The challenge, of course, is that the payload is sitting on the equivalent of eight times its weight in high-grade explosives. Of course, the explosives are carefully engineered to not all go off at once so that the smooth and controlled rocket flight is obtained (this is the rocket science of rocket science!), but it is still dangerous enough that most rocket launches require observers to be several kilometers away when viewing in person.

Are there more efficient ways to get to orbit? Some enterprising inventors and designers have proposed such fanciful ideas as using a device called a mass driver which uses electromagnetic forces to accelerate a payload in a sort of souped up version of a maglev monorail. Sometimes combined with an engine called a scramjet which combusts the air at supersonic speeds, such systems would conceivably allow for launch into space without the need for enormous amounts of fuel, though, due to conservation of energy, it would require the same amount of energy although it could perhaps be sourced more sustainably than rocket fuel. These technologies are still in the development phase and it is not clear whether or how they might be practical for launches.

Another proposal which is perhaps even more fanciful but may be maximally efficient is a space elevator. This idea requires building a tower or cable that stretches well beyond low-Earth orbit to some 35,000 km distance in order to be dynamically stable. At the top of the space elevator, a counterweight would have to be fastened which would keep the structure secured through tension through the centrifugal force on the counterweight. Payloads could be sent up the space elevator using a variety of techniques — one more astonishing proposal suggests using lasers to push the payload up the space elevator. Once the payloads reached the top, they would be traveling at orbital speed simply by following the rotation of the Earth (this is so-called “geostationary orbit”). At that point the payload could be released and given a little push away from the space elevator and it would remain in space. The material stress on the space elevator’s material would be so strong, however, that there is currently no known substance that could withstand it.

For the last twenty years, the International Space Station has been serving as a laboratory for testing what it is like to have a permanent human-presence in orbit. Using orbital platforms as staging grounds for further space flight beyond the Earth has often been suggested as a way to assemble larger space ships without having to launch them all at once from the ground. While the ISS has not yet been used for this purpose, there are those who think it could serve such a function for future crewed missions to the Moon or Mars.

Challenge #2: Microgravity

The weight of anything in orbit which is the force felt in response to the force of gravity is zero. This is because when in orbit the force accelerates the orbiting body and is not opposed (this is so-called “free fall“). In a massive space ship, there is a tiny amount of gravitational pull that the ship will exert, but those forces are small enough that NASA calls such a scenario “microgravity” to indicate that the gravity is reduced by factors of millions compared to what is experienced on Earth.

Microgravity requires certain practical considerations to be taken into account. Astronauts need to alter their eating, drinking, and hygiene habits to account for a lack of gravity. Simple manual tasks become more of a challenge: tools need to be tethered or they drift away, people need to strap in to sleeping sacks or they can hurt themselves or smash into equipment when they’re unconscious. While the story of NASA spending one million dollars to develop a pen that would work in microgravity because most pens use gravity to pull the ink from the well is not true (the story goes that when the Soviets were asked how they solved the problem and replied that they used a pencil), it is true that a private company developed a “space pen” in the 1960s that forces the ink out using pressurize gas, these pens can be bought for a few dollars each.

 

The health effects of microgravity have now been extensively studied as astronauts have spent months and years aboard the International Space Station. Once key issue is loss of muscle tone and bone density since those are both maintained on Earth by bodyweight resistance that does not exist in microgravity. Thus, astronauts are tasked with exercising for some 2.5 hours each day and even still will experience adverse effects that make it difficult to adjust to the living with their weight upon their return.

Challenge #3: Life Support

Once leaving the confines of Earth, certain requirements of life become scarce. At minimum, we require provisions of 4 kg of water, 3 kg of oxygen gas, and 2 kg of food each day to survive. Water and oxygen can be recycled after they are consumed by humans by condensing and distilling water humans release through sweat and urination and recreating oxygen from the carbon dioxide that humans exhale. The ISS is able to recycle 90% of the water and 40% of the oxygen consumed by humans with the losses re-provisioned by the periodic resupply missions that also bring new food, replacement crews, and equipment. On proposed long duration space flights well beyond low-Earth orbit, such resupply missions will not be so routine, so care will need to be taken in closing the recycling loop as much as possible. In principle, food can be grown on trips that are long enough and plants also provide an efficient ecosystem balance for repurposing the carbon dioxide that humans exhale. Optimizing plant growth for food and air recycling remains one of the research challenges taken on by current space missions and may be useful for future crewed missions to Mars or beyond.

