Frequently Asked Questions
(Updated Dec. 28, 2015)
There are just a few questions and answers.
Why is there a difference of a few seconds between MSL Curiosity mission time and Local Mean Solar Time (LMST) at the Curiosity landing site?
When the Mars Science Laboratory project was being planned, a landing site at longitude at about 137.42°E was targeted and the mission clock was based on Local Mean Solar Time (LMST) at that longitude. Sol 0 would commence at LMST midnight for that location, the night before landing was to occur. The landing site coordinates were later refined after course corrections were made while MSL was in-flight to Mars and the rover touched down somewhat "long" of the final target coordinates, so the landing site turned out to be at 137.441635°E. The difference in Local Mean Solar Time for these two longitudes is about five seconds.
As the discrepancy is not important to mission operations, but making changes to mission timing tools (even before landing occurred) could potentially cause more trouble than any benefit that might be gained, mission planners continued to use a mission clock based on LMST at longitude 137.42°E.
Why is there a difference of about three and a half minutes between Mars Phoenix mission time and Local Mean Solar Time at the Phoenix landing site?
Although much larger than the difference for MSL Curiosity, the reason is essentially the same. During mission planning, it was expected that Phoenix would land at about longitude 233.35°E and the mission clock was based on LMST for that longitude. However, the choice of landing site was not yet final. Two months before landing, the Phoenix team decided that the landing should occur at 233.975°E. At this longitude, midnight before landing would occur two and half minutes earlier. Even though there were two months to go before landing, re-programming mission timing tools was not considered to be worth the effort.
Moreover, when Phoenix landed, the actual location was at 234.24845°E, about a quarter degree (or 5.5 km) east of the target. This added another minute to the difference between landing site LMST and the mission clock, for a total of three and a half minutes.
Why is there a difference of 41 minutes between MER-A Spirit mission time and Local Mean Solar Time (LMST) at the Spirit landing site?
During mission planning, the MER-A team at the NASA Jet Propulsion Laboratory specified that the mission clock for each MER lander should (1) use mean Mars time units as the clock ticks, but (2) be roughly aligned with Local True Solar Time at the middle of the originally planned mission duration of 90 sols. This second requirement, in other words, means that on about the 45th sol of the Spirit mission, noon on the mission clock was within 30 seconds of true solar noon at longitude 184.5215°W.
(Note: Spirit operated for over 2200 sols, and as in January 2015, Opportunity reached 3900 sols. The planned mission durations were greatly exceeded!)
The difference between mission time and LMST at the landing site is quite large because when Spirit landed on Mars, the planet was at the point in its orbit where the difference between Local True Solar Time (LTST) and LMST — what the technical notes call the Equation of Time — was near its maximum value. So although on the day of landing there was a 10-Mars minute difference between the mission clock and LTST, the difference between the mission clock and LMST was over 41 Mars minutes.
Why do the lander mission times for Spirit and Opportunity differ by 12 hours plus/minus a minute and 10 seconds?
As noted above, the mission times defined by the JPL planners for the Mars Exploration Rovers were defined so that at about the middle of each rover's planned mission, the individual mission clocks would approximately sync up with Local True Solar Time at the lander sites. This means that the mission times are calculated separately and have no direct relation to each other.
Unfortunately, although the mission times for the two landers turned out to be very close to being 12 Mars hours apart — which is roughly what one expects; the two landers are almost on opposite sides of the planet — mission planners at JPL did not decide to simplify matters and choose a mission clock scheme which would make the difference exactly 12 Mars-hours.
How do I set Mars24 to show me what Mars looks like from Earth?
Perform the following steps in the settings window:
- Select the sunclock display and set it to use the "Orthographic" map projection.
- Set the "center of map" to use the landmark pop-up menu.
- Select "Sub-Earth point" in the landmark pop-up menu.
If you watch the sunclock for few minutes, you will occasionally see a slight shift in the graphics as Mars rotates within the view.
If you're trying to compare the resulting image to what you might see through a telescope, keep in mind that the display does not account for the time it takes light to get from Mars to Earth. Depending on where the two planets are in their orbits, this can take from three to 20 minutes, and during that time Mars will rotate from one to five degrees.
Remember also that Mars24 sunclock uses stock images with enhanced or false color and enhanced contrast. The realistic "VIS_MDIM_Map" and "MOC_MOLA_NGS_Map" images included in Mars24 do not include any seasonal effects that alter the appearance of Mars, such as dust storms and growth/contraction of the polar caps. So even if you had a super high resolution telescope like the Hubble, Mars probably would not look quite like the Mars24 display.
Why does the table on the panorama display say that the Sun had an elevation of -0.2° at sunrise and sunset? Shouldn't it be 0.0°?
On Mars, just like on Earth, the Sun is not so far away that it is a pinpoint light source. Depending on where Mars is in its orbit, the Sun has an apparent radius between 0.16° and 0.19°. Mars24 defines sunrise and sunset as occurring when the edge of the Sun crosses the horizon, which is to say, when the center of the Sun is at some elevation between -0.16° and -0.19°. Rounded off to a single digit after the decimal character, this is displayed as -0.2°.