Reaching light

Reaching the Speed of light

Is interstellar travel doomed to remain in the realm of science fiction with faster than light travel and infinite improbability drives? Never mind warp speed, is light speed even possible? Technically no Light is massless and travels a little over 1 billion km/h Since spacecraft are not massless you need energy to accelerate as you keep gaining speed, you’ll need increasingly larger amounts of fuel. Eventually, even tiny gains in acceleration require huge amounts of energy Getting to exactly one hundred percent light speed would require an infinite amount of energy which is why it’s an impossible goal Before we look at how close we can get to the speed of light .

Let’s consider what we’ve accomplished so far When it comes to space travel, everything starts with a rocket still the best way we know of to literally get our feet off the ground rockets give spacecraft that initial massive boost to escape Earth’s gravity t’s the first step in getting people to the moon sending probes to the planets our sun and even the edge of our solar system apollo 10, the dress rehearsal for Apollo 11 never landed on the moon, but at reentry it did set the world record for the fastest manned spacecraft at just under 40,000 km/h in order to get all the way to the moon, it needed the help of Saturn V still the most powerful rocket ever launched The New Horizons probe, hitching a ride on the Atlas V holds the record for fastest launch velocity at over 58,536 km/h that was also the first spacecraft to reach Pluto in 2015 sending back the first detailed images ever taken of Pluto.

Launched in 1977, Voyager 1’s mission was the exploration of Jupiter and Saturn and once complete to leave the solar system entirely It couldn't do it with rocket power alone so it used gravity assists at Jupiter to slingshot to Saturn and again at Saturn to fling itself out to the edge of the solar system Voyager 1 reached a top speed of 62,000 km/h but it still took over 30 years to reach interstellar space in 2012. In 1976, the Helios-2 probe set off to study the sun and the interplanetary medium To do so they were put in highly elliptical orbits with the Sun at one end and all the way out to the Earth's orbit at the other Each time it approached the Sun the massive gravitational force speed it up to a record setting speed of 253,000 km/h the fastest any spacecraft has ever travelled Solar Probe Plus is study the corona and outer atmosphere of the sun.

Over the course of seven years multiple gravity assists will be used at Venus to bring it into an orbit around 7 times closer to the sun than Helios With that close of an orbit it will crush the Helios record with a top speed of just under 725,000 km/h even still, that’s just a meager .07% the speed of light and would take over 6,000 years to travel 4 and a quarter light years to get to Proxima Centauri the closest star to our own In 2016 a planet, Proxima b, was discovered around the star. It’s probably rocky like the Earth and in the right orbit to be potentially warm enough for liquid water on the surface if it has any. This makes it an excellent target for the first interstellar spacecraft staying in the realm of current or near future technology only can we improve on that speed?

Ion propulsion is currently employed in some satellites but most notably, the Dawn spacecraft which studied the asteroid Vesta and the dwarf planet Ceres Inside the thruster, electrons bombard neutrally charged atoms ausing them to lose electrons and become positively charged ions Thrust is produced as they are shot out in an ion beam the resulting thrust is miniscule but since it’s fuel supply can last for years, unlike a rocket that tiny amount of thrust keeps compounding on itself and in time, it can potentially reach speeds of up to 324,000 km/h, although no spacecraft currently has while not the fastest it’s a practical method to study multiple celestial bodies in one mission an exciting potential for solar system exploration.

In the 1970s, the British Interplanetary Society conducted a study Project Daedalus for an unmanned fusion propelled interstellar spacecraft With help from the Tau Zero foundation in 2009 hey initiated Project Icarus to update the concept with the Ghost team’s design winning the internal competition in 2013 for propulsion, Ghost uses Inertial Confinement Fusion Small fuel pellets are shot into a fusion chamber Lasers bombard each pellet from all sides, compressing it then one final laser pulse shoots into the core, igniting the fusion process The resulting plasma is expelled from the ship, producing thrust The older Daedalus design would have reached a remarkable 12% light speed Ghost can only get to 2.33% light speed reaching Proxima Centauri in around 186 years while much slower,  Ghost allows for deceleration at the star for scientific study using it’s own fusion engine and a magnetic sail 400 km in diameter. 

Ghost also uses a more practically sourced fuel Deuterium and Tritium while Daedalus used the more efficient combination of Deuterium and Helium-3. Unfortunately, Helium-3 is too rare and inaccessible on Earth You would have to mine a gas giant like Jupiter or possibly the moon to get enough fuel. Though it’s the fusion process itself that may be the biggest challenge We have yet to achieve a sustained fusion reaction in the lab until we figure that out, these ships won’t leave the ground weighing in at 1.4 million tonnes and over a kilometer long Ghost couldn’t launch from Earth it would have to be assembled in parts.

This is no small task as The International Space Station, the largest spacecraft ever built is 450 tonnes and 109 meters long, peanuts compared to Ghost Since almost all of the mass of a fusion spacecraft is fuel imagine not having to carry any with you Enter the solar sail IKAROS is a small solar sail spacecraft built by the Japanese space program JAXA Launched in 2010, it was the first successful demonstration of solar sail propulsion similar to ion propulsion sunlight results in only a small amount of thrust but it keeps compounding on itself and only stops when the spacecraft is too far from the sun. The total thrust is determined by the surface area of the sail versus the mass of the spacecraft With a big sail you can catch more light With less mass, the light can push you faster There’s also more solar energy imparted on the spacecraft the closer you can get to the sun as long as you don’t burn up in the process.

Unfortunately this may not be good enough to get to another star within a lifetime It would take a 1 kg spacecraft with a 1km x 1km sail travelling extremely close to the sun about 100 years to reach Proxima Centauri But we may be able to improve on this Breakthrough Starshot is another proof-of concept project to design an unmanned laser propelled spacecraft. A mothership containing thousands of nanocrafts will be launched into orbit This allows for redundancy in case of failure and a variety of payload options These tiny, gram-scale nanocraft have a sail only 4 meters to a side and just a few hundred atoms thick.

The Light Beamer is a ground based array of lasers that combine together in a single beam to propel each nanocraft to 20% light speed within minutes. At that speed, they could reach Proxima Centauri in as little as 20 years While just a flyby mission they could get close enough to Proxima b to take pictures with enough detail to see continents and oceans if it has any While the team doesn’t see any deal breakers that could stop the project that’s not to say there won’t be any challenges especially building the light beamer at 100 gigawatts, the array would be 100 times more powerful than a nuclear power plant It will also need to contend with the scattering effects of the atmosphere as an orbital laser of this scale is too expensive Speaking of which, the projected cost is nearly 10 billion dollars about half of NASA’s 2016 budget But it does start to look like a bargain compared to the $100 billion dollar cost of the International Space Station Breakthrough Starshot may represent not only the most practical interstellar spacecraft but at 20% light speed it would be the fastest