Due to further analysis, the mission profile has changed.
The Mars Insight Lander followed the recent NASA model for stationary landers. This included the standard 6 (or 7) minutes of terror when the probe entered the atmosphere, used the heat shield and then discarded it, used the supersonic chute and then the final descent was via rocket engines on the lander. There are many stages in this process where a single fault could result in total loss of the mission. But the folks at NASA seem to have this down after a second successful landing.
Adding more risks to this already terror filled descent seems unwise. Deploying the balloon while descending introduces many more points of failure that could end the mission. Instead, deploying the balloon after landing and a thorough checkout seems much more prudent.
This does introduce the need for a full blown lander, or at least most of a lander like the Insight one. The balloon and payload would replace the instruments on the Insight lander. The helium tank for the balloon would likely be under the top deck, and the top deck would be used for the payload and balloon, with associated cables needed.
Here is an artist conception of the Insight lander.
Previous post on mission:
The Mars Balloon (or Aerobot) is proposed to be launched from a low cost system, such as the Pegasus rocket. While using Pegasus means less infrastructure is needed, it also limits the size of the capsule and cruise stage combination.
The capsule plus cruise stage will then follow a trajectory to Mars. Upon reaching Mars, the capsule will detach, enter the atmosphere, slow down, inflate the balloon, and the probe is on it’s way. To be worth the expenditure, the mission should be operational for at least ten days. On Earth, long duration balloon flights have lasted 100 or more days. That would produce an awesome science return from such a modest investment.
The entry sequence would be very similar to that shown here.
Rather than using airbags or skycranes, the aerobot will inflate a 10 meter (or slightly larger) balloon while descending slowly using a parachute. The balloon will then float for 10 or more days over the surface, imaging the surface, taking meteorological readings, and also checking for atmospheric gasses.
Data from the aerobot will be relayed to the orbiting probes such as the Mars Reconnaissance Orbiter and Odyssey. The MRO is equipped with the UHF radio to talk to rovers (and balloons!) and the Xband radio to talk t the DSN on Earth.