great for compressing lots of information into a small amount of space, but they are also short-lived—how many people do you know who could play a VHS tape today? If you want to create a library for humans thousands or millions of years in the future, your best bet is to keep it analog.

But analog storage takes up a lot of room. So sending the bulk of human knowledge to space will require a lot of compression. To do this, Spivack tapped Bruce Ha, a scientist who developed a technique for engraving high-resolution, nano-scale images into nickel. Ha uses lasers to etch an image into glass and then deposits nickel, atom by atom, in a layer on top. The images in the resulting nickel film look holographic and can be viewed using a microscope capable of 1000x magnification—a technology that has been available for hundreds of years.

The lunar library on the Beresheet lander consisted of 25 layers of nickel, each only a few microns thick. The first four layers contain roughly 60,000 high-resolution images of book pages, which include language primers, textbooks, and keys to decoding the other 21 layers. Those layers hold nearly all of the English Wikipedia, thousands of classic books, and even the secrets to David Copperfield’s magic tricks.

The lunar library includes thousands of high-resolution images of book pages compressed into just a few square inches. Bruce Ha A copy of the first layer of the lunar library on the Beresheet lunar lander. The actual lunar library has the center image removed. Bruce Ha

Spivack had planned to send DNA samples to the moon in future versions of the lunar library, not on this mission. But a few weeks before Spivack had to deliver the lunar library to the Israelis, however, he decided to include some DNA in the payload anyway. Ha and an engineer on Spivack’s team added a thin layer of epoxy resin between each layer of nickel, a synthetic equivalent of the fossilized tree resin that preserves ancient insects. Into the resin they tucked hair follicles and blood samples from Spivack and 24 others that he says represent a diverse genetic cross-section of human ancestry, in addition to some dehydrated tardigrades and samples from major holy sites, like the Bodhi tree in India. A few thousand extra dehydrated tardigrades were sprinkled onto tape that was attached to the lunar library.

The promising thing about the tardigrades, says Spivack, is that they could hypothetically be revived in the future. Tardigrades are known to enter dormant states in which all metabolic processes stop and the water in their cells is replaced by a protein that effectively turns the cells into glass. Scientists have revived tardigrades that have spent up to 10 years in this dehydrated state, although in some cases they may be able to survive much longer without water. Although the lunar library is designed to last for millions of years, scientists are just beginning to understand how tardigrades manage to survive in so many unforgiving environments. It’s conceivable that as we learn more about tardigrades, we’ll discover ways to rehydrate them after much longer periods of dormancy.

Spivack says that adding the DNA-filled resin to the lunar library at the last minute was a major risk, because any mistakes in how it was incorporated could have ruined the nickel engravings. In retrospect, however, it may have been what saved the library from destruction. The resin layers added a significant amount of strength to the lunar library, which made it less likely to break apart upon impact. Moreover, Spivack says that the heat generated by the impact wasn’t high enough to melt the nickel layers, which were themselves encased in several protective layers to block radiation. “Ironically, our payload may be the only surviving thing from that mission,” Spivack says.