By Seth Mayo, Curator of Astronomy
When you gaze up at the sky on a clear night, the enormity of the universe is in full view. Even though most of those celestial objects are at distances well beyond the scope of human comprehension, a floating telescope - merely 340 miles above the Earth - has been able to bring these far-off wonders closer to home. This amazing piece of technology is none other than the orbiting observatory, the Hubble Space Telescope.
2020 marks 30 years since the launch of Hubble, when it hitched a ride aboard the Space Shuttle Discovery in 1990. Since this school-bus-sized telescope was placed into low Earth orbit above our light-distorting atmosphere, the observatory has made more than 1.3 million observations, been involved in over 15,000 scientific papers, and helped humanity understand the universe from the Solar System, all the way out to the farthest objects ever observed.
Even though Hubble is associated with the discovery that our universe is expanding due to its naming in recognition of Edwin Hubble - the early 20th-century astronomer who discovered the ever-enlarging state of space - this great observatory has revealed so much more, covering a broad spectrum of astronomical subjects.
Although there are tremendous accomplishments made by Hubble these past three decades, the mission has not been without great challenges to overcome. Through the years, scientists and engineers dealt with a flaw in the large light-gathering mirror that was incorrectly ground during manufacturing, the failures of gyroscopes as part of the pointing control system, and some of its primary cameras failing or malfunctioning. Thanks to the five servicing missions during NASA's Space Shuttle program and vigilant maintenance efforts made by engineers on the ground, it has been possible to run an observatory that is twice as old as most people's cars.
With 30 years behind us, let's take a look at some of the major achievements that this electronic eye on the universe has accomplished.
With a fairly large 7.8-foot diameter mirror that serves as its primary light-gathering source, Hubble is not only suited to observe extreme distances billions of light-years away but has also been extraordinary at peering at objects within our own Solar System.
In 1994, just after the completion of Servicing Mission 1 by the Space Shuttle Endeavour mission (STS-61) that fixed the manufacturing flaw in the primary mirror and restored clear vision to Hubble, one of the first major observations was made in our planetary neighborhood.
This was the impact of a sizable comet known as Shoemaker-Levy 9 in the colorful gas clouds of Jupiter. Before Hubble made this fortuitous observation of the collision, a huge impact such as this had never been witnessed, especially as it was occurring.
Jupiter's large gravitational field draws in many asteroids and comets into close proximity to the planet, but only once or twice a century does a major collision occur. Hubble was able to watch this event unfold over six days in July of 1994 and revealed 21 massive dark splotches that were created by the impacts. Shoemaker-Levy 9's collision revealed to scientists for the first time the chemicals that are found underneath Jupiter's uppermost cloud layer as it was churned upwards from the cometary bombardment.
Jupiter has continually been a reliable target due to its dynamic atmosphere. On numerous occasions, the telescope has focused its attention on the Great Red Spot - a massive swirling storm as large as two to three Earth's.
Hubble's cameras have shown that the Great Red Spot is continually shrinking - a process that is still a mystery to scientists today. These cloud observations have not only helped us understand the atmospheres of Jupiter and other gas planets but also have aided in our ability to forecast and model storms that form on our own planet Earth.
Looking farther out into the Solar System, the Hubble Space Telescope has also narrowed in on the dwarf planet Pluto from time-to-time.
Before 2015, when NASA's New Horizons spacecraft flew by Pluto taking the first up-close images, Hubble had the most detailed picture of this tiny, very dim world. Hubble's observations gave us a general understanding of Pluto's size and a very rough image of its geologic features. This, in turn, helped mission planners on the New Horizons team to determine how the spacecraft flyby would pan out after the long nine-year journey to the Kuiper Belt Object.
Hubble's arguably most significant contribution to the understanding of Pluto is its discovery of four of its five moons. Up until 2005, the only moon known around Pluto was Charon, discovered in 1978. Charon is half the size of Pluto with a very tight orbit, making it an object easier to find early on. In 2005, astronomers found two more moons, Hydra and Nix, using the powerful optics of Hubble. Subsequently, in 2011, Hubble was used to discover Kerberos, and in 2012, the Moon Styx.
