So why not a blimp? I did not see any mention why they did not consider this though they did mention the Soviets using balloons on Venus. Is the air too thin to support any sort of lighter than air vehicle?
On Mars, gravity + thin air would likely limit the usefulness. But you're spot-on with Venus. Blimps would be great there. Hell, there were articles floating a couple of years ago suggesting you could park blimp on Venus above the corrosive layers of atmosphere, and conditions there could actually support a small base that would extract necessary resources from the atmosphere, and where humans would only need light shielding (+ breathing apparatus). I'm not saying building a cloud city on Venus is a good idea now, but I do think a long-duration mission to study the atmosphere would be, as data from Venus is also useful for studying climate change on Earth.
It sounds like it might be feasible: the atmosphere on the surface of Mars is described as the equivalent of Earth at "100,000 feet" (30 km) but the record for a weather balloon is 53km, and balloons carrying people have gone as high as 40km.
The helicopter being proposed is "the size of a softball", though, and not the main focus of the mission. The high altitude weather balloons have been much bigger and would probably need a dedicated mission.
Forgive my ignorance, but I'd think the lower gravity would actually reduce buoyancy, so a lighter than air vehicle would have less upward force, while being at the mercy of high winds. At the same time, less gravity makes it easier for a helicopter because less lift would be required for a heavier than air vehicle.
Not having read the article, I am sure that they already did. Nothing gets sent beyond Earth's orbit without having gone through as much testing as is humanly possible on Earth. The cost of failure on Earth is several orders of magnitude less expensive than the cost of failure elsewhere.
I would guess that sending a test helicopter along with another rover would be to do a feasibility test for larger flight-based probes in the future. Test it now with a small helicopter, and you use 10-20 pounds of payload in order to do better wind and atmospheric studies. On the other hand, if you send a probe that uses flight as its primary mode of transportation, and some unknown unknown causes it to fail, you've scrapped the entire mission.
> They might not have, because it might just fall out of the sky. It's built to overcome Mars' gravity, not Earth's.
A video has them flying that thing inside a pressure chamber without payload, so at least in this configuration it has enough power to overcome Earth's gravity. Probably wouldn't lift off on Earth with the payload, though.