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BepiColombo to swing by Mercury for the sixth time
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By NASA
6 min read
Preparations for Next Moonwalk Simulations Underway (and Underwater)
The SWOT satellite is helping scientists size up flood waves on waterways like the Yellowstone River, pictured here in October 2024 in Montana. SWOT measures the height of surface waters, including the ocean, and hundreds of thousands of rivers, lakes, and reservoirs in the U.S. alone.NPS In a first, researchers from NASA and Virginia Tech used satellite data to measure the height and speed of potentially hazardous flood waves traveling down U.S. rivers. The three waves they tracked were likely caused by extreme rainfall and by a loosened ice jam. While there is currently no database that compiles satellite data on river flood waves, the new study highlights the potential of space-based observations to aid hydrologists and engineers, especially those working in communities along river networks with limited flood control structures such as levees and flood gates.
Unlike ocean waves, which are ordinarily driven by wind and tides, and roll to shore at a steady clip, river waves (also called flood or flow waves) are temporary surges stretching tens to hundreds of miles. Typically caused by rainfall or seasonal snowmelt, they are essential to shuttling nutrients and organisms down a river. But they can also pose hazards: Extreme river waves triggered by a prolonged downpour or dam break can produce floods.
“Ocean waves are well known from surfing and sailing, but rivers are the arteries of the planet. We want to understand their dynamics,” said Cedric David, a hydrologist at NASA’s Jet Propulsion Laboratory in Southern California and a coauthor of a new study published May 14 in Geophysical Research Letters.
SWOT is depicted in orbit in this artist’s concept, with sunlight glinting off one of its solar panels and both antennas of its key instrument — the Ka-band Radar Interferometer (KaRIn) — extended. The antennas collect data along a swath 30 miles (50 kilometers) wide on either side of the satellite.CNES Measuring Speed and Size
To search for river waves for her doctoral research, lead author Hana Thurman of Virginia Tech turned to a spacecraft launched in 2022. The SWOT (Surface Water and Ocean Topography) satellite is a collaboration between NASA and the French space agency CNES (Centre National d’Études Spatiales). It is surveying the height of nearly all of Earth’s surface waters, both fresh and salty, using its sensitive Ka-band Radar Interferometer (KaRIn). The instrument maps the elevation and width of water bodies by bouncing microwaves off the surface and timing how long the signal takes to return.
“In addition to monitoring total storage of waters in lakes and rivers, we zoom in on dynamics and impacts of water movement and change,” said Nadya Vinogradova Shiffer, SWOT program scientist at NASA Headquarters in Washington.
Thurman knew that SWOT has helped scientists track rising sea levels near the coast, spot tsunami slosh, and map the seafloor, but could she identify river height anomalies in the data indicating a wave on the move?
She found that the mission had caught three clear examples of river waves, including one that arose abruptly on the Yellowstone River in Montana in April 2023. As the satellite passed overhead, it observed a 9.1-foot-tall (2.8-meter-tall) crest flowing toward the Missouri River in North Dakota. It was divided into a dramatic 6.8-mile-long (11-kilometer-long) peak followed by a more drawn‐out tail. These details are exciting to see from orbit and illustrate the KaRIn instrument’s uniquely high spatial resolution, Thurman said.
Sleuthing through optical Sentinel-2 imagery of the area, she determined that the wave likely resulted from an ice jam breaking apart upstream and releasing pent-up water.
The other two river waves that Thurman and the team found were triggered by rainfall runoff. One, spotted by SWOT starting on Jan. 25, 2024, on the Colorado River south of Austin, Texas, was associated with the largest flood of the year on that section of river. Measuring over 30 feet (9 meters) tall and 166 miles (267 kilometers) long, it traveled around 3.5 feet (1.07 meters) per second for over 250 miles (400 kilometers) before discharging into Matagorda Bay.
The other wave originated on the Ocmulgee River near Macon, Georgia, in March 2024. Measuring over 20 feet (6 meters) tall and extending more than 100 miles (165 kilometers), it traveled about a foot (0.33 meters) per second for more than 124 miles (200 kilometers).
