FIELD JOURNAL FIELD JOURNAL FIELD JOURNAL FIELD JOURNAL
Ready or Not, Here They Come!by Matt Wallace
June 24, 1997
We just finished a couple of sets of operational readiness tests. We had a full-up test about a week-and-a-half ago, where we had all the participating scientists and all the operations engineers come to JPL where we went through four days of operational testing, simulating the first four days on Mars. It was really exciting! We have a sandbox here that simulates the Martian environment, and hardware that looks and acts just like our flight hardware. We have a rover that's really a flight spare, so it's very high fidelity. We did a lot of really good testing, which included waking up the rover and letting it go through its self-assessment and health checks and making an assessment from the lander images to determine if it was safe to deploy the ramps. Once we made that assessment we deployed the ramps and the rover stood up and locked into an upright position. We took a few more pictures to make sure the ramps unfurled and then the rover traversed down the ramps onto the surface of Mars. This test went better than any of the tests we've done so far. This really was a good experience!
Pathfinder lands on Mars at about 10 a.m. local time on July 4. We get our first transmission session at about 2 p.m. on the low-gain antenna (rather than the high-gain), so our downlink period will be rather slow. But during that session, which lasts about 50 minutes, we'll get information from the rover. It'll wake up when it's commanded by the lander and it'll go through a self-assessment and transmit the data to the lander, which in turn will relay it back to us during that session. Somewhere around 2:45 p.m. we'll see the first rover telemetry data.
We'll have a bit of a wait before we can turn around and send our first set of instructions because the lander will have to go through a few of its operations. Shortly after landing it will do a sun search with the camera so it can figure out its orientation and point the high-gain antenna. That will take about three hours. Sometime around 4 p.m. we will receive our first high-gain session and will activate a sequence to stand the rover up.
There is a whole list of things we have to look at before we'll tell the rover to egress down the ramp. First we'll look at the tilt of the ramp to make sure it's not too steep for the rover to drive down. Then we'll look to make sure the ramp isn't twisted because if it is, the rover could fall off the sides. We'll check to make sure the airbag material has properly retracted. If it puffs up and gets up around the sides of the ramp, there's a potential for the rover's wheels to snag on material while egressing down the ramp. We'll have to look for rocks that may impinge on the ramp and cause it to twist. We'll look for rocks down at the base of the ramp to make sure that once the rover actually gets down to the surface, it can go somewhere! The rover can traverse over small rocks, lower than the height of its wheels, which is about 8 cm. Once the rocks get much than that they become hazards rather than obstacles.
During our first week on mars, we intend for the rover to stay very close to the lander, just because we'll have a better idea of the obstacles in the terrain of the lander images and the area directly around the lander. I think we're going to find that there are a lot of very interesting rocks and soils within just a couple of meters. If the mission lasts several months there's a possibility that we'll traverse outside the visual range of the lander.
The rover carries three experiments: a spectrometer, a dust experiment and a wheel-abrasion experiment. The primary instrument is the spectrometer, which is mounted on the rover's back. We'll use it to determine the elemental composition of the soil and the rocks by putting the spectrometer down on the soil and the rocks. The dust experiment sits on top of our solar panel where there is a small sensor with a coverglass that can be moved back and forth to determine how much dust is building up.
The wheel-abrasion experiment is on the rover's right, middle wheel. There is a strip of metal that contains different coatings and metals. We're going to turn that wheel while holding all of the others in a fixed position. We can turn that wheel and look at the way in which the metal coatings abrade. This will give us an indication of the roughness and granularity of the soil and sand.
Another whole set of experiments we're going to do is the soil mechanics experiment. It's similar to the wheel-abrasion test, but slightly different in that we'll turn the rover's front and rear brake wheels one at a time, a number of revolutions, while holding the other wheels fixed. This will dig a wheel down into the soil and by looking at that hole, geologists and scientists can determine a lot about the internal coefficient of friction and cohesion. It's an interesting way to use the rover's hardware to get good science data!
The rover will not specifically look for life on Mars. The Mars Pathfinder program was designed before the Mars meteorite rock was found, so it was not specifically designed to look for life. But it will tell us quite a bit about the geology and the effects that water may have had on the planet. It will also tell us a lot about the elemental composition of the soil and the rocks and whether or not those conditions were actually conducive to life at some point in Mars' history.
The rover's primary mission will last seven days, but if we go seven days there's a good chance we'll last at least 30 days, which is the primary mission span of the lander. The 30 days are limited by the lander battery. The rover's seven-day primary mission is a function of the thermal cycle and the unknown effects of thermal cycling on the rover. It gets extremely cold on Mars--down to -120 C at night and as high as 20 C during the day. That's a pretty big swing! There's a possibility that the mission will continue longer than 30 days. It will definitely go on as long as the hardware survives. We've even got some long-range plans out to a year!