Al Stahler: On Mars, tomorrow
Life appeared on Earth four billion years ago. What was happening on Mars back then? When we send robot geologists to study the rocks of Mars, they also look for evidence of ancient biology.
Adding interest to the search is the possibility that life evolved first on Mars, and was then transplanted to Earth … which would make all of us the descendants of Martians.
Planets play catch. Planets play catch, not with balls, but with rocks. When a humongous meteor smashes into Mars, the smash-up gouges out a crater, and busts up rocks. Most of the broken rock falls back to the ground, but a few – a very few – are blown out into space. Of those very few, a very, VERY few fall to Earth … giving us a couple-dozen free samples of planet Mars, without having to leave home.
The game of catch goes both ways – every now and then, we Earthlings fling a spacecraft out toward Mars. This Thursday, it will be Mars’s turn to catch.
To throw something across the solar system, you’ve got to throw hard. Last July, a six-wheeled robot geologist – a rover – sat bolted into the nose of an Atlas V rocket. The countdown reached zero, the engines ignited, and pushed their cargo upward … a thousand miles an hour … five thousand … ten thousand … fifteen thousand miles an hour.
The rover, and the equipment needed for its journey, weighed way over a ton. Fifteen thousand miles an hour was all the Atlas V could muster. But fifteen thousand miles an hour is not fast enough to remain in space.
The rover quickly detached itself from the Atlas V … and ignited another, much smaller rocket. As the Atlas V fell back to Earth, the smaller rocket boosted the rover to sixteen, then seventeen thousand miles an hour … fast enough to remain in space … though only in orbit.
The rover is now a satellite of Earth … but not for long. It’s orbit is not permanent, but just a “parking orbit.” Half-way around the world, the rover re-ignites its rocket engine.
From seventeen thousand miles an hour, the rover reaches twenty thousand … twenty-two thousand … twenty four thousand … close to twenty five thousand miles an hour.
Twenty-five thousand miles an hour is too fast for Earth’s gravity to hang on to anything. Twenty-five thousand is escape velocity from Earth.
Having reached escape velocity, the rover – named Perseverance – heads out for planet Mars.
Perseverance will reach Mars this Thursday, a bit after noon, California time.
For the spacecraft to escape from Earth … for the spacecraft to cover hundreds of millions of miles, to reach Mars … it had to be truckin’. But now, to land softly on Mars … to get down in one piece … the spacecraft must shed all of that momentum, and slow itself down … to zero.
Water is soft and squishy … until we do a belly-flop. Unable to squish out of the way, water then feels almost solid.
If we come in fast enough, air, too, can also feel almost solid.
The rover – encased in a protective shell – orients itself to belly-flop into the Martian atmosphere. Unable to squish out of the way, the Martian air is scrunched – squeezed so hard, it glows, red-hot. Plowing through the Martian sky, the spacecraft becomes a meteor … a shooting star.
The belly-flop maneuver slows the spacecraft greatly … but not enough to land. It’s still plummeting downward at roughly a thousand miles an hour.
A mortar is a cannon, invented in the early Renaissance for attacking castles – for lobbing cannonballs, not long distances, but in high arcs, to get them over castle walls.
A mortar is bolted to the upper part of Percy’s aeroshell … not to attack Martian castles, but to lob a parachute, more-or-less straight up. The parachute is made, not of the traditional silk, but of Kevlar … The fabric that goes into bulletproof vests. The chute’s been tested under a load of thirty-plus tons – it’s got to slow the spacecraft – down from a thousand miles an hour – fast.
If Mars had a thick atmosphere, like Earth, the parachute could deliver the rover to the ground. But Martian air is thin.
Having begun its journey with rockets, so the rover ends its journey. Roughly a mile above the surface … falling 200 miles an hour … the aeroshell releases the parachute, signaling the “sky crane” to ignite its rockets. The sky crane slows the rover’s fall, down to a hair over one mile an hour. Hovering over the Martian landscape, the sky crane gently lowers Percy, by cable, to the ground.
Compared to probes to other planets, success on Mars has proved challenging … thus the motto, “Mars is hard.” Having spoken with many people involved with this and past Mars missions, I’m sure that more than one will be wearing his or her lucky hat … lucky shirt … lucky underwear.
Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at email@example.com.
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