To survive, humans also require temperature and atmospheric pressure control. The ISS maintains a temperature of normally 22° C (72° F) with temperatures adjustable between 18° and 26° C, well within the range of 4° to 35° C range to which humans can survive prolonged exposure. The space station has an atmospheric pressure the same as Earth’s, but as long as the gaseous concentrations are adjusted, there is, in principle, no upper limit for humans to survive higher pressures since it is pressure differences rather than pressure itself which causes harmful effects. For low-pressure environments, the famous Armstrong Limit (named after US Air Force Major General Harry Armstrong) at 1/16 standard atmospheric pressure is the point below which the water in a human’s circulatory system will start to boil with bruising and embolisms resulting. Experiments with test pilots suggest that without a pressure suit or oxygen, you would only have “useful consciousness” for a matter of seconds if in an environment below the Armstrong Limit. Contrary to some popular accounts in fiction, human beings do not explode if exposed to the vacuum of space.

Challenge #4: Radiation

The Earth’s atmosphere and its magnetic field provide a shield against much of the most harmful radiation that permeates outer space. In the immediate environment surrounding the Earth, the major source of such danger comes from the solar wind and cosmic rays which have their origin in highly energetic processes beyond the solar systems and, for the most energetic particles, even beyond our galaxy. The most harmful form of radiation is so-called “ionizing radiation” which consists of particles with enough energy to ionize atoms and therefore disrupt the chemical equilibrium in cells. Short term effects from exposure to such radiation include various acute radiation sicknesses and longterm carcinogenic and degenerative effects.

After about six months in low-Earth orbit with the same level of shielding as provided by the ISS, humans receive the equivalent dose of radiation to ten CT-scans which is close to five times the occupational safety level as recommended by health agencies. The increased risk associated with this exposure is one of the major long-term health risks of space flight. In addition to exposure rates, Space Agencies sets career limits on the total amount of radiation exposure which range from 5 to 20 years aboard the ISS (younger people have lower career radiation limits as some of radiation’s long-term cancer and central nervous system effects are shown to arise after many decades). This is at about the same level of radiation exposure that an astronaut would receive on one round-trip to Mars. For planned longer stays on the Red Planet, radiation levels are approximately three times that on the ISS.

For space travel outside of the Earth’s protection, space weather becomes a major consideration.

It is for these reasons that developing technology that can shield humans from radiation for long space voyages is one of the major research programs of the world’s space programs. A variety of ideas have been proposed, from a straightforward solution of placing a significant amount of mass between the astronaut and the radiation source to new specialized materials with radiation-absorbing properties or artificial magnetospheres that are mini-versions of the one that protect the Earth.

Challenge #5: Distance and Time

When the Apollo astronauts traveled to the Moon, the trip from Earth to the lunar orbital insertion took two days. The fastest space missions to Mars take months and follow somewhat surprising trajectories in the form of transfer orbits which allow the spacecraft to travel with the aid of two brief rocket thrust maneuvers. Other more energy-efficient proposals to get to Mars have included using the gravitational field of Venus to slingshot spacecraft at the expense of more time needed to travel slowly to Venus.

Using the existing technology to travel to other stars would result in travel times in excess of thousands of years. This is obviously prohibitive for human lives, and so a variety of possible solutions have been proposed to allow for people to get to the other potentially habitable places beyond our Solar System.

Interstellar Space Flight Proposals

In spite of the challenges mentioned above, there are some feasible ways to send spacecraft to nearby stars. The Voyager 1 and Voyager 2 spacecraft are the products of human technology that have travelled the farthest from the Earth — they are both currently coasting through interstellar space. However, they were not designed to study anything beyond our outer solar system. Although Voyager 1 has travelled over 15 billion miles away from the Earth, keep in mind that this is a distance of just 0.00255 light years and the nearest star to the Sun, Proxima Centauri, is 4.24 light years away.

Generation Ships

A concept sometimes used in science fiction are so-called “generation ships” which travel at speeds similar to those we can currently achieve and would arrive at distant stars many thousands of years in the future. The idea is to provide enough material and people to sustain a journey of such a length. Such ships are sometimes envisioned as entire self-sustaining ecosystems that are designed to last far longer than most human technologies survive. Whether such a generation ship would be feasible is a question that would need considerable input from ecologists and social scientists.

Light Sails

Solar Sails

Mass (and its relativistic twin, energy) poses a huge challenge for getting a spacecraft to travel close to the speed of light. One way to get around this is send out the absolutely smallest mass possible and use a sustainable source of energy to accelerate the craft, ideally with the energy source separated from the craft to minimize the total mass. An obvious choice for energy is the Sun, which will keep shining for another 5 billion years. In principle, the power from the Sun could be used to push on a sail that could travel out of our solar system. A good analogy for this, on an earthly scale, is a sailboat that is propelled by wind (which itself is powered by the Sun). With a solar sail, the light from the Sun (photons) directly push on the sail. While photons have no mass, they still impart a quantity called momentum that can speed up the sail. This force or push from the Sun is small — we don’t feel it pushing on us — so it takes time for the lightweight sail to speed up. That’s the basic idea, and it’s really quite simple. In fact, Johannes Kepler thought of the idea as he contemplated how sunlight pushes on the material in a comet (Halley’s comet, to be specific, which came by Earth in 1607) to create the comet’s long tails. Kepler wrote in a letter to Galileo in 1610:

“Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.”

