NASA’s Hubble Detects First-Ever Spin Reversal of Tiny Comet - UNIVERSE

Astronomers using NASA’s Hubble Space Telescope have found evidence that the spinning of a small comet slowed and then reversed its direction of rotation, offering a dramatic example of how volatile activity can affect the spin and physical evolution of small bodies in the solar system. This is the first time researchers have observed evidence of a comet reversing its spin. 

The object, comet 41P/Tuttle-Giacobini-Kresák, or 41P for short, likely originated in the Kuiper Belt, and was flung into its current trajectory by Jupiter’s gravity, now visiting the inner solar system every 5.4 years.

After its 2017 close passage around the Sun, scientists found that comet 41P experienced a dramatic slowdown in its rotation. Data from NASA’s Neil Gehrels Swift Observatory in May 2017 showed the object was spinning three times more slowly than it had in March 2017 when it was observed by the Discovery Channel Telescope at Lowell Observatory in Arizona.

A new analysis of follow-up Hubble observations has shown the spin of this comet took an even more unusual turn.

Hubble images from December 2017 detected the comet spinning much faster again, with a period of approximately 14 hours, compared to the 46 to 60 hours measured by Swift. The simplest explanation, researchers say, is that the comet continued slowing until it almost stopped, and was then forced to spin in the near-opposite direction by outgassing jets on its surface.

The science paper detailing this finding published Thursday in The Astronomical Journal.

This artist’s concept depicts comet 41P, a tiny Jupiter-family comet, as it approached the Sun and frozen gases began to sublimate and shoot material off into space.

Illustration: NASA, ESA, CSA, Ralf Crawford (STScI)

Small, temperamental nucleus

Hubble also constrains the size of the comet’s nucleus, measuring it at around 0.6 miles across (about a kilometer), or about three times the height of the Eiffel Tower. 

This is especially small for a comet, making it easy to torque, or twist.

As a comet approaches the Sun, heat causes frozen ices to sublimate, venting material into space. 

“Jets of gas streaming off the surface can act like small thrusters,” said paper author David Jewitt of the University of California at Los Angeles. “If those jets are unevenly distributed, they can dramatically change how a comet, especially a small one, rotates.”

The comet was originally spinning in one direction, but gas jets pushing against that motion gradually slowed it down. Because the jets kept pushing, they ultimately caused the comet to start rotating in the opposite direction.

“It’s like pushing a merry-go-round,” said Jewitt. “If it’s turning in one direction, and then you push against that, you can slow it and reverse it.”

Evidence of rapid evolution

The study also shows that the comet’s overall activity has declined significantly since earlier returns. During its 2001 perihelion passage, 41P was unusually active for its size. By 2017, its gas production had decreased by roughly an order of magnitude.

This change suggests that the comet’s surface may be evolving quickly, possibly as near-surface volatile materials become depleted or covered by insulating dust layers.

Most changes in comet structure occur over centuries or longer. The rapid rotational shifts observed in comet 41P provide a rare opportunity to witness evolutionary processes unfolding on a human timescale. 

Modeling based on the measured torques and mass loss rates suggest that continued rotational changes could eventually lead to structural instability for comet 41P. If a comet spins too rapidly, centrifugal forces can overcome its weak gravity and strength, potentially causing fragmentation or even disintegration.

“I expect this nucleus will very quickly self-destruct,” said Jewitt.

Yet, comet 41P has likely occupied its present orbit for roughly 1,500 years. 

This artist’s concept depicts comet 41P as it approached the Sun and frozen gases began to sublimate off the comet’s surface. This animation only depicts one jet, but this comet may have multiple streams of material ejecting into space. This jet is pushing against the comet’s spin, then forcing it in the opposite direction. Small fragments of the comet are also shown spewing into space.

Animation: NASA, ESA, CSA, Ralf Crawford (STScI)

Archival find

Hubble has been collecting imaging and spectroscopic data from across the cosmos for over 35 years, and all of those observations are available in the Mikulski Archive for Space Telescopes, a central repository for data from more than a dozen astronomical missions, including Hubble.

Jewitt found these observations while browsing the archive, and realized they were yet-to-be analyzed. 

By making NASA’s science data open to all, observations made years, or even decades ago, can be revisited to answer new scientific questions. In many cases, scientists continue to make discoveries not just with new observations, but by mining the archive built over decades of space exploration.  

Source: NASA's Hubble Detects First-Ever Spin Reversal of Tiny Comet - NASA Science

North Sea wind farms may be reshaping sediment flows by 1.5 million tons a year - Earth - Earth Sciences - Environment

Credit: Unsplash/CC0 Public Domain

Offshore wind farms are an important pillar of the European Union's strategy for renewable energy—by 2050, the EU aims to increase capacity in the North Sea more than tenfold. A new study by the Helmholtz-Zentrum Hereon shows that the expansion of wind farms can alter the natural transport and deposition of sediments on a large scale and over the long term. The German Bight is particularly affected. The researchers have published their findings in the journal Nature Communications Earth & Environment.

Redistribution of sediment and carbon

Suspended particles are constantly being moved around the North Sea, originating from the stirring-up of local seabed sediments by waves and currents, as well as from material carried in from the Atlantic Ocean through the English Channel, or from rivers. This material travels through repeated cycles of settling and resuspension until it finally accumulates as mud in calmer areas where currents are weak.

Offshore wind turbines are barriers in the air and in the water. They influence the stratification of the sea into warmer and colder water layers and slow down currents over a wide area throughout the North Sea. These factors determine how mud and organic particles are transported through the ocean and where they deposit. Hereon researchers have now discovered that existing wind farms in the North Sea are already causing a significant spatial redistribution of these sediments. This affects up to 1.5 million tons of mud annually—and the carbon bound within it.

Sediments consist in part of the remains of dead marine animals and plants. This organic material contains particulate organic carbon (POC), which sinks to the seafloor with the particles and can be stored there for centuries. The seafloor is therefore referred to as a carbon sink. Oceans thus make an important contribution to global carbon sequestration and help mitigate climate change.

German Bight is particularly affected

The researchers used a new computer model that, for the first time, combines calculations of the atmosphere, waves, currents, and sediment transport in the North Sea. The data is based on previous Hereon studies on the impact of offshore wind turbines on air and ocean currents.

"Our simulations suggest that these amounts will accumulate increasingly over the coming decades as offshore wind farms expand. This could affect the long-term functioning of the ecosystem and carbon storage in the North