The NASA/ESA Hubble Space Telescope has been orbiting the Earth well above our planet's atmosphere for over 25 years.
Editor's note: As part of Science Channel Weekend, Discovery Channel will premiere "Telescope" on Feb. 20 at 9 p.m. ET/PT, a "dynamic journey behind the scenes of the next step in the evolution of telescopes: NASA's James Webb Space Telescope," directed by Oscar®-nominated filmmaker Nathaniel Kahn. The James Webb Space Telescope will be 100 times more powerful than the NASA/ESA Hubble Space Telescope (pictured here) and "Telescope" will tell the story of this epic mission and the history of astronomy to this point.
Discovery News will be following this event closely and, in the run-up to the premiere, will feature related articles and galleries based around the history of astronomy and the incredible feats the Hubble continues to achieve over 25 years after launch. Read on for a rundown of our top 10 moments in astronomical history that have helped us reach the construction of NASA's next generation space telescope...
In January and February 2015, all five naked-eye planets were visible in the night sky.
Our earliest ancestors depended on the sky to track the changing of the seasons. It told them when certain kinds of animals would be available for hunting, for example. When agriculture developed, civilizations such as those in ancient Egypt used the stars to predict when to plant crops and when to harvest them. We used the sky as a giant clock to tell time during the year. When phenomena such as eclipses or comets appeared, they were seen as unpredictable events guided by the gods. Today, we know these occur due to orbits and gravitational interactions in space.
Over time, some clever people noticed there were stars that moved in predictable ways across the sky. They traveled the same track as the sun and moved among the background stars. We now know these objects as planets (loosely translated from the Greek word "wanderers"). In many cultures, these planets were given the names of gods. Even the English names of Mercury, Venus, Mars, Saturn and Jupiter -- the five planets known by the ancients -- are named after supreme beings.
This graphic shows the position of all the planets in our solar system, but is not to scale and includes some stylistic additions (such as the galaxy at lower right).
Early beliefs (based on religion) often said the Earth was at the center of the universe. While early astronomers could observe the sky, there were many things they didn't understand. Why did Mars, for example, sometimes reverse course in the sky and then resume marching in the same direction as other planets? Some astronomers came up with elaborate geometrical constructions called epicycles, which were supposed to predict the planets' seemingly chaotic movements.
A simpler solution was proposed by Nicholas Copernicus in the 1500s, who published material showing the sun at the center of the universe, with the Earth revolving around it just like other planets. (This also was proposed by Aristarchus of Samos in Greece in the third century, but his writings were not well-known to the Western world at the time). This solved the problem of the epicycle, and it was backed up with other evidence. For example, Galileo's discovery of moons orbiting Jupiter in 1610 showed that not everything revolved around the Earth. Religious authorities were not pleased, but with time the evidence outweighed the arguments.
As telescopic technology advanced, we eventually learned the sun is not the center of the universe, either. In the 1750s, it was believed that the Milky Way was a large collection of stars with its own center. By the early 1900s, observations of novas (starbursts) in other galaxies showed that they were further away than the Milky Way. Eventually, astronomer Edwin Hubble found evidence that the universe was expanding equally in all directions, with no true center.
Artist's impression of the gravity field around Earth as well as Gravity Probe B, a spacecraft intended to measure space-time (height, width, length and time).
While we could see the planets moving, what caused them to move was poorly understood for millennia. This changed in the 1600s when Sir Isaac Newton began to apply mathematical theory to observations of the universe. He calculated three basic laws of motion as well as a law of universal gravitation, which says that any two things in the universe attract each other. Planets have large gravitational attractions, while pebbles in the ring of Saturn would have small ones.
In the early 1900s, our understanding of gravity changed with observations by physicists such as Albert Einstein, who found that time can change depending on your reference frame. If you're travelling close to the speed of light, your sense of time slows down compared to somebody living on Earth. Time was eventually treated as a fourth dimension (after width, height and length) and led to a better understanding of the extreme gravitational conditions around black holes and other massive gravitational objects. Gravity of an object was then treated as a "warp" in the universe of space-time, with objects within the warp becoming influenced by gravity.
In early 2016, gravitational waves were detected by the Laser Inteferometer Gravitaional-Wave Observatory (LIGO). These are essentially ripples in space-time caused by massive objects orbiting each other, such as black holes. Einstein predicted their existence and astronomers have been trying to track them down for 50 years.