Science-fiction writers thought about this, too. In 1964, American writer Arthur C. Clarke wrote the short story “The Sunjammer,” which imagines “sun yachts” racing through the sky as a sport. An idea for a solar sail to visit Halley’s Comet on its return in 1986 was seriously considered by NASA and the Planetary Society, which had been founded in 1980 by Carl Sagan. The first solar sail, called IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun; also a nod to the mythological Icarus who flew a little too close to the Sun) was launched in 2010 by JAXA, the Japanese air and space agency. The goal of IKAROS was to show that solar sails could be launched and work successfully. The sail for IKAROS was only 7.5 μm thick and was a 14 meter by 14 meter square (196 m² total, which is about half the area of a basketball court); it weighed about 2.2 kg. Two successful solar sails were launched in 2015 and 2019 by the Planetary Society, named LightSail 1 and LightSail 2.

Using sunlight to propel sails works well for visiting places in our solar system. However, as you get further away from the Sun, the sunlight gets very spread out and the sail receives less and less of it. By the time sunlight reaches the Earth, we receive 1,360 watts for every square meter (1360 W/m²). So, the 196 square meter sail for IKAROS received about 270,000 W of power from the Sun (14 m × 14 m = 196 m² and 196 m² × 1360 W/m² = 266,560 W). A way around this is to use a different source of energy that can provide power at this level or much, much higher. This can in theory be done with lasers.

Breakthrough Starshot

The Breakthrough Starshot initiative has the goal of sending a light sail to take pictures of Proxima b, which is the closest known exoplanet to our solar system. The Breakthrough Starshot initiative received $100 million in funding from entrepreneur Yuri Milner in 2016 (Milner also funded the Breakthrough Listen initiative, which is focused on SETI). The project aims to create a fleet of nanocrafts that are made up of a small chip to take images of the planet that is attached to an extremely thin and lightweight sail. The sails would be pushed on their journey by an array of powerful lasers that are on the Earth’s surface. The goal is to send as many nanocrafts as possible, as some fraction are expected to fail somewhere along the way or after arriving at Proxima b.

There are many engineering challenges to overcome, such as designing and building an array of lasers on the Earth that can send out up to 100 GW of power (a GW is a billion watts; sci-fi fans may recognize “1.21 giga watts” as the power needed for the flux capacitor in Back to the Future). The chips, which include the camera and other communication equipment, need to be extremely light, on the order of just a few grams (about the mass of a few paperclips), and around the size of a postage stamp. The sail also needs to no more than a few grams total and made of a reflective material, while still being large enough — perhaps a few meters on each side — to function properly. That all said, progress has been made on the project. Some prototypes, with much smaller sails than what the final version will have, were launched into the Earth’s atmosphere and took pictures of the Earth.

Relativistic Advantage and Constant Acceleration

Einstein’s Theory of Relativity offers a few fantastical but physically plausible mechanisms for allowing for interstellar flight in ways that could in principle allow for a human being to explore the any location in the observable universe in one lifetime. To accomplish this incredible feat, a spacecraft could take advantage of time dilation and length contraction.

Einstein’s Theory of Special Relativity is based on two assumptions: the laws of physics are universal in all reference frames (a reference frame is defined by a speed), and the speed of light is constant in all reference frames. Taking these two assumptions to their logical conclusions implied that different reference frames will measure lengths and durations differently depending on the speed. In particular, the faster a frame of reference is moving, the shorter the distances in the direction of motion will physically become. This is length contraction. Likewise, time moves more slowly in moving frames of reference such that less time will pass in a moving frame than in a stationary one.

A key factor used to calculate these relativistic effects is called the Lorentz Factor

where v is the speed  and c is the speed of light. Relativity requires this factor be multiplied or divided by measurements of space or time in different reference frames to account for the effects of the constant speed of light in all reference frames. The Lorentz Factor, then, shows immediately why the speed of light is the universal speed limit since a speed of v = c gives a gamma factor of infinity.

If you know the Lorentz Factor of a frame of reference, that is the amount by which times () are dilated (increased) can be related to each other by first considering all clocks that are moving with the reference frame to have a time () and then asking how the time measured on the same clock would be observed to pass when compared to a clock that was not moving (). Using the Lorentz Factor to relate these two different time measurements is done with the equation:

.

Thus, if a spacecraft is traveling with a speed v = 0.6c, it will have a Lorentz Factor of and therefore a stationary observer would observe that one hour passes on the moving spacecraft for every one hour and fifteen minutes elapsed on their stationary timepiece.

