Space travel, the act of sending objects and humans beyond Earth's atmosphere, includes both orbiting Earth and traveling far beyond Earth's gravity. The fringes of outer space were explored as early as 1949, but only since 1957 have humans had the ability to send objects permanently beyond our atmosphere. Controlled motion in space requires the accurate use and measurement of time; the laws of physics dictate that there is only one best path from one place to another, a path available only by using precise timing. Thus, to travel successfully in space requires both an exact departure time and a way to measure time intervals with great precision.
To travel in space we must leave behind our Earth-centric concepts of time intervals and grapple with the concepts of relative planetary time and absolute time. The common definitions humans use for the passing of time on Earth change depending on the location in space. When communicating between planets, the orbits and rotations of each planet affect the success of those communications. The timing of those motions is critical to find the optimum moment to send and receive messages.
The distance between the planets of the solar systems places them minutes and hours apart. When communicating with objects in space, near-instantaneous communication changes with distance to asynchronous messages and responses. The universe is so immense that time in the form of the light year is used as a measure of distance in space travel. To travel beyond our solar system, it will take a very long time to get any place if we cannot travel nearly as fast as light.
Although at extremely high speeds the theory of relativity is invoked, the basic motions of space travel are governed by Newton's laws of the motion of bodies and Kepler's laws of planetary motion. Newton and Kepler showed that orbital motions are predictable both backward and forward through time. Through careful recording of where objects in space have been, it is possible to plan the optimum moment for a future launch of a space vehicle.
Space travel is possible because of the “clockwork” nature of the universe in classical physics. Everything in the universe interacts primarily via gravity. All destinations of any significant size are traveling in specific regular orbits. These orbits behave for the traveler as asynchronous clocks. Each object revolves in a regular orbit with a regular period, but every object has its own period. Navigation in space is therefore concerned with finding the optimal time to travel from one orbit to another.
Orbital mechanics dictates that there are an infinite number of possible free-flight paths between two objects. Space navigators are concerned with the trade-off between the path that requires the least fuel and time spent in transit. Operating within these limitations has meant that, from the beginning, space travel has been associated with certain time-specific terms such as countdown, launch window, and mission elapsed time.
A practical example of the timing needed would be predicting the optimum time to leave Earth for the Moon. This is referred to as a launch window or orbital window. The Moon revolves around the Earth in an elliptical orbit, so there is an optimal time of the month to launch when the Moon's orbit brings it closest to Earth. The calculation also should consider that the Earth rotates on its axis. Every 24 hours there is an optimum time for the rocket to leave Earth pointing at the moon. The interaction of two cyclic clocks gives a best time and day every month to launch. The planner also needs to account for the motion of the Moon during the rocket's time of transit, aiming for where the Moon will be when the rocket reaches lunar orbit path.
To go to another planet in the solar system, the planner must consider the orbital periods of different objects around the sun. For a launch to Mars the best orbital window occurs only once every 2.2 years when both Mars and Earth are on the same side of the sun. Also, most orbits are elliptical, and so where each planet is at its apogee (greatest distance) and perigee (closest distance) in relation to the sun also can dictate the optimum time to travel the least distance. The Voyager probes in the 1970s took advantage of a so-called grand alignment, when Jupiter, Saturn, Uranus, and Neptune were all on the same side of the sun. The fortunate timing of the “window” is evident when you consider this alignment only takes place every 176 years, because these planets' orbital clocks all run at such different rates.
As we continue to travel in space, other cyclic phenomena will influence the timing of rocket launches. A short list includes the approximately 11-year solar cycle of flare activity, the orbits of various comets and asteroids, the behavior of planets with multiple moons, and the motions of other solar systems around the core of the Milky Way.
A last, now common, use of time and predictable orbital motion in space is in global positioning system (GPS) satellites. The fundamental properties of a satellite regularly orbiting Earth allow an observer on the planet to determine his or her location. An observer with an accurate clock and the knowledge of the transit time for signals from four satellites can determine location. The transmitter compares its internal clock with the ultra stable and accurate atomic clocks on each of the four satellites. The difference between the five clocks is influenced by the motion and location of the satellites and can be used to calculate a location and altitude on the planet very accurately.
Space travel shows that many of our common time measurements are relative to Earth. As humans travel in the solar system, they will need to redefine what terms like noon, day, month, and year mean. The case of Mars is particularly illustrative because we contemplate colonization there at some future time. Travelers in space will face the problem of choosing their own definitions. Whether the defined terms will be of where they departed, their destination, or some standard time is still not clear.
