The future of deep space exploration holds the promise of solving the many mysteries that lie beyond our solar system. But despite renewed interest in again visiting the moon and a fevered pitch to reach Mars, deeper adventures into the galaxy will depend on the realization of light-speed travel.
To the disappointment of many, the chances of going that fast, as any scientist will attest, are currently as cold and dark as space.
"Light-speed travel is out of the question," said Dan Batcheldor, a professor and the head of the Florida Institute of Technology's Aerospace, Physics and Space Sciences department.
"The only things that can travel at that clip are photons, packets of light. Anything that has mass, as it accelerates and once it gets to a very high speed - ten thousand kilometers per-second - has to leave behind Newtonian philosophies and that beautiful theory of general relativity that Albert Einstein put together."
The paradox of human-led travel to the far reaches of space is that a journey using contemporary technology would last longer than the life of the travelers. That's why any conversation about deep-space travel inevitably circles back to the question of light speed, which offers two irrefutable answers.
First, no man-made object can yet travel that fast. Second, scientists can't predict the precise biological dangers of moving that fast and far, effectively removing humans from the conversation, at least until a real-life equivalent of the fictional Millennium Falcon makes light-speed, deep-space travel as safe and fun as Han Solo and Chewbacca had it in "Star Wars."
Not Quite Ready for Complete Light Speed
"Cracking the speed of light means cracking the laws of physics," said Stephen Holler, an associate professor of physics at Fordham University.
"Right now, we are limited by these laws, but there may be new physics out there that we have not yet discovered." Crossing vast interstellar distances would require a manipulation of space-time - folding space and jumping - rather than the easy-to-visualize method of accelerating from one point to another at a superluminal speed, he said.
Holler added: "The distances are just so vast that even at ten times the speed of light, it would still take more than 10,000 years to go from one end of the Milky Way to the other, never mind intergalactic travel."
Nevertheless, scientists and engineers persist - mostly in theory. As Holler notes, the many ideas behind interstellar travel are all theoretical. But they are still worth reviewing. One popular notion is using ramjet technology to "scoop up" hydrogen in space and use that as fuel, Holler said.
Another concept has solar sail technology using radiation pressure from perhaps a powerful laser to push the spacecraft. "At best, these would get somewhere between 10 percent and 50 percent of the speed of light," he said. At that rate, it would take decades for a craft to reach another space system.
According to University of Richmond physics professor Jack Singal, the fastest sustained long-term speeds of deep-space probes are those of Pioneer 11 (launched in 1973 and still traveling but no longer sending back information), Voyager 1 and 2, (both launched in 1977 and still collecting data) and New Horizons (launched in 2006 and the first probe to study Pluto, the planet that's no longer a planet).
These probes are currently moving between 16,000 and 32,000 miles-per-hour away from the sun as they exit the solar system, he said. That, Singal added, is "less than 0.01 percent of the speed of light." (If you're reading this on Earth, the speed of light is 186,282 miles per second.)
Perhaps future probes venturing into deep space won't be as large as the Voyager crafts or the Parker Solar Probe but will be something a scientist could accidentally step on. According to Batcheldor, of Florida Institute of Technology, scientists are looking at the possibility of using micro-satellites the size of a computer chip to conduct small-data collections of space. "It might be able to send back some details about another space system that we might not get with our own telescopes," he said.
Yet, even a micro-satellite would travel at no more than a quarter of the speed of light, Batcheldor estimated. It would take decades to reach the next space system at that speed, he said.
Overcoming Known and Unknown Hurdles
Short of a scientific and technological miracle, it's clear humans can't and won't venture beyond Mars anytime soon, assuming space flights there are a lock. "Exploring any place beyond Mars wouldn't be possible," Batcheldor said.
"The time scale says a trip from Earth to Jupiter and back would take the lifetime of a human," he said. "Of course, our imaginations and dreams of going beyond that will make us run wild. But it's not going to be possible unless we will be able to travel the speed of light - which we will not."
Still, a scientist can dream, or at least theorize. For the sake of scientific conjecture, and maybe even fun, Singal posits that if humans could travel at light speed, instead of taking thousands of years to reach a nearby star, they instead would get there in just a dozen years.
Yet, if that dream became reality, there still would be far too many unknowns, too many questions and concerns, about how an individual could adapt and even survive the inescapable elements of quantum physics, not to mention radiation and other dangers of space exploration.
Time dilation, for one, would be an inescapable hurdle, Singal said. "Less time would be perceived as passing on the spaceship than on Earth," he said. "This opens up all sorts of possibilities for the survival of instruments or even biological beings during such a journey."
Batcheldor agrees. Experiments on astronaut Scott Kelly showed that even spending one year in the low orbit of the International Space Station can affect human physiology, he said. "There were time-warping effects on that mission. We have no idea what would happen during extended space travel."
Creating a Home Away From Home
Despite the uncertainties, impracticalities and possible dangers, the academics interviewed for this article want scientists, engineers and society at large to stay open, even excited, about the future of space exploration.
The professors are happy about the plans of NASA, other countries' space agencies and global private companies to not only again step foot on the moon but also explore it for clues about the formation of the solar system. Also, they're optimistic that the poles of the moon could be mined for water that could be used to make fuel for longer space missions.
Even though light-speed travel seems like a remote possibility, the space community still needs to plan for the many practical needs of deep-space exploration, according to Ondrej Doule, the director of the Human Spaceflight Lab at Florida Institute of Technology, where he specializes in the architecture of space.
His lab aims to convince space agencies and companies to consider the functions that will make space travel and the habitation of the moon and Mars amenable to human comfort. For instance, spacecraft and habitats should achieve artificial gravity so that travelers and settlers will feel the comforts of Earth and not experience the long-term effects of weightlessness, such as muscle and bone mass loss. And explorers will need sufficient and consistent protection from the radiation of space, he said.
"If we are going to colonize different parts of the universe, we are going to need everything that's necessary to survive over the long term," Doule said. "This is the direction where we are heading. I don't think the Earth is our home planet. I think the universe is our home."
Indeed, us earthlings have barely started to learn about our solar system, let alone our galaxy and the entire universe, lending weight to the notion that traveling at light speed will always be the primary key to deep-space exploration.
"The universe is pretty big. Our galaxy is more than 100,000 light years across, and the distance to Andromeda, the nearest galaxy to our own, is some 2.5 million light years," said Fordham University's Holler. "At light speed, covering these vast distances would take an inordinate amount of time, far in excess of the human lifespan. Travel to Andromeda would require more time than Homo sapiens have walked the Earth."