[Update: Recent images of Charon, a moon of Pluto, from the New Horizons mission indicate that Charon may be home to the tallest cliff in the solar system. See: A ‘Super Grand Canyon’ on Pluto’s Moon Charon]
In the woods near my childhood home there was a cliff. I suppose it is more accurate to say there is a cliff — cliffs don’t move much on a scale of decades — but ‘was’ seems more appropriate because the actual size of the cliff does not represent the size that existed in my young mind. The fear of standing near the edge prevented any reasonable calculation of height.
But I could count. I could throw a rock and listen for how long it took for the ‘crack‘ sound to echo up from the canyon below. Being able to count out whole numbers before hearing the rock strike was almost as frightening as looking over the edge. Almost.
Despite what I may have imagined as a child, that cliff in the woods near home is not, in fact, the tallest cliff in the world. The greatest purely vertical drop is the 1.25 kilometer cliff on the side of Mount Thor in Canada. The greatest nearly vertical drop is a 1.34 km fall from the Trago Towers, a group of rock towers in the self-governing Pakistani territory of Gilgit-Baltistan. A fall from either would take approximately twenty seconds.
Not only is that an excruciatingly long time to fall, it is more than enough time to reach the human body’s terminal velocity in Earth atmosphere of 200 kph. [This depends, of course, on your preferred method of falling. A speed of 200 kph assumes a horizontal alignment. If you go nose-first, you can probably get it up to about 320 kph, assuming you have a particularly compelling reason to hit the Earth face-first at bullet-train speed.]
Neither Thor nor the Trago Towers come close to being the tallest cliff in the Solar System. The tallest is the Verona Rupes on Miranda, a moon of Uranus. If you were to look down from the sudden, shear edge of the rupes (Latin for “cliff”), you’d see a vertigo-inducing ten-kilometer drop.
Just why there is this giant cliff on Miranda is still studied and debated. It shouldn’t be there, given that Miranda is one of the smallest objects in the Solar System to be spherical under its own gravity. Yet Miranda is covered in mountains and cliffs. Perhaps it’s the result of crust rifting or cryovolcanic eruptions of icy magma. Or perhaps there was a single, massive collision with another moon, tearing Miranda asunder before reassembling into its current shape.
Whatever process created the massive mountain, a jump from the cliff’s edge would take a long, long time to complete. Thanks both to the distance and Miranda’s very, very low gravity, the fall would take a full eight minutes. Partially this is because of how slow the fall would be at the beginning.
Gravitational acceleration on Miranda is 0.079 meters per second squared. If you dove out from the cliff, it would appear momentarily as if you were hovering, floating still with the great cyan orb of Uranus above. The Sun would look like the star that it is – a bright star, certainly, but just a star. If you looked hard enough you could probably pick out Uranus’s other four large moons: Ariel, Umbriel, Titania, and Oberon.
Then the fall would start.
Slow at first. It would take about twelve or thirteen seconds just to get to the lazy pace of one meter per second. But with no air resistance to speak of, you’d just keep getting faster and faster, and ten kilometers is a long way to go. Faster and faster. By the time you’re near the ground, you’d be traveling at over 144 km per hour (90 mph).
Survival is possible. After all, 144 kph is fast, but not as fast as you’d be traveling during the much shorter fall from Mount Thor. All that’s needed is something that could cushion a 90 mph impact. A parachute wouldn’t help, as there’s no atmosphere to catch with it. A large, quick-inflating airbag might suffice. Or some kind of retrorocket boots, like an Iron Man-type thing.
Terminal Velocity – NASA
Voyager, Uranus Images – NASA
Photograph No. 1: Miranda from Voyager 2 spacecraft, Jan. 24, 1986; Photograph No. 2: Verona Rupes from above, Voyager 2 spacecraft, Jan. 24, 1986