(Usually. But not this year. This year, the crowds were nice, the questions were good, people were pleasant and interested and positive. They weren't the problem. I am the problem. I'm burned out. Intellectually, I see that as stupid. I have my dream job: I have a robot on another planet, and I get to drive it around and make it do stuff. And they pay me. But it's just not working for me any more.)
(But I digress.)
For the record, here are the answers to about 90% of the questions I get:
- Is this as big as the real rovers? Yes, the demo rover you're seeing is the same size and general overall appearance as the real rovers.
- Extended answer: we built this model to practice getting off of our lander, the spaceship we rode to Mars, onto the Martian surface. You don't want to go three hundred million miles ... and then trip. Now that she's done that job, we retired her and just bring her out once a year to say hi to all you nice people. Hi, nice people! (If there are enough kids in the crowd at this point, I wiggle the high-gain antenna so it's like the rover's waving. They practically always laugh and wave back.)
- Extended answer involves joking that they are the fastest cars on the planet, that there aren't any cops around to pull us over for speeding, etc. Sometimes that takes the form of mock outrage: why do the rovers go so slowly? You mean, why do the rovers go so fast?
- The question is sometimes phrased as, How fast do the real rovers go? And I point at the model and answer: that fast! And then I do the above spiel.
- This is also often a good time to talk about Earth vs. Mars gravity. I usually phrase this as "The Martian Diet," and discuss how well it compares to the Atkins Diet.
- Other useful lines: everyone who's bet against the rovers so far has lost, so I don't recommend it. Never bet against the rovers!
- Extended answer: That light-time delay drives everything we do. We'd like to drive the rovers with a joystick -- push forward on the joystick, and the rover goes forward; pull back and it stops. But the light-time delays make that impossible. Imagine trying to back your car out of your driveway like that. You look over your shoulder, hit the gas ... and nothing happens for four minutes. And when it does happen, you don't know about it for another four minutes, because your back windshield hasn't gotten the return signal yet. If we tried to drive the rovers like that, then by the time we saw the cliff coming and pulled back on the joystick to stop (I mime all of this as I'm talking, and pretend panic when I'm talking about seeing the cliff and needing an emergency stop) -- we'd be too late, the rovers would already have gone over the cliff.
So we don't do that. Instead, we just command the rovers once a day. As the sun goes down in the Martian sky, and our solar-powered rover has to go to sleep for the night, it sends us pictures and other data telling us where it is now. Then the rover goes to sleep, and we go to work. We look at all the data, use a simulation to figure out what to make the rover do next, and build up a list of commands to script out the rover's whole next day. Then, as the sun comes back up in the Martian sky and the rovers are waking up, we send them those new commands. Then the rovers go to work, and we go home and go to sleep.
- Extended answer: this year, we had a huge dust storm that blocked out about 99% of the direct sunlight. Fortunately, the rovers survived that storm, but when the dust settled out of the atmosphere, it settled onto our solar panels. On Spirit, especially, the solar panels are dustier than they've ever been -- about 70% of the light coming from the sun is blocked by dust on the solar panels, and that's going to make it a tough winter for Spirit.
- Extended answer: yes, we could have added something that would blow the dust back off the solar panels, but if we added that, we'd have had to take off a science instrument. Given that we're more than four years past our warranty, it's hard to argue with that judgment, no matter how nice it would be to have now.
- This big antenna is called our "low-gain" antenna. It communicates at only 10 bits per second, but it works even if the rover has lost track of time, doesn't know where the Earth is, or whatever. So it's the one the rover uses if it panics.
- This lollipop-like antenna is called the "high-gain" antenna. It swivels back and forth like this, and rotates like this, and that lets us point it at the Earth no matter where the Earth is in the Martian sky. This one's a lot faster, about as fast as a slow modem -- up to about 28 kilobits per second. And this is the one we normally send commands to.
- But this little, unimpressive-looking antenna, this one has sent back more than 90% of the data that the rovers have sent to Earth. That's because it talks to an orbiter in the sky over Mars, called Mars Odyssey, and Odyssey then sends the data back to Earth from there. It doesn't have to send a signal all the way back to Earth, so it can transmit a lot faster. This one's about as fast as a DSL connection, up to 256 kilobits per second.
- They're where we tied the rover to the spaceship she rode to Mars, so she wouldn't bounce around inside too much.
