What inspired the circular panels on the Phoenix Lander and InSight rover?

Question

The solar panels on the Phoenix Lander and InSight rover seem to take on an iconic new look of circular solar panels. As far as I could see solar panels on landers and rover took up the maximum surface area that they could be packaged into. For Pathfinder, that meant three triangular panels, for its accompanied rover, Sojourner, that was a simple rectangle on its top.

The Mars Exploration Rovers has a slightly more difficult design of Solar Panels, but ended up with a similar triangular wing shaped design to that of Pathfinder (as it used a similar tetrahedral lander to Pathfinder, I believe).

However, with Phoenix (2007) and InSight (2018) a circular solar panel arrangement has been used for whichever reasons. A similar design can also be seen Enhanced Cygnus spacecraft, as well as some mock-ups of the Orion spacecraft, where it is almost expected (by amateurs/ the casual public) to have rectangularsolar arrays.

I'm curious as to what led to the decision of the use of circular arrays in the Phoenix Lander and InSight Rover as opposed to trying to pack in as much solar array area as possible.

NB: I'm marginally aware that the Enhanced Cyngus uses circular panels to reduce its weight and increase its max payload by ~700kg ¹

Answer 1

Good catch noting that Cygnus has the same solar panel design! Orbital ATK, developer of Cygnus, builds these panels under the "Ultraflex" and "Megaflex" brands, and did indeed supply them to JPL for two Mars missions:

• Mars Phoenix Lander
• Mars InSight

The Mars Polar Lander program successfully qualified the panels for flight; but were not used on that particular mission.

You can find more about the panels in this PDF by Orbital-ATK. With panels on SpaceX's Dragon, where you have a number of rectangular panels that expand individually, this necessitates the need to have at least $n-1$ hinges or joints for $n$ panels. Additionally, on a platform such as a Mars lander, these hinges will be operating in the direction of gravity, which is much harder than operating perpendicular to the force of gravity. A way to solve this is to have a hinge able to pivot the entire array at the body of the lander, but this just means you now have $n$ hinges for $n$ panels.

With a Ultraflex panel, the entire boom can be mounted statically, and a single motor can unfurl the entire panel, as seen in this deployment annotation:

These panels had enough determined about their reliability that NASA felt confident in using them, again, reading from the Orbital ATK PDF:

• Lightweight: ⅓ total mass of a rigid solar array of the same power

• High strength: > 10x max on-orbit acceleration

• High deployed stiffness: 3-8x higher 1st mode

• Compact: ¼ stowage volume and footprint compared to a rigid solar array of the same power

• Reliable: flight proven; deployment powered by a single redundant motor.