Release the beasts!
- By Ira Thorpe
- February 29, 2016
- Comments Off on Release the beasts!
Captain A. G. Lamplugh, a British pilot from the early days of aviation once famously said “Aviation in itself is not inherently dangerous. But to an even greater degree than the sea, it is terribly unforgiving of any carelessness, incapacity or neglect.” Space flight is less forgiving still. A single small mistake like a sticky bearing or a mis-typed line of code can cripple a billion-dollar spacecraft – and perhaps the careers of the scientists and engineers who built it. Except for a few notable exceptions, there is no possibility of correcting a mistake once the space craft is launched. Instead, spacecraft and their scientific instruments are relentlessly tested, tweaked, and re-tested on the ground in an effort to expose all of the flaws before the launch. This is much of the reason for the high cost of space flight. But in the end, the spacecraft must finish with rehearsals and take the stage.
In-orbit repair of spacecraft, such as the 2009 servicing of the Hubble Space Telescope shown here, is the exception rather than the rule. Failure in a critical subsystem can be a mission-ending event. Credit: STS-125 Crew, NASA
Strangely enough, it is often the secondary things that give you the most trouble. Such is the case with the LISA Pathfinder mission, a technology demonstrator for a future gravitational wave observatory in space which launched last December. The measurement concept for LISA Pathfinder ties back to some of Einstein’s fundamental ideas about gravity itself.
“How can gravity be measured?” Ask this question to most people and you might get the answer “with a bathroom scale”. Of course what the scale is really measuring is the force the floor is exerting on your feet to keep you from falling. We equate that with a gravitational force because we learned from Isaac Newton that a body at rest must have no net force applied to it – the forces must be balanced. But what if we put that scale in an elevator and started accelerating upwards? The floor would have to exert an additional force to accelerate you and the reading on the scale would increase. Similarly, if the elevator started accelerating downwards, your weight would decrease [Note: for real life elevators, this effect is thankfully tiny so don’t bother trying to use it to meet your New Year’s resolutions]. At the extreme if you just let the elevator drop in free fall, the reading on the scale would drop to zero. This is precisely what is happening with the astronauts on the International Space Station – they are continually falling towards the Earth and hence are “weightless”.
Einstein’s Equivalence Principle states that for a single location in space, the effects of gravity are identical to the effects of acceleration. It is only by comparing different locations in space that spacetime curvature, or gravity, can be measured. Picture courtesy James Overduin.
So how can you tell the difference between gravity and an acceleration that seems to mimic it? It turns out you can only do so by making comparisons over some distance. For example, if you release two objects in space at the same altitude but separated by some distance, you’ll notice that as they fall towards the Earth, they move towards one another because the directions each experiences as “down” are not exactly parallel. Both are falling towards the center of the Earth and they slowly come together. These types of effects, which measure the relative motion of two objects in free-fall, are what we use to measure gravity – what Einstein viewed as the curvature of space and time.
Gravitational waves can also be detected by measuring the curvature of spacetime using pairs of objects in free-fall. Last week, the LIGO project made history by announcing the first direct detection of gravitational waves from colliding black holes using their twin interferometers in Louisiana and Washington. For LIGO, the objects used to measure spacetime curvature are mirrors suspended in sophisticated pendulums. While this arrangement does not leave them free fall in the vertical direction, they are free to swing in the horizontal direction and hence able to respond to passing gravitational waves.
The Laser Interferometric Gravitational-Wave Observatory (LIGO) announced the historic first direct detection of gravitational waves from a pair of merging black holes (left). LIGO detected the passing gravitational waves by measuring the distance between carefully suspended mirrors (right). Courtesy Caltech/MIT/LIGO Laboratory
In space, it is possible to let the object be in true free fall. The idea is to take your reference objects, in LISA Pathfinder’s case two 46mm cubes of a Gold-Platinum alloy, and let each drift freely in its own hollow cavity within the spacecraft. Since the spacecraft and test masses are all in free-fall in the same orbit, their relative positions should remain fixed. However there are non-gravitational forces, such as the pressure from sunlight on the spacecraft, that will eventually disturb this balance and cause the spacecraft to drift into the test masses. LISA Pathfinder addresses this challenge using a technique called drag-free control in which the spacecraft tracks and follows the motion of the test masses. In essence, it functions as a flying shield that protects the test mass from external forces while simultaneously providing a platform for the equipment required to track the motion of the test masses.
This approach requires several interesting and challenging technologies: systems for precisely measuring the position and attitude of the test masses, a microthruster system capable of finely guiding the spacecraft, and a control system to keep it all in sync. But it also requires something else: a mechanism to hold the test masses during launch and release them once on orbit. This is a component that is not part of the essential measurement – it only has to do its job once. But it’s critical that it performs.
It turns out the engineering requirements for this mechanism are tough: it must apply high forces to keep the test masses secure during launch, it must precisely position the test masses inside their housings prior to release, it must release the test masses with the slightest residual velocities so that the control system can get to work. It also cannot stick to the soft test mass and no lubricants are permitted as they would disturb the sensitive electronics and optics.
The Gold-Platinum cubes that serve as LISA Pathfinder’s test masses (left) have two sets of geometric features associated with the caging mechanism. The corner features were used by a high-force spring-loaded mechanism to secure the mass during launch. The central feature is used by the grabbing, positioning, and release mechanism (right), which can precisely position and release the test mass so that it can be captured by an electrostatic control system. Copyright RUAG Space, Switzerland.
The LISA Pathfinder team struggled through several designs and re-designs during the development of the project and ultimately settled on a two-stage design. The first stage provided the high forces needed to survive launch while the second stage provided the precise positioning and release function. On February 3rd, the first stage of the mechanism was successfully released, leaving the test masses resting on the second stage. After several days of carefully testing several critical subsystems the team issued the command on February 15th to release one of the test masses, christened “Jake”. The following day, its twin brother “Elwood” was released as well. Yes, we’ve named the test masses after the Blues Brothers. Who says scientists don’t have a sense of humor?
LISA Pathfinder’s test masses ‘Elwood’ and ‘Jake’ we’re released from their caging mechanisms on Feb 15th and 16th with residual velocities of a few tens of microns per second. Neither was pulled over for speeding. Picture courtesy of Martin Hewitson.
