Quote:
Originally Posted by EricRobodox
Its not really a rover, so there is no need for it not to be careful about a drive system. Airbags would be useful for that. But if its like my mission, Mars Science Laboratory, it may be too heavy for airbags to work. And thrusters allow it to land wherever it would like, rather than less predictability from the bouncing of the airbags. Many reasons probably why they did a thruster landing. Clean launch though, very cool.
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Actually, the rationale for the use of a propulsive landing system rather than an airbag system for Phoenix is pretty straightforward. It has nothing to do with whether or not their is a rover on board. Phoenix was originally proposed as the first flight project for the Mars Scout program, which was envisioned as a series of (relatively) low-cost flight projects for Mars exploration. To keep the proposed project under the cost cap (yes, space flight projects have cost limits imposed on them too!), the spacecraft was designed to re-use several hardware items scavenged from prior spacecraft. For example, the arm on the Phoenix lander is the flight spare from the 1999
Mars Polar Lander. The main structure and landing system for Phoenix is actually the hardware originally built for the 2001
Mars Surveyor Lander project, which was cancelled before it ever flew. So Phoenix is using a propulsive landing system because they are using the hardware from the 2001 lander, and that is the method that the earlier mission was designed to use. But why wasn't the 2001 mission designed to use airbags instead of propulsion?
The answer to that is revealed when you think about the timelines involved. Airbags were used successfully for the first time on the
Mars Pathfinder lander, which touched down on Mars on July 4, 1997. Up until that time, airbags were an untested, unproven technology. The Mars Surveyor Lander mission was intended to launch in April of 2001. But that meant that the spacecraft was actually being designed in the mid-1990's. Rather than potentially risk the 2001 mission on a technology that no one had seen work, in 1995-1996 the designers chose to stick with a propulsive landing system similar to that used on prior missions like the
Viking 1 and Viking 2 landers. By the time airbags were shown to actually work in 1997, the construction of the 2001 lander was so far along that it would have been much more expensive to stop the project and re-design for the use of airbags than to just finish building what had already been designed.
-dave
p.s. This was not the only place that unused 2001 Lander project hardware showed up on other missions. The "Athena" science package that was orginally slated as the primary payload of the lander was brought forward and became the primary payload carried aboard the
Spirit and Opportunity rovers launched in 2003.