These moon discoveries made around Pluto indicated that this system was much more complex and interesting than previously thought.
Some of the most iconic images and observations by Hubble have come from the study of the starry neighborhood of our own Sun.
Within the numerous pockets of stars spread out over thousands of light-years away, giant clouds of gas and dust - also known as nebulae - imbue the universe with these painting-like structures.
One of the most famous and studied of these gas clouds is the Orion Nebula, that sits just below the belt of the constellation of the same name. In 2006, the most detailed view from Hubble of the Orion Nebula was released, providing the best and closest example of a star-forming region to Earth.
At close to 1,500 light-years away (about 8.8 quadrillion miles) the complex layers of gas clouds captured by Hubble lie home to thousands of stars that are formed by the gravitational collapse of material within the nebula. Most notably, a small collection of stars in the heart of the nebula that is shaped like a trapezoid - aptly named the Trapezium - are new and radiating intensely in ultraviolet light. This light exerts an extremely powerful stellar wind, carving out and pushing on the gas around the young, luminous stars, and creating a cavernous landscape in the surrounding region. This has given scientists a snapshot of the violent nature of young stars as they push their constituent gas away.
A closer inspection of this very active star-forming region has given light to dusty disks that have formed around numerous new stars. Hubble revealed these disks - known as protoplanetary disks ot "proplyds" - to be the primordial materials that would likely form into planets. Our own Solar System is thought to have formed in this way, and Hubble's detailed view of these proplyds has helped us to understand this planetary forming process.
As with life, eventually comes death, and this is no different for stars in the universe.
Hubble has captured the death throes of stars on numerous occasions - none more famous than the stellar remnant known as the Crab Nebula.
The twisted, almost knotted up appearance of chaotically stretched out bands of gas reveal the extremely violent explosion of a star, an event known as a supernova. This was seen as an oddly bright spot in the sky by the Chinese about 1,000 years ago and is located about 6,500 light-years away from Earth. The exploded remnants of a star much larger than our Sun is stretched over six light-years across the image, and with many of these nebulous observations, the materials within the gas clouds have been enhanced with color to differentiate the elements found within the Crab Nebula. In Hubble's image, blue represents neutral oxygen, green as singly ionized sulfur, and red as doubly ionized oxygen. Many of these heavy elements found throughout the universe and even in our own bodies come from these supernovae.
Embedded deep within the Crab Nebula, is the monumentally dense core of the dead star left behind - a spinning neutron star known as a pulsar. Astronomers utilized Hubble along with another orbiting telescope, the Chandra X-ray Observatory, to combine optical and x-ray observations to better understand this extremely dense, rotating ball of matter. The pulsar is as massive as our Sun, but squished down to the size of Manhattan, all while spinning 30 times per second.
Even though gas clouds like the Crab Nebula, and countless other stellar remnants imaged by Hubble, are fraught with violence and destruction, they have also been found to kick start new star and planet formation as the expanding material develops shockwaves that spark gaseous collapse.
Perhaps the grandest and most fundamental discoveries made by the Hubble Space Telescope involved the farthest and oldest objects ever observed, and the scale of the expansion and size of the universe.
One tradition of the Hubble team is to point the orbiting observatory at a small and dark region of the sky and taking very prolonged exposures, collecting a great deal of light from the most distant galaxies.
This has culminated in the release of the "Deep Field" images, the first set revealed to the world in the mid-1990s. Initially released were the Hubble Deep Field North and South, with each exposure taken in one area of the northern and southern celestial hemisphere skies over an approximately 10-day period. What was revealed was an abundant collection of diverse galaxies spread across the expansive images. And with all telescopic viewings, the farther away the celestial object, the further back in time that can be seen. The images went so far back in time that many of the galaxies observed were smaller and much more irregular shaped, indicating the state of the youngest galaxies that formed after the beginning of the universe.
