Investgators into the accident quickly focused their attention on events that transpired during the launch of Columbia on 16 January 2003. About 80 seconds after liftoff, at least one chunk of foam from the Shuttle's External Tank was observed to break off the tank and impact near the leading edge of Columbia's left wing. Photos of this dislodged foam and the post-impact debris are shown below.
Investigators into the Columbia accident have estimated that the dislodged foam was about 19 by 11.5 by 5.5 inches (48 by 29 by 14 cm), weighed about 26.7 ounces, or 1.7 lb (0.75 kg) and impacted the Shuttle at nearly 530 mph (850 km/h). For the sake of a rough comparison, this block of foam would be about the same size and weight as a large loaf of bread.
However, no one had ever suspected that a piece of foam this size could actually do any significant damage to the sturdy reinforced carbon-carbon (RCC) material out of which the Orbiter's wing leading edges are composed. This material is one of the strongest used anywhere on the vehicle and is designed for great resistance to the intense heating encountered during re-entry. While much of the rest of the Orbiter is also covered in thermal protection tiles, the white and black tiles that cover most of the surface are made of a ceramic material that is easily damaged by impact. The gray RCC material used on the leading edges, however, is a much thicker and heavier substance with significantly greater resistance to impact damage.
Yet photographic evidence indicates that a large chunk of foam broke away from the External Tank and impacted somewhere along these RCC panels. To investigate the damage this kind of impact might have, RCC panels from the Orbiter Discovery were placed in a NASA test facility and subjected to a series of impacts similar to those it is believed Columbia experienced. After firing a 1.7 lb (0.75 kg) block of foam from a nitrogen gas-powered cannon at a speed of 530 mph (850 km/h), the foam slammed into the slate gray RCC panels with a force of 4,500 lb (20 kN). The first test created a 3-inch (7.6 cm) crack extending from the surface of the panel, through the 1/3-inch (8.5 mm) thick RCC material, into a T-seal joint that seals the gap between adjacent panels, and through a rib reinforcement that attaches to the wing structure.
That brings us to the heart of your question--how could a piece of the Shuttle break off and accelerate so quickly that it would impact the Orbiter at a speed of more than 500 mph? Your speculation that the piece of foam has an initial forward velocity equal to that of the Shuttle does have some merit to it. However, what you are forgetting is the concept of momentum. Momentum is defined as the product of an object's mass and velocity (i.e. p=mv). In other words, momentum can be thought of as "mass in motion." An object may have a very high speed, but if it is very lightweight, its momentum will be relatively small. That principal is key to understanding why the foam accelerated in a new direction so quickly relative to the moving Shuttle.
The Shuttle was traveling at a speed of about 1,570 mph (2,525 km/h) when the foam broke away, and that speed would have been the initial "upward" velocity of the foam. Even so, the material would have had a relatively small forward momentum due to its low weight. As a result, the upward motion was rapidly decelerated by the high speed airflow impacting against the foam. This airflow quickly began accelerating the foam in the opposite, downward direction. Under the influence of the local aerodynamic flowfield, the foam was then directed into Columbia's left wing at a speed of about 530 mph.
Had the foam instead been a much heavier item, its upward momentum would have been much greater and it would have continued to travel upward for a much longer period before aerodynamics and gravity reversed its motion. An example that illustrates this point is the release of a bomb from an aircraft. Consider a plane flying straight and level at a given velocity. When a bomb is released, it will also continue traveling forward in the same direction as the plane. However, the "downrange" distance it travels is directly related to its weight, or its momentum. This factor must be taken into account by the pilot (or aircraft software) or the bomb will not be able to reach its intended target. If released too early, the bomb will run out of momentum and fall short. If released too late, the bomb's momentum will carry it beyond the target. In addition, a heavy, 2,000 lb (905 kg) bomb has far more momentum than a light 500 lb (225 kg) weapon. In other words, the lighter bomb is decelerated by the opposing airflow much more rapidly and cannot fly as far as the larger bomb. Therefore, the pilot must release the heavier bomb at an earlier time in his bombing run than he would release the smaller one.
In order to relate this explanation to objects we are familiar with in our daily lives, let's briefly return to the
loaf of bread analogy drawn earlier. Consider for a moment that you are driving your car down the highway at
65 mph (105 km/h). You roll down your window and gently toss out a loaf of bread. Now does that loaf match your
speed and continue traveling forward at the same rate as your car? Of course not. It is rapidly slowed by the
oncoming air flow and then accelerated backwards in the opposite direction. The same principal holds true in the
case of the foam shed by the Space Shuttle. In that case, however, the Shuttle was traveling nearly 25 times
faster than your car was zooming down the highway, so that acceleration will obviously be much greater.
- answer by Greg Alexander, 6 July 2003
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