How Far can Human Travel Safely

We humans are obsessed with speed. Recent months, for instance, brought news that students in Germany have broken the record for the fastest accelerating electric car, and that the US Air Force plans to develop hypersonic jets that would travel at more than five times the speed of sound – that’s speeds in excess of 3,790mph (6,100km/h).
Those jets would carry no crew – but not because humans can’t travel at such high speeds. In fact, humans have already travelled many times faster than Mach 5. Is there some limit, however, beyond which hurtling bodies can no longer bear the strain of speed?
The current human speed record is shared equally by the trio of astronauts who flew Nasa’s Apollo 10 mission. On their way back from a lap around the Moon in 1969, the astronauts’ capsule hit a peak of 24,790mph (39,897km/h) relative to planet Earth. “I think a hundred years ago, we probably wouldn’t have imagined a human could travel in space at almost 40,000 kilometres per hour,” says Jim Bray of the aerospace firm Lockheed Martin.
But we could beat that record relatively soon. Bray is the director of the Orion crew module project for America’s space agency, Nasa. The Orion spacecraft is intended to carry astronauts into a low Earth orbit, and is a good bet for the vehicle that will break the 46-year-old record for the fastest we’ve ever travelled.
The Space Launch System, a new rocket that will ferry the Orion spacecraft aloft, should have its first crewed mission in 2021 – a flyby of an asteroid captured in lunar orbit – with a months-long mission to Mars then in the offing. At present, designers envision Orion’s typical maximum velocity in the neighbourhood of 19,900mph (32,000km/h). But the Apollo 10 speed record could be surpassed, even just sticking with Orion’s base configuration. “Orion is designed for many different destinations over its lifetime,” says Bray. “Its speed could well go a lot higher than we plan now.”
Even Orion won’t represent the peak of our speed potential, though. “There is no real practical limit to how fast we can travel, other than the speed of light,” says Bray. Light zips along at about a billion kilometres per hour. Can we hope to safely bridge the gap from 40,000kph to those speeds?
Surprisingly, speed – defined as a rate of motion – in of itself is not at all a problem for us physically, so long as it’s relatively constant and in one direction. Therefore, humans should – in theory – be able to travel at rates just short of the “Universe’s speed limit”: the speed of light.
But assuming we can overcome the considerable technological obstacles in building faster spacecraft, our fragile, mostly-water bodies will have to contend with significant new hazards that come with such high-speed travel. Speculative dangers could arise, too, if humans achieve faster-than-light travel, either by exploiting loopholes in known physics or through paradigm-shattering discoveries.

Withstanding G-forces
However we attain speeds in excess of 40,000kph, we will have to ramp up to (and down from) them patiently. Rapid acceleration and deceleration can be lethal to the human organism: witness the bodily trauma in car crashes as we go from a mere tens-of-kilometres-per-hour clip to zero in the span of seconds. The reason? A property of the Universe known as inertia, whereby any object with mass resists change to its state of motion. The concept is famously expressed in Newton’s first law of motion as “an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an outside force”.
“For the human body, constant is good,” explains Bray. “It’s acceleration we have to worry about.”
About a century ago, the invention of sturdy aircraft that could manoeuvre at speed led to pilots reporting strange symptoms related to speed and directional changes. These included temporary vision loss and the sensation of either leadenness or weightlessness. The cause is G-forces, otherwise called gravitational forces, or even simply Gs. These are units of accelerative force upon a mass, such as a human body. One G is equal to the pull of Earth’s gravity toward the planet’s centre at 9.8 metres per second squared (at sea level).
G-forces experienced vertically, from head to toe or vice versa, are the ones that can be truly bad news for pilots and passengers. Blood pools in the heads of those undergoing positive Gs, from toe to head, causing an engorged sensation like when we do a handstand. “Red out” sets in as blood-swelled, translucent lower eyelids rise up to cover the pupils. Conversely, when acceleration is negative, from head down to foot, the eyes and brain become starved of oxygen as blood collects in the lower extremities. Dimmed vision called “grey out” initially occurs, followed by total vision loss, or “blackout”. These high Gs can progress to outright faints, dubbed G-induced loss of consciousness (GLOC). Many aviation deaths result from pilots blacking out and crashing.
The average person can withstand a sustained force of about five Gs from head to toe before slipping into unconsciousness. Pilots wearing special high-G suits and trained to flex their torso muscles to keep blood from whooshing out of their heads can still operate their aircraft at about nine Gs. “For short periods, the human body can take much higher than nine Gs,” says Jeff Sventek, the Executive Director of the Aerospace Medical Association, based in Alexandria, Virginia. “But to sustain that for long period of time, not too many humans can do it.”
If only for mere moments, we humans can tolerate way stronger Gs without grievous injury. The record for momentary Gs is held by Eli Beeding Jr, an American Air Force captain. He rode a rocket-powered sled backwards in 1958 and recorded a pummelling 82.6Gs on his chest accelerometer as the sled accelerated to about 34mph (55kph) in one-10th of a second. Beeding blacked out but suffered little more than back bruises, in a remarkable demonstration of the body’s resilience.

