50 years after reaching the moon, dream of sending man to Mars may not be best course

Mars was supposed to be next. Surely the moon was just a steppingstone in the conquest of space. For many people who came of age during the Apollo era, it seemed reasonable to assume that in short order the entire solar system would be our stomping ground. Eventually we’d be visiting stars. “Star Trek,” which debuted in 1966, seemed a plausible vision of human destiny.

Half a century after Apollo 11, we have been forced again and again to recalibrate our expectations.

The exploration of deep space by flesh-and-blood human beings no longer looks inevitable. It doesn’t look especially affordable under plausible government budgets in the post-Space Race era, and private-sector dreams may never quite pencil out, as they say. Space travel remains dangerous; the catastrophic loss of two space shuttle crews proved that.

There’s also been a more subtle revelation from half a century’s experience with spaceflight. Going into space has given us a greater appreciation of our connection to the Earth.

The human body goes haywire when hurled into space and away from the familiar environment of the Earth’s surface. We learned this by doing it. And our innate terrestrial nature is both biological and psychological: Astronauts in orbit spend a lot of their free time looking out the window, toward home.

Maybe the most important thing we’ve learned from the Space Age is that we’re Earthlings.

When astronaut Scott Kelly went into orbit in 2015 for a nearly year-long mission, his immune system initially went bonkers. It acted as if under attack by a virus. At the cellular level, his body was screaming: Where’s the gravity?

The fluids in his body wound up in the wrong places, an occupational hazard for astronauts. The effects include insomnia and blurred vision. And although his genetic code didn’t change, his gene expression — the creation of proteins that are the workhorses of the body — did undergo pronounced changes, with some genes turning off and others turning on.

Astronauts adapt to zero gravity and perform their jobs well. But then they face another jolt when they return to Earth. Kelly suffered from painful rashes, swollen legs, nausea and flulike symptoms. His gene expression mostly returned to its normal state, but not entirely. Kelly said he didn’t feel quite right for about eight months.

While Scott orbited the Earth, his twin brother, Mark, went about his business on the surface, pausing to let researchers sample his blood, urine, etc., for comparison with his sibling in space. NASA said its Twins Study revealed no showstoppers — nothing that would prevent an eventual human mission to Mars, the agency’s long-term goal.

But the study provided a reminder that space travel is brutal on human bodies, which are adapted for life on this particular planet. Our bones lose density. Muscles can atrophy. Astronauts have to exercise two hours a day to keep from wasting away. We can live in space, but that environment doesn’t really agree with us.

“We are fairly exquisitely designed for this planet, and we’re fairly fragile physiologically when you get off this planet,” study co-author Christopher Mason, a geneticist at Weill Cornell Medicine, told The Washington Post.

In the Hollywood version of spaceflight, no one ever worries about the composition of the air in the spaceship. But as Scott Kelly can attest, the air in the International Space Station can be a bit off. Kelly says high levels of carbon dioxide can cause malaise, especially in areas of poor air circulation.

The Kelly brothers participated in a recent NASA-sponsored media teleconference to discuss the Twins Study, during which one of the researchers, Stuart Lee, said the air on the ISS “is very close to what we have on the ground.” The CO2 levels, he went on, are 0.3 percent of the air, compared to about 0.03 percent on the Earth’s surface.

Scott Kelly quickly chimed in: “So it’s 10 times higher, Stuart. Right?”

“So, yeah, it’s 10 times higher,” Lee acknowledged.

Then there’s radiation. Earth’s magnetic field protects the ISS from much of the radiation of space. But an astronaut journeying to Mars would not have that protection and would be particularly vulnerable to “cosmic rays,” which are elementary particles of galactic origin that travel at nearly the speed of light and could potentially cause cancer, genetic damage and acute radiation sickness.

A human being is a composite organism — a collaboration involving trillions of microbes, most of them residing in our gut. The microbes emerge from, and have intimate connections to, the Earth. Mars not only doesn’t have the kind of air, water, gravity and radiation that we’re used to, it also, almost surely, doesn’t have the right kind of bacteria.

None of this prevents a human mission to Mars or other places in space. Never underestimate the ingenuity of future generations. But the greatest technological leaps since Apollo have occurred in the realm of 0’s and 1’s — the digital revolution. Robots do well in space. Any mission to the moon or Mars has to justify a human presence.

NASA plans to put another rover on Mars soon, one designed to obtain soil samples that can someday be sent back to Earth robotically. It’s a scientific echo of what the Apollo astronauts did. The difference this time: No humans in the loop.

Alexa, bring us a Mars rock.