Humans have a local interpretation of when noon is. We tend to think of noon as “solar noon, when the sun is highest in the sky. That also matches with” mean noon' (what the clock says) within plus or minus 16 minutes because Earth has a very regular orbit around the sun. We experience only minor differences throughout the year. Mars, however, has an orbital eccentricity that is considerably larger, which means that the lengths of various Martian seasons differ considerably. On Mars the time of day when the sun is highest in the sky varies throughout the year. On Mars, because of its orbit, mean noon and solar noon can be plus or minus 51 minutes apart through the seasons.
A sidereal day on Earth is 23 hours and 56 minutes, and on Mars it is 24 hours 37 minutes. For an explorer on Mars this will require only a moderate adjustment. When operating missions remotely from Earth, however, more planning will be required. During missions that last weeks and months, the days on Earth will become out of sync with those on Mars. The Jet Propulsion Laboratory team running the Pathfinder missions had to modify their schedules to the Martian day in order to coordinate probe observation times with daylight on Mars, regardless of what the daylight time was on Earth. They coined the term sols to describe mission times and events on Mars. Any future residents of Mars will have to develop their own clocks to cope with this subtle change.
On Earth a month is defined by the motion of the Moon around the Earth every 27.3 days. With 12 to 13 orbits per year, months on Earth are 1/12 of a year. Other planets, however, may have no moon, or several moons. Residents will need to create a new calendar, with new definitions of the length and name(s) of the months. What will define a month on Mars? It has two moons, Deimos and Phobos, that whip around it every 1.3 and 0.3 days, respectively, making for very frequent months.
We define a year as 365 days, 5 hours, 48 minutes, 46 seconds (the time for Earth to revolve around the sun). In our solar system, years vary in length more than 1,000 times, from 88 Earth days on Mercury to 90,777 days on Pluto. Living year-round on Mars would require adjustment for residents to the Martian year of 687 days. Because we measure our lives in these “Earth standard” units of time, how will such basic concepts as birthdays and age change? Exploration in space will mean either being tethered to a single planet's definitions or developing new ways of naming how time passes.
Communication distances in space are another way that time and space travel are tied together. Objects in space are separated by huge distances, which translate into being separated by time. We need precise clocks to communicate across these distances.
Radio waves, like all forms of electromagnetic waves, travel at the speed of light. The time it takes for a message to travel a given distance can be calculated using the equation t = d/v (t = time, d = distance, v = speed of light). Our Moon is so far from Earth that light (and radio waves) takes 1.3 seconds to get there. Communications between these bodies have this 2.6-second gap between when a message is sent and when the reply is received. For human conversation this is merely annoying, but for an operator remotely driving a robotic vehicle, this 2.6-second gap could be disastrous.
An additional problem is that Earth is constantly revolving and traveling in its orbit. These motions move the transmitters and receivers apart, so multiple sets are needed over the surface of the Earth to have continuous communication with an object in space. The problem is increased when communicating with probes on or orbiting around other planets which are moving too.
Consider, as an example, a robot explorer on Mars. The round trip for communications to Mars when Mars is closest to Earth is 20 minutes. When the Earth and Mars are at opposite sides of the Sun, the round trip is 40 minutes. Additionally, both Earth and Mars revolve on their respective axes, so the transmitters and receivers on their surfaces are moving constantly. Similar to orbital windows for launching, there are optimal communication windows when the communicators are facing each other. The clocks between these transmitters and receivers need to be precisely synchronized. For most space missions there are communication blackout times when communication is impossible because the transmitter and receiver do not have “line of sight.”
Each planet exists within a “time island,” separated from the others by the gap it takes communications to go between them. This time gap functions as an isolating element when doing realtime tasks like communicating between computers. If we choose to explore the outer solar system, the gap for round-trip communication between Earth and a moon of Saturn would be 2 hours and 12 minutes. In our world of networked computing and Internet communications, this is an unseen barrier to human colonies in the solar system. Each colony will need its own computer network and will communicate with outside systems though a built-in delay. This has had implications for remote control of exploratory probes, leading to an increase in the ability of probes to be autonomous of a human controller. For time-critical maneuvers, like entering or leaving orbit, instruction sets are sent for the robot to carry out, and highly accurate clocks are required.
