Perseverance ejected two 70 kg "cruise balance masses" just before it became an airplane, to move its center of mass off axis and increase its angle of attack from 0° to about 16° to make it steerable.

Shortly before it stopped being an airplane, it ejected six more tungsten "balance masses," in something called the Straighten Up and Fly Right (SUFR) maneuver. This reduced its AoA to less than 5° before deploying its parachute.

  1. How many kg were those six masses?
  2. Why were that many needed?
    If for sequential deployment to damp oscillations, why were so few needed? Why not spray a thousand, or even spray a heavy liquid?

Its 2012 predecessor Curiosity had 2x 75 kg and 6x 25 kg masses.

  • 2
    $\begingroup$ Thank you for putting Curiosity's masses for comparison. Nice question! $\endgroup$
    – DrSheldon
    Commented Mar 8, 2021 at 21:18
  • 1
    $\begingroup$ related and video (but doesn't answer this) Is Perseverance's heat shield's thickness radially symmetric? and screenshot: i.sstatic.net/V2Pcb.png which says 75 kg x 2 for Perseverance, though I don't think that's meant to be a precise authoritative number. And to think that here I was worried about a measly 6 kg of dead weight, now they're throwing 300 kg of "sand bags" overboard! $\endgroup$
    – uhoh
    Commented Mar 8, 2021 at 22:06
  • $\begingroup$ Damping oszillations by adding masses may be a bad idea. For a mass spring system oscillation frequency gets lower by adding masses but a damper is some thing different. To damp oscillations energy must be consumed but not stored as done by masses or springs. $\endgroup$
    – Uwe
    Commented Mar 9, 2021 at 1:43
  • $\begingroup$ @Uwe, the same total kg, only reduced more gradually. If damping even is the reason. $\endgroup$ Commented Mar 9, 2021 at 3:37
  • 1
    $\begingroup$ related: Spin-stabilization of a NASA rover en route to Mars. What and why? $\endgroup$ Commented Mar 7, 2022 at 19:30

1 Answer 1


The link in BrendanLuke15's comment just now eventually led me to an answer.

The six masses were each 25 kg, the same as Curiosity's, according to p. 3 of JPL's 2019-03-02 technical report Mars 2020 Entry, Descent, and Landing system overview:

Finally, the vehicle executes a maneuver called ”Straighten Up and Fly Right” (SUFR), ejecting six 25 kg balance masses to set the angle of attack to 0.

Other publications hint, but only hint, that they were deployed simultaneously.
Their full name is Entry Balance Masses (EBMs), if that helps someone else figure out why six were needed.

  • 1
    $\begingroup$ The descent stage landing radar (see figure 2) "sticks out" towards the side with 6 masses (right side), maybe the radar creates a volume crunch that required 6 smaller masses rather than 2 bigger ones (see figure 22 for a "front view" of that side) $\endgroup$ Commented Mar 8, 2022 at 0:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.