Out into space
Astronauts, depending on their vehicle, have also experienced fairly high Gs – between three and eight on takeoffs and atmospheric re-entries, respectively. These G-forces are mostly benign front-to-back Gs, thanks to the smart practice of strapping spacegoers into seats facing their direction of travel. Once at a steady cruising speed of about 16,150mph (26,000kph) in orbit, astronauts no more feel their speed than do passengers on a commercial airplane.
If G forces aren’t a problem for Orion’s longer duration missions, small space rocks – “micrometeoroids” – might be. These grain-size bits can reach impressively devastating speeds of nearly 186,000mph (300,000km/h). To protect the vessel and its crew, Orion has a protective outer layer varying in places from 18 to 30cm thick, plus other shielding and clever equipment placement. “So we don’t lose a critical flight system, for the entire spacecraft we have to look at which angle a micrometeoroid can come from,” says Bray.
To be sure, micrometeoroids are not the only hindrance to future space missions where higher human travel speeds would likely come into play. On a Mars mission, other practical issues will need to be addressed, including the crew’s food supply and their increased lifetime cancer risks from cosmic radiation exposure. Shortening travel times, though, would mitigate these issues, making a go-faster approach very desirable.

Space travel, the next generation
This need for speed will pose fresh obstacles. The new Nasa vessels that might threaten Apollo 10’s speed record will still rely on tried-and-true, chemical rocket propulsion systems, used since the very first space missions. But such systems have severe speed limitations because of the low amounts of energy they release per unit of fuel.
So, in order to achieve significantly faster travel speeds for humans bound for Mars and beyond, scientists recognise that new approaches will be required. “The systems we have today are going to be good enough to get us there,” says Bray, “but you would like to see a revolution in propulsion.”
Eric Davis, a senior research physicist at the Institute for Advanced Studies at Austin and contributor to Nasa’s Breakthrough Propulsion Physics Programme, a six-year-long research project that ended in 2002, outlines three of the most promising means – assuming conventional physics – for getting humanity up to reasonable interplanetary travel speeds. In brief, they are the energy-releasing phenomena of fission, fusion and antimatter annihilation.
The first method is the splitting of atoms, as is done in commercial nuclear reactors. The second, fusion, combines atoms into heavier atoms – the reaction that powers the Sun, and a technology that remains tantalisingly out of reach; “always 50 years away”, as an old industry motto goes.
“These technologies are advanced,” says Davis, “but they’re conventional physics and have been well-established since the dawn of the Atomic Age.” Optimistically, various propulsion systems based on fission and fusion concepts could theoretically accelerate a vessel up to 10% of the speed of light – a cool 62,000,000mph (100,000,000km/h).
The far-and-away best case for powering fast spacecraft is antimatter, the doppelganger to regular matter. When the two matters make contact, they obliterate each other as pure energy. Technologies to generate and store (admittedly minuscule) quantities of antimatter exist today. Yet production of antimatter in useful amounts would need dedicated, next-generation facilities, and engineering challenges galore would loom for the intended spacecraft. But Davis says plenty of good ideas are on the drawing board.
With antimatter-fuelled engines, spacecraft could accelerate over periods of months or years to very high percentages of the speed of light, keeping Gs to a tolerable level for occupants. These fantastic new speeds, however, would usher in fresh dangers for the human body.
An energetic hail
