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LISA is a proposed space probe designed to measure gravitational waves. It aims to measure gravitational waves directly by using laser interferometry. It uses a drag-free satellite design to protect the interferometers from confounding acceleration due to solar wind and light pressure.

According to Wikipedia,

drag-free satellites are satellites where the payload follows a geodesic path through space only affected by gravity and not by non-gravitational forces such as drag of the residual atmosphere, light pressure and solar wind.

A zero-drag satellite has two parts, an outer shell and an inner mass called the proof mass. The proof mass floats freely inside the outer shell, while the distance between the outer shell and the proof mass is constantly measured. When a change in the distance between the outer shell and the proof mass is detected, it means that the outer shell has been influenced by non-gravitational forces and moved relative to the proof mass. Thrusters on the outer shell will then reposition the outer shell relative to the proof mass so that its distance is the same as before the external influence changed it.

The outer shell thus protects the proof mass from nearly all interactions with the outside that can cause acceleration, except those mediated by gravity, and by following the proof mass, the outer shell (which is to say, the rest of the spacecraft, carrying instruments, etc.) itself follows a geodesic path.

Examples include Gravity Probe B , DECIGO , LISA and STEP .

Ground based gravity wave observatories such as LIGO measure spatial distortions of 10^-18m over a path length of 4 km, a fraction of the diameter of a proton. The interferometer must be protected from spurious accelerations, even ground vibration from distant road traffic.

LISA will require power to run the interferometer lasers and detectors. The interferometers themselves must be "surrogate proof masses" (their trajectory must exactly mirror the proof masses). If interferometers were connected to the outer shell, they would be subjected to the same accelerations as the outer shell as it adjusts its trajectory to match the "designated" proof masses (gold cubes).

LISA needs its interferometers to be active components requiring power. How is this power supplied without transmitting spurious accelerations from the shell spacecraft thrusters?

Addendum: I was incorrect in my assumption that the interferometer was separated from the spacecraft bus. This paper, (from a comment by @jpa), https://iopscience.iop.org/article/10.1088/1742-6596/154/1/012021/pdf specifically states that

The S/C (spacecraft) bus structure is designed seamlessly with the scientific payload (interferometers).

I had assumed the interferometers needed to be following a free-fall geodesic trajectory, The “shell” spacecraft around would shield it from solar wind and light pressure. The “shell” would sense relative motion of the 2 proof masses and use thrusters to compensate. Intermittent thruster firing would impose acceleration on the shell. These motions would be transmitted to the interferometer if the two were mechanically connected by power lines.

However, LISA's design does not use intermittent thrusting.

LISA requires micronewton thrusters to provide the fine spacecraft attitude and position control for drag free flight and beam pointing to the distant spacecrafts. The thrusters are operated continuously during science operations with their thrust levels set by the disturbance reduction system control loops.

So the "shell" spacecraft continuously follows the geodesic trajectory of the proof masses without intermittent corrections. There are no intermittent thruster effects transmitted to the interferometers.

Not my first error. Not the last, either.

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    $\begingroup$ Not cool to narrow the focus of your question after an answer has been posted which very well addresses the original version. aka "moving the goal posts" $\endgroup$ May 1 at 20:46
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    $\begingroup$ @OrganicMarble ... the question was intended to be about LISA and how the interferometers are powered, not about drag free satellites where the "payload" is the proof mass itself. The edit was apparently required for clarification. Coolness was not my motivation. $\endgroup$
    – Woody
    May 1 at 20:48
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    $\begingroup$ @Woody Out of curiosity, do you happen to have a reference for the interferometers being in the inner mass? In this admittedly old 2009 summary it seems the interferometers would be on outer shell, and specific control methods would be used to track the proof masses with sufficient accuracy (i.e. so that the acceleration of outer and inner mass is sufficiently equal). $\endgroup$
    – jpa
    May 2 at 9:08
  • $\begingroup$ @jpa ... thanks for the link. $\endgroup$
    – Woody
    May 2 at 15:13

2 Answers 2

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Nothing is mounted on the proof mass. The proof mass is unpowered, ideally touches nothing, and is designed to be as featureless as possible. The ones used in Gravity Probe B are the most perfectly spherical things ever manufactured, consisting of 4 cm diameter quartz balls coated in niobium. They do their own thing, and the rest of the spacecraft maneuvers to stay around them:

A primary concern was minimizing any influence on their spin, so the gyroscopes could never touch their containing compartment. They were held suspended with electric fields, spun up using a flow of helium gas, and their spin axes were sensed by monitoring the magnetic field of the superconductive niobium layer with [Superconducting Quantum Interference Device]s.

