“Twenty years from now you will be more disappointed by the things that you didn’t do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover.” -Mark Twain
You’ve all heard of Kepler before, including the exoplanet-finding spacecraft named after the famed 17th Century astronomer. Johannes Kepler is probably most famous for his laws governing the motion of the planets, which cemented the heliocentric model — and not the geocentric model — as the best description for the motions of objects in our Solar System.
But Kepler — born in 1571 — had an even earlier fascination than the planets. When Kepler was only 6 years old, the Earth received a remarkable visitor.
The Great Comet of 1577 lit up the night sky, instantly fascinating watchers of the heavens. And Tycho Brahe — Kepler’s future employer and mentor — scientifically and meticulously recorded his observations of it.
And as you yourself will notice — despite Tycho placing the Earth at the center — Tycho paid careful attention to the tail of this comet! Now Kepler not only had access to it, but comets are fairly plentiful, and Kepler got to observe many of his own. You can easily find some in your skies, too, if you’re willing to wait.
Here’s a picture of Comet Hyakutake courtesy of NASA, and you can immediately notice its spectacular tail! Part of the tail looks white, while farther back, it looks blue. Do all comets look this way?
Well, Halley’s Comet — shown above from 1986 — has a white part and a blue part, but they don’t line up as well, do they? Let’s look at an extreme example:
Comet West, imaged by John Laborde, clearly has a big separation between the white part and the blue part. What’s going on here? There are two separate parts to every comet’s tail: a gas part, which points directly away from the Sun, and a dust part, which curves as the comet moves through its orbit.
But, perhaps more significantly, you’ll notice from the diagram above, as well as from Tycho’s notebook drawings, the tail of the comet always points away from the Sun!
Kepler, realizing this, theorized that the Sun must be exerting some type of outward solar pressure, or some type of ‘breeze’.
In fact, Kepler suggested that we could, someday, use this for travel through space, stating
…ships and sails proper for heavenly air should be fashioned.
Well, it officially happened! NanoSail-D deployed, in orbit around Earth, on January 20th, and is successfully using light from the Sun to power it, controlling its flight by sailing, just as Kepler imagined!
And for the next three months, it will sail around the Earth, being periodically visible through its — what else — reflected sunlight! There’s currently a contest going on to photograph it, for those of you interested, before it re-enters Earth’s atmosphere and burns up. But how amazing is this; 400 years later, and Kepler’s original vision is coming true!