As you’ve hopefully all heard by now, the Mars Phoenix lander made a perfect
landing over the weekend, and is already returning images. NASA managed to not
only achieve a perfect landing, but to use Mars reconnaissance orbiter to catch a
picture of the Phoenix descending with parachutes deployed!
Alas, NASA’s Phoenix press people aren’t nearly as good as its technical people. As an alert reader pointed out, in their press release about capturing
the photo of the probe with parachute deployed, that they said the following:
Phoenix released its parachute at an altitude of about 12.6 kilometers (7.8 miles) and a velocity of 1.7 times the speed of sound.
That looks relative innocuous, right?
Wrong.
The statement about velocity is meaningless.
The speed of sound isn’t a constant. It varies, enormously, depending
on the medium. In air, its speed is dependent on the chemical makeup of
the air, and on its density, temperature, and pressure – among other factors. So what speed of sound are they talking about?
The speed of sound where the probe was entering the Martian atmosphere? That would make sense as a measurement, but be totally uninformative to us back on
earth, since we don’t know the speed of sound in the upper atmosphere of Mars.
The speed of sound on earth? That would be informative to us – since we
have an idea of the speed of sound here, but it wouldn’t make much sense as a measurement there – the point of using the speed of sound would seem to
be related to giving us a sense of the kind of forces acting on the Phoenix
as it decelerates. But the speed of sound on earth doesn’t tell us that – because the kind of shock waves we would expect is dependent on the speed of sound in the atmosphere it’s passing through.
You can talk about speeds compared to the speed of light – because there’s a meaningful upper bound – the speed of light in a vacuum. And that’s what we usually mean when we talk about the speed of light. But with sound, that’s not
true. The speed of sound can vary quite dramatically in different mediums. It’s a big enough difference that it’s part of a common experiment done by
elementary school students! (I can remember doing an experiment in fourth grade science with a wall, where we were measuring when you could hear a rock hit a wall; one person had their ear against the wall; the other was standing a couple of feet away from the wall, and the person with the rock was about 10 feet away. The time difference was noticeable. It was very small – but distinctly noticeable. The speed of sound in air is 1260 feet per second; so a sound takes roughly 1/10th of a second to move 100 feet. The speed of sound in stone is in the range of 21,000 feet per second – which is virtually instantaneous to a human being at a range of 100 feet. So you’re looking at a roughly 1/10th second difference.)
So how fast was the Phoenix moving when it deployed its parachute? I haven’t
a clue. My best guess would be around 580 meters per second – assuming that
they were using the speed of sound in earth atmosphere at standard temperature and pressure. The speed of sound in the Martian atmosphere – which is quite a lot thinner than earth’s – would be slower, so 580 m/s is a decent upper-bound estimate.