“We’re learning more about the shape and speed of flow waves, and how they change along long stretches of river,” Thurman said. “That could help us answer questions like, how fast could a flood get here and is infrastructure at risk?”
Complementary Observations
Engineers and water managers measuring river waves have long relied on stream gauges, which record water height and estimate discharge at fixed points along a river. In the United States, stream gauge networks are maintained by agencies including the U.S. Geological Survey. They are sparser in other parts of the world.
“Satellite data is complementary because it can help fill in the gaps,” said study supervisor George Allen, a hydrologist and remote sensing expert at Virginia Tech.
If stream gauges are like toll booths clocking cars as they pass, SWOT is like a traffic helicopter taking snapshots of the highway.
The wave speeds that SWOT helped determine were similar to those calculated using gauge data alone, Allen said, showing how the satellite could help monitor waves in river basins without gauges. Knowing where and why river waves develop can help scientists tracking changing flood patterns around the world.
Orbiting Earth multiple times each day, SWOT is expected to observe some 55% of large-scale floods at some stage in their life cycle. “If we see something in the data, we can say something,” David said of SWOT’s potential to flag dangerous floods in the making. “For a long time, we’ve stood on the banks of our rivers, but we’ve never seen them like we are now.”
More About SWOT
The SWOT satellite was jointly developed by NASA and CNES, with contributions from the Canadian Space Agency (CSA) and the UK Space Agency. NASA’s Jet Propulsion Laboratory, managed for the agency by Caltech in Pasadena, California, leads the U.S. component of the project. For the flight system payload, NASA provided the Ka-band radar interferometer (KaRIn) instrument, a GPS science receiver, a laser retroreflector, a two-beam microwave radiometer, and NASA instrument operations. The Doppler Orbitography and Radioposition Integrated by Satellite system, the dual frequency Poseidon altimeter (developed by Thales Alenia Space), the KaRIn radio-frequency subsystem (together with Thales Alenia Space and with support from the UK Space Agency), the satellite platform, and ground operations were provided by CNES. The KaRIn high-power transmitter assembly was provided by CSA.
News Media Contacts
Jane J. Lee / Andrew Wang
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-0307 / 626-379-6874
Written by Sally Younger
2025-074
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Last Updated May 21, 2025 Related Terms
SWOT (Surface Water and Ocean Topography) Jet Propulsion Laboratory Explore More
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By European Space Agency
Video: 00:01:20 Listen to the ESA/JAXA BepiColombo spacecraft as it flew past Mercury on 8 January 2025. This sixth and final flyby used the little planet's gravity to steer the spacecraft on course for entering orbit around Mercury in 2026.
What you can hear in the sonification soundtrack of this video are real spacecraft vibrations measured by the Italian Spring Accelerometer (ISA) instrument. The accelerometer data have been shifted in frequency to make them audible to human ears – one hour of measurements have been sped up to one minute of sound.
BepiColombo is always shaking ever so slightly: fuel is slightly sloshing, the solar panels are vibrating at their natural frequency, heat pipes are pushing vapour through small tubes, and so forth. This creates the eerie underlying hum throughout the video.
But as BepiColombo gets closer to Mercury, ISA detects other forces acting on the spacecraft. Most scientifically interesting are the audible shocks that sound like short, soft bongs. These are caused by the spacecraft responding to entering and exiting Mercury's shadow, where the Sun's intense radiation is suddenly blocked. One of ISA's scientific goals is to monitor the changes in the ‘solar radiation pressure’ – a force caused by sunlight striking BepiColombo as it orbits the Sun and, eventually, Mercury.
The loudest noises – an ominous ‘rumbling’ – are caused by the spacecraft's large solar panels rotating. The first rotation occurs in shadow at 00:17 in the video, while the second adjustment at 00:51 was also captured by one of the spacecraft’s monitoring cameras.
