NASA’s Cassini spacecraft has been studying Saturn, it’s rings and moons for nearly six years, despite the original mission being slated to expire after only four years. But, due to better than expected performance and lower than expected fuel consumption, NASA has extended the mission for an additional seven years, to 2017.
In order to negotiate the flight path for the next seven years, the engineers in charge of planning the orbital maneuvers consulted with the five science teams affiliated with the project. Each team is assigned to study different things: Saturn, Titan (Saturn’s largest moon), the rings, the icy satellites, and the magnetosphere. Each team presented their wish list for the places they’d like to see over the next seven years, and the engineers got busy running the numbers:
The first time [the engineers] met with the discipline teams, they offered three possible tours. The next time, they offered two, and, in January 2009, the scientists picked one of them. Last July, after six months of tweaking by [the engineers], the final “reference trajectory” was delivered. It now includes 56 passes over Titan, 155 orbits of Saturn in different inclinations, 12 flybys of Enceladus, 5 flybys of other large moons — and final destruction.*
In essence, this team of engineers had to balance the wishes of the five research teams with the remaining fuel and gravity boosts available. The approach they took was to present alternatives and iterate on them until they found the best solution, given the requirements and the constraints:
“It’s not like any problem set you get in college, because you have so many factors pulling in different directions,” Mr. Seal said. “The best way to measure it is to look at how much better the next iteration is than the previous one” until “you’re only making slight improvements.” Then you stop.*
I can’t think of a better way to describe the iterative development process espoused by most agile software development methodologies, including Scrum and XP. You know when to stop when the remaining improvements are no longer worth the investment.
I wonder how many of our customers would like to see three options, then two, then one…