Auroras sparkle in these images of Uranus captured by the Hubble Space Telescope in 2011.
The telescope revealed many faint objects beyond the reach of human eyes. William Herschel discovered Uranus in 1781 inadvertently when he was cataloging all the stars he could find that were magnitude 8 (fainter than the human eye's capability) or brighter. That's when he spotted Uranus moving among the background stars. He planned to name it after King George III, but other astronomers decided to name the planet after a god, just like the other ones.
Other discoveries swiftly followed: Ceres (then called an asteroid, not a dwarf planet) was found in 1801, the first of now thousands known. Neptune was found in 1846, and Pluto (first considered a planet) in 1930. The solar system was a much vaster place than previously imagined. Over time, models predicted that comets likely resided beyond the orbit of Neptune in a place of icy objects called the Kuiper Belt. In the early 2000s, several new Pluto-sized objects were found in the Kuiper Belt, prompting the International Astronomical Union to create a new category of objects called "dwarf planets" and place Pluto and Ceres within this category.
No less astounding was the discovery of planets beyond our own solar system. First, astronomers found 3 planets orbiting pulsar PSR B1257+12 in 1992, then a huge exoplanet orbiting the main-sequence star 51 Pegasi in 1995. We now know of more than 1,000 confirmed planets outside of the solar system, with thousands yet to be confirmed. The bulk of them have been discovered using NASA's Kepler space telescope, which was launched in 2009.
The speed of light puts a serious damper in our plans to visit the Andromeda Galaxy, which is 2.5 million light-years away from Earth. Inset is a possible black hole.
We use the speed of light as one way to measure the universe. We've clocked it accurately for centuries, and know that the speed is about 300,000 kilometers (186,000 miles) per second in a vacuum. The sun is about eight light-minutes away from Earth. The closest star system (Alpha Centauri) is roughly four light-years away, and the closest large galaxy (Andromeda) is about 2.5 million light-years distant.
While we have "Star Trek" dreams of warping across the universe to these distant destinations, at least for now it appears we are limited by physics. Another of Einstein's discoveries was how mass and energy are equivalent, which is shown in the equation E=mc2. As you fly at speeds close to the speed of light, the energy required to get you there increases the mass of you (and the vehicle you are travelling in). At a point just before breaching the speed of light, mass becomes infinite. In other words, it's impossible to travel any faster.
There are some clever shortcuts proposed, however. Perhaps the universe has wormholes where you can travel seamlessly through space and time. Perhaps there are ways of at least communicating faster than the speed of light, since entangled quantum particles can communicate instantaneously, regardless of how far they're pulled apart (this is often called "spooky action at a distance"). But as far as we know it for now, the speed of light is as fast as anything can travel.
The cosmic microwave background is a remnant of the energy of the Big Bang.
If the universe began as a singularity and then expanded outwards -- a phenomenon known as the "Big Bang" -- it must have been an environment of unimaginable energy. Over time, as the universe got bigger, that energy dissipated, cooled and condensed into the matter that fills the cosmos.
We have now observed the remnant of that immense explosion following an accidental discovery in 1965. While the background radiation was first predicted by Ralph Alpher in 1948, two researchers at Bell Telephone Laboratories only found it decades later when they discovered interference on a new radio receiver they were building. Arno Penzias and Robert Wilson made their discovery just as another team was trying to find it, which resulted in two papers (one from each team) being published in the Astrophysical Journal in 1965.
Astronomers are now aware there are tiny temperature fluctuations (called anisotropies) in the cosmic microwave background (CMB) that reveal very slight density fluctuations in the early universe. These minuscule fluctuations can be detected by very sensitive instruments, such as the space-based NASA Wilkinson Microwave Anisotropy Probe (WMAP) and European Planck space telescope. It is believed these variations reveal much about how the early universe formed, the large scale structure of the universe and the nature of the earliest galaxies to form.
Artist's impression of the Big Bang believed to have started the universe, and the universe's subsequent expansion.
