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Continuing with the theme from last week Mars has been the source of legend and myth since mankind first looked at the night sky in detail. However, until recently little was really known about the planet and it has proven to be somewhat of an unwilling subject. In the 1960's NASA and the Russian space agency launched some 13 missions to explore the red planet with all but NASA's Mariner 4, 6 and 7 failing. 
The next twenty years 1970 to 1989 were somewhat more successful but even so 9 of the 13 missions failed to operate to their full potential. However, NASA's Viking 1 and 2 proved to be highly successful landing two craft on the surface and one in orbit. The primary mission of Viking 1 and 2 was to look for any sign of life and the initial tests seemed to confirm that there was some form of life present on the red planet. Later detailed examination revealed that the test being used to detect the presence of life was fundamentally flawed leaving the question of Martian life still unanswered.
The embarrassing failure of the Mars Climate Orbiter in 1999 due to mix up between metric and imperial measurements capped ofF  yet another decade of failure. Six of the 8 missions failed, but again there was an outstanding success with the Mars Pathfinder mission landing the first successful rover on the surface.
So far the 21st century has shown a turn around in the luck with only the UK's Beagle 2 apparently crashing while attempting to land on Mars. The Phoenix Lander and Dawn Spacecraft fly by are still on route. To date the two outstanding results have been the Mars Exploration Rovers MER-A and MER-B as depicted in the artist's impression above and to the right,
MER-A or Spirit (see mission patch on right) left Earth on its 207 day journey to Mars on 10th June 2003. On 4th January 2004 it successfully landed about 15° south of the Martian equator in Gusev Crater.
 After surveying the surrounding terrain as seen in the image above, and carrying out system checks, it commenced its wandering exploration of the Martian surface.
MER-B or Opportunity (see mission patch on the right) departed on its slightly shorter 202 day trip to Mars on 7th July 2003 successfully landing 2° south of the Martian equator in Meridiani Planum. It is, however, on the opposite side of Mars to its twin rover Spirit.
Again, after surveying its landing site as seen in the image below and carrying out system checks it too started off on its wandering mission of exploration.
The chronicle of failed missions testifies to the difficulty of sending an interplanetary mission to Mars but getting there is only the first step and there are numerous other stumbling blocks that have been the undoing of several missions. Unlike the Moon Mars has an atmosphere so any lander is going to need some sort of heat shield to prevent it from burning up when it enters the atmosphere.
However, it doesn't stop there as the atmosphere is not dense enough to realistically utilize aerodynamic devices like parachutes to slow the descent rate sufficiently to perform a soft landing. In 1976 the Viking 1 lander utilized small rocket braking systems for the final descent but the size of MER landers was too large to use this method.
In 1997 the Mars Pathfinder mission landed the Sojourner rover on the surface of Mars and while Spirit and opportunity were considerably larger engineers believed that the system was sound and could be scaled up to work on the new landers. Basically it was a three stage system as follows:
- Heat Shield: The first stage was the use of an ablative heat shield that both protected the rover and associated equipment and reduced the speed enough to utilize aerodynamic devices to continu the deceleration.
- High Speed Parachute: The next stage was the use of parachute to scrub off as much of the speed as possible. This was no ordinary parachute as the relatively thin atmosphere meant the terminal velocity of the lander after jettisoning the heat shield would tear a normal parachute to shreds. Designing this parachute turned out to be problematic with the original design failing catastrophically at opening. After the opening problems were solved further testing revealed the design has serious stability problems. Time was running out and at one point it looked like the parachutes would not be ready in time for the 2003 launch window but persistence paid off and the deadline was met.
- Rocket Motors: The mass of the landers meant that using only rocket and parachutes would not be practical, but they were not done away with completely.
- Gas Bags: The final stage is the source of the term "Bouncing" in the title and involved the use of several large gas bags. When the craft was close enough to the ground the parachute was released and the lander was cocooned inside several large gas filled bags. These let the lander bounce several times and once the craft came to a complete stop the bags were deflated. Again, during testing the bags proved to be problematic with several unexpected failures but after some redesign and further testing a working system was developed.
Having a failure rate of nearly 55% is bad enough but operating an unmanned roving vehicle on the surface of another planet is even more difficult, complex and literally fraught with mission destroying pitfalls. Real time data and image transmission not only requires considerable band width but the power consumption would be far greater than the primarily solar powered rovers and orbiting Mars Surveyors could supply.
The other major problem is the delay in getting signals to and from Mars. At best the round trip takes 6½ minutes and by the time Mars reaches conjunction this delay has stretched out to almost 45 minutes. Delays like this in critical control and decision making loops would never work as the rover could end up being a pile of debris at the bottom of a cliff for three quarters of an hour by the time the message to stop reached it. The only answer is to build a rover that is for the most part autonomous and capable of critical decisions without input from Earth.
Spirit and Opportunity were scheduled to explore the area around their landing sites for signs of water, life and other geological features for approximately 90 sols (Martian days). However since they were operating well at the end of the initial 90 sols the mission has been extended on several occasions and as of Monday 17th December Spirit was on sol 1,406 while Opportunity had reached sol 1,386.
The Martian year is 668.6 sols long and as its axis is inclined to the orbital plane by 25.19° Mars experiences seasons in a way similar to Earth except lasting for around twice as long. The initial 90 sol missions were during the summer months at the landing sites and little attention was paid to what would happen when the sunlight diminished as the seasons moved towards autumn and winter. The mission controllers knew that the solar cells that supplied the rovers with their energy would not be able to sustain the rovers during the southern Martian winter. The rovers were parked in a sheltered well lit area and all but the most critical systems were shut down. When the Sun started to get higher in the sky the rovers were carefully started up again and to everybody's relief both came back to life.
The rovers have since been hit with dust storms, have recorded miniature tornado like atmospheric phenomena, explored craters, looked for signs of life, found mineral deposits that seem to indicate there was water once present on Mars but to date no sign of life.
The Spirit and Opportunity have been many times more successful than anybody could have believed and are currently in a race against time to reach the place they will spend their second Martian winter and with a little luck they will both come back to life at the end of the southern Martian winter.
As usual you can read more on the subject by following the links below. I highly recommend visiting the NASA MER Archive page as it has links to a phenomenal amount of information on the Spirit and Opportunity missions.
Once again the end of year is rapidly approaching and MASU is going to take a short break so there will be no new threads for a couple of weeks. I will be checking for new posts in existing threads from time to time. In the meantime I hope you all have an enjoyable and fulfilling festive season and a prosperous and rewarding new year.
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