Mars Spirit and Opportunity rovers exceeded expectations by working for many years than the planned 90 day mission. But why was Nuclear Battery System not used and Solar panels were used for power generation?
Spacecraft design is based on conservative, pessimistic estimates, "stuff that's proven to work". When those estimates are (usually) exceeded, the next generation "faster horse" can be designed with the updated proven estimates as guidelines. This is the very careful way of doing things.
As such, this starts out with the first rover on Mars, Sojourner.
- Solar powered
- Planned 7 sols mission
- Lasted 83 sols.
Solar power had then proved itself. If you have got 83 sols worth of time, you can design the next generation of rover to fill all that time with objectives, instead of just 7 sols.
The next pessimistic estimate was for these next generation rovers where a low 90 sols. Good enough for the extended lifetime Sojourner had demonstrated.
Spirit and Opportunity where thus designed to be as likely as possible to achieve their goals, even in the face of very pessimistic estimates.
When Sprit and Opportunity eventually managed to run for years and years, the goalpost for the next generation rover was moved proportionally. When operational lifetime becomes this long, one begins to look out for more stable power sources like RTGs.
In hindsight, one may question the performance of solar power, now that years of operational life time is a fact, not wishful thinking. But the rovers where not designed based on wishful thinking, they were designed to last for a ~90 sol operation. For that goal, solar power was a working, proven, technology.
Mars Spirit and Opportunity rovers exceeded expectations by working for many years than the planned 90 day mission.
Ninety days was not the planned mission. It was instead the bare minimum amount of time the rovers had to work for the mission to be deemed a "success".
Mechanical and electrical equipment tends to have a high failure rate early on, a lowish failure rate after that "infant mortality" period has passed, and much later, a high failure rate as components inevitably wear out. If the Mars rovers did get past the initial high "infant mortality" period, they had a good chance of continuing for years -- which they did.
(Why) was Nuclear battery system not used for Mars Spirit and Opportunity rovers?
At the time the mission was planned, neither NASA nor the Jet Propulsion Laboratory had yet to prove themselves worthy of the large expense and extreme rarity of an RTG. NASA, along with other space exploration agencies, had instead experienced a long history of mostly failures with regard to exploring Mars. That string of mostly failures started to change with the Mars Pathfinder / Sojourner mission. There was no knowing at the time that that was the inflection point.
The Sojourner rover had just proven itself at the time work was started on the Mars Exploration Rovers. The Sojourner rover was a very low cost (in NASA terms) technology demonstrator add-on to the low cost Mars Pathfinder mission. The Mars Exploration Rovers greatly extended the limited capabilities of the Sojourner rover. (Sojourner cost a mere 25 million US dollars (1997 dollars). It did not have many capabilities.)
It would have been madness to outfit the Mars Exploration Rovers with RTGs.