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The NASA News item NASA's Perseverance Mars Rover Gets Balanced says:

Just as an auto mechanic places small weights on a car tire's rim to bring it into balance, the Perseverance team analyzed the data and then added 13.8 pounds (6.27 kilograms) to the rover's chassis.

6+ kg of ballast out of 1025 kg according to the article is only 0.6 % and doesn't sound like much until once considers strict weight budgets; perhaps some scientific instrument like a methane detector or enhanced selfie cam or a next generation sundial or some other capability could have been added or enhanced instead of dead weight.

Question: Why did they just add dead weight (assuming that's what happened) rather than just slightly "move stuff around a little"? I'm thinking slotted mounting holes and/or spacers or risers for the RTG or other heavy components.

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    $\begingroup$ I'll add the "Slotted mounting holes would be poor engineering practice" comment before anyone else does; but an answer that expands on that and explains why would be great! $\endgroup$ – uhoh Apr 27 at 23:28
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    $\begingroup$ Other references to dead weight added to spacecraft: this answer to Is there any money in space? and answers to Where are all those blocks of steel and concrete now? $\endgroup$ – uhoh Apr 27 at 23:37
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    $\begingroup$ Shuttle had ballast as well "Lead ballast in the nose wheel well and on the X o 378 bulkhead provides weight and center-of-gravity control. The nose wheel well will accommodate 1,350 pounds of ballast, and the X o 378 bulkhead will accommodate a maximum of 2,660 pounds." science.ksc.nasa.gov/shuttle/technology/sts-newsref/… $\endgroup$ – Organic Marble Apr 27 at 23:50
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    $\begingroup$ @OrganicMarble Was the Space Shuttle balanced experimentally before flight? $\endgroup$ – uhoh Apr 28 at 0:08
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    $\begingroup$ Also "Columbia typically carried 1000 pounds of ballast or more within boxes in the aft compartment, but during orbiter processing for STS-93 both the ballast and the ballast boxes were removed." nasaspaceflight.com/2019/07/… $\endgroup$ – Organic Marble Apr 28 at 2:17
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Unfortunateley no direct answer for the Mass Rover, but for space craft overall by using the Rosetta Lander (Philae) as an example:

I was listening to a presentation of the Rosetta Lander Mechanical Engineering Team Leader some years ago. He spoke about "dead mass" on the Lander. So as far Philae is concerned. It was not like they have been happy about having to add balancing mass.

The mass became necessary because of last minute mass changes of some instruments. Some teams could not meet the masses they hoped they could achieve. Some other projects failed so they had to use replacements having different masses. In the end, they HAD to use balancing mass. But nobody wanted, the mass margins for the whole lander had to be fit.

On the other side you cannot move around instruments onboard. Every Instrument place is tested for thermal and electromagnetic disturbances. So they all have a spot and they know, the heat and the waves coming from the instruments do not affect other instruments. They even had magnets on bord to change the magnetic fields. (Source for all of this is the Philae presentation, too). Moving around instruments would actually result in redesigning the whole space craft and you do not have the time at some point of the development.

So in the end you are forced to put some dead V2A onboard.

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    $\begingroup$ Can you add a link to that presentation of the team leader of the Rosetta lander ? $\endgroup$ – Cornelisinspace Apr 28 at 9:20
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    $\begingroup$ Sry, I saw the Presentation years ago live. I do not have nor found it after a quick Google Research. $\endgroup$ – CallMeTom Apr 28 at 9:27

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