Tag the Sky

How about a constellation of small satellites that serve as pixels in an orbiting display?  We could use that for skywriting in orbit.

It turns out that you can see an LED shining from space, at least with binoculars. FITSAT-1, designed by students at Japan’s Fukuoka Institute of Technology, proved it. That spacecraft was an elegantly designed project that shows how clever use of commercially developed technology can inform completely new ideas in spacecraft architecture. In fact, the FITSAT mission opens up the possibility of low-cost optical communications for the masses. At one end—the technological leading edge—NASA’s recent success with the Lunar Laser-Communications Demo sets a high bar for deep-space communications with light. At the other, FITSAT shows what you can do with some elbow grease and some smart students. And I have a feeling they’ll do even better in the years to come.

In fact, FITSAT wasn’t the first to write on the cosmos. Two months before FITSAT launched in 2012, NASA’s Mars Science Lab rovers imprinted the red planet with Morse code that spells out J-P-L. And a decade before MSL, many of us sent our names to NASA for inclusion on the Cassini mission, which carried them to Saturn on a DVD. Maybe it was Carl Sagan and Ann Druyan who started it, sending that gold record into space on Voyager, with the thought that some distant civilization might find it and learn something about our race, by that time likely long gone. But however we might trace this history of writing in space, FITSAT brought home to us the idea that personalizing space and communicating through light is within reach of all of us.

Sure, it’s true that these bright LEDs are a form of light pollution, and astronomers probably aren’t keen on that. However, I would point out that the diodes can be chosen to emit light in very narrow bands, and even in regularly spaced pulses like FITSAT did. That sort of specificity would provide astronomers with a digital means to filter out the LED signals, if there’s a risk that they’ll compromise science. But to the naked eye, these flashing lights can be much more than noise. They can be art.

With dimensions of 10x10x10 cm, and a mass of only 1 kg, many CubeSats can be launched at once. FITSAT itself was part of a group of three 1U CubeSats launched from the International Space Station in one go. This feature confers a very powerful advantage: a constellation of CubeSats can be launched and can begin performing formation maneuvers almost instantly, without needing to catch up with one another the way they would if members of the constellation were launched separately. Some initial activities—maybe a day’s worth or two, at most—and the spacecraft would be ready for operations.

Let’s create a constellation that is basically a matrix of dots, and we’ll take as our guide the venerable Epson MX-80 dot-matrix printer. Its print head consisted of a 9×9 pixel array. So, we’ll need 81 CubeSats for 1980s-caliber printing. How far apart? The Moon subtends an angle of 0.54 deg. in the night sky. And I think you’ll agree that the moon is plenty noticeable. Let’s go for half that width, about 1/4 deg. So, in LEO—specifically, at the ISS altitude of 325 km—the spacecraft would be about 158 m apart.

At this altitude, the constellation flies overhead quickly. The CubeSats orbit the Earth in about 90 minutes, and they’re visible from a given city for not much more than 10 minutes. So, it is tempting to consider what such spacecraft would look like if they were in geostationary orbit (also known as GEO). At that altitude, 35,768 km above the surface, spacecraft in orbit travel as slowly as the Earth rotates. So, this constellation would remain fixed overhead. That’s convenient, but for the LEDs to seem as bright as they do in LEO, they would have to put out over 12,000 times the power. I suppose one would simply use 12,000 as many LEDs, and the spacecraft bus to power them would resemble a high-end geostationary communications satellite. Remember, though, that we are contemplating 81 of these things. At well over a hundred million dollars each, not including launch cost, a constellation of GEO spacecraft would be dauntingly expensive. It would cost about as much as the James Webb Space Telescope, with hardly that level of scientific impact. So, I think LEO is the way to go.

The tricky thing about formation flight is that you can’t have an arbitrary arrangement of satellites travelling with exactly the same velocity. Three subtleties come into play. (1) The spacecraft need to orbit the Earth in the same time; if they don’t have the same orbital period, the constellation drifts apart. But since orbital period isn’t affected by (2) inclination (tilt) or (3) eccentricity (non-circularity) of the orbit, we can use those parameters to define the shape of the constellation.

