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What Does the Milky Way Weigh? Hubble and Gaia Investigate
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By NASA
Explore Hubble Hubble Home Overview About Hubble The History of Hubble Hubble Timeline Why Have a Telescope in Space? Hubble by the Numbers At the Museum FAQs Impact & Benefits Hubble’s Impact & Benefits Science Impacts Cultural Impact Technology Benefits Impact on Human Spaceflight Astro Community Impacts Science Hubble Science Science Themes Science Highlights Science Behind Discoveries Hubble’s Partners in Science Universe Uncovered Explore the Night Sky Observatory Hubble Observatory Hubble Design Mission Operations Missions to Hubble Hubble vs Webb Team Hubble Team Career Aspirations Hubble Astronauts News Hubble News Hubble News Archive Social Media Media Resources Multimedia Multimedia Images Videos Sonifications Podcasts e-Books Online Activities Lithographs Fact Sheets Posters Hubble on the NASA App Glossary More 35th Anniversary Online Activities 5 Min Read 20-Year Hubble Study of Uranus Yields New Atmospheric Insights
The image columns show the change of Uranus for the four years that STIS observed Uranus across a 20-year period. Over that span of time, the researchers watched the seasons of Uranus as the south polar region darkened going into winter shadow while the north polar region brightened as northern summer approaches. Credits:
NASA, ESA, Erich Karkoschka (LPL) The ice-giant planet Uranus, which travels around the Sun tipped on its side, is a weird and mysterious world. Now, in an unprecedented study spanning two decades, researchers using NASA’s Hubble Space Telescope have uncovered new insights into the planet’s atmospheric composition and dynamics. This was possible only because of Hubble’s sharp resolution, spectral capabilities, and longevity.
The team’s results will help astronomers to better understand how the atmosphere of Uranus works and responds to changing sunlight. These long-term observations provide valuable data for understanding the atmospheric dynamics of this distant ice giant, which can serve as a proxy for studying exoplanets of similar size and composition.
When Voyager 2 flew past Uranus in 1986, it provided a close-up snapshot of the sideways planet. What it saw resembled a bland, blue-green billiard ball. By comparison, Hubble chronicled a 20-year story of seasonal changes from 2002 to 2022. Over that period, a team led by Erich Karkoschka of the University of Arizona, and Larry Sromovsky and Pat Fry from the University of Wisconsin used the same Hubble instrument, STIS (the Space Telescope Imaging Spectrograph), to paint an accurate picture of the atmospheric structure of Uranus.
Uranus’ atmosphere is mostly hydrogen and helium, with a small amount of methane and traces of water and ammonia. The methane gives Uranus its cyan color by absorbing the red wavelengths of sunlight.
The Hubble team observed Uranus four times in the 20-year period: in 2002, 2012, 2015, and 2022. They found that, unlike conditions on the gas giants Saturn and Jupiter, methane is not uniformly distributed across Uranus. Instead, it is strongly depleted near the poles. This depletion remained relatively constant over the two decades. However, the aerosol and haze structure changed dramatically, brightening significantly in the northern polar region as the planet approaches its northern summer solstice in 2030.
The image columns show the change of Uranus for the four years that STIS observed Uranus across a 20-year period. Over that span of time, the researchers watched the seasons of Uranus as the south polar region darkened going into winter shadow while the north polar region brightened as northern summer approaches. NASA, ESA, Erich Karkoschka (LPL) Uranus takes a little over 84 Earth years to complete a single orbit of the Sun. So, over two decades, the Hubble team has only seen mostly northern spring as the Sun moves from shining directly over Uranus’ equator toward shining almost directly over its north pole in 2030. Hubble observations suggest complex atmospheric circulation patterns on Uranus during this period. The data that are most sensitive to the methane distribution indicate a downwelling in the polar regions and upwelling in other regions.
The team analyzed their results in several ways. The image columns show the change of Uranus for the four years that STIS observed Uranus across a 20-year period. Over that span of time, the researchers watched the seasons of Uranus as the south polar region (left) darkened going into winter shadow while the north polar region (right) brightened as it began to come into a more direct view as northern summer approaches.