At several hundreds of millions of kilometres per hour, every mote in space, from stray hydrogen gas atoms to micrometeoroids, becomes in effect a high-powered bullet ploughing into a ship’s hull. “When you’re going at high speeds, that’s equivalent to a particle moving at you at high speeds,” says Arthur Edelstein. He worked with his late father, William Edelstein, a professor of radiology at the Johns Hopkins University School of Medicine, on a 2012 paper exploring the effects of cosmic hydrogen atoms on ultrafast spaceflight.
Although only present at a density of around one atom in a cubic centimetre, the cosmos’s ambient hydrogen would translate into a bombardment of intense radiation. The hydrogen would shatter into subatomic particles that would pass into the ship, irradiating both crew and equipment. At speeds around 95% of light, the exposure would be near-instantly deadly. The star ship would heat up, too, to melting temperatures for essentially any conceivable material, while water in the crew’s bodies would promptly boil. “These are all nasty problems,” quips Edelstein.
He and his father roughly estimated that barring some sort of conjectural magnetic shielding to divert the lethal hydrogen rain, star ships could go no faster than about half of light speed without killing their human occupants.
Marc Millis, a propulsion physicist and the former head of Nasa’s Breakthrough Propulsion Physics Programme, cautions that this potential human travel speed limit remains a distant worry. “Based on the physics that has already been accrued, velocities beyond 10% the speed of light will be very difficult to achieve,” Millis says. “We are not in danger yet. To use an analogy, we don’t need to worry about drowning if we can’t even get to the water yet.”

Faster than light?
Assuming we do learn to swim, so to speak, might we also someday learn how to surf spacetime, to extend the analogy, and travel at faster-than-light (superluminal) speeds?
The inherent survivability of the superluminal realm, though speculative, isn’t without some educated shots in the dark. One intriguing faster-than-light scenario works like the “warp drive” of Star Trek. Called an Alcubierre drive, it involves compressing the normal spacetime described by Einsteinian physics in front of a star ship, while expanding it behind. In essence, the ship resides within a chunk of spacetime – a “warp bubble” – that moves faster than the speed of light. The ship, however, remains at rest within its pocket of normal spacetime, avoiding any violation of the universal light-speed limit. “Instead of swimming through the water” of normal spacetime, says Davis, the Alcubierre drive “will carry you like a surfer riding on the crest of wave on a surfboard”.
The catch: the concept requires an exotic form of matter possessing a negative mass to contract and expand spacetime. “Physics doesn’t forbid negative mass,” says Davis, “but there are no examples of it and we’ve never seen it in nature.” The other catch: a 2012 paper by University of Sydney researchers suggests that the warp bubble would gather up high-energy cosmic particles as it inevitably interacted with the Universe’s contents. Some particles would leak into the bubble itself, blasting the ship with radiation.

Stuck at sub-light?
Are we forever stuck at sub-light velocities because of our frail biology? The answer matters not just for setting a new human world (galactic?) speed record, but for the prospect of our species ever becoming an interstellar society. At the half-light speed limit that Edelstein’s research places on our bodies, a voyage to the nearest star is more than a 16-year round-trip. (Time dilation effects, wherein less time would pass for the hurtling star ship crew with their reference frame than for people back home on Earth in a different reference frame, would not be a dramatic effect at half-light speed.)
Millis holds out hope. Seeing as humanity has invented high-G suits and micrometeoroid shielding to allow safe travel at terrific speeds in the great blue yonder and the star-studded blackness of space, he thinks we will devise ways to survive whatever velocity frontiers we face next.
“The kind of technologies that could enable unforeseeable new transit speeds, if future physics finds out that such technology is possible,” Millis says, “would also give us new, unforeseen possibilities for protecting crews.”