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Gravity Probe B Webcast - Spacecraft Q&A
Question and Answer Board

Blaxe from Deruyter
What is the aerodynamic of the space craft and how did you know what was the right design?
During the trip to orbit, our spacecraft is actually sheltered in the nose cone of a Boeing Delta II rocket... How do we know it's right? Rockets of this design have flown many payloads in the past and so we expect a safe trip into orbit.

This brings up another very interesting point, which is that when you're up in space, you think there is no air and so aerodynamics don't matter. But actually, there is a very minute amount of air -- something like a hundred million times less, maybe a billion times less -- but a very small amount of air, and that'll tend to gradually slow down our spacecraft.

This happens to all spacecraft, but for us this actually matters. So we have to fire very gentle, almost like a human breath, jets of helium gas to exactly compensate for the effect of the slowing down by the spacecraft in the small amount of air at that altitude.
Ang Yi Ci from Singapore
Will the spacecraft and the telescope be damaged easily and why did you made it to near round shape?
The spacecraft and the telescope and the rest of the hardware were constructed to very exacting standards. There's a whole lot of testing and design work to build them to the robust standards so they can withstand a trip to orbit. There's a lot of vibration and things that happen on the trip up.

So in the end, we have a very durable design and also one that's very well tested, so the telescope and hardware are in fact very durable.

The second part of the question, why did we make it near-round...I'm not exactly sure for this question exactly what part of our spacecraft they're talking about, but of course the thing we're most proud of in terms of GP-B is how round the spheres are in the gyroscope.

They're the most round things ever made by man, they're about the size of a ping pong ball. Why are they round? That's a good question. To help understand it, I think I'm going to go to the most simple type of gyroscope that people seem to have some kind of intuitive feel for, and that's a simple children's top.

When you spin the top up, you put it on the table and it starts to wobble almost immediately, and that wobbling is interfering with its ability to be a good gyroscope. So it's important for us to have as small as possible a wobble as can be achieved, and there are two ways to do that. Number one, you can eliminate the forces on the top, and we do that by going into space. That's why it's necessary to do GP-B in space, so you can get rid of the force of gravity.

The second part is you can make the design of the top symmetrical in such a way so there's not much for those forces to pull on to cause a wobble. So, it's the beautiful symmetry of the sphere which makes it very robust with respect to external forces causing a wobble.
Rich from Elk Grove, Illinois
How many gyroscopes are in the GPB? What is the procedure if one or more of the gyroscopes would fail?
We have four gyroscopes in GP-B. Now each of those gyroscopes is capable of performing the science measurement. Each of them by itself will perform a measurement to the accuracy needed to verify Einstein's theory of relativity or to disprove it, if that's how the data comes out. We have times for redundancy for that.

With that said, you can combine the results from the different gyroscopes and achieve a slightly more accurate test, so we obviously don't expect the gyroscopes to fail. We have something like 100,000 hours of test data across a variety of gyroscopes, flight design or close to flight design. That's about 50 man years, so we have high confidence in the design of the gyroscopes, but if something were to happen, there are times for redundant.
John from Miami
When and if it accomplishes its mission will it be used for other missions?
GP-B, after the completion of its mission, will not be used for another mission. But interestingly enough, there are several other experiments that we can do at the same time we're taking the relativistic data.

For example, in the process of doing the experiment it will map out the Earth's gravitational field to incredible accuracies to tell you things about the Earth's shape, its density variation, so it turns out those things are interesting to scientists here on Earth.

Another experiment it will do which astronomers are interested in is, we look at our guide star for a year. There's never been a time when someone has trained a telescope and looked at the same star for a year, so people are interested, astronomers are interested in that data.

I'll also point out that the technology we developed is being used for a variety of other experiments. I won't go into the details, but if people want to look up on the Web or something, they can look at STEP, or Satellite Tests of the Equivalence Principle antenna. There's a variety of ways the technology is being used for further science.
Geoffrey from Hong Kong
When will this mission end and where it will fly?
We have a 16-month mission. That's two months of initial setup, about 13 months of science data and one month for post-science calibration. After that, there should be another few months of helium left in the dewar so we can do some more experiments.

Where does it fly? Right now, it's in orbit around the Earth in a polar orbit about 650 kilometers up. So what is a polar orbit? It orbits the Earth this way so that it goes over the North Pole and under the South Pole
Heidi from Raleigh
How long is your probe going to be in space and how does it land back on Earth?
We actually don't land back on Earth. After the experiment is over, it will continue to orbit the Earth and eventually, of course, because of this force of atmospheric drag I talked about earlier, it'll slow down gradually and like all satellites around Earth over the decades it will eventually break up and reenter and become a series of shooting stars. That's decades away, but eventually that will happen.
Fabian from Bronx
How will the GPB collect its data and how will the data prove or disprove the theories being tested?
Good question. The way we collect the data in GP-B is we have these four spheres that are spinning and you measure the direction of the spin axis, or the line about which the sphere is spinning, and you compare that line to the line of sight to a distant guide star.

So we can tell the spin axis of the gyroscope using one of the most sensitive detectors ever built called the SQUID-magnetometer -- SQUID for Superconducting Quantum Interference Device -- and the line of sight to the guide star is measured with a telescope that's at about two degrees Kelvin or two degrees above absolute zero.

So you measure those two things and you difference them, and you can tell the angle that the gyroscope's spin access makes to the guide star. Einstein's theory says that over time, that will change in a very particular way. So we compare our measure with what the theory says and that proves or disproves the theory.
Kevin Lindsey from New Mexico
How will the Gravity B spacecraft's mission change how we view time and space? How long will it take before the result are given to the public?
I think I'm going to answer the second part first. As I said, we've got about a 16-month mission. After that, it's going to take about a year to perform all the analysis, so at the end of the year, the results get published and the data becomes public. So about two years to two and a half years or something like that from now.

The first part, how will it change our views of space and time... You know, Einstein's theory says some pretty funky things. It says that the Earth is warping the fabric of space and time and it's twisting it a little bit as it's rotating, and that twisting part has never been observed in any controlled experiment before. So that's one of the very interesting things we'll see.