The top row, in visible light, shows how the color of Uranus appears to the human eye as seen through even an amateur telescope.
In the second row, the false-color image of the planet is assembled from visible and near-infrared light observations. The color and brightness correspond to the amounts of methane and aerosols. Both of these quantities could not be distinguished before Hubble’s STIS was first aimed at Uranus in 2002. Generally, green areas indicate less methane than blue areas, and red areas show no methane. The red areas are at the limb, where the stratosphere of Uranus is almost completely devoid of methane.
The two bottom rows show the latitude structure of aerosols and methane inferred from 1,000 different wavelengths (colors) from visible to near infrared. In the third row, bright areas indicate cloudier conditions, while the dark areas represent clearer conditions. In the fourth row, bright areas indicate depleted methane, while dark areas show the full amount of methane.
At middle and low latitudes, aerosols and methane depletion have their own latitudinal structure that mostly did not change much over the two decades of observation. However, in the polar regions, aerosols and methane depletion behave very differently.
In the third row, the aerosols near the north pole display a dramatic increase, showing up as very dark during early northern spring, turning very bright in recent years. Aerosols also seem to disappear at the left limb as the solar radiation disappeared. This is evidence that solar radiation changes the aerosol haze in the atmosphere of Uranus. On the other hand, methane depletion seems to stay quite high in both polar regions throughout the observing period.
Astronomers will continue to observe Uranus as the planet approaches northern summer.
The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
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20 Years of Uranus Observations
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Last Updated Mar 31, 2025 Editor Andrea Gianopoulos Location NASA Goddard Space Flight Center
Contact Media Claire Andreoli
NASA’s Goddard Space Flight Center
Greenbelt, Maryland
claire.andreoli@nasa.gov
Ann Jenkins
Space Telescope Science Institute, Baltimore, Maryland
Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
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By NASA
Explore This Section Perseverance Home Mission Overview Rover Components Mars Rock Samples Where is Perseverance? Ingenuity Mars Helicopter Mission Updates Science Overview Objectives Instruments Highlights Exploration Goals News and Features Multimedia Perseverance Raw Images Images Videos Audio More Resources Mars Missions Mars Sample Return Mars Perseverance Rover Mars Curiosity Rover MAVEN Mars Reconnaissance Orbiter Mars Odyssey More Mars Missions Mars Home 3 min read
Visiting Mars on the Way to the Outer Solar System
Written by Roger Wiens, Principal Investigator, SuperCam instrument / Co-Investigator, SHERLOC instrument at Purdue University
A portion of the “Sally’s Cove” outcrop where the Perseverance rover has been exploring. The radiating lines in the rock on the left of the image may indicate that it is a shatter cone, showing the effects of the shock wave from a nearby large impact. The image was taken by Mastcam-Z’s left camera on March 21, 2025 (Sol 1452, or Martian day 1,452 of the Mars 2020 mission) at the local mean solar time of 12:13:44. Mastcam-Z is a pair of cameras located high on the rover’s mast. This image was voted by the public as “Image of the week.” NASA/JPL-Caltech/ASU Recently Mars has had a few Earthly visitors. On March 1, NASA’s Europa Clipper flew within 550 miles (884 kilometers) of the Red Planet’s surface on its way out to Jupiter. On March 12, the European Space Agency’s Hera spacecraft flew within about 3,100 miles (5,000 kilometers) of Mars, and only 300 kilometers from its moon, Deimos. Hera is on its way to study the binary asteroid Didymos and its moon Dimorphos. Next year, in May 2026, NASA’s Psyche mission is scheduled to buzz the Red Planet on its way to the metal-rich asteroid 16 Psyche, coming within a few thousand kilometers.
Why all these visits to Mars? You might at first think that they’re using Mars as an object of opportunity for their cameras, and you would be partially right. But Mars has more to give these missions than that. The main reason for these flybys is the extra speed that Mars’ velocity around the Sun can give them. The idea that visiting a planet can speed up a spacecraft is not all that obvious, because the same gravity that attracts the spacecraft on its way towards the planet will exert a backwards force as the spacecraft leaves the planet.