In 2004, Hubble made an even longer and impressive exposure, known as the Ultra Deep Field. This revealed even more galaxies - close to 10,000 - and further back in time. This image was taken over 11.3 days, and at the time, revealed the oldest galaxies ever seen, some dating back to when the universe was about 800 million years old. As with earlier observations, the youngest galaxies are smaller and irregular, while the newer galaxies are more well defined with prominent spiral and elliptical shapes.
Looking even deeper, the Hubble eXtreme Deep Field was released in 2012 and was created by the combination of other Deep Field observations through the years. This "combo" image equated to 22 days of exposure and became one of the most detailed views of the galactic landscape of our universe.
These types of observations have led us to refine the age of the universe to the highest accuracy available and to determine its future.
When Edwin Hubble discovered the true scale of the universe and its expanding nature - as described in his Hubble constant in the 1920s - the discovery fundamentally changed our understanding of cosmology.
Before the era of the Hubble Space Telescope, astronomers believed that the universe was anywhere from 10 to 20 billion years old after the initial Big Bang that started everything.
Hubble's comprehensive study of Cepheid stars - variable stars that shift up-and-down in brightness in a predictable manner - has led them to be used as standard candles (an object with known brightness that is used for astronomical distance measuring), which have helped scientists constrain the expansion rate of the universe, along with other cosmological studies. This has helped us to refine the Hubble constant, giving us the age of the universe at just under 13.8 billion years old.
Even more startling was the revelation on how the expansion of the universe was progressing. In 1998, astronomers from the High-z Supernova Search Team and the Supernova Cosmology Project published their results on their study of what are called Type Ia supernovae, a type of stellar outburst that has a predictable brightness, also used as a standard candle to compare objects at different distances and time periods.
This led to a Nobel prize in 2011 for three members of these supernovae teams, and surprisingly, Hubble's observations revealed that the universe was not only expanding but accelerating in its expansion. This helped astronomers to form the idea that there is some type of repelling force by something that cannot be measured with current tools. As a placeholder to describe this unknown repelling force, the name dark energy has been given to this utterly mysterious thing. It turns out that whatever dark energy actually is, it happens to make up 70 percent of all the energy and matter in the universe.
This has continued to be one of the greatest mysteries in all of modern cosmology, and Hubble has led the way in helping us get closer to an understanding.
With 30 years of astounding science by Hubble, this observatory is still heavily relied upon by the astronomy community today. Even though it is high above most of Earth's atmosphere, there is still a very small amount of gas molecules that imports some drag on Hubble over time. Without constant orbital boosting, the space telescope's orbit will slowly degrade until it re-enters Earth's atmosphere.
Barring any kind of systems or technological problems that may occur, Hubble may be able to continue its amazing exploration of the universe for a good part of the next decade.
Looking toward the future, Hubble may be the last of its kind of orbiting observatories that studies a wide swath of wavelengths of light, from infrared to ultraviolet parts of the spectrum.
Currently, many ground-based telescopes can counteract the effects of the atmosphere by employing a technique called adaptive optics, avoiding the need to put them in space. This has allowed for much bigger and more cost-effective observatories on the ground, some being as large as 40 meters across (about 130 feet) or more.
In more specific wavelengths of light, such as infrared, the next space-based telescope that is thought of as a successor to Hubble is NASA's James Webb Space Telescope.
James Webb has a light-gathering mirror about 2.7 times the size of Hubble at 21 feet in diameter and will be placed a million miles from Earth.
This new generation of observatory will primarily see the universe in infrared light and will be able to look even further out into the universe and further back in time. It will be able to image even younger galaxies and stellar objects than ever before, reveal more star-forming regions in unprecedented detail and observe the atmospheres of exoplanets beyond our Solar System.
James Webb was slated for launch back in 2018, but due to engineering issues and delays, the space telescope is now planned for a 2021 launch.
For its profound exploration of the universe, the Hubble Space Telescope will be included with some of the greatest scientific tools humanity has ever created.
30 years may seem like a long time in human terms, but its legacy and contributions to our understanding will last for ages, well beyond our time.