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Scientists were just as elated over what the astronauts had left behind. Performing well at the Ocean of Storms base, ALSEP (for Apollo Lunar Surface Experiments Package) had begun transmitting valuable data even before the astronauts left the moon. For the time being, earth controllers commanded two of the instruments—designed to investigate any traces of lunar atmosphere—to go into a stand-by mode; that would allow earthly gases left within them to bake out during the torrid two-week-long lunar day (maximum temperature: 240° F.). Once freed of these vapors, which interfere with their high-voltage power supply, the instruments will be switched into full operation.
The other three instruments were anything but idle. Radioing data constantly, ALSEP's magnetometer indicated that the moon's magnetic field—which could offer important clues to the lunar interior—may be considerably stronger than had been believed. Palmer Dyal, one of the magnetometer experimenters, had an esoteric, but speculative explanation: after a period of vulcanism, the moon cooled more rapidly than scientists had heretofore thought, thus preserving a larger portion of its primordial magnetic field.
Echo Chamber. The solar-wind spectrometer was also working well, even though it had, for the moment, little to detect; the moon was passing through the earth's magnetic tail (April 22, 1966), which shielded the lunar surface from the high-velocity solar particles that normally bombard it. Meanwhile, the seismometer had recorded an unexplained, two-minute tremor. And scientists were still trying to explain the strange vibrations recorded for 55 minutes by the instrument immediately after Intrepid's ascent stage impacted into the Ocean of Storms.
Although these bell-like reverberations were unlike any seismic event on earth, Columbia University Geophysicist Gary Latham offered a plausible explanation. The effect may have been caused, he said, by a layer of rubble or fractured rock sandwiched between bedrock in the floor of the Ocean of Storms and a solid cover of fine material deposits above. Lacking dampening fluids or gases, the layer of rubble may have acted as an echo chamber in which the seismic waves reverberated. If so, the next big seismic event on the moon should be a scientific spectacular; the third-stage rocket of Apollo 13's Saturn 5 will be sent crashing into the lunar surface, creating an impact equivalent to the explosion of 8½ tons of TNT.
The astronauts' own splashdown was far more gentle. At the end of a three-day homeward flight that was uneventful to the point of boredom, Yankee Clipper dropped into the choppy Pacific 405 miles southeast of American Samoa—exactly 10 days 4 hr. and 36 min. after its lightning-marred launch from Cape Kennedy. The landing was only 13 seconds off schedule and only 2.6 miles from its target near the bow of the Hornet. Even so, there was a moment of tension. Drifting down under its three big orange-and-white chutes in full view of a worldwide TV audience, Yankee Clipper suddenly seemed to be billowing smoke—a sight that was ominously reminiscent of the fatal Apollo fire in 1967. In this case, however, Skipper Conrad was simply venting surplus fuel, an operation usually performed at a higher altitude.