NASA’s astrophysical observatories have provided many significant advances in solar system exploration. Telescopes like Hubble and Spitzer have led directly to new discoveries and also enhanced the productivity of planetary missions. For example, monitoring of Mars led to insights on ideal landing sites for Martian missions, and recent observations revealed Kuiper Belt targets for the New Horizons mission to investigate.
Scheduled for launch in 2018, the James Webb Space Telescope is poised to revolutionize many areas of astrophysical research, including solar system science. Webb is many times more powerful than the Hubble and Spitzer observatories. It has greater sensitivity, will resolve smaller features on the planets, and will view celestial objects in specific colors of light.
Examining planetary systems and the origin of life is one of Webb’s core goals. The telescope will observe Mars and the giant planets, minor planets like Pluto and Eris, and even the small bodies in our solar system: asteroids, comets, and Kuiper Belt Objects. Webb will help us to understand Mars' atmosphere, and conduct studies that verify findings of the Mars rovers and landers. In the outer solar system, Webb's observations will be used with Cassini's Saturn observations to give us a better picture of the seasonal weather on our giant planets. Finally, asteroids and other small bodies in our solar system possess features that Earth-based observatories are blind to, but that Webb will be able to identify by analyzing those objects’ light. Webb will also help us learn more about the mineralogy of these rocky objects.
Water on Mars now and in the past is one of the most pressing scientific objectives for scientists studying Mars. Does the atmosphere of Mars reveal a more habitable past? Is it habitable now? Are there unidentified sources of water on Mars? How wet was – or is – Mars? What processes altered the chemical stability of its atmosphere? Besides searching for water, geologists will use Webb to characterize the formation and evolution of global dust storms and cloud systems over dormant volcanoes, and search for traces of changing atmospheric chemistry.
Beginning in 2018, and after the end of the Cassini mission in 2017, Webb will provide an important tool for keeping track of the changing seasons on Saturn’s moon, Titan. As Saturn slowly circles the Sun, Titan undergoes an annual cycle of 29.5 Earth-years, and will be in northern summer when the telescope begins observations. Webb will reveal the interplay of chemistry and atmospheric dynamics in response to the change of Titan’s seasons. To probe Titan’s complex chemistry, Webb will investigate atmospheric composition, watch clouds, track hazes, and monitor surface changes due to rainfall, geologic activity or sea shrinkage.
Webb also offers unprecedented observing opportunities in the near- and mid-infrared for Jupiter, Saturn, Uranus, and Neptune. Potential groundbreaking investigations of these planets include studies of how their aurorae emerge, tracking atmospheric changes in the aftermath of comet or meteor strikes, and more. Webb's infrared capabilities can be used to probe different depths in these atmospheres, mapping cloud structures and major storm systems, as well as their evolution, with finer detail than previously possible.
Webb will enable mapping of organic molecules -- called hydrocarbons -- across the disks of Uranus and Neptune for the first time, providing insight into the thermodynamics, chemistry, and global circulation within these two atmospheres. Such investigations will reveal whether such changes are consequences of local weather, or tied to solar activity or the seasonal cycle. Webb will be a workhorse in deepening our understanding of processes in the giant planet atmospheres.
Webb complements Hubble and other solar system missions, including those observatories on the ground, orbiting Earth, and in deep space. Data of different wavelengths and from different sources can help build a broader, fuller picture of the objects in our solar system, especially with the help of Webb’s unprecedented improvements in sensitivity and resolution.
Many rightfully think of astronomical phenomena as occurring over hundreds of millions of years. But in the last 25 years, the Hubble Space Telescope has kept a watchful eye on events within our own solar system, which happen on the timescale of days, weeks, and years. The short-term phenomena Hubble has witnessed on other planets includes the weather -- watching storms arise and dissipate across the faces of other worlds.
Among these constantly shifting weather patterns are dust storms on Mars. Working with the eagle-eyed Mars Global Surveyor (MGS) spacecraft in orbit around Mars in 2001, Hubble observed Mars’ biggest dust storm in decades. The Martian storm raised a cloud of dust that engulfed the entire planet for several months. Hubble’s Earth-orbit perspective allowed it to view the entirety of the global storm, while its long-term presence in space continues to allow it to monitor changes in Mars’s seasons over months and years.
In the outer solar system, turbulent storms dot the atmospheres of the giant planets, allowing Hubble to become an expert storm tracker. For instance, Hubble has observed the downsizing of Jupiter’s most famous feature, the spinning, cyclone-like storm known as the Great Red Spot.
When 17th-century astronomers first turned their telescopes to Jupiter, they noted a conspicuous reddish spot on the giant planet. Three hundred years later, we know a lot more about the Great Red Spot, the largest known storm in the solar system and home to winds that reach speeds of about 270 mph (435 kph). For instance, we know that the Red Spot changes shape, size, and color, sometimes dramatically. Measurements made by Hubble in May 2014 revealed that the Red Spot is slightly larger than the width of Earth, with a diameter of 10,250 miles (16,500 km) across. But during Voyager flybys in 1979, it was almost twice the size of Earth and one-sixth the diameter of Jupiter itself. This indicates that some unknown activity in the planet's atmosphere may be draining energy and weakening the storm, causing it to shrink.
