Space Shuttle Science
by Richard A. Muller
Technology for Presidents
from Technology Review Online
February 10, 2003
The tragic loss of Space Shuttle Columbia and its crew startled the nation. Why? Not "why was it lost?" Rather, why did its destruction startle the nation?
The dangers of Shuttle flight launch are obvious. No one within hearing distance will ever forget the devastating roar. Even the unadorned numbers are staggering. At launch, the booster carries 1.8 kilotons of liquid hydrogen and oxygen, the energy of 4 kilotons of TNT, roughly 20 percent of that of the bomb that destroyed Hiroshima. The astronauts surf into space on top of an enormous continuing explosion. Could that be made safe? The simple answer: no.
The reentry numbers are equally impressive. The Shuttle reenters at Mach 18--that’s 18 times the speed of sound. If all the energy of the air pushed out of the way by the shuttle were converted to heat, it would reach a temperature equal to the local atmospheric temperature (250 degrees Kelvin) multiplied by 18 squared; that's 80,000 K. But not all the energy is converted to heat; the actual temperature is "only" 2000 K. Yet even that is about a third of the temperature of the surface of the Sun. Hobbyists who photograph the reentry see a beautiful bright trail, like the fireball of a meteor, made of ions created from the intense heat of air impacting the Shuttle. Astronauts peering out their windows have been equally entranced (and maybe a bit frightened) as the dark sky of space is hidden by the glowing hot plasma. Can such a reentry be made safe? No.
Only the tiles, primarily on the bottom of the Shuttle, are designed to take such heat, and even they can take it only for a limited time. During re-entry, the impact of Mach 18 air will destroy the craft unless a careful balance is maintained and the heat confined to these tiles. If the Shuttle tumbles, air quickly weakens and destroys the entire craft. We don't yet know what caused the Shuttle to tumble on February 1, and even when we do, it will be impossible to guarantee that it will not happen again. To me, the remarkable achievement of the space program is that only two Shuttles have been lost in 113 missions.
There are few jobs more dangerous than that of an astronaut. But most of the public doesn't know that--or rather, didn't. The Challenger disaster had been largely forgotten. The misimpression that the Shuttle is safe is largely the work of NASA public relations, which has strived to make the flights sound routine. The shuttle is so safe that we can fly senators and schoolteachers. Shuttle flights are so routine that we can use them to perform experiments suggested by high school students.
There is another major misconception that NASA has more actively spread, perhaps because many in the agency have convinced themselves that it is really true. Here it is: the primary goal of the Space Shuttle is the advancement of scientific knowledge. Most scientists roll their eyes when they hear this claim. Don't misinterpret what I say: there is good, even great science launched into space on the Shuttle. Results from the Hubble Space Telescope excite physicists and astronomers as much as the general public. But a case can be made that Hubble could have been launched at a lower cost by non-reusable rockets. The United States military prefer to launch their spy satellites -- close cousins of the space telescopes -- with unmanned vehicles. Such satellites don't have to be man-qualified, that is, they don't require the extra engineering costs to make them absolutely safe to be in space alongside humans.
True, Hubble was defective, and required repair by Shuttle astronauts. But the military loses its spy telescopes too, and its response is to launch a replacement. Launching two completely new Hubble telescopes--the original and a replacement, with neither qualified for human servicing (and therefore cheaper)-- would arguably have been less expensive in the long run.
When it comes to the science itself, the Space Shuttle is a poor choice of platforms. Humans are a source of noise -- vibrational, infrared, gravitational. Sensitive experiments must get away from this. Just flying an experiment on a manned mission automatically raises the experiment’s costs. Many scientists moan privately about scientific missions that were delayed and made more costly because they were moved off unmanned launch vehicles and forced to become part of NASA’s scientific justification for the Shuttle.
Hubble aside, what would you name as the really glorious achievements of NASA in the last 20 years? My favorite: the discovery that every moon of every planet is significantly different from every other moon, a result completely unanticipated and still not understood. One might also pick the amazing success of weather satellites. Or the remarkable pictures you get from your satellite TV system. Those in the know might pick our space spy systems. Then there’s GPS -- the Global Positioning System, used to guide airplanes, boats, hikers, automobiles as well as soldiers and smart weapons. These projects have one thing in common: they were all unmanned.
After the Columbia catastrophe, Senator John McCain said, "Space exploration is a mission the U.S. will not abandon." I hope he is right. But (and here Senator McCain might differ), such exploration in the next decade or two could perhaps be better done with unmanned vehicles. We are already sending robots to Mars. We have plans to bring back samples. Some day we may even send astronauts there. But let's not be in a hurry to do that. New telescopes and unmanned instruments will tell us more about space than orbiting astronauts. There is a future for humans in space. Eventually hypersonic flight will be perfected, and we will be able to ride airplanes into orbit. But nobody knows when the required "scramjet" engines will be deployable; they are still in the R&D stage, with strong emphasis on the R. Tests reveal critical new physics with each added Mach number, and hypersonic prototypes have barely reached Mach 5. Orbit requires greater than Mach 18.
Maybe a different approach will succeed first. The development of ultra-strong carbon fibers may make the dream of a space elevator (formerly called "skyhook") real, allowing us to ride slowly and safely up and down. But the science isn't here yet. In 20 years -- who knows?
Many in NASA believe that the future of their agency depends on continuing and extending human space flight. Without astronauts, the argument goes, the public will lose interest in space. I think this argument is wrong. Few Americans even knew the Space Shuttle was in orbit -- until the astronauts were killed. When the students I know at Berkeley put space posters on their walls, they are not posters of astronauts. They are images of regions where stars are being born, of exploding stars, of extremely distant and hauntingly beautiful fields of galaxies. They were all taken with instruments that didn't need humans to get into space, and required their absence in order to take stable pictures.
The Space Shuttle is not safe and it will not be safe in the foreseeable future. Is using it worth the lives lost? We must be honest. Perhaps the Shuttle missions should continue, but if they do, let us do it in full public realization that the chance of death on each mission is about 2 percent. Soldiers going into battle often accept risks even higher than that. The astronauts always knew this danger, and they chose to accept it. Can the public accept such a high level? I don't know. But whatever the decision is, it should be made in candor and in truth. The Space Shuttle is big engineering; it is the dream of man in space; it is an adventure. But it is not safe, it cannot be made safe, and it is not big science.
Richard A. Muller, a 1982 MacArthur Fellow, is a professor in the Physics Department at UC-Berkeley where he teaches a course entitled, "Physics for future Presidents." He is also a faculty senior scientist at Lawrence Berkeley Laboratory.