Lightspeed: Taming the Galaxy
Reaching new worlds in the Milky Way means we’ll need to push our limits. How fast can we get?
At this point, every fan of science fiction, and all casual viewers of Star Wars and Star Trek, have seen images of stars suddenly stretching out as a spaceship races forward at the speed of light or greater (Warp 9, anyone?).
Beyond the fact that we actually can’t ever reach the speed of light, because to do so we would also be approaching infinite mass, as we approach lightspeed we wouldn’t actually see the above image. A study published in 2013 in the Journal of Physics Special Topics by a team at the University of Leicester determined that what a witness would actually see is little more than a white blur.
Basically, everything currently visible would disappear and be replaced by a Doppler blueshift that shortens light wavelengths so much they move out of the visible spectrum and into X-rays, which we can’t see. At the same time, the cosmic background microwave radiation is shifted into the visible spectrum, so the observers would only see a hazy orb of light.
The speed of light is a physical limit for anything with mass, and has been theoretically accepted as true ever since Albert Einstein penned his famous equation E = MC². Light itself doesn’t have mass, and travels at over 1 billion km/hr, or 186,000 miles per second. Spaceships, on the other hand, and the people and cargo inside them, have a lot of mass. It takes energy to accelerate, so the continue accelerating more energy needs to be expended. This means more fuel. At some point, the smallest gains in acceleration will need huge amounts of fuel expenditure — the law of diminishing returns. Reaching the speed of light would eventually require an infinite amount of energy, which leads us into the brick wall of impossibility.
Einstein’s special theory of relativity proved that the passage of time is affected by how fast an object moves “relative” to what’s around it. The faster an observer travels through space, the more the observer’s perception of time changes. This is called time dilation.
For the sake of explanation, say we can travel very close to C — perhaps 99.5% or so. If an astronaut takes off at this speed to go to Alpha Centauri, 4.37 light years distant, it would take about 8.5 years to go there and return to Earth. That is how long it would seem to people on Earth, anyway. To the astronaut, only a little over ONE year would have passed.
So, what happens as we approach C?
Mass goes up.
Time slows down.
So far, we have come very far in our ability to go faster. Our greatest speed accomplishment so far is in the form of rockets. Rockets are basically controlled explosive devices, filled mostly with highly volatile fuels that are ignited and directed in such a way as to propel the rocket’s mass upward against the force of gravity. Right now, rockets are our only way to get up and out of Earth’s gravity well. We’ve reached orbit, the Moon, and even sent probes on journeys outside of the Solar System all due to rockets.
Apollo 10, the practice run for the first Moon landing, set a record for manned vehicular speed, just a bit under 40,000 km/hr on May 26, 1969. It got to the Moon with a mighty shove from the Saturn V rocket, the most powerful rocket to ever be used in the 20th century. This rocket holds the record for greatest velocity of an unmanned launch: over 58,000 km/hr.
In 1977, Voyager I was launched on its mission to explore Jupiter and Saturn, and then continue on into interstellar space. The probe’s NASA team gave it a couple boosts of acceleration by using the gravity of the two gas giants to slingshot the probe on to its next goals. Using those gravity assists, Voyager I hit a top speed of 62,000 km/hr.
62,000 km/hr…and we’re only at less than a ten-thousandth the speed of light.
The Helios II probe, setting off in 1976, however, zipped around and around the Sun numerous times, ever increasing its velocity. Ultimately, Helios II reached over 252,000 km/hr. This is the top speed so far achieved by ANY man-made object.
Planned for an August of 2018 launch, the Parker Solar Probe is meant to study the corona of the Sun. This probe will end up in an orbit around the Sun that is 7 times closer than Helios II’s orbit. Such a near orbit will allow the Parker Solar Probe to reach speeds of at least 725,000 km/hr — Not quite a thousandth the speed of light, .07 percent C.