On Neptune, Hubble has captured the most insightful images to date of a planet whose blustery weather bewilders scientists. Neptunian winds blow at an average of 900 miles per hour (1,450 kph), and huge storms — some the size of Earth itself — come and go with regularity. Hubble’s observations captured springtime on Neptune for the first time, tracking waves of massive storms -- each one larger than the distance from Kansas to New York -- with temperatures colder than -350 F (210 C).
Hubble revealed Uranus, once considered one of the blander-looking planets, as a dynamic world with the brightest clouds in the outer solar system. The clouds are probably made of crystals of methane, which condense as warm bubbles of gas well up from deep in the planet's atmosphere.
Hubble’s ability to see ultraviolet, infrared, and visible light make it the ideal meteorologist for the solar system, allowing it to see below the cloud tops and investigate the massive storms on distant planets. As Hubble continues its mission, we will surely learn more about the wild weather of the other planets in our solar system, reminding us that these aren’t just placid chunks of rock or balls of gas orbiting the Sun, but changing, evolving, dynamic places with unique seasons and climates that we’re just beginning to understand.
We think we know Pluto. It's the smallest styrofoam ball in our solar system models. It’s a chunk of ice, rock, and hydrocarbons that drifts 4.67 billion miles (7.5 billion km) from Earth at its orbit’s farthest point. It's the tiny former planet that stirred up controversy when it was reclassified several years ago as both a dwarf planet and a member of the collection of icy cosmic objects we call the Kuiper Belt.
But Pluto – and the Kuiper Belt -- are really still giant question marks, yet to be visited by planetary spacecraft. The New Horizons mission is on its way to Pluto, scheduled to fly through the dwarf planet's system and beyond in July 2015. In the meantime, astronomers on Earth have relied on other ground- and space-based observatories, including the Hubble Space Telescope, to investigate those distant reaches of our solar system.
Initially, astronomers studying Pluto with Hubble were not met with much scientific enthusiasm, given the potential science return from other observational targets and priorities. Hubble did observe Pluto a handful of times in the 1990’s, but many of the early requests to observe Pluto were rejected. Astronomers did get some additional amount of time to observe the Pluto system with Hubble in 2005. These observations revealed two never-before-seen moons: Nix and Hydra. Six years later, in 2011, Hubble's keen vision found another moon, designated Kerberos, while searching Pluto for rings. This discovery expanded the size of Pluto's known satellite system to four moons, including its largest moon, Charon, which was discovered in 1978 and first imaged by Hubble shortly after launch in 1990.
Kerberos has an estimated diameter of 8 to 21 miles (13 to 34 km). By comparison, Charon is 746 miles (1,200 km) across, and the other moons, Nix and Hydra are in the range of 20 to 70 miles in diameter (32 to 113 km).
Just months later, a team of astronomers using Hubble reported the discovery of a fifth moon orbiting Pluto. The moon, named Styx, is estimated to be irregular in shape and 6 to 15 miles (10 to 25 km) across. With five moons now known in the Pluto system, scientists are intrigued that such a small planet can have such a complex collection of satellites. The discovery provides additional clues for unraveling how the Pluto system formed and evolved. The dwarf planet's entire moon system is believed to have formed by a collision between Pluto and another planet-sized body early in the history of the solar system. The smashup flung material into orbit around Pluto, which then coalesced into the family of satellites now seen.
The discovery of four additional moons in the Pluto system has lent planning support to the New Horizons mission, launched in 2006. The myriad of moons means there could be smaller objects out there too, items that could pose hazards for the spacecraft during its flythrough. For this reason, the New Horizons team is using Hubble's powerful vision to scour the Pluto system to uncover potential hazards to its spacecraft. Moving past the dwarf planet at a speed of 30,000 miles per hour, New Horizons could be destroyed in a collision with an unknown moon.
In addition to supporting the flyby of the New Horizons mission, Hubble has uncovered three Kuiper Belt Objects (KBOs) the New Horizons spacecraft could potentially visit after passing Pluto. The KBOs that Hubble found are each about 10 times larger than typical comets, but only about 1-2 percent of the size of Pluto. Unlike asteroids, KBOs have not been heated by the Sun, and are thought to represent a pristine, well preserved, deep-freeze sample of what the outer solar system was like following its birth 4.6 billion years ago. The KBOs found in the Hubble data are thought to be the building blocks of dwarf planets such as Pluto.
The New Horizons team started to look for suitable KBOs in 2011 using some of the largest ground-based telescopes on Earth. They found several dozen KBOs, but none were reachable within the fuel supply available aboard the New Horizons spacecraft. The New Horizons team was awarded Hubble telescope time and identified one KBO that is "definitely reachable" and two other potentially accessible KBOs that will require more tracking. The three KBOs identified are each a whopping 1 billion miles beyond Pluto. Two of the KBOs are estimated to be as large as 34 miles (55 kilometers) across, and the third is perhaps as small as 15 miles (25 kilometers). NASA will decide on whether or not to extend the New Horizon’s mission for a KBO fly-by after a proposal is submitted in 2016. Accomplishing such a KBO flyby would substantially increase the science return from the New Horizons mission.
