Artist's conception of the entry of Curiosity into the atmosphere of Mars. NASA/JPL-Caltech image PIA 14835.
Deceleration as a function of time since atmospheric entry (gray line) during the arrival of Pathfinder at Mars in 1997. The black line shows an exponential fit to a portion of the gray curve. This fit, which has a characteristic timescale of 4.3 s, illustrates that Eq. (5) holds well for this mission. In the data, a maximum deceleration of 155 m/s2 occurs at 114 seconds, parachute deployment occurs at 209 s, and first impact occurs at 336 s. (Data source: NASA Planetary Data System dataset MPAM_0001.)
Prior to parachute deployment or retrorocket ignition, the speed of a spacecraft of mass m depends on the mass of atmospheric gas that is swept up by the spacecraft. The six solid lines show how the ratio of speed v to initial speed vu depends on altitude z (in units of scale height H) for different values of . Here MA is the mass of a vertical atmospheric column above altitude z = 0 that has an area equal to the spacecraft area, and is the angle between the spacecraft velocity and the vertical, so that is the mass of atmospheric gas swept up by the spacecraft by altitude z = 0.
A view from Mars orbit of Curiosity descending on its parachute. NASA/JPL/University of Arizona ESP_028256_9022.
Closeup view of the damaged Genesis capsule after its return to Earth. NASA/JPL-Caltech.
The altitude z and speed of a spacecraft descending on a parachute at terminal velocity are related to the atmospheric scale height H, the terminal velocity at parachute deployment vp , and the time since parachute deployment. Here zp is the altitude of parachute deployment so is the change in altitude since parachute deployment; similarly, tp is the time of parachute deployment so is the time since parachute deployment. We plot dimensionless altitude [ , solid line] and dimensionless velocity ( , dashed line) as functions of dimensionless time since parachute deployment [ ].
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