Even at that speed, it would take over 6000 years for such a craft to make the 4.25 light year voyage to Proxima Centauri, our nearest interstellar neighbor, and its potentially habitable exoplanet Proxima Centauri B. 6000 years is far too long… By the time an exploratory probe or human colony seedship arrived on Proxima B, there may no longer even be any civilization remaining on Earth. How do we improve on this speed?
Ion propulsion is one modern technology that might do it. The Dawn spacecraft actually employs this. Dawn studied the dwarf planet Ceres and Vesta asteroid. Within the ion thruster, neutrally charged atoms are hit by electrons which cause the atoms to shed their own electrons and become positively-charged ions. These are pushed out in a concentrated beam to produce thrust. It’s rather weak, but the fuel lasts a long time (years), which allows acceleration to constantly pile up. This might lead to speeds upward of 320,000 km/hr. This isn’t great enough to help with interstellar travel and getting us to Proxima B, but it will be sufficient for exploration within a solar system. Beginning a mission with slingshots around the sun and then adding on to that acceleration with ion propulsion might get us to 1,000,000 km/hr.
There are some possibilities for interstellar speeds, though. The British Interplanetary Society sponsored a competition called Project Icarus to develop an unmanned fusion powered spacecraft. The winning team’s concept, chosen in 2013, is known as Ghost.
The Ghost team’s concept utilizes inertial confinement fusion, in which fuel pellets are shot into a fusion chamber where lasers strike the pellets from all angles to compress the pellet. A final laser shot fires into the dense pellet and ignites the fusion process. Plasma is created and expelled from the ship, harnessed by magnetic coils, to create thrust. This can accelerate the Ghost ship to over 23 million km/hr, or 2.3 % C, which could transport a payload to Proxima B in about 186 years. Though promising, there are numerous hurdles. The fuels themselves (deuterium, helium 3, and tritium) are expensive, and in the case of helium 3, can only be found by mining Saturn and Jupiter. Physicists also have not yet been able to manage a sustained fusion reaction in a lab. These fusion ships are also, as conceived, too large to launch from Earth, at dozens of times the size of the International Space Station, and would require assembly and launch in orbit. In addition, most of the mass of these ships would be made of the fuels themselves, which is rather inefficient for any kind of travel (just like our modern rockets).
Another project called Icarus was launched in 2010 by the Japanese space agency JAXA. This craft, however, had no fuel to carry and relies on a solar sail for its propulsion. Solar sails are a lot like ion propulsion: not much thrust is generated at any given time by the force of stellar particles, but it compounds over time. However, because of this low power we can’t consider solar sails as a way to transport something of high mass very quickly. Even a 1 kilogram payload with a solar sail of 1 square km would take 100 years or more to reach Proxima B.
The Breakthrough Starship project would improve on this by propelling a craft with lasers. This project will accelerate tiny gram-sized probes with 1 meter square sails to roughly 20% of C by hitting them with dozens of laser beams all focused into one 100-gigawatt pulse. Again, the payloads are only big enough to allow for exploration and not actual transport of anything useful, but we will be able to reach Proxima B much faster and possibly receive large amounts of useful data and pictures within our lifetimes.
If ever we do approach substantial fractions of C, the dangers of cosmic radiation and particulate matter in space increase exponentially. Oleg Semyonev, in his paper Radiation Hazard of Relativistic Interstellar Flight (highly recommended for those of you who would like to dive deeper into this topic) discusses some possibilities for protection from this. Extreme shielding methods include building a ship inside of an asteroid or comet, or encasing it in up to a 5 meter+ depth of water. Electromagnetic shielding could also redirect radiation around a ship much like the magnetic fields do for Earth.
Unfortunately, actually transporting objects via a traditional mode of accelerated spacecraft of human-scale mass and greater may never be truly practical. Our imagined visions of outer space filled to the brim with spaceships that tread the interstellar reaches may never really come to fruition. Our ultimate ability to travel across the vast gulf between the stars likely depends on whether we can maneuver through the laws of physics another way, using hypothetical constructs such as an Alcubierre warp drive to collapse space in front of us, or quantum teleportation to send copies of ourselves to other worlds.
Thank you for reading and sharing!