SPACE CENTER, HOUSTON (AP) -- Apollo 13's astronauts today sped quietly and flawlessly toward a Tuesday rendezvous with the moon while ground controllers tracked a rocket stage headed for an explosive impact on the lunar surface.

James A. Lovell Jr., Fred W. Haise Jr. and John I. Swigert Jr. retired early today for 10 hours rest. Mission Control planned to let them sleep past noon.

Medics reported that Lovell, the only one of the three wired for medical monitoring slept soundly.

Trailing about 900 miles behind Apollo 13 was the third stage of the Saturn 5 rocket that hoisted the astronauts away from planet earth Saturday.

Signals form the ground shifted the 61-foot stage to a collision course with the moon. Mission Control said tracking data today shows it will strike the surface about 8:10 p.m. (EST) Tuesday about 105 miles west of a seismometer left on the moon by the Apollo 12 astronauts in November.

That is about half an hour after Apollo 13 is to fire into moon orbit. The astronauts won't see the impact because they will be on the far side of the moon.

Seismic experts estimate the 15-ton stage will smash with a force equal to 11 tons of TNT, and that it will gouge a crater 100-120 feet deep. The resulting tremors, recorded by the seismometer should tell scientists much about the internal structure of the moon.

The Apollo 12 spacemen deliberately crashed their lunar lander after they left the moon and the impact vibrated the seismometer a surprising 55 minutes, indicating the subsurface material is broken up. A similar explosion on earth would reverberate only a few minutes.

The Apollo 13 astronauts were relaxing on the three-day outward journey in preparation for four grueling days in the vicinity of the moon.

Preparation for Wednesday's lunar landing begins tonight when Lovell and Haise crawl through a connecting tunnel to inspect the lunar craft they call Aquarius.

They'll check communications, propulsion and other systems to make sure the spindly legged lander is ready for the difficult and dangerous descent to the moon's ancient Fra Mauro highlands Wednesday night.

In this rugged area on the eastern shore of the Ocean of Storms, Lovell and Haise plan to deploy a nuclear-powered science station, drill 10 feet beneath the lunar crust and trek nearly two miles on a geology field trip. They hope to gather rocks dating back some five billion years.