That may change if the Bush administration succeeds in its hotly debated plan to develop space-based missile defense systems. As the Drell Lecture promotes examination of issues in international policy, Ride took the opportunity to explore technical issues that might arise if weapons were put into orbit.

"The policy questions going forward might be sort of simplistically stated as: Does it make sense for us to put weapons into space? Does it make sense to develop anti-satellite weapons, or A-SATs?"

A-SATs would be "disastrous," she said, because the space debris they would create could damage the many satellites traveling in low-Earth orbit -- 150 to 400 miles above the Earth. "It's a very, very typical orbit," Ride said. It is where the space shuttle, the space station, photoreconnaissance satellites and some navigation satellites circle.

"Anything in low-Earth orbit is traveling really, really, really fast," she said. "The space shuttle travels 17,500 miles per hour, and anything in that same orbit has to, Newton says, travel at that same speed."

Ride asked: "What if anti-satellite testing proceeds and we start testing rockets that clobber satellites and explode them in space?

What if enough of that goes on that there's the equivalent to a test range up in low-Earth orbit?" Each explosion would create billions of pieces of space debris orbiting at 5 miles per second just waiting to run into a satellite.

Ride recalled a run-in with space debris on her first shuttle flight.

"About halfway through the flight there was a small pit in the window of the space shuttle and we didn't know what it was. An awful lot of analysis was done while we were in orbit to make sure that the strength of the window would sustain reentry. It did. We were all fine.

"But the analysis afterward showed that our window had been hit by an orbiting fleck of paint, and the relative velocities were enough that the paint actually made a small but visible gouge in the window. Well, a fleck of paint is not the same as a small piece of metal traveling at that same speed. So, as soon as you start increasing the amount of junk in a low-Earth orbit, you have an unintended byproduct that starts putting some of your own quite valuable satellites at possible risk."

Today, satellites provide communication, reconnaissance, navigation and monitoring. Intelligence agencies and the military currently use about 100 satellites as part of our national security, Ride said.

They capture images during the day in visible light, at night in infrared light or through clouds using radar. They monitor global hotspots, pinpoint missile launch points for early warning, communicate enemy activity and allied actions. A satellite that in peacetime uses a global positioning system for navigation may in wartime target bombs or remotely piloted vehicles.

"The current landscape is that the United States has an absolutely huge advantage over every other country in space capabilities," Ride said. "It's hard getting to space. It's hard developing things that work in space, and it's really, really hard to get things to work reliably over long periods of time in space."

On the other hand, many countries have sophisticated rockets that can launch objects into space. Many operate satellites. And China expects to launch a person into space within the next year.

Ride attended Stanford, where she received bachelor's degrees in physics and in English in 1973, and master's and doctoral degrees in physics in 1975 and 1978, respectively. She flew aboard the space shuttle Challenger in 1983 and 1984, and served on the presidential commission investigating its subsequent explosion in 1986. During the lecture, she gave a quick overview of the space race, shared her space photos and addressed the capabilities and limitations of satellites and other space assets used for national security.

Her photos unflinchingly looked down the eye of a typhoon in the Indian Ocean, chronicled massive erosion in Madagascar and showed dramatic differences in water reclamation at the Egyptian-Israeli border. But one photo, of the Earth's horizon, was striking because it revealed stakes that transcend the security of any one nation.

"If you take a look at where the blackness of space ends and Earth begins," Ride pointed out, "you'll see a very, very, very thin royal blue line. That thin royal blue line is Earth's atmosphere. That's all there is of it. That's all that separates everything we know on our planet from the vacuum of space. And it's a very, very striking sight to every astronaut the first time he or she looks off toward the horizon because it just drives home that we live on a planet, and our planet is very fragile."

The Drell Lecture is named for physicist and arms control expert Sidney Drell, with whom Ride worked as a CISAC Science Fellow when she left NASA in the late 1980s to reenter academia.

Ride currently is a physics professor at the University of California- San Diego, director of the University of California's Space Institute, and president and chief executive officer of Imaginary Lines, a company she founded to support girls' natural interests in science and technology.

The thin blue line during the Cold War

Above the thin blue line is where the military has cast its eyes ever since 1957, when the Soviets launched the Space Age with the first satellite, Sputnik, a celestial basketball containing little more than a radio transmitter. It beeped incessantly as it circled the Earth every 100 minutes or so -- a constant reminder to Cold War America that the archenemy it considered technologically inferior was not.

Sputnik had a huge psychological impact on the United States, Ride said. "It had enormous consequences for the course of the country and also for the course of our national security."

Sputnik was launched by a powerful and sophisticated rocket, and once a country developed such a rocket, it had the makings of an intercontinental ballistic missile. "When you lift something with a rocket and push it into orbit, all you need to do is give it a little bit lower push. Instead of going into orbit, it'll land on your adversary half the world away. That fact was not lost on anyone in the United States."

Awakened to their vulnerability, Americans responded with an urgent call for science education reform, the creation of NASA and new space-oriented military programs.

Space became the measure of a superpower, Ride said, and the Russians continued to pull off a few consecutive firsts: Sputnik II quickly followed the first Sputnik with the launch of a dog. Meanwhile, Americans suffered "a series of reasonably spectacular and very public failures," Ride said. Before a television audience, rockets blew up sensationally on the launch pad.

The first U.S. satellite finally went into orbit in 1958; it discovered Earth's radiation belts. But that achievement was quickly eclipsed when Soviets launched the first person, Yuri Gagarin, into space on April 12, 1961. When Americans launched Alan Shepard into space a month later, the Soviets were quick to point out that Shepard's journey was only a suborbital flight, as the rocket that launched him wasn't powerful enough to put him into orbit.

President Kennedy responded by announcing a bold plan for leapfrogging the Soviet Union: America was going to send a man to the moon and return him safely before the decade was out. He made that announcement before America had ever put a single person into orbit.

"It was really a point in time where space was taking center stage," Ride said. "You definitely had the national security and military overtones around the whole experience."

In the early 1960s, reconnaissance satellites reassured America that a "missile gap" did not exist between it and the Soviet Union.

Surveillance satellites looked for signs of nuclear explosions, as atmospheric testing had caused communications blackouts that lasted for hours and paralyzed satellites. Anti-ballistic missile defenses were proposed to use nuclear-tipped rockets to intercept incoming missiles in outer space. Programs were established for military pilots ("space bombers"), military astronauts and a military space station. "The astronauts were selected," Ride said. "They were trained for a few years. There was actually hardware built, but of course it was never launched."

That blue line probably appeared thinner than ever, but thankfully the rule of law prevailed. In 1963, international treaty banned nuclear testing in the atmosphere and in space, and in 1967 outlawed weapons of mass destruction in space. Military bases on the moon were banned. Other treaties prohibited interfering with "national technical means" -- spy satellites that were considered stabilizing by both sides.

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