PowerSource

Jun 18 2008 4:17PM | Permalink | Email this | Comments (1) |
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Here’s one last post for this week on Mars, this time only peripherally related to power, but again sourced by the prolific Bill Yalen. I downloaded and tried the Mars24 sunclock, and it works great. So, take it away, Bill:

There is a wonderful downloadable application called Mars24, from the NASA Goddard Institute for Space Studies (GISS) at Columbia University in New York City, available here:  http://www.giss.nasa.gov/tools/mars24/ . From the Mars24 Webpage:

“Mars24 is a Java program which displays a Mars sunclock, a graphical representation of the planet Mars showing its current sun- and nightsides, along with a numerical readout of the time in 24-hour format. Other displays include a plot showing the relative orbital positions of Mars and Earth and a diagram showing the solar angle for a given location on Mars.”

Among other things, you can see a view of Mars from any angle, with user-selectable landmarks and landing sites highlighted. It is clear at a glance what is in sunlight or dark, or in LOS with Earth, etc. The app can be set to the current time or any user-selected date & time in the past or future. You can also switch from the planet view (looking down from space) to a panoramic view from a selected location on the surface (e.g., Phoenix) looking out & up toward the horizon & sky. That view shows the path of the sun and Earth in the sky, along with a tabular display of sunrise, sunset, elevation angle, etc.

From that I was able to deduce that Phoenix should now be in the period of maximum solar energy, with the noontime sun rising to its highest elevation, of 47.0 degrees. With a Martian axis tilt of 25 deg., and a landing site latitude of ~68 degrees N, one would expect max sun elevation to be 90-68+25=47, so that makes sense.

Based on plugging in various dates into Mars24, the max noon sun elevation of 47.0 deg. runs from 6/15 – 7/4, implying that the summer solstice would occur on 6/24 or 25. Sure enough, this is confirmed by http://www.planetary.org/explore/topics/mars/calendar.html showing that Ls = 90°, the northern summer solstice, on 2008 Jun 25. (Interesting that, in this year of Phoenix, Earth & Mars happen to have closely synchronized summer solstices).

So it appears that the mission planners set up the timeline very well to:

-land shortly after the landing site had entered its period of uninterrupted arctic summer sunlight;

-then have sun energy building, with its peak coming at the one-month point;

-at two months the sun situation should be back to about the way it was at the start; and

-thereafter sun energy will gradually reduce (though still not quite setting) through the remainder of the 90-day baseline mission.

Soon after that, the sunrise/sunset cycle should resume, with the nights being short at first but gradually lengthening. As the days shorten and the peak noontime sun elevation angle decreases, both ambient temperature and available solar power will drop. Eventually, we’ll have the answer to your question about how long the batteries and the rest of the systems can continue to operate.