I saw this project linked within this [excellent answer][1]. Why is this proposed balloon (assisting in deceleration of the proposed Martian atmosphere entry) donut-shaped? Are there aerodynamic, stability and/or weight advantages? 

Also, can it be articulated for steering by adjusting lengths of the lines?

I think the word ballute is used in the video (balloon parachute) but that seems to be something different. See for example NASA Tech Memo TM X-1773 [Flight-Test Results from Supersonic Deployment of an 18-foot-diameter (5.49-meter) Towed Ballute Decelerator][4] (figure shown at the end).

See the Purdue University website page for the project at https://engineering.purdue.edu/AAECourses/aae450/2015/spring

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[![enter image description here][2]][2]

https://youtu.be/CX93F0qqAUQ

[![enter image description here][3]][3]


  [1]: https://space.stackexchange.com/a/23685/12102
  [2]: https://i.sstatic.net/D5Sf7.jpg
  [3]: https://i.sstatic.net/OQ4vf.png
  [4]: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690017080.pdf