[meteorite-list] How Hubble Has Survived a Decade of Impacts

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:54:10 2004
Message-ID: <200202261620.IAA14592_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/astronomy/hubble_impact_020226.html
      
How Hubble Has Survived a Decade of Impacts
By Robert Roy Britt
space.com
26 February 2002
              
With little more protection than what your family car would provide,
the Hubble Space Telescope has endured 12 years of bombardment by
tiny bits of debris, leaving the craft covered with hundreds of
little divots and one gaping hole that illustrate how hostile space
can be.
 
Photographs taken during a 1999 servicing mission and released last
month reveal how frequently Hubble is peppered by tiny specs of space
dust and, more often, bits of metal, paint and other garbage left
behind by dead spacecraft and previous missions.
 
On average, every square meter of Hubble gets hit by about 5
sand-grain-sized bits each year, says debris expert Nick Johnson of
NASA's Johnson Space Center. Such debris is too small to see and
avoid.
 
So far, the impacts have not threatened the telescope's ability to
function.
 
"We're talking about little tiny imperfections that really have no
negative impact on the vehicle," Johnson said in a telephone
interview. "You could bring Hubble back and put it in the
Smithsonian, and it will look good. But if you look up close there
will be all these little dings in it."
 
There are no plans just yet to stick 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 and take more
pictures of its pit-riddled exterior.
 
Interestingly, Hubble has survived the slew of minor impacts in spite
of the fact that it carries little in the way of special protection.
The International Space Station and the shuttles are equipped to
handle marble-sized impacts, in order to protect their human cargo.
But like most spacecraft not designed to carry a crew, Hubble's skin
is a lot like a well-built car.
 
"The metal in an old Mercury is as good as many spacecraft," Johnson
says.
 
Tally of impacts
 
Pictures released by NASA in a January newsletter show some of the
571 dings that were counted in the most recent photographic survey,
taken by shuttle astronauts during the 1999 servicing mission. Most
of the pockmarks are less than 3 millimeters across, about the size
of this "o" on your computer screen.
 
Johnson said there are probably more than 1,000 such blemishes, an
estimate that reflects the fact that not all areas of the spacecraft
were photographed. In addition, many, many smaller dings and
scratches surely cover the spacecraft, but they are not visible with
the camera equipment used, he said.
 
Johnson said the worst impact so far was spotted during the first
Hubble servicing mission, back in 1993. "There was a good-sized hole
in the high-gain antenna," he said. "Fortunately it went right on
through like a cookie cutter" and the antenna continues to work.
 
Astronauts got a photograph of that hole, and it was subsequently
measured to be three-quarters of an inch in diameter, a hole shot
through a more than a quarter-inch of Space Age honeycombed composite
material with a graphite-epoxy face sheet. Ouch.
 
NASA officials do not discount the threat of impacts on their
spacecraft. But it's the larger stuff that most frightens them. And
they call the growing sea of manmade junk -- caused by explosions,
spacecraft impacts, and even nuts and bolts left behind during
missions -- an increasing threat. The space agency estimates there
are 4 million pounds of junk orbiting Earth, including more than
100,000 objects 1 centimeter (about 0.3 inches) and larger.
 
An impact by one of these larger objects could destroy or disable a
satellite like Hubble.
 
A fast moving object can vaporize on impact, generating a cloud of
plasma, or electrically charged particles. An electrical current can
then flow from one part of the craft, through the plasma cloud, and
then destroy an instrument on another part of the craft -- similar to
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 any blemish larger than half a
millimeter or so.
 
But the odds of that happening to Hubble are extremely small, Johnson
said. A long tube designed to shade the Sun also protects the lens. A
particle would have to enter the tube at a perfect angle to make it
all the way to the lens. Experience has taught the telescope's
operators which angles to avoid.
 
All spacecraft at risk
 
Hubble isn't the only spacecraft that runs into debris. NASA's fleet
of shuttles are hit dozens times on every mission, Johnson said.
"They typically leave a little scratch or a crater much smaller than
the pebble crater you'll see on your automobile windshield."
 
One or two exterior windowpanes are typically replaced after each
mission, however, and dozens of windows have been replaced over the
years.
 
Larger debris sometimes forces mission planners to change the orbital
position of a spacecraft to avoid a potentially mission-ending
collision.
 
Johnson and his colleagues study the photos of Hubble and the actual
impacts on space shuttles for clues about what causes them. More than
half the impacting debris is manmade, he said. The stuff moves fast,
travelling at about 10 kilometers per second (22,370 mph). Natural
debris typically races along at twice that speed.
 
Other worries
 
Photo surveys of Hubble were initially begun so engineers could
examine its overall health. Mission officials worry more about the
effects of freezing temperatures and long exposure to the Sun than
about impacts.
 
"There is a cumulative effect caused by solar radiation and solar
wind particles, plus the extreme heat and cold which has degraded the
thermal insulation covering the telescope," said David Bretz, a
scientist at Hernandez Engineering who also works at JSC. "Each
servicing mission is an opportunity to photograph and evaluate the
degradation and cover or replace weakened blankets."
 
If time allows, the crew slated to head to Hubble later this week
will place additional shielding on four of Hubble's equipment bays,
Bretz said.
Received on Tue 26 Feb 2002 11:20:52 AM PST