NASA TV

Russian spacecraft stick up from their docking ports on the International Space Station during Saturday's encounter with a piece of space junk. Spacefliers took shelter in their Russian Soyuz lifeboats as a precaution.

The International Space Station's crew members took shelter in their Russian Soyuz lifeboats as a precaution during Saturday's totally harmless passage of a piece of space debris.

In a series of Twitter updates on Friday night, NASA said a hunk of junk from a Russian satellite was projected to fly past the space station at an estimated distance of 14.8 kilometers (9 miles), at around 2:38 a.m. ET Saturday. That was within the zone that required precautionary measures to be taken. The zone is called a "pizza box" because of its shape: 50 kilometers (30 miles) on a side, and 750 meters (a half-mile) above and below the plane of the station.

The appointed time came and went without incident. "Nichevo ... Nothing," one of the Russian cosmonauts said. The spacefliers had hoped to catch a glimpse of the object, but no visual sighting was reported.

NASA said the relatively small piece of debris was a leftover from the 2009 collision involving an Iridium telecommunications satellite and Russia's Cosmos 2251 military communications satellite. It was detected by radar on Friday, sparking the alert.

"Everything went by the book," NASA spokesman Rob Navias said after the all-clear was sounded. He said the station's controllers followed a "precautionary and conservative" approach by ordering the crew to take shelter.

The station currently has six crew members aboard: two Americans (Don Pettit and Dan Burbank), three Russians (Anton Shkaplerov, Anatoly Ivanishin and Oleg Kononenko) and Dutch astronaut Andre Kuipers. The spacefliers were awakened a little more than an hour earlier than scheduled and put the station's control systems into standby mode. Then they took their places in the two Soyuz craft docked to the station and closed the hatches.

The crew members were prepared to descend back down to Earth if the piece of debris had collided with the 450-ton space station and dealt such a serious blow that the orbital outpost had to be abandoned. Instead, they merely reopened the hatches, returned the control systems to their regular settings and resumed a "normal and relaxing weekend," Navias said.

He said it was "serendipitous" that the precautionary measures were taken on the astronauts' day off, meaning that there would be "no impact to scientific research or any other crew work."

Not the first time, or the last
NASA issued a similar collision alert back in November, but called off the alert even before the astronauts' appointed time to get into the Soyuz space capsules. Last June, the crew actually did get into the Soyuz craft due to a collision threat, but the space junk whizzed past at a distance of 850 feet (260 meters). Astronauts took similar precautions in April 2009 and November 2008.

On other occasions, the space station has changed its orbital path slightly to eliminate the risk of collision with space debris. That's how NASA dealt with potential collision threats in January, involving debris from the Iridium satellite as well as from a Chinese satellite that was smashed up in 2007. But in order to use that option, the crew needs more than a day of advance warning.

Experts say there are more than 20,000 pieces of orbiting space junk more than 10 centimeters wide — that is, bigger than a softball. Lots more pieces are smaller, down to the size of a marble. "More than 500,000 pieces of orbital debris are tracked," NASA noted Friday night.

These bits of debris zip around the planet at speeds of 17,500 mph relative to Earth, and could cause serious damage if they were to hit the space station just wrong. NASA and the Defense Department keep close track of the bigger pieces, but the experts are worried that the space-debris problem will only get worse in the years ahead.

All sorts of schemes have been proposed to address the problem, including the idea of shooting water guns or lasers at pieces of space junk, or throwing nets over them. Last month, a Swiss venture announced that they were developing a "janitor satellite" to sweep up the trash. Do you have a better idea? Share it as a comment below. 

Update for 11:05 p.m. ET March 23: I originally said the "pizza box" zone was 25 kilometers on each side, but what I meant to say was that it extends 25 kilometers out from the space station on each side. That means the total dimension of the box is 50 by 50 by 1.5 kilometers, with the station in the center, as NASA explains.

Update for 12:55 a.m. ET March 24: When the space station crew was awakened this morning, Mission Control told NASA astronaut Dan Burbank that the debris was projected to come within 9 miles, which is closer than the initial estimate of 14.3 miles. I updated the figures to reflect that, but even the updated estimates had a measure of uncertainty. That's why the spacefliers took shelter.

More about space debris:

• Junk-tracking 'Space Fence' passes key test
• To control space junk, remove 5 pieces a year
• Time running out to salvage doomed satellite
• Europe creating its own space-debris tracker

Last updated at 3 a.m. ET March 24.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.