Faint sounds like wind being picked up in a phone call, which grow more audible around 30 seconds into the video, are caused by Mercury's gravitational field pulling the nearest and furthest parts of the spacecraft by different amounts. As the planet's gravity stretches the spacecraft ever so slightly, the spacecraft responds structurally. At the same time, the onboard reaction wheels change their speed to maintain the spacecraft's orientation, which you can hear as a frequency shift in the background.
This is the last time that many of these effects can be measured with BepiColombo's largest solar panels, which make the spacecraft more susceptible to vibrations. The spacecraft module carrying these panels will not enter orbit around Mercury with the mission's two orbiter spacecraft.
The video shows an accurate simulation of the spacecraft and its route past Mercury during the flyby, made with the SPICE-enhanced Cosmographia spacecraft visualisation tool. The inset that appears 38 seconds into the video shows real photographs taken by one of BepiColombo's monitoring cameras.
Read more about BepiColombo's sixth Mercury flyby
Access the related broadcast quality video material.
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By European Space Agency
Video: 00:01:36 Fly over Mercury with BepiColombo for the final time during the mission’s epic expedition around the Sun. The ESA/JAXA spacecraft captured these images of the Solar System's smallest planet on 7 and 8 January 2025, before and during its sixth encounter with Mercury. This was its final planetary flyby until it enters orbit around the planet in late 2026.
The video begins with BepiColombo's approach to Mercury, showing images taken by onboard monitoring cameras 1 and 2 (M-CAM 1 and M-CAM 2) between 16:59 CET on 7 January and 01:45 CET on 8 January. During this time, the spacecraft moved from 106 019 to 42 513 km from Mercury's surface. The view from M-CAM 1 is along a 15-metre-long solar array, whereas M-CAM 2 images show an antenna and boom in the foreground.
After emerging into view from behind the solar array, Mercury appears to jump to the right. Both the spacecraft and its solar arrays rotated in preparation for passing through Mercury's cold, dark shadow.
For several hours after these first images were taken, the part of Mercury’s surface illuminated by the Sun was no longer visible from the M-CAMs. BepiColombo's closest approach to Mercury took place in darkness at 06:58:52 CET on 8 January, when it got as close as 295 km.
Shortly after re-emerging into the intense sunlight, the spacecraft peered down onto the planet's north pole, imaging several craters whose floors are in permanent shadow. The long shadows in this region are particularly striking on the floor of Prokofiev crater (the largest crater to the right of centre) – the central peak of that crater casts spiky shadows that exaggerate the shape of this mountain.
Next, we have a beautiful view of Mercury crossing the field of view from left to right, seen first by M-CAM 1 then by M-CAM 2 between 07:06 and 07:49 CET. These images showcase the planet's northern plains, which were smoothed over billions of years ago when massive amounts of runny lava flowed across Mercury's cratered surface.
The background music is The Hebrides overture, composed by Felix Mendelssohn in 1830 after being inspired by a visit to Fingal’s Cave, a sea cave created by ancient lava flows on the island of Staffa, Scotland. Similarly shaped by lava is Mercury's Mendelssohn crater, one of the large craters visible passing from left to right above the solar array in M-CAM 1's views, and at the very bottom of M-CAM 2's views. The Mendelssohn crater was flooded with lava after an impact originally created it.
The end of the video lingers on the final three close-up images that the M-CAMs will ever obtain of Mercury. The cameras will continue to operate until September 2026, fulfilling their role of monitoring various parts of the spacecraft. After that point, the spacecraft module carrying the M-CAMs will separate from BepiColombo's other two parts, ESA's Mercury Planetary Orbiter (MPO) and JAXA's Mercury Magnetospheric Orbiter (Mio). MPO’s much more powerful science cameras will take over from the M-CAMs, mapping Mercury over a range of colours in visible and infrared light.
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By European Space Agency
On 8 January 2025, the ESA/JAXA BepiColombo mission flew past Mercury for the sixth time, successfully completing the final ‘gravity assist manoeuvre’ needed to steer it into orbit around the planet in late 2026. The spacecraft flew just a few hundred kilometres above the planet's north pole. Close-up images expose possibly icy craters whose floors are in permanent shadow, and the vast sunlit northern plains.
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