NASA鈥檚 Gravity Probe B mission has blasted off successfully and has begun a 44-day period in which it gears up to test Einstein鈥檚 theory of gravity with exquisite precision.
The three-tonne probe survived the riskiest part of its mission 鈥� the launch 鈥� when a Delta II rocket lifted off from Vandenberg Air Force Base in Southern California at 1757 GMT on Tuesday. The original launch date on Monday was postponed for a day due to uncertainties about high-altitude winds.
The craft鈥檚 solar panels unfolded about 71 minutes after launch, and the probe separated from the rocket鈥檚 second stage, which had been propelling it upward, about four minutes later.
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鈥淭he solar arrays are deployed, and we have received initial data that indicates all systems are operating smoothly. We are very pleased,鈥� said NASA mission manager Rex Geveden.
Frame dragging
According to the theory of general relativity, the gravity of a massive, spinning body can drag space-time along with it. Gravity Probe B houses four gyroscopes 鈥� the most perfect spheres ever created 鈥� that could measure the 鈥渇rame dragging鈥� caused by Earth. The effect would be seen as small changes in the gyros鈥� spin axes relative to a distant guide star.
About a day after launch, the probe will begin to lock on to its guide star using software and thrusters. Over several days, the thrusters will also place the craft in a circular, polar orbit 640 kilometres above the Earth鈥檚 surface.
But the most time-consuming part of the approximately 44-day 鈥渋nitialisation and orbit checkout phase鈥� involves the precise calibration of the gyros.
Beginning 22 days after launch and lasting for several weeks, bursts of helium gas will set each of the gyros spinning at ever faster rates, culminating at a rate of about 9000 rotations per minute.
Burned off
On day 38 after launch, the gyros鈥� spin axes will be aligned toward the guide star, a process that could take five days.
The final step before the spacecraft is ready for its year-long science phase requires the helium in the gyros to be removed, leaving the instruments in a vacuum. The gas is 鈥渂urned鈥� off by briefly raising the temperature of the gyros from -271掳 Celsius to -266掳C.
鈥淭his is a great moment and a great responsibility, the outcome of a unique collaboration of physicists and engineers to develop this near-perfect instrument to test Einstein鈥檚 theory of gravity,鈥� said Francis Everitt, a physicist at Stanford University in California who has led the project over its record 40-year gestation.
The $700 million mission will send back information about the gyros spin axes and spacecraft performance at least twice daily, and scientists expect it will take at least a year to analyse the data when the mission is complete.