As we move farther from Earth the gaps become more significant. Human and robot explorers become travelers isolated from timely assistance or instructions. An explorer on Pluto would have a round-trip communication gap of more than 9 hours. Exploration beyond the solar system will have communication gaps of weeks and months. Travelers to the nearest star (4.2 light years away) would experience a gap of almost 8 1/2 years between a sent message and a reply. Any meaningful dialogue will be reduced to asynchronous communication spread over decades. As we contemplate life in other solar systems, this gap suggests we may never be able to have a dialogue with other intelligent life, only a series of exchanged monologues.
Time is used in space travel as a measure of distance. The known universe is so vast that we need a very long measuring stick to conceptualize it. The commonly used measure of space distances is the speed of light. Light is the fastest phenomenon we know of, moving at approximately 30 centimeters (approximately 1 foot) per nanosecond. In fact, the current definition of the meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second. In a year (31,557,600 seconds) light travels 9.46 × 1012 kilometers or about 5.88 × 1012 miles. We can conceptually measure the relative size of space with light. In our solar system, the Moon is 1.3 light seconds from Earth, the sun is 8.3 light minutes away, and Pluto is 4.4 light hours away. The nearest star to Earth, Alpha Centauri, is 4.3 light years away (273,000 times the distance from Earth to the Sun), the center of our galaxy is 32,000 light years from Earth, and the nearest galaxy to Earth is 75,000 light years away. These distances remind us that the events we see in space are minutes, hours, and years separated from what is actually happening at a given location.
Distances between solar systems are so large that there have been only a few objects sent beyond the solar system. Pioneer 10 was launched in 1972 and left the solar system in 1983, aimed at the star Aldebaran, about 65.1 light years away. At its present rate of speed it will take it more than 2 million years to reach there. Voyagers 1 and 2, launched in 1977, have also left the solar system. Because of their gravitational assists during their mission, they are traveling at a much higher velocity than is
Pioneer. Voyager 1 is traveling at a speed of 61,799 kilometers per hour (.000057 the speed of light) covering about 538,465,972 kilometers a year. Even at those speeds, as of February 2008, Voyager 1 was “only” 15.7 billion kilometers from the sun.
Voyager 1 is traveling faster than any other spacecraft humans have launched, yet it will not be “near” another solar system for 6,600 years when it passes 4 light years from Barnard's Star. The closest Voyager 1 will get to another solar system in the next 300,000 years will be 1.7 light years, 38,000 years after launch. From that point, at its present speed if it could fly straight to the system, it would take it an additional 30,000 years.
The distances in space are daunting without some way to reduce the travel time. Velocity in space is related to the amount of force one can produce and the amount of time needed to produce it. Space travel is essentially frictionless, so once a velocity is achieved, an object will continue at that speed indefinitely. An important implication is that the object will not slow down at its destination without the application of an equal and opposite force.
One solution would be greater rocket velocities with improved rocket propellants. Rocket propel-lants are compared by the specific impulse (Isp) they provide. Specific impulse is measured in “seconds,” or the number of seconds during which a rocket engine can produce 1 pound of thrust from 1 pound of propellant. The velocity a vehicle can achieve is directly proportional to the specific impulse. Current chemical rockets such as the NASA Space Shuttle have a specific impulse of 450 seconds (burning liquid hydrogen and liquid oxygen). To achieve greater velocities for longer times, we will need to explore experimental propulsion systems like nuclear (900-1,200 seconds) and ion propulsion, which has a specific impulse of 2,000 to 20,000 seconds. The ultimate would be an engine with a specific impulse of 30,000,000 seconds (approximately the speed of light).
One of the fascinating outcomes of relativity theory is the idea that, at very high speeds, time is distorted. The amount of time that passes for a passenger is reduced as the velocity of the spaceship is increased to larger fractions of the speed of light. This effect is of interest because the time it would take to reach another solar system would be reduced for the passenger. If we could attain greater speeds, we could achieve a kind of time travel for passengers, allowing them to experience less time passing than would observers on Earth.
The second way to tackle the long to extreme travel times between planets and solar systems is to slow the passage of time's effects on human passengers. There is speculation that the existence of “hibernation” genes within mammals or the development of “suspended animation” technologies would allow the engineering of humans to travel in a state of reduced metabolism. This suspension would allow essentially little or no time to pass for the traveler as well as reduce the number of supplies the craft would need to carry or recycle. It also may be possible through advances in biotechnology to engineer humans for extended life spans allowing “near-by” flights within a human life span. Finally, science fiction has explored the concept of “colony” ships, which would carry humans for several generations.
Light, Speed of, Newton, Isaac, Spacetime, Curvature, Time, Planetary, Time, Relativity of, Time Travel, Twins Paradox, Wormholes
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