Scientists consider how to visit the closest exoplanet
By now, you probably have heard that astronomers have discovered a new exoplanet orbiting the star closest to our sun. That star, Proxima Centauri, is 4.2 light-years away. In miles, that’s like 25 trillion.
The new planet’s name is Proxima b. Because its star is so relatively close, some people wonder whether it would be possible to go there. And it is possible, in a way. But it would take a very long time. Even at the speed of light, it would take more than four years to go from Earth to the new planet (and no spaceship can go nearly that fast). New Horizons is the spacecraft that flew past Pluto. At 16 kilometers (10 miles) per second, it’s the fastest vehicle that humans have launched. That was good enough to get to Pluto in nine years. But at that speed it would take almost 80,000 years to get to Proxima b.
In science fiction stories, of course, a warp drive or some other invention allows a spaceship to go faster than light. Then you could get to Proxima b easily. But in real life, nothing can go faster than light, and the best rockets can’t even come close.
Scientists do have ideas for building ships that could do better than those available today. Rockets work by expelling their fuel’s exhaust behind the vehicle. That propels the rocket forward because for every action there is an equal and opposite reaction. (That’s Isaac Newton’s third law of motion.) To go forward faster, then, you need to expel your fuel’s exhaust faster backward. One plan to do that would use electrically charged atoms to create a plasma engine. Its exhaust could speed away at 50 kilometers (about 30 miles) per second. That would make it a few times faster than the New Horizons probe.
Another very new idea would use radioactive matter. Many unstable atoms produce a specific form of radioactivity known as alpha particles. When an atomic nucleus spits out alpha particles, they can zip away 300 times faster than the exhaust of a plasma engine.
But to power a spaceship with alpha particles, you’d need a good supply of their radioactive source. Uranium might work. It comes in different forms, called isotopes. Some of them would be better than others. Perhaps the best of these, uranium-232, is not found in nature. It can be produced from other elements, however, by carrying out the right nuclear reactions.
Alpha particles usually fly off in all directions. So to power a ship, you would need a material that stops the particles flying off in one direction. Those free to fly the other way (the exhaust) would drive the ship forward (via that opposite but equal reaction, again).
Because alpha particles are tiny, an alpha-powered ship would start out slow. (In fact, you would have to shoot it into space to begin with, powered by an ordinary rocket.) But in space, with nothing to slow it down, the alpha ship would gradually gain speed. Eventually it might reach 200 to 300 kilometers (some 125 to 185 miles) per second.
In a scientific report about this plan, researchers from China say an alpha-powered ship could reach Proxima Centauri in 4,000 to 9,000 years. The exact time would depend on how much weight was onboard compared with how much fuel the ship carried. That’s much faster than other propulsion systems. Still, nobody onboard the ship when it departed would still be alive when it arrived.
It might be possible, though, to send tiny spacecraft to Proxima Centauri much more quickly. And even a craft too small to carry passengers might still be big enough to hold cameras that snap pictures of the new world.
One group has proposed such a plan. A wealthy Russian named Yuri Milner and the famous physicist Stephen Hawking announced a project to study whether very small “nanocraft” could be sent to the Alpha Centauri system. (Proxima Centauri is one of three stars in that system.)
Their plan would be to build tiny cameras and lasers onto a wafer weighing about as much as a dime. Attached to the wafer would be a very thin material, sensitive to laser light. That would act as its sail. Pressure from a powerful laser beam could propel the sail-wafer. Scientists working on the plan think they might be able to drive a fleet of such wafers toward Proxima Centauri at one-fifth the speed of light. At that rate, the wafer crafts might reach Proxima b in 20 years or so. Then they could use their laser to send data about the planet back to Earth.
Nobody knows for sure whether this plan will work. Even if it does, building a ship that fast — but big enough to carry people — would be much, much harder. And it might not be such a good idea. A real ship (say the size of the space shuttle) flying at a fifth the speed of light would be very dangerous. If something that size crashed into Earth, it would release the same amount of energy as millions of atomic bombs. If any intelligent beings living on Proxima Centauri saw such a ship heading their way, they might get pretty nervous.
