Brad Fitzpatrick (brad) wrote,
Brad Fitzpatrick

More Mars Craziness
To land in a precise location on Mars after traveling over 300 million miles, navigators at the Jet Propulsion Laboratory (JPL) had to overcome the head-spinning challenges of calculating the exact speeds of a rotating Earth, a rotating Mars, and a rotating spacecraft, while they all simultaneously are spinning in their own radical orbits around the Sun.


, but even the seemingly insignificant solar radiation pressure and thermal radiation forces acting on the spacecraft to a level equal to less than a billionth of the acceleration of gravity one feels on the Earth need to be taken into account. Without knowing the acceleration error to that degree, the spacecraft would have moved off course by 3.7 km (2.3 miles) over 10 days.


"We had to know everything from how the iron molten lava in the center of the Earth was churning to how plate tectonic movements were affecting the wobble of the Earth to how the plasma in the atmosphere delayed the radio signals to and from the Deep Space Network stations,


Three DSN sites are roughly equally spread around Earth's globe at 120-degree intervals, so that antennas are pointed toward Mars at any given time as the Earth turns. If the exact location of any of these antennas is incorrect by just 5 centimeters (2 inches) on the surface of Earth, that math error builds over the 150 million kilometers (90 million miles) distance between Earth and Mars, creating a 1500-foot (0.3-mile) location error at Mars.


The navigation team also used a tongue-tying tracking technique called spacecraft-quasar delta differential one-way range or DDOR (pronounced "Delta Door"), which utilized their knowledge of locations of quasars to a few billionths of a degree to help locate the spacecraft's motion in the "up or down" direction in the sky.
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