The speed of light is a big constraint when it comes to space exploration. It's the rate at which light travels in a vacuum, precisely 186,282 miles per second (29,9792,458 metres, or 300,000 kilometers, per second). It's unchangeable, and as far as we currently know, nothing can exceed that speed. That may seem pretty fast, but compared to the sheer size of space, it's like trudging through molasses.
For scale, the moon is approximately 238,855 miles (384,400 km) from Earth; It takes about 2.51 seconds for light to make a round trip from the Earth and back. Mars is 3 minutes and 2 seconds for a one-way trip, or six minutes and four seconds both ways. It would take 4.6 hours for light to reach Pluto, and another 4.6 hours to bounce back. Finally, it would take us 4.24 years traveling at the speed of light to reach Alpha Centauri Bb—the closest exoplanet from Earth. That's traveling at top speed.
Not So Fast
Unfortunately, technology hasn't evolved enough to where we can put people in spaceships and propel them through space at the speed of light. If we were able to do so, the astronauts would experience time in a strange way; for the astronauts, time would appear to move at a different speed than on Earth. Five years on a ship traveling at around 99 percent the speed of light corresponds to roughly 50 years on Earth However, Einstein's special theory of relativity tells us we have some obstacles to overcome before we can even make light-speed travel viable for humans.
Once an object reaches the speed of light, its mass becomes infinite. Naturally, it then requires an infinite amount of energy to move forward, effectively putting a speed limit on how fast an object can move. We would literally need to bend the laws of physics in order to create an object that could travel at the speed of light, or faster.
Leading the Way
Some of the top astrophysicists in the world have been working on a solution to our predicament, proposing many different theoretical engines and propellants that could possibly move at light speed. One of the most promising devices could use fusion power to drastically cut down on traveling times. Physicists working on the project estimate that we could travel to Titan—one of the most interesting places in our solar system (and roughly a billion miles from Earth)-in just two years!
Called a direct fusion drive (or DFD), this piece of conceptual technology would rely on a similar process through which our Sun is powered. We have already begun coming up with ways to harness the heat and energy byproducts of nuclear reaction for power. In fact, scientists are reportedly close to releasing the first commercial fusion reactor.
Fusion power has long been a source of research for astrophysicists and engineers who are working to develop space flight technologies. A fusion reactor essentially capitalizes on the energy released from two lighter atoms combining into a heavier nucleus—a process constantly going on in the center of stars. Imagine bottling up the immense power of a nuclear fusion bomb—which gets its momentum from forcing isotopes of hydrogen together at extremely high temperatures to form helium, in the process causing a violent release of energy, and using it to propel you through space. Sounds wild, right?
Well, there's a reason the world isn't powered by fusion reactors. They make an ideal power source because the reactions are theoretically more stable than fission reactors, therefore there's less risk of a meltdown like the Chernobyl and Fukushima nuclear disasters. There's also no nuclear waste or toxic byproducts. The problem? To start the reaction, we'd need to heat hydrogen up to temperatures that exceed 6 times that of the Sun's core—or 180 million degrees Fahrenheit (100 million degrees Celsius). That is extremely hot, and any reactor would need to be built out of material that can stand up to the intense heat of plasma(an ionized gas consisting of positive ions and free electrons at very high temperatures), which would need to be kept at extremely high temperatures under massive pressure for months at a time.
Direct Fusion Device:
All hope is not lost. Physicists are now working on what was once thought to be next to impossible: direct fusion drives (DFDs). In fact, scientists from the Princeton Plasma Physics Laboratory have been working on a DFD that could potentially take us to Titan-Saturn's arguably most interesting moon-in just two years of travel time. This device is known as the Princeton Field Reversed Configuration-2 Reactor (PFRC-2). Researchers hope it will one day become the main device used to launch satellites and probes into space, and it could one day take humans across space.
“DFD employs a unique plasma heating system to produce nuclear fusion engines in the range of 1 to 10 MW, ideal for human solar-system exploration, robotic solar-system missions, and interstellar missions,” development researchers from the Princeton Plasma Physics Laboratory said in 2019.
“[T]he engine itself exploits many of the advantages of aneutronic fusion, most notably an extremely high power-to-weight ratio,” the October 2020 press release reads. “The fuel for a DFD drive can vary slightly in mass and contains deuterium and a helium-3 isotope. Essentially, the DFD takes the excellent specific impulse of electric propulsion systems and combines it with the excellent thrust of chemical rockets, for a combination that melds the best of both flight systems.”
An FRC reactor employs a linear solenoidal, magnetic-coil array to confine the plasma, and operates at higher plasma pressures than other designs, providing higher fusion power density for a given magnetic field strength than other magnetic-confinement plasma devices.
In general, fusion systems are thought to be ideal for interstellar missions because they use hydrogen as fuel, and it happens to be the most plentiful element in the universe-so the ships won't require substantial reservoirs of gas to refuel. This could work out great for probes, too. Unfortunately, with current technology, it takes approximately 7 years to traverse the 2 billion miles between Earth and Saturn. The famous Cassini—Huygens probe was launched in 1997 and arrived at its destination in 2004—traveling at 42,500 mph (or 68,397 km/h) and encountering temperatures between 266 degrees F (130 degrees C) and -346 degrees F (-210 degrees C). New Horizons, the first mission to Pluto, took nearly 10 years to complete.
Interestingly, it's important when launching probes for astronomers to consider the alignment of planets. The Cassini-Huygens probe, for instance, was launched during a window where Venus was relatively close to Earth. It traveled from Earth to Venus, back to Earth, to Jupiter, and then Saturn. Each time, the probe got a little gravitational assist from each planet, which increased Cassini's speed.
"In order to map the best route to Saturn's biggest moon, the Italian team collaborated with the DFD's developers at PPPL and were granted access to performance data from the test engine. They then pulled some additional data on planetary alignments and started working on orbital mechanics. This resulted in two different potential paths, one where constant thrust was only applied at the beginning and the end of the journey (called a thrust-coast-thrust—TCT—profile) and one in which the thrust was constant for the duration of the journey."
"Both journeys involved switching the direction of thrust to slow the spacecraft down to enter into the Saturnian system. Providing constant thrust would put the journey at a little less than two years, while the TCT profile would result in a total trip duration of 2.6 years for a spacecraft much larger than Cassini. Both of those paths would not require any gravity assists, which spacecraft traveling to the outer planets have regularly benefited from."
If this device doesn't pan out, there are several other devices that are in the works, but this remains one of the most promising ideas. However, as the next good window for travel to Titan occurs in 2046, researchers have about 30 years to get the DFD up and going.