The Lorentz Factor is likewise the factor by which lengths () are contracted (decreased). Similar to the consideration of time dilation, an observer can either measure the length between two locations when traveling at the same speed as the common reference frame between the two locations (), or one can measure the same length when traveling at a different speed (). Using the Lorentz Factor to relate these two different length measurements is done with the equation:

.

Using the example of a 100-meter-long spacecraft traveling at 60% the speed of light, a stationary observer will observe the spacecraft’s length to be a mere 80 meters. At the same time, passengers traveling on the spacecraft, will see the length contraction occurring in their direction of travel between two points in the same reference frame that appear to the observers in the spacecraft to be traveling at 60% the speed of light in the opposite direction compared to the spacecraft’s travel. This means that there will be literally less space to traverse than if they had measured the proper distance to the destination from a reference frame where everyone was at rest.

This can have dramatic effects at speeds close to the speed of light. For example, traveling at 80% the speed of light will result in a Lorentz Factor of which means that distances to all the stars are reduced by 40% with a comparable reduction in travel time. Traveling at 99.5% the speed of light results in Lorentz Factor of with a 90% reduction in distances and travel times.

Although no spacecraft can reach the speed of light, spacecraft speeds, in principle, can get arbitrarily close to it with arbitrarily large Lorentz Factors. In such situations, the distances between two points become shorter and shorter and the amount of time it takes to get between two locations becomes smaller and smaller. Move fast enough and you can get to the edge of the universe in as short amount of time as you want.

The trade-off is that that the closer the speed gets to the speed of light, the more energy that is needed to increase the speed by the same amount. Roughly, for speeds that are close to the speed of light the amount of energy needed scales as the Lorentz Factor, so to boost a spaceship to the speed of light will require an infinite amount of energy. Therefore, these speculations require something close to an inexhaustible supply of energy to achieve these fantastic effects. Even the most conservative approaches require energy budgets that are well in excess of the entire energy budget used by humankind each year. Getting to speeds that will make the journey between Earth and any distant location as short as possible will require developing entirely new sources of energy that are currently well beyond human technology.

Assuming that such an energy source (matter-antimatter, for example) could be harnessed, the planning of the journey has some additional complications. For example, speeding up to high speeds requires accelerating the spaceship which, as anyone who has ever been on a rollercoaster can attest, could result in some discomfort if the acceleration is large enough. A proposed solution that has been offered as a thought experiment is to accelerate the spaceship at a constant 1 g of acceleration at the equivalent to Earth’s gravitational acceleration of 9.8 m/s². In such a scenario, the spaceship occupants would feel the same gravity as they did on Earth, and after one year at this constant acceleration the spacecraft will reach 77.5% the speed of light.

Constant acceleration space travel, then is a fairly astonishing physical scenario. A successful trip might entail accelerating towards the destination for the half the trip, and then decelerating for the second half so that the spacecraft arrives in the same frame of reference as the target. This allows for arrivals at even incredibly distant destinations within human-realizable timeframes. Under such scenarios, it takes 3.5 years to reach Proxima Centauri, just under 20 years to reach the galactic center, and just under 50 years to travel the furthest distances observable from Earth.

One complicating matter is that as you start to move at relativistic speeds, the ambient electromagnetic radiation becomes blueshifted into energy realms that are lethal and highly destructive. Likewise, colliding with the particles that might be along the trajectory would be the equivalent of an atom smasher. The spacecraft would have to survive such an onslaught, so some sort of shielding would be required to protect from the exposure.

Returning home brings on other difficulties. While the time passed on the spacecraft is relatively short, the time elapsed on Earth will be substantially greater. This is due to the so-called “Twin’s Paradox” which relates that in the context of relativity, an observer who accelerates to a boosted reference frame and then decelerates back will experience less time than an observer who remains in the initial reference frame. This has the effect that while the spacefarer in a constant accelerating spacecraft experiences a 7 year round trip to the nearest star, everyone who remains on Earth will experience 12 years. The astronaut who travels to the center of the Milky Way will return to an Earth that is 55,000 years in the future while the voyages that go to the edge of the universe will return to a Milky Way well after the Sun has become a white dwarf, billions of years in the future.

In spite of all these uncomfortable complications, the very fact that such a journey in a human lifetime is physically possible is somewhat remarkable. While we are nowhere near being able to build such constant acceleration spacecraft engines, there is no law of physics which prevents such a thing from being constructed, though it would be on a scale much more massive than anything currently possible.

 

Review Questions

Summary Questions

1. What is the main reason that space travel from Earth is so challenging?
2. What is microgravity?
3. Why does microgravity pose challenges for astronauts?
4. Why is radiation from space is harmful to life?
5. How do the large distances to other stars pose a challenge in traveling to them?
6. What are some ways to deal with the vast distances involved in interstellar travel?

Exercises