CAPE CANAVERAL, Fla. -- An American expedition to Mars is the Holy Grail of U.S. human spaceflight, but more than a half-century after the dawn of the Space Age, the reality is this: NASA is "no-go," at least for now.
To send an expedition to Mars today, NASA would have to knowingly expose astronauts to cancerous, or even lethal, levels of space radiation. It's an ethical quandary for those involved in NASA's renewed push toward deep-space exploration. And it's being explored by some of the most distinguished scholars, scientists, engineers, health professionals and ethicists in the nation.
It's "the elephant in the room," NASA Chief Astronaut Robert Behnken recently told a National Academy of Sciences committee.
"We're talking about a lot of ionizing radiation, almost a guarantee for cancer, and you are really close to the edge of the range for lethal exposure," said Kristin Shrader-Frechette, a University of Notre Dame professor and a specialist in ethical issues that arise in scientific research and technology development. "If we can't get shorter transit times in space, and we can't get better shielding, then we really can't do (a Mars) spaceflight."
The situation is prompting a wholesale reconsideration of how much space radiation astronauts can be exposed to and whether those limits should be eased to enable deep-space exploration. .
The prospect raises scores of thorny questions, including these:
• Should astronauts be allowed to volunteer for a flight when NASA knows space radiation exposure limits almost surely will be exceeded?
• Should a true understanding of the risk, and informed consent, be enough for someone to volunteer for a Mars mission?
• Should NASA's chief astronaut or chief medical officer be given authority to grant individual waivers to the limits?
• Should NASA, Congress or the White House have the authority to knowingly approve a waiver, or going even further, a one-way mission to the red planet - one that includes no plans for a return to Earth?
The jury is still out. The National Academy's Institute of Medicine took up the issue this year and is deliberating. A report is due in April 2014.
NASA's Astronaut Office has already weighed in.
"The Astronaut Office actively supports readdressing the policies for crew health standards for exploration missions ... primarily because NASA will likely exceed the current medical standards to effectively pursue exploration" beyond low Earth orbit, Behnken said.
Here's the situation:
On a 500-day round trip to Mars, astronauts would fly outside the Earth's magnetic field, which largely protects International Space Station crews and the planet from deadly forms of space radiation.
Those flying beyond Earth orbit would face consequential radiation risks, including exposure to:
• Solar energetic particles generated by solar flares or coronal mass ejections from the sun.
• Galactic cosmic rays from exploding stars, quasars and gamma ray bursts outside our solar system.
Shielding and sheltering measures can protect crews from solar energetic particles, but new breakthroughs in lightweight materials are needed to make deep-space missions possible.
Galactic cosmic rays, however, are penetrating and can cause acute radiation sickness. Exposure also can cause circulatory and neurological diseases, and can result in latent cancer effects - astronauts could get cancer and die earlier than they otherwise would.
NASA follows standards established by the National Council of Radiation Protection and Measurement. Models based partly on data collected in the aftermath of the 1945 Hiroshima atomic bombing were used to set those standards.
Astronauts are not allowed to accumulate a career radiation dose that would exceed a 3 percent increase in lifetime risk of developing a fatal cancer.
In considering an easing of the limit, the Astronaut Office says other issues also should be addressed. NASA analyses show current limits also would:
• Ostensibly discriminate against women.
• Reduce the pool of astronauts who qualify for mission assignments.
Male and female astronauts face the same level of risk - the 3 percent increase in what NASA calls "Risk of Exposure-Induced Death" for fatal cancer.
But women have a lower threshold for space radiation exposure than men, largely because the increased risk from breast, ovarian and uterine cancers.
Consequently, the exposure limit for women is set 20 percent lower than the comparable limit for men. That effectively means women qualify for as few as 50 percent of potential mission assignments when compared to men, former NASA Chief Astronaut Peggy Whitson said.
"So in particular, in my case, if I had a Y chromosome, I would be qualified. But because I have two Xs, I'm not," she said.
Also a problem: The number of astronauts qualified for particular mission assignments.
Case in point: the selection of Scott Kelly to fly a yearlong ISS mission with Russian cosmonaut Mikhail Kornienko beginning in the spring of 2015.
Many factors were considered in the selection process. Among them: Command experience, Russian language proficiency and expertise in spacewalking and robotic arm operations.
But the pool dried up when space radiation exposure limits also were considered.
"We were effectively limited to three candidates for two positions (prime and back-up)," Behnken said.
In this case, Kelly was a standout candidate anyway. But in the future, the best pick for a particular mission - say, a Mars expedition - could be sidelined by space radiation exposure limits, Whitson said.
As far as a one-way trip to Mars?
"I think across the board the one-way mission needs to have a very high reward aspect to it - the national objective sort of award," Behnken said. "There are many things we do in wartime situations - that we ask people to step up and accomplish for us. We need that level of reasoning from the Astronaut Office perspective."
That said, Behnken added this: "The one-way mission to Mars is not one I would see the Astronaut Office having a line of people signing up for."