The key is in the direction that it approaches and leaves the planet. If the spacecraft leaves Mars heading in the direction that Mars is traveling around the Sun, it will gain speed in that direction, slingshotting it farther into the outer solar system. A spacecraft can typically gain several percent of its speed by performing such a slingshot flyby. The closer it gets to the planet, the bigger the effect. However, no mission wants to be slowed by the upper atmosphere, so several hundred kilometers is the closest that a mission should go. And the proximity to the planet is also affected by the exact direction the spacecraft needs to go when it leaves Mars.
Clipper’s Mars flyby was a slight exception, slowing down the craft — by about 1.2 miles per second (2 kilometers per second) — to steer it toward Earth for a second gravity assist in December 2026. That will push the spacecraft the rest of the way to Jupiter, for its 2030 arrival.
While observing Mars is not the main reason for their visits, many of the visiting spacecraft take the opportunity to use their cameras either to perform calibrations or to study the Red Planet and its moons.
During Clipper’s flyby over sols 1431-1432, Mastcam-Z was directed to watch the skies for signs of the interplanetary visitor. Clipper’s relatively large solar panels could have reflected enough sunlight for it to be seen in the Mars night sky, much as we can see satellites overhead from Earth. Unfortunately, the spacecraft entered the shadow of Mars just before it came into potential view above the horizon from Perseverance’s vantage point, so the sighting did not happen. But it was worth a try.
Meanwhile, back on the ground, Perseverance is performing something of a cliff-hanger. “Sally’s Cove” is a relatively steep rock outcrop in the outer portion of Jezero crater’s rim just north of “Broom Hill.” Perseverance made an approach during March 19-23, and has been exploring some dark-colored rocks along this outcrop, leaving the spherules behind for the moment. Who knows what Perseverance will find next?
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Last Updated Mar 28, 2025 Related Terms
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Video: 00:05:23 For over a decade, ESA’s Gaia mission has mapped our galaxy with stunning precision—rewriting the story of the Milky Way. As its mission enters a new phase, we look back at its most groundbreaking discoveries.
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Hubble Spots a Chance Alignment
This NASA/ESA Hubble image features the spiral galaxy NGC 5530. ESA/Hubble & NASA, D. Thilker The subject of today’s NASA/ESA Hubble Space Telescope image is the stunning spiral galaxy NGC 5530. This galaxy is situated 40 million light-years away in the constellation Lupus, the Wolf, and classified as a ‘flocculent’ spiral, meaning its spiral arms are patchy and indistinct.
While some galaxies have extraordinarily bright centers that host a feasting supermassive black hole, the bright source near the center of NGC 5530 is not an active black hole but a star within our own galaxy, only 10,000 light-years from Earth. This chance alignment gives the appearance that the star is at the dense heart of NGC 5530.
If you pointed a backyard telescope at NGC 5530 on the evening of September 13, 2007, you would have seen another bright point of light adorning the galaxy. That night, Australian amateur astronomer Robert Evans discovered a supernova, named SN 2007IT, by comparing NGC 5530’s appearance through the telescope to a reference photo of the galaxy. While it’s remarkable to discover even one supernova using this painstaking method, Evans has in fact discovered more than 40 supernovae this way! This particular discovery was truly serendipitous: it’s likely that the light from the supernova completed its 40-million-year journey to Earth just days before Evans spotted the explosion.
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Last Updated Mar 28, 2025 Editor Andrea Gianopoulos Location NASA Goddard Space Flight Center Related Terms
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By European Space Agency
The European Space Agency (ESA) has powered down its Gaia spacecraft after more than a decade spent gathering data that are now being used to unravel the secrets of our home galaxy.
On 27 March 2025, Gaia’s control team at ESA’s European Space Operations Centre carefully switched off the spacecraft’s subsystems and sent it into a ‘retirement orbit’ around the Sun.
Though the spacecraft’s operations are now over, the scientific exploitation of Gaia’s data has just begun.
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