The launch of Columbia on STS-107 in January. (credit: NASA/KSC)

Weighing the risks of human spaceflight

by Jeff Foust
Monday, July 21, 2003

Society’s approach to the risk of injury and death often appears contradictory. In some respects, we have become increasingly risk-averse, taking steps to reduce or otherwise mitigate the dangers posed by everything from automobiles to household appliances. The litigious nature of contemporary American society has certainly played a role in this, as lawyers seek to place blame and seek compensation for even minor accidents. The fact that nearly every cup of coffee purchased today carries a disclaimer printed on the side that “the beverage you are about to enjoy is extremely hot” is testament to this trend.

At the same time, though, other evidence points to a willingness by at least some people to participate in risky activities. The last decade has seen a surge in popularity in extreme sports from rock climbing to snowboarding to mountain biking. While one can debate how risky such activities truly are, they at least appear more dangerous than a game of softball or a round of golf. In addition, despite the harrowing tales of mountain climbing in books like Jon Krakauer’s Into Thin Air, mountain climbing is as popular as ever. In the week leading up to the 50th anniversary of Sir Edmund Hillary’s ascent to the summit of Mount Everest in late May, a record number of people—137—reached the peak of the world’s tallest mountain.

To compound these contradictions are misperceptions people hold about the risks of various activities. Some people perceive flying as a far more dangerous activity than driving. Yet, over the course of a lifetime a typical American has a far greater risk of dying in a car crash than in a plane crash. According to the National Safety Council, an American has about a 1-in-80 chance of dying in an automobile accident of some kind in his lifetime, versus 1-in-4,608 for flying accidents. You’re still far more likely, though, to encounter someone with a fear of flying than a fear of driving.

How we perceive risk—and react to it—plays a critical role now in the aftermath of the Columbia accident. NASA officials, space program advocates, and some members of Congress have argued that NASA needs to get right back to flying the shuttle as soon as the fixes recommended by the Columbia Accident Investigation Board (CAIB) are implemented. That view is shared by a majority of Americans, according to a poll completed earlier this month by Zogby International for the Houston Chronicle: 68% said they thought the benefits of manned spaceflight outweighed the benefits. Some critics, however, have argued that it is too risky to fly the shuttle, at least with crews onboard. “An accident rate of one every 62.5 [sic] missions, through which 14 American astronauts have lost their lives, is simply not acceptable,” said Congressman Joe Barton in May. “If we had the same accident rate in our commercial aviation industry, thousands of people would be killed each day—which the citizens of this nation simply would not stand.”

This argument about the risks of human spaceflight carries over into the discussion about the proposed Orbital Space Plane (OSP). Barton, for example, thinks the OSP will be a safer alternative to the shuttle, even though no one yet knows exactly what the spacecraft will look like. “We need to spend the money on building an advanced orbiter or space plane, of the best and safest technology,” he said. An OSP, though, would be launched on an expendable booster—an Atlas 5 or Delta 4—that has been in service less than a year and may require hundreds of millions of dollars of additional engineering work to achieve the reliability required to carry a manned spacecraft.

The latest salvo in this debate came last week, when NASA published a document titled “Human-Rating Requirements and Guidelines for Space Flight Systems”. The document describes how the agency plans to certify the safety of future human spacecraft, such as the OSP. While the document outlines the procedures NASA would follow to human-rate a spacecraft, and includes some specific requirements (such as the need for abort and crew rescue systems), the document does not specify what level of risk is acceptable for future manned spacecraft. “Program management… shall establish, assess, and document the program requirements for an acceptable life cycle cumulative probability of safe crew and passenger return,” the document states. It does note that while future manned missions beyond Earth could have acceptable success probabilities of as low as 0.99, “considerably better performance, on the order of 0.9999, is expected for a reusable ETO [Earth to Orbit] design that will fly 100 or more flights.”

The question of what amount of risk is acceptable for human spaceflight—and who should make that determination—remains open. Answering those questions will have an effect on not just the future of the shuttle and the OSP, but also on future commercial suborbital and orbital spaceflight.

page 2: one former astronaut’s perspective >>