With little more protection thanwhat your family car would provide, the Hubble Space Telescope has endured 12years of bombardment by tiny bits of debris, leaving the craft covered withhundreds of little divots and one gaping hole that illustrate how hostile spacecan be.

Photographs taken during a 1999servicing mission and released last month reveal how frequently Hubble ispeppered by tiny specs of space dust and, more often, bits of metal, paint andother garbage left behind by dead spacecraft and previous missions.

On average, every square meter ofHubble gets hit by about 5 sand-grain-sized bits each year, says debris expertNick Johnson of NASA's Johnson Space Center. Such debris is too small to seeand avoid.

So far, the impacts have notthreatened the telescope's ability to function.

"We're talking about littletiny imperfections that really have no negative impact on the vehicle,"Johnson said in a telephone interview. "You could bring Hubble back andput it in the Smithsonian, and it will look good. But if you look up closethere will be all these little dings in it."

There are no plans just yet tostick Hubble in a museum. Instead, a fourth servicing mission to the telescope,planned for liftoff later this week, will add a new camera to the telescope andtake more pictures of its pit-riddled exterior.

Interestingly, Hubble has survived the slew of minor impacts in spite ofthe fact that it carries little in the way of special protection. TheInternational Space Station and the shuttles are equipped to handlemarble-sized impacts, in order to protect their human cargo. But like mostspacecraft not designed to carry a crew, Hubble's skin is a lot like awell-built car.

"The metal in an old Mercuryis as good as many spacecraft," Johnson says.

Tally of impacts

Pictures released by NASA in a January newsletter show some of the 571dings that were counted in the most recent photographic survey, taken byshuttle astronauts during the 1999 servicing mission. Most of the pockmarks areless than 3 millimeters across, about the size of this "o" on yourcomputer screen.

Johnson said there are probably more than 1,000 such blemishes, anestimate that reflects the fact that not all areas of the spacecraft werephotographed. In addition, many, many smaller dings and scratches surely coverthe spacecraft, but they are not visible with the camera equipment used, hesaid.

Johnson said the worst impact so far was spotted during the first Hubbleservicing mission, back in 1993. "There was a good-sized hole inthe high-gain antenna," he said. "Fortunately it went right onthrough like a cookie cutter" and the antenna continues to work.

Astronauts got a photograph ofthat hole, and it was subsequently measured to be three-quarters of an inch indiameter, a hole shot through a more than a quarter-inch of Space Agehoneycombed composite material with a graphite-epoxy face sheet. Ouch.

NASA officials do not discountthe threat of impacts on their spacecraft. But it's the larger stuff that mostfrightens them. And they call the growing sea of manmade junk -- caused byexplosions, spacecraft impacts, and even nuts and bolts left behind duringmissions -- an increasing threat. The space agency estimates there are 4million pounds of junk orbiting Earth, including more than 100,000 objects 1centimeter (about 0.3 inches) and larger.

An impact by one of these largerobjects could destroy or disable a satellite like Hubble.

A fast moving object can vaporizeon impact, generating a cloud of plasma, or electrically charged particles. Anelectrical current can then flow from one part of the craft, through the plasmacloud, and then destroy an instrument on another part of the craft -- similarto the damage caused by a lightning strike.

A telescope's lens, of course,would not fare will if hit. A camera lens or mirror can be compromised with anyblemish larger than half a millimeter or so.

But the odds of that happening toHubble are extremely small, Johnson said. A long tube designed to shade the Sunalso protects the lens. A particle would have to enter the tube at a perfectangle to make it all the way to the lens. Experience has taught the telescope'soperators which angles to avoid.

All spacecraft at risk

Hubble isn't the only spacecraft that runs into debris. NASA's fleet ofshuttles are hit dozens times on every mission, Johnson said. "They typically leave a little scratchor a crater much smaller than the pebble crater you'll see on your automobilewindshield."

One or two exterior windowpanes are typically replaced after eachmission, however, and dozens of windows have been replaced over the years.

Larger debris sometimes forces mission planners to change the orbitalposition of a spacecraft to avoid a potentially mission-ending collision.

Johnson and his colleagues study the photos of Hubble and the actualimpacts on space shuttles for clues about what causes them. More than half theimpacting debris is manmade, he said. The stuff moves fast, travelling at about10 kilometers per second (22,370 mph). Natural debris typically races along attwice that speed.

Other worries

Photo surveys of Hubble were initially begun so engineers could examineits overall health. Mission officials worry more about the effects of freezingtemperatures and long exposure to the Sun than about impacts.

"There is a cumulative effect caused by solar radiation and solarwind particles, plus the extreme heat and cold which has degraded the thermalinsulation covering the telescope," said David Bretz, a scientist atHernandez Engineering who also works at JSC. "Each servicing mission is anopportunity to photograph and evaluate the degradation and cover or replaceweakened blankets."

If time allows, the crew slated to head to Hubble later this week willplace additional shielding on four of Hubble's equipment bays, Bretz said.