- You can see how the wheels are open on the inside, to save mass. (I demonstrate by sticking my whole fist in there.) That means the rovers can pick up rocks and dirt as they drive around. The holes give the rocks and dirt a chance to fall back out as we drive. Otherwise, too much stuff would build up in the wheels and clog 'em.
- When we look back at pictures we take of our tracks, we can look at the pattern the holes make. If they're crisp and well spaced, that means we're on good driving soil; if they're mushy and too close together, that means we're bogged down.
- (Note: we don't really do the analysis that way so much any more, but we used to do that a lot and sometimes still do.)
- Extended answer: Although these aren't real Martian rocks, there are some real Martian rocks here on Earth. They probably got kicked here by an asteroid: a big rock falls on Mars, makes a huge explosion, and a few of the rocks that get kicked up by that explosion end up here on Earth. Doesn't happen often, but we have billions of years to play with. We know they're from Mars because we've carefully analyzed gases trapped in the rocks; they don't look like what we know about the early Earth atmosphere, and they do look like what we know about the early Martian atmosphere, so it seems reasonable to conclude that they must be from Mars.
- Extended answer: you notice that the rover's cameras come in pairs. That's so that the rovers can do what you and I do, build up a 3-D picture of the world. The rovers can then reason about what they see in that picture -- they can tell if a rock is too big, or if there's a cliff, and steer away from it.
Those are the most common ones, by far. I got some questions about the drive and steer actuators, but just two or three. One thing I noticed was that some questions that were routine in past years, weren't asked at all this year. In particular, nobody asked me how far the rovers have driven. (Nearly 8km for Spirit, over 11km for Opportunity.)
You have to scale these answers up or down to fit the questioner and the crowd. If it's a kid who's asking, I usually kneel so I'm about their height and try to couch the answers in terms I think are age-appropriate.
Let's see ... another thing that came up repeatedly was the whole bit about how the rovers were all folded up for their trip to Mars, and had to unfold themselves once they got there. About halfway through the day I got the idea of calling the rover a Transformer; the kids dug that. And another thing was, every time Mike just dragged the rover where he wanted it, I made a rueful face and observed aloud that there were many times we've wished we could do that with the real ones.
Mars is normally down by the 303 cafeteria, but this year, we were set up by the front entrance for the first time. So as fresh batches of people came in, I would say this: "Howdy, folks, welcome to JPL, welcome to Mars. If you're just arriving, I'd like to make a recommendation. Go ahead and pick up your goody bag at the entrance, then stand right about where you're standing ... for eight hours. And then go home. Because nothing else you're gonna see today is as cool as these rovers." And that would get a laugh, and then I'd go on and talk about the model and solicit questions.
There are also several types of questioners. Among them:
- The Boy/Girl Scout. No, I mean literally. They come in with a list of questions they're supposed to ask as some sort of troop activity. Didn't happen this year (or not that I saw, though I saw some scouts there), but it's been a fixture of previous years.
- The super-bright kid. They talk with a kid's diction but an adult's understanding, or something a lot closer to it than you'd expect from their ages. These are my absolute favorites, because they mix an adult's intelligence with a kid's enthusiasm. And they're always, always jazzed about being there. I had two of them this year.
- One was an Asian kid named Max, who was incredibly excited to be there. Max told me he was five years old; he said this, and everything else, with an enthusiasm kids normally reserve for new Harry Potter releases. Max is going to be an astronaut, and his sister likes robots more than he does; he's really into airplanes. And did I mention he's going to be an ASTRONAUT! Max was awesome. Max is, he really is, going to be an astronaut.
- The other was a white kid, I'd say about eight or 10 years old. I didn't talk to him for very long, but he had an astute observation. "You said this is a rover just like the real rovers," he said, looking at me narrowly. And he's right, that's how I'd phrased it that time. "But the real rovers have explosive pyros for cutting the cables [he gestured to show me how the pyros work], and that rover has something on it that looks like Velcro." So I explained that this was a demo rover and we made it cheap because it just had to look and act like the real thing, but he was right that a lot of the parts weren't like the real parts. He nodded in understanding, satisfied, and moved on.
There. Having said all that ... maybe I can feel okay about not volunteering next year. I'll just drop off a sign with the URL for my blog, so people can read this if they want to know anything. I'm sure that'll be fine.