The spacecraft in row 5, column 5 of this 9×9 matrix is in the middle. Let’s say that this single spacecraft travels in a circular reference orbit. The others are orbiting at the same period, either ahead or behind the reference orbit, above or below it (in a north-south sense) or inside/outside of it (in an altitude sense). With the right combination of slightly perturbed, non-circular and/or inclined orbits, we can create a constellation that resembles our desired shape. Many spacecraft will be switching places once per orbit, some of the constellation seeming to rotate around a line from the center of the Earth to the reference orbit.

This dance is a straightforward consequence of the Clohessy-Wiltshire equations, which describe the motion of an orbiting body relative to a circular reference like the one we have here. The CW equations describe relative orbital motion, which we perceive as an interaction among spacecraft that happen to have the same orbital period, although in fact there is no gravitational attraction among these satellites that causes their motion.

The figure above suggests a matrix of such CubeSats (larger than they would appear, given their relative spacing). Their attitude need not be particularly well-controlled as long as the LEDs emit a wide beam, and neither is their relative position. As shown, the 9×9 matrix is arbitrarily rotated and is displaying a symbol. Each needs to know its position in orbit so that the constellation, collectively, can produce the image that ground operators have sent it. That is, each one would illuminate its LEDs, or not, depending on its position and the time at which the constellation is supposed to create a certain image.

Flight software would make all this possible. A key ingredient is position knowledge, which would be available from GPS measurements. For example, at Cornell we built CUSat, a pair of satellites capable of detecting their absolute position in orbit to within a few meters. Their relative position would have been known to within a centimeter or so, had the flight computer not overheated and ended the mission prematurely. All that position detection took was a couple of homemade GPS boards, courtesy of the late Paul Kintner‘s lab, and some software conceived by Shan Mohiuddin, one of Professor Mark Psiaki’s students.

We’ll also need some propulsion. Options include electrodynamic tethers, cold-gas propulsion (prohibited by the CubeSat spec, for the most part), and electric propulsion. There are many forms of electric propulsion, but among those I am partial to solutions that involve ionic liquids for their power efficiency and scalability. Or, for a much lower-cost solution, Rodrigo Zeledon has figured out an innovative way to use water for propulsion: the ultimate cheap, green propellant, and compatible with the CubeSat spec.

Having a constellation that can illuminate part of the sky on command offers many interesting possibilities. Here are a few:

• More is better. A much larger constellation—higher resolution—takes us from dot-matrix characters toward a jumbotron or video billboard in the sky.
• The opportunities for art are legion. I’ll suggest only one. What if the constellation could behave in a way that seems to interact with the stars as it passes them? The constellation’s light might shudder, change color to blue, or twinkle as it passes an exoplanet. It might warm to the rising sun by changing color, or it might show a sequence of images that resemble Apollo’s chariot. In the case of higher resolution, maybe our formation responds to passage through an astronomical constellation by acting out some scene from Greek mythology.
• A news crawl in the sky? There might be a business case for this concept.
• Astronomy lessons: as the constellation passes a particular celestial object, seen from a specific region on the ground, it identifies the object and offers information about it. In fact, if we use the FITSAT trick of high-frequency modulation of the LEDs, we might be able to transmit simultaneously in different languages, asking only that the user look through binoculars with blinking apertures (like 3D shutter glasses for some home TVs), unique to his or her language.
• A game in which people on the ground can aim a laser pointer at the constellation—not recommended when aircraft are present—with the goal of interacting with the light: turning on or off LEDs, or changing their color. The constellation would be a blank canvas, and we Earthlings could paint on the night sky.
• Interaction like this raises the possibility of a game—maybe celestial 囲碁 (Go), requiring only two colors of LED, to be played in tag-team fashion by those who see the game board pass overhead.

I take the FITSAT existence proof to be enough to convince folks that a 1U CubeSat could be capable of certain aspects of this mission. Propulsion and additional power may require another 2U worth of spacecraft bus volume and mass. At $100K to launch these 3U CubeSats, it’s over $8M simply for the launch cost. Traditionally, one might double that cost as a very rough estimate that accounts for the space hardware. Include the resources for ground stations, and let’s guess a $20M investment is needed for a commercial enterprise that could tag the night sky with Earthlings’ celestial musings.