Once the the proof masses are on orbit and spinning, the electric fields (used to keep the spheres from touching the walls of their container during storage and launch) and gas jets are turned off, there is no further power provided to the proof masses. Their only "communication" with the rest of the vehicle is naturally producing a magnetic field (since they're rotating superconductors), which is measured by sensors on the main vehicle, causing the vehicle to maneuver to keep itself centered around the free-falling proof mass(es).

There are lots of sources of noise in this system, since the signals sought are extremely small. This is why it took 40 years for GPB to go from first NASA funding to launch, and why it took another 10 years to get to final publication of the results. Making things that precise, and figuring out exactly how to calculate and cancel the systematic errors, are very difficult. A good source for many of the relevant pre-launch historical papers (1959 to 2003) is https://einstein.stanford.edu/content/sci_papers/ .

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    $\begingroup$ Gravity Probe B had the luxury of using inert test masses. For LISA, the interferometers themselves must be "surrogate test masses". If interferometers were connected to the outer shell, they would be subjected to the same accelerations as the outer shell as it adjust its trajectory to match the "designated" test mass (gold cubes). LISA will require its surrogate test masses (interferometers) to be active components requiring power.. $\endgroup$
    – Woody
    May 1 at 19:45
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    $\begingroup$ Because a number of problems occured during comissioning, gravity probe B did not fly round a free-falling test mass (primary drag free mode). It used backup mode, which centres the test mass electrically in its housing, and then flies to zero the suspension signal, a similar effect but different noise contributions. The sensing of the test mass position is always electrical, and magnetic sensing only monitors the spin axis. $\endgroup$
    – Neil_UK
    May 2 at 13:06
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The interferometers themselves must be "surrogate test masses"... LISA will require its surrogate test masses (interferometers) to be active components requiring power.

I can't find any confirmation of these statements, so it would be interesting to see where you got them from. I can't find the phrase "surrogate test mass" anywhere other than in your original question, though admittedly I didn't look very hard.

Consider Advanced drag-free concepts for future space-based interferometers: acceleration noise performance (2014):

This is the current baseline design for LISA: two cubical sensors per SC, a combination of capacitive sensing and optical sensing (optical sensing only in the direction of the two lines of sight of the LISA constellation) to measure relative position and attitude of the test masses with respect to the SC for drag-free control

(SC here meaning "spacecraft")

The proof-mass lies within a reference housing. The received light is measured with respect to the housing, while a separate interferometer measures the displacement between the housing and the proof-mass.

The gravitational reference sensor is a capacitive sensor that features a cubical TM made of an alloy of Au-Pt, and surrounded by a configuration of gold-coated electrodes for capacitive sensing and electrostatic actuation.

(TM here meaning "test mass", synonymous with "proof mass" in this context)

This very strongly implies that the LISA drag-free control system uses inert test masses to provide reference signals to other parts of the system. Trying to mount active hardware on the proof mass would not only drastically increase its size, complexity and cost, but I rather suspect that it would have serious negative effects on its thermal and magnetic properties and probably also spoil its physical symmetry.

Instead, inter-spacecraft interferometry is used to measure the distances between two spacecraft shells, and separate in-spacecraft interferometry is used to measure the relationship between those shells and the test masses, and the measurements are combined to eliminate errors caused by non-gravitational forces acting on the spacecraft.

The paper goes on to talk about alternative versions of the LISA experiment and the different designs of drag-free sensors and feedback that might be used, and none consider the possibility of active equiment on the proof mass.

Elsewhere you can find stuff about the LISA pathfinder mission that was launched in 2015 and prototyped some of the technologies the real thing would need.

LISA Pathfinder was launched on December 3, 2015 as a proof-of-concept that tests that the noise characteristics of free-floating test masses within the spacecraft are small enough compared to an expected gravitational wave signal.

and

The plot shows the measured level of imperfection from pure free-fall of the two gold-platinum test masses.

are probably the relevant bits. No indication that active equiment was used or considered on the the test masses, or any implication that this would need to be designed and tested for the LISA mission.


Note that you don't need to install active equipment on the test mass to use it as part of an interferometer! From LISA Technology Package System Design And Operations (2006) and related work like The Disturbance Reduction System: Testing Technology for Drag-Free Operation that you can use laser interferometry to measure the relative positions of the two test masses and the spacecraft shell. This simply relies on the test masses being able to reflect the appropriate wavelengths of light. I don't believe there was any suggestion that the LISA inter-spacraft laser beams be bounced of the test masses, however.

disturbance reduction system block diagram showing inert cubic test mass positions and orientations being measured using interferometry

(terrible JPEG artefacts present in original Disturbance Reduction System PDF linked above)

rendering of LTP, showing pair of sensors with inert masses and inertial sensors, and a laser interferometer measuring the relationship between them and with the device shell

(from LISA Technology Package System Design And Operations)

This feels like it should put the idea of active test masses to rest: they're simply not necessary.

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