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Phil Plait's Bad Astronomy's Fly Me to the Moon and Then to a Near-Earth Asteroid describes Near Earth Asteroid Scout (NEA Scout, or just Scout) a 6U cubesat that will be deployed from Artemis somewhere in cis-lunar space, deploy a solar sail, then sail to a near Earth asteroid:

Don’t let the size fool you. It uses four telescoping aluminum rods that will extend to 6.8 meters long (the height of a two-story house), which will unfurl a solar sail made of a kind of fancy plastic (aluminized polyimide) that is extremely lightweight because it’s only a stunning 2.5 microns thick. For comparison, a typical human hair is 100 microns thick. So, wow.

Once deployed, the sail will use sunlight as propulsion. Although photons have no mass, they do have momentum, so when they hit the sail they give it a kick. This is why the sail is so thin but large (85 square meters) and the spacecraft small and lightweight; the lower the mass the higher the acceleration. Although the acceleration is incredibly low, it’s continuous. That adds up, so while you don’t move fast initially, you can build up quite the velocity over weeks and months.

The asteroid target is not set yet, because the launch date isn’t secure and everything in space moves. After launch it will be placed into orbit around the Sun (what’s called a heliocentric orbit). After that, Scout will use a cold gas thruster to position itself, unfurl the sail, and then away it goes.

The cubesat will have both a large solar sail and solar panels. It will also need an attitude determination and control system for transit and to implement it's asteroid acquisition, approach and observation sequences as it flies past it.

The solar sail structure will be low mass and therefore somewhat delicate, and also subject to excitation into oscillation modes that might be a problem during the observation phase.

Reaction wheels can gently and smoothly change attitude, but the will usually need some kind of momentum unloading scheme.

Question: How will NEA Scout control its attitude during deep space flight and keep its camera steady without causing any vibrations or damage to its 85 square meter solar sail?

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The NEA Scout contains multiple items aimed at controlling attitude:

  • An inertial measurement unit that contains rate gyros that sense the vehicle's rotation rate with respect to inertial,
  • A star tracker that senses the vehicle's orientation with respect to inertial,
  • A set of large reaction wheels for coarse attitude control,
  • A set of smaller reaction wheels for fine attitude control,
  • A set of cold gas thrusters for angular momentum dumping (and also for translational control), and
  • An Adjustable Mass Translator (AMT) (or Active Mass Translator (AMT); some NASA documents use one name while others use the other).

The last one, the AMT, is a rather cool concept. Had NASA designed the non-array / non-sail part of the vehicle to be rigid, the effective of force at center of pressure from solar radiation would oftentimes not be in line with the center of mass. This would result in a torque on the vehicle against which the reaction wheels would have to fight. This in turn would have resulted in reaction wheel saturation that the cold gas system would have to fight.

NASA instead designed the vehicle so that a good portion (40% of the vehicle mass) can tractor itself to move side to side in both the X and Y directions. (The Z axis, "up and down", is normal to the sail plane.) This means the vehicle can trim itself so as to make the line from the center of solar radiation pressure to the center of mass be close to parallel to the solar radiation force vector.

How will NEA Scout control its attitude during deep space flight and keep its camera steady without causing any vibrations or damage to its 85 square meter solar sail?

The NEA Scout will only have to keep itself fairly steady for brief moments of time. While this is speculation on my part, it is a widely used concept where very fine control is required: Temporarily disable any attitude controls that might cause unsteady rotational motion. That would certainly include the cold gas reaction control system, and probably would also include the AMT tractors.

The cold gas reaction control system used for momentum dumping presents challenges with respect to potential damage to the vehicle. That the trusters have a tenth of a second minimum on-time and minimum off-time has the potential of exciting solar sail flex modes. Flex mode excitation is something to be avoided in any space vehicle. (Flex mode excitation is also something to be avoided with bridges. A couple of examples are the Tacoma Narrows Bridge that infamously was torn apart due to winds, and the Broughton Suspension Bridge that troops tore apart by marching in rhythm whilst crossing it. Every spacecraft structural engineer needs to know about the Tacoma Narrows Bridge and the Broughton Suspension Bridge.) NASA claims to have analyzed and addressed flex excitation issues with regard to NEA Scout.

References:

Near-Earth Asteroid Scout
Flex Dynamics Avoidance Control of the NEA Scout Solar Sail Spacecraft's Reaction Control System
Solar Sail Attitude Control System for the NASA Near Earth Asteroid Scout Mission
Solar Torque Management for the Near Earth Asteroid (NEA) Scout CubeSat Using Center of Mass Position Control

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  • $\begingroup$ Heaton is the author of the abstract in the 2nd NTRS link, and that name appears twice in the conference proceedings jsforum.or.jp/ISSS2017/papers thought a title with "Flex Dynamics" is not there. There is discussion of avoiding feedback; flexing modes getting amplified by the attitude control response loop, but mitigating the excitation of the flex modes by the thruster impulses themselves doesn't seem to be discussed. $\endgroup$
    – uhoh
    Aug 22 at 13:24

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