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alpha particle A type of ionizing radiation, physically equivalent to the nucleus of a helium atom (because it consists of two protons and two neutrons). Alpha particles carry away some of the energy that had been in the nucleus from which it had been emitted. That energy will be deposited into any material with which it collides. If alpha particles collide with living tissue, they can be very biologically damaging. Alpha particles are released by some radioactive atoms, including plutonium, polonium, radium and uranium.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
atomic Having to do with atoms, the smallest possible unit that makes up a chemical element.
element (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
engine A machine designed to convert energy into useful mechanical motion. Sometimes an engine is called a motor.
exhaust (in engineering) The gases and fine particles emitted — often at high speed and/or pressure — by combustion (burning) or by the heating of air. Exhaust gases are usually a form of waste.
exoplanet A planet that orbits a star outside the solar system. Also called an extrasolar planet.
isotopes Different forms of an element that vary somewhat in weight (and potentially in lifetime). All have the same number of protons but different numbers neutrons in their nucleus. As a result, they also differ in mass.
laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.
light-year The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.
matter Something which occupies space and has mass. Anything with matter will weigh something on Earth.
nuclear reaction Events that physically alter the nucleus of an atom. (This is in contrast to chemical reactions that affect the electrons orbiting an atom.) Some nuclear reactions will transmute an atom, change it into a different chemical element, such as through fission (also known as atom splitting). Others may involve the capture of energy by bombardment with electromagnetic radiation or subatomic particles. Nuclear reactions are not affected by temperature and pressure (as chemical reactions may be). Instead, they are driven primarily by the energy of the particle that hits them or by the intensity of the radiation prompting the reaction.
nucleus Plural is nuclei. (in physics) The central core of an atom, containing most of its mass.
particle A minute amount of something.
physicist A scientist who studies the nature and properties of matter and energy.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
plasma (in chemistry and physics) A gaseous state of matter in which electrons separate from the atom. A plasma includes both positively and negatively charged particles. Pluto A dwarf planet that is located in the Kuiper Belt, just beyond Neptune. Pluto is the tenth largest object orbiting the sun.
pressure Force applied uniformly over a surface, measured as force per unit of area.
propulsion The act or process of driving something forward, using a force. For instance, jet engines are one type of propulsion for keeping airplanes aloft.
radioactive An adjective that describes unstable elements, such as certain forms (isotopes) of uranium and plutonium. Such elements are said to be unstable because their nucleus sheds energy that is carried away by photons and/or and often one or more subatomic particles. This emission of energy is by a process known as radioactive decay.
rocket Something propelled into the air or through space, sometimes as a weapon of war. A rocket usually is lofted by the release of exhaust gases as some fuel burns. (v.) Something that flings into space at high speed as if fueled by combustion.
science fiction A field of literary or filmed stories that take place against a backdrop of fantasy, usually based on speculations about how science and engineering will direct developments in the distant future. The plots in many of these stories focus on space travel, exaggerated changes attributed to evolution or life in (or on) alien worlds.
speed of light A constant often used in physics, corresponding to 1.080 billion kilometers (671 million miles) per hour.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Or a sunlike star.
trillion A number representing a million million — or 1,000,000,000,000 — of something.
uranium The heaviest naturally occurring element known. It’s called element 92, which refers to the number of protons in its nucleus. Uranium atoms are radioactive, which means they decay into different atomic nuclei.
Journal: W. Zhang et al. Revisiting alpha decay-based near-light-speed particle propulsion. Applied Radiation and Isotopes. Vol. 114, August 2016, p. 14.
Press Release: Internet investor and science philanthropist Yuri Milner & physicist Stephen Hawking announce breakthrough starshot project to develop 100 million mile per hour mission to the stars within a generation. Breakthrough Initiatives. April 12, 2016.
Press Release: VASIMR® VX-200 meets full power efficiency milestone. Ad Astra Rocket Co. November 23, 2010.