Corona Light


Spacecraft extend human consciousness into space. We see galaxies through their optics, touch other planets with their robotic arms, and dance virtually through the solar system as they propel themselves ever farther from Earth. Communications technology connects us to these spacecraft. Images of Earth, Mars, what have you, reach us by radio, usually encrypted. Without this flow of data between Earth and our distant robotic avatars, the spacecraft might as well not be there. At least, that’s the principle that governs the design of most contemporary spacecraft.

There is another approach. In the early 1960s, three decades before President Clinton declassified the existence of the National Reconnaissance Office, the United States operated the Corona series of earth-imaging satellites. The spacecraft would fly overhead, take pictures on Eastman Kodak film, and release a canister of this film–called a “bucket”–to reenter the Earth’s atmosphere. As it fell, an aircraft captured it. The film was developed and images printed in basically the same way that it had been done since the 19th century. Not only was this Corona architecture an elegant adaptation of the photographic technology of the time, it was also quite secure. No one could intercept Corona’s images simply by listening in.

Let’s consider a spacecraft that communicates its discoveries Corona style. And not just any spacecraft. Let’s look at a spacecraft that does science at an exoplanet. Cornell astronomy professor Jamie Lloyd and I have been thinking about this idea lately.

Our version of the Corona architecture begins with a spacecraft-on-a-chip: like a Sprite but a single application-specific integrated circuit (ASIC). Really. A single silicon or gallium arsenide chip that weighs only milligrams. Most of a Sprite’s electronic guts reside in a single IC already: the wonderful CC430 chip by Texas Instruments. So, it’s not asking too much for the rest of the circuitry to be collapsed into a single semiconductor device. If an ASIC proves to be too hard, maybe we would use a Field Programmable Gate Array (FPGA), which is a sort of all-purpose reconfigurable IC that can be naturally radiation tolerant. Tiny solar cells, nanowire antennas, and a CMOS camera round out the key subsystems.

Traditional radio communication from such a tiny device is hopeless on the scale of solar system distances, let alone interstellar distances. There just isn’t enough power available through solar cells or within an on-board energy-storage technology to transmit a signal powerful enough to be received back on Earth. For example, the current Sprite can manage roughly 1 bit per second from low Earth orbit for very low power, thanks to a subtle trick known as matched filtering. Even if continuous communications at this rate were possible from much farther away, downlinking every bit of this SD card’s data would take 31 millennia. Hence the Corona architecture. Let the golden light radiating from its angelic countenance illuminate our imaginations.We’ll save the data on a memory device and retrieve it physically.

Thanks to 21st century technology, our Corona Light can carry a few milligrams of silicon to house terabytes of information. Back in 2010 SanDisk offered a 2 terabyte SD card, a version of which serves as Corona Light’s bucket. We’ll count on this SD card, or something like it, to retain the spacecraft’s discoveries.

We would speed Corona Light on its way out of the solar system. How? Well, that’s another post. No more than one miracle per week, I think. But to fix ideas, let’s say we’ll accelerate it out of an orbiting railgun. The U.S. Navy apparently can launch 3.2 kg to 2.6 km/sec from its experimental railgun. They continue to make progress. So, let’s say they’ll exceed 700 megaJoules some day. Although the dynamics of launching something tiny differ, that energy would propel a 30mg spacecraft-on-a-chip to 7,000 km/sec, i.e. 2.3% of the speed of light. The spacecraft will need that sort of speed if it is to make it to Alpha Centauri, about 4.3 light years away, on a human time scale. The simplest version of the celestial mechanics involved says that the spacecraft reaches the exoplanet NASA has found there in 184 years. Maybe we can do better, but (again) that’s for a future post.

The trick is to get this spacecraft-on-a-chip back to Earth, or at least within the solar system, where we can pop the cap on this tiny satellite and drink in its knowledge. It will need a beacon, and I’ll propose that we simply replicate what we know we can do on Sprite today: a specific pseudorandom noise sequence that we would be looking for (accounting for all that Doppler shift, of course) when the spacecraft completes its ring around our spiral arm of the Milky Way and returns home.

Sometime in year 184, Corona Light performs a fly-by of Alpha Centauri. As it approaches this star, the power available to the spacecraft through its tiny solar cells increases, enabling some very minimal science: capturing a few photons from the environment and recording a time stamp in the form of counts of radioactive decay of a small on-board sample of an isotope. It would be carbon-dating itself, in some sense. A realtime on-board clock would be asking a lot—power that requires mass, slowing it down. But if possible, that would be great; these days an atomic clock on the scale of a microchip is mainstream technology. But the isotope-decay trick trades precision for even lower mass, which I think we want.

This fly-by puts the spacecraft on a trajectory that uses the pull from the galactic center to redirect the spacecraft back into our solar system. Make no mistake: this maneuver is not easy. Missing the Earth is likely. Fortunately, Corona Light is cheap, a few pesos each. We’ll fabricate thousands, maybe millions of them (they would fit in a suitcase) and fire them toward Alpha Centauri as a halo of tiny exploration vehicles. If one makes it back, we will have visited the nearest star. The SD card with a couple of centuries of data will represent our most distant physical interaction with the cosmos. Let’s toast to the original Corona mission and raise our bottles of Mexican beer to the possibility of interstellar discovery.


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