Sun setting on Phoenix
Published on Oct 29, 2008 at 7:18 pm.
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The Sun rises and sets. That’s a fact of life that is an everyday occurrence to most of us. But, for much of the Phoenix lander’s life on the surface of Mars, sunrises and sunsets didn’t happen. At a Martian latitude of 68° 13′ N, the Sun does not set near the summer solstice. But, earlier this year, Phoenix got to experience sunrises and sunsets. While that might be interesting to us humans, it is a problem for a solar powered spacecraft. At night, Phoenix has to rely on its batteries. Since the solar panels were providing more power during the day than the spacecraft needed to operate, the extra power was used to charge the batteries.
Unfortunately, those days are now long gone. With each passing day, the Sun appears further south in the Martian sky. Come December, the Sun will be on the Martian celestial equator. That will be the Martian Autumnal Equinox. On that day, the Sun will shine for half of the day. But, already, Phoenix is having difficulties. The spacecraft is so far north that the Sun never really gets very high in the sky, not even near the Summer Solstice. Right now, the Sun doesn’t get any higher than a bit over 34° above the southern horizon. That’s about how high the Sun gets here in Texas at noon on the Winter Solstice! By the time that we get to the Martian Winter Solstice, the Sun will not even rise at Phoenix’s location.
As the Sun sinks lower in the sky, the weather turns colder. That means that the on board heaters must work harder to keep instruments and electronics warm, and that uses more electricity. At night, that puts a drain on the batteries. But, the increase in power usage also comes at a time when power generation is dropping. The lower the Sun is in the sky, the less power the solar panels generate. The lower the Sun is in the sky, the shorter time that it is above the horizon, too. So, the solar panels generate the lower power for a shorter period of time. These two factors quickly get to the point that the survival of the lander becomes in jeopardy. We are now near the point where the heaters and instruments are using more electricity during a single sol than the solar panels will be able to supply. When that happens, the batteries wind up not being able to keep up with the demand. The Phoenix lander will die.
The good news, though, is that we can prolong that death. By selectively turning off instruments and heaters for those instruments, NASA can cut the power drain. That will allow the lander to operate for more days. The first such cut is now upon us. Mission controllers are now cutting off the heater to the lander’s robotic arm and the camera on that arm. The arm has been placed in a position where its soil probe is stuck in the ground to measure thermal and electrical conductivity. That instrument does not require heating. There will be no more soil samples, and one set of ovens used to bake the soil to analyze it will no longer be used. So, that will save a lot of energy. Further heaters will be turned off one by one, rendering different instruments inoperable.
One difficult phase of the mission will occur in about the first two weeks of December. During that time, Mars will be almost directly opposite the Sun from Earth. That will put the Sun too close to the line of site between Earth and Mars for communications to occur. The spacecraft will be on its own. Unfortunately, that will also be about the time that the final heaters will likely need to be turned off. If it gets too cold, then the lander might die. But, if the batteries run down too much, it will also die. Unfortunately, the lander will be out of communication at about that time, so we might not know to tell it to turn off another heater before it runs down the power too much. Conversely, we don’t want to turn off the heater earlier than we have to, since we might lose scientific data that could otherwise be measured. So, the mission team has some tough decisions ahead on what to do with Phoenix before we lose contact for two to three weeks. And, there is always the possibility that the spacecraft might not be in any shape to resume contact once Mars sweeps past alignment with the Sun. I can imagine that it will be a tough few weeks on the mission team.
But, even if we do lose contact with Phoenix, we should remember that it has already lasted far longer than it was designed to last. Phoenix was supposed to last for 90 days. It has been there for 157 days. It will almost surely have lasted over double its design life before it finally meets its end.
-Astroprof
Image courtesy NASA , UA (Corby Waste)
CCPhysicist on November 1, 2008 at 8:37 pm: 1
That drawing is nice, but it would be so much better if it had the ice patch and trenches, etc, around the lander.
What is even better are the pictures from the last month or so. I was speechless at the video of clouds moving across the sky overhead and dust devils moving by in the distance. It is out of the news now, but I thought those pictures of actual weather on Mars were stunning.