In the years following the New Horizons Pluto flyby, astronomers plan to use the infrared vision of Hubble's planned successor, NASA's James Webb Space Telescope, for follow-up observations. The Webb telescope will be able to measure the surface chemistry of Pluto, its moons, and many other bodies that lie in the distant Kuiper Belt along with Pluto.
On July 7, 2007, NASA launched its Dawn spacecraft on a four-year journey to the asteroid belt. Dawn's mission was to asteroid-hop, going into orbit around Vesta in 2011 and Ceres in 2015, investigating up close what scientists had previously only seen as fuzzy, far-flung images.
To prepare for the Dawn spacecraft's first stop at Vesta, astronomers used Hubble's Wide Field Planetary Camera 2 in 2007 to do some reconnaissance on the geology and topology of the asteroid. Astronomers mapped Vesta's southern hemisphere, a region dominated by a giant impact crater formed by a collision billions of years ago. The crater is 285 miles (456 km) across, which is nearly equal to Vesta's 330-mile (530-km) diameter. The impact broke off chunks of rock, producing more than 50 smaller asteroids (nicknamed "vestoids") and may have blasted through Vesta's crust.
In this set of images, Hubble's sharp "eye" can see features as small as about 37 miles (60 km) across. The images show the difference in brightness and color on the asteroid's surface. These characteristics prepared planetary scientists for the important up-close observations of large-scale features when the Dawn spacecraft arrived at Vesta in July 2011, helping them select the most scientifically productive regions to observe.
Hubble's view of Ceres, taken with the Advanced Camera for Surveys/High Resolution Channel in 2004, reveals bright and dark regions on the asteroid's surface that could be topographic features, such as craters, and/or areas containing different surface material. Large impacts may have caused some of these features and potentially added new material to the landscape. The Texas-sized asteroid holds about 30 to 40 percent of the mass in the asteroid belt. Hubble's images gave rise to the questions guiding planetary scientists as Dawn approaches Ceres: Does Ceres have a rocky inner core, an icy mantle, and a thin, dusty outer crust? Does it have water locked beneath its surface?
On March 6, 2015, the Dawn spacecraft entered orbit around Ceres. From its new position, Dawn can now see the surface with more clarity than Hubble. As Dawn studies the asteroid's surface details, the questions posed by the Hubble images will begin to be answered.
Before Dawn reached Vesta and Ceres, Hubble provided our best look at each asteroid. Hubble's supporting observations for spacecraft missions to minor planets do not end at the asteroid belt, however. In fact, Hubble has been observing the Pluto system in anticipation of the New Horizon mission, scheduled to fly by Pluto in July 2015. Using Hubble, astronomers have found new moons in the Pluto system, have created surface maps, and have found potential targets for New Horizons to explore after it visits Pluto. Soon, New Horizons will achieve "Better Than Hubble" resolution, and the Hubble team could not be happier about it!
The James Webb Space Telescope will continue to investigate small bodies in our solar system starting in 2018, since the infrared is particularly useful for studying the surfaces of planets. Webb will support an entirely new generation of solar system exploration!
Over the last 25 years, the Hubble Space Telescope has tracked many comets on their journeys through the inner solar system and traced their orbits to their farthest reaches. These icy wanderers, remnants of the debris cloud that once encircled our newborn Sun, give astronomers clues to the formation and evolution of our solar system.
Most comets spend their lives beyond the orbit of Neptune, where they were pushed by gravitational interactions with the newly formed giant planets during the early development of the solar system. Occasionally, gravitational interactions with one another result in the orbit of one of these objects being perturbed until it swings into the inner solar system. When a comet gets within the orbit of Mars, the Sun's light warms the comet's ices. The ices begin to evaporate directly into a gas, and the comet brightens. At that point, the Hubble Space Telescope can observe the comet, detecting changes in brightness, noting expulsion of gases, and analyzing its composition.
Scientists can learn much about the building blocks of our newborn solar system by studying the composition of comets, but they can also examine interactions between comets and other celestial bodies to glean clues about planet formation and composition. For example, Hubble observed Comet Shoemaker-Levy 9 impact Jupiter in 1994. A series of Hubble observations spanning the year after impact revealed some surprising results, including unexpectedly low amounts of water in Jupiter's atmosphere.
Hubble has continued to observe comets as they travel through our solar system, bearing witness to the eventual destruction of those that edge too close to the Sun. Hubble observed Comet ISON as it made its first voyage to the inner solar system, contributing its study of the comet's activity to a wealth of worldwide observations. Although the breakup of Comet ISON was too close to the Sun for Hubble to observe, the telescope has observed the disintegration of other comets, including Comet LINEAR (C/1999 S4) and Comet 73P/Schwassmann-Wachmann 3. Most recently, Hubble created a striking image of Comet Siding Spring's flyby of Mars. When the James Webb Space Telescope comes online later this decade, it will continue Hubble's legacy of solar system observations.
Watch throughout the year for more articles on Hubble's 25 years of discovery.