Bam!—Apollo 15 Lands on the MoonThe first word spoken on the surface of the Moon after Apo...

Bam!—Apollo 15 Lands on the Moon

The first word spoken on the surface of the Moon after Apollo 15 landed on July 30,1971, was “Bam!” This was James Irwin’s involuntary reaction to their rather bone-jarring touchdown. “We did hit harder than any of the other flights!” says Irwin. “And I was startled, obviously, when I said, “Bam!’˝

The reason for the “firm touchdown” of Apollo 15, as pilot David Scott later characterized it, was that the rocket engine was shut off a bit earlier than planned, when the lander was still 4.30 ft above the lunar surface and moving downward with a speed of 0.500 ft/s. From that point on the lander descended in lunar free fall, with an acceleration of 1.62 m/s2. As a result, the landing speed of Apollo 15 was by far the largest of any of the Apollo missions. In comparison, Neil Armstrong’s landing speed on Apollo 11 was the lowest at 1.7 ft/s—he didn’t shut off the engine until the footpads were actually on the surface. Apollos 12, 14, and 17 all landed with speeds between 3.0 and 3.5 ft/s.

To better understand the descent of Apollo 15, we show its trajectory during the final stages of landing in Figure 2-37 (a). In Figure 2-37 (b) we show a variety of speed-versus-time plots.

FIGURE 2-37

Suppose, instead of shutting off the Engine, the astronauts had increased its thrust, giving the lander a small, but constant, upward acceleration. Which speed-versus-time plot in Figure 2-37 (b) would describe this situation?

A B C D