The InSight mission just successfully landed on Mars after its roughly eight-month journey beginning aboard an Atlas V rocket launched from Vandenberg Air Force Base on March 5, 2018. On Quora, someone recently asked why an Atlas V rocket was used in favor of something like the new Falcon 9 Heavy. I’m sharing my answer here:
When you took your last plane flight, what type of aircraft were you on? Was it a Boeing 737? Why not a 757? Why not a 777? Why not an Airbus A320? 330? 340? Are you getting where I’m going with this? The aircraft chosen for a given route is primarily chosen by the availability and capabilities of that aircraft matched with the parameters of the route itself (passenger requirements, range vs. route length, airports, etc.)
Like there are a myriad of aircraft available to use for commercial travel routes. there are a myriad of launch vehicles available for taking payloads from Earth into orbit and beyond. The one used depends on a number of things including launch vehicle availability, launch location, launch location capabilities, payload mass capacity, payload size, reliability, cost, etc.
Currently, there are dozens of launch vehicles available to choose from. The United States alone has the Atlas V, Antares, Falcon, Falcon Heavy, Minotaur V, Minotaur C, Pegasus, Delta IV, Delta IV Heavy, Electron, SLS, and a number under development. Which one to choose for InSight?
InSight’s launch weight was 694 kg. There are a number of launch vehicles that are acceptable for this. As others have said, the Falcon 9 Heavy with its nearly 64,000 kg launch capability to low-Earth orbit (LEO) is overkill. It would be like using a 747 on a commercial air route from Atlanta to a regional airport like Raleigh. Additionally, when Insight had launched (May 5, 2018), the Falcon 9 Heavy had only its well-publicized inaugural launch under its belt. Missions are in planning and integration for years, which means the InSight planners would have had to choose to use Falcon 9 Heavy before it had even made a single successful flight. Would you risk years worth of work on an untested launch system? The one thing that separates the Atlas from the rest is its success rate. It has had 79 launches with 78 successes. Even the Delta IV doesn’t match this at 37/36.
I don’t have any firsthand knowledge of why an Atlas V was chosen as the launch vehicle for InSight, but I can make an educated guess that it was due to a combination of cost, reliability, and availability.