In 1929, astronomer Edwin Hubble discovered the universe is expanding. He was a busy and diligent observer, using a new 100-inch telescope at Mt. Wilson in California to make many discoveries, such as the true distance to galaxies. He did so by looking at novas in these galaxies, estimating their brightness and then calculating how much the brightness dimmed with distance. Then, based on work from astronomer Vesto Slipher, Hubble measured the motion of galaxies and published a paper that eventually showed that the universe is expanding.
This discovery was astounding enough, but the astonishment continued when in the late 1990s, the universe was found to be accelerating in its expansion. Astronomers measuring supernovas in distant galaxies found that the supernovas were less bright than would be predicted by their redshifts (indicating they are moving away from us). This discovery eventually netted the key researchers a Nobel Prize.
This Hubble Space Telescope image shows evidence of a gravitational lens at its center (the faint circle surrounding the middle galaxies). In some cases, gravitational lenses could be evidence of dark matter. In this case, however, a galaxy cluster likely created the distortion.
The universe's accelerated expansion was a mystery to astronomers, but they proposed that there must be some sort of force pushing it along. A leading theory today is that there is a force called dark energy that can't be directly detected with current astronomical techniques.
There are a few theories as to what this dark energy could be. It might be a property of spacetime itself. As space expands, more of this dark energy is created, pushing the expansion even further. Another possible explanation stems from the quantum theory of matter, which theorizes that particles appear and disappear and create energy.
Dark energy is believed to make up around 68 percent of the mass in the known universe, with dark matter coming in at around 27 percent. Scientists aren't sure what dark matter is, but they are aware that their gravitational calculations show that there is a large chunk of the universe that we can't see. We can indirectly observe dark matter, though, by its effect on gravity. One example is seeing how it bends light, through a phenomenon known as gravitational lensing. The rest of the universe, less than 5 percent, is made up of the energy and matter that we can see with telescopes.
Geysers erupt on Saturn's icy moon Enceladus, as viewed by the Cassini spacecraft in false color.
Water is considered a key ingredient of life, and over time we have come to learn that it is a universal element in the solar system and, indeed, the universe at large. Early observations by spacecraft in the 1970s and 1980s revealed icy worlds beyond Earth. The discovery of icy moons near Jupiter, Saturn and beyond came as a surprise since we are so used to observing the airless moon near Earth, but over time we have learned to appreciate their complex chemistry.
These worlds (such as Jupiter's Europa and Saturn's Enceladus) are considered the best promise of life beyond Earth that we know of in our own solar system. Of note, water may also survive in liquid form inside these moons. One example is Saturn's Titan, which is replete with hydrocarbons (one of the building blocks of life) and, models indicate, could host a liquid ocean underneath its surface.
More advanced observations in the 1990s and beyond revealed water ice in some surprising places. It turns out that water ice can survive on the airless moon and even on Mercury -- the closest planet to the sun -- as long as it lies within permanently shadowed craters from the sun, or under a protective layer of dust. Mars also has polar caps that are partially made of water ice. Water ice is also present on comets and on certain small worlds, such as the dwarf planet Ceres.
Artist's impression of the James Webb Space Telescope, which is expected to launch in 2018.
Astronomy is only going to get more interesting as telescopes improve to probe the origin of the universe, and look for other worlds besides our own. One example of telescopes being planned include NASA's James Webb Space Telescope (a successor to the Hubble Space Telescope), which is scheduled to launch in 2018. It is expected to look at the origin and evolution of galaxies, how stars came to be, and the circumstances around "first light" -- when light first shone in the universe shortly after the cosmos was formed.
The European Extremely Large Telescope, expected to be completed in 2024, will check out the universe's secrets from the ground. It is planned to look at exoplanets, the early days of the universe, supermassive black holes and the mysterious nature of dark matter and dark energy. E-ELT, James Webb and other next-generation telescopes will also attempt to search for Earth-like planets in other solar systems, including examining their atmospheres, orbits and origins.
The recent discovery of gravitational waves, a key component of Einstein's general relativity predictions from 100 years ago, could also spawn a new type of astronomy dubbed "gravitational wave astronomy." Not dependent on the electromagnetic spectrum (i.e. visible light, X-rays and infrared), gravitational wave astronomy could measure the ripples in spacetime itself, revealing massive objects that would have otherwise remained invisible.
Remember to tune into Discovery Channel's premiere of "Telescope" on Feb. 20 at 9 p.m. ET/PT.