Surface Magnetometer (LSM). The ALSEP was powered by a SNAP-27 radioisotope thermoelectric generator, developed by the Atomic Energy Commission. The PSE added to the network of seismometers left by Apollo 12, 14 and 15. NASA intended to calibrate the Apollo 16 PSE by crashing the LM's ascent stage near it after the astronauts were done with it, an object of known mass and velocity impacting at a known location. However, NASA lost control of the ascent stage after jettison, and this did not occur. The ASE, designed to return data about the Moon's geologic structure, consisted of two groups of explosives: one, a line of "thumpers" were to be deployed attached to three geophones. The thumpers would be exploded during the ALSEP deployment. A second group was four mortars of different sizes, to be set off remotely once the astronauts had returned to Earth. Apollo 14 had also carried an ASE, though its mortars were never set off for fear of affecting other experiments. The HFE involved the drilling of two holes into the lunar surface and emplacement of thermometers which would measure how much heat was flowing from the lunar interior. This was the third attempt to emplace a HFE: the first flew on Apollo 13 and never reached the lunar surface, while on Apollo 15, problems with the drill meant the probes did not go as deep as planned. The Apollo 16 attempt would fail after Duke had successfully emplaced the first probe; Young, unable to see his feet in the bulky spacesuit, pulled out and severed the cable after it wrapped around his leg. NASA managers vetoed a repair attempt due to the amount of time it would take. A HFE flew, and was successfully deployed, on Apollo 17. The LSM was designed to measure the strength of the Moon's magnetic field, which is only a small fraction of Earth's. Additional data would be returned by the use of the Lunar Portable Magnetometer (LPM), to be carried on the lunar rover and activated at several geology stops. Scientists also hoped to learn from an Apollo 12 sample, to be briefly returned to the Moon on Apollo 16, from which "soft" magnetism had been removed, to see if it had been restored on its journey. Measurements after the mission found that "soft" magnetism had returned to the sample, although at a lower intensity than before. A Far Ultraviolet