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The Webfooted Astronomer - November 2000
The Story of L-DEF
By Pat Lewis
SIXTEEN years ago, in April 1984, NASA launched into orbit a big satellite carrying several kinds of experiments for exposure to space conditions. One group of experiments was to test the effect of radiation on millions of plant seeds, living biological tissue. Another group tested construction materials and painted surfaces, because NASA was in process of designing a project then called Space Station Freedom. Engineers knew that space environment is harsh. Which materials would endure it best?
The Earth's atmosphere extends upward, becoming thinner and thinner, for about 200 miles. Spacecraft orbiting above that height are generally free from friction with air, but a few molecules do get scattered higher. Among these are molecules of oxygen, O2. Strong ultraviolet light from the Sun splits these molecules into individual oxygen atoms, which are highly reactive and can attack metallic and painted surfaces. In 1984 this chemical hazard was known but not well understood. In addition, satellites are threatened by radiation, meteoroids, temperature changes, and debris in space. NASA needed to test all kinds of materials.
To carry the test samples, a large satellite was built, the Long-Duration Exposure Facility, L-DEF, pronounced ell-deaf. It was fairly inexpensive as spacecraft go, costing only $14 million, but it was 30 feet long and as big as a city bus. It would just fit into the cargo bay of a space shuttle. Shaped like a cylinder with flat panels along the sides, it carried dozens of removable trays to hold samples. It had a socket that could be grasped in space by the robot arm of the shuttle. Loaded with over a hundred different kinds of seeds, and panels of all sorts of building materials, L-DEF was taken up into space and released at an altitude of 295 miles above the Earth.
The original plan was to keep it in orbit for a year, then send a shuttle to retrieve it. But some shuttle flights had to be postponed, and L-DEF was still in orbit on that terrible day in 1986 when the Challenger blew up. After that the space program was badly disrupted. No more shuttles flew for over two years. Stranded in space, the big satellite orbited on for over five years. It was okay, although the seeds and samples were getting a lot more exposure than anyone had planned. But then a crisis arose, threatening the mission from an unexpected source, the Sun.
Our slightly variable star has a repeating cycle of activity. Every 11 years it reaches a maximum, with an increase in solar storms and emissions that affect Earth's atmosphere. In 1989 the Sun erupted in violent storms. Earth's upper atmosphere became heated, and expanded upward to slow L-DEF with friction. It began to sink earthward. By January it was losing half a mile a day. Scientists calculated that only a few weeks were left before it would plunge down and burn up in the atmosphere. All the precious experiments would be lost.
A rescue mission was scheduled, and the shuttle Columbia went up to rendezvous with L-DEF at an altitude of only 205 miles. As the astronauts approached the satellite they had to avoid contaminating it with exhaust from the shuttle's engines. They flew above it, turned upside down and maneuvered gently closer. Through the portholes the crew saw an amazing sight. The satellite showed the effects of a terrific beating. Small craters riddled the panels. Sunshields hung in tatters. One solar panel was broken completely off, and loose pieces drifted in space. The astronauts reached out with the shuttle's robot arm and delicately made contact. L-DEF was captured. Soon, enclosed in the shuttle's cargo bay, it was back safe on Earth.
In a special clean room in Florida, technicians unloaded it. Wearing elaborate operating-room suits, they first removed trays of seeds that had been exposed to 5,000 times normal radiation in almost six years. There were 12½ million tomato seeds. Three million school children all over the country were primed to receive these seeds and plant them that spring, watching for any mutations that might have been caused by radiation. To the surprise of scientists, almost no changes were observed. Just one plant developed variegated leaves partly deficient in chlorophyll. When the plants produced tomatoes, many growers were afraid to eat them, but a hog fed on them won first prize in a show in Turlock, California.
Especially interesting were the effects on construction materials – various thermal-control coatings, plastic and resin films and composites. Some resins and plastics were completely eaten away, probably by atomic oxygen. Many samples became cracked, peeled or discolored, diminishing their ability to resist heat. Maintenance of thermal control is essential in a spacecraft; without it, the craft might just keep getting hotter and finally burn up. The engineers now know some materials to avoid. Other experiments gave light on one of the most troublesome problems of space flight – collisions with cosmic particles and man-made space debris. Panels were pitted with tiny craters. Dust-sized particles were caught in collectors built by Dr. Don Brownlee of the University of Washington.
High-energy cosmic rays, fast-flying nuclei of heavy elements, are a hazard to astronauts. L-DEF carried more than 50 canisters designed to intercept cosmic rays and investigate where they came from, the relative abundance of these nuclei in space, the processes in stars by which they are created, and how to reduce their danger to humans.
Among displays at the University of Washington's 1999 Astronomy Open House, I saw a piece of aluminum with a little crater displayed under a microscope. It was a thrill to realize that the bit of metal had been exposed for almost six years in space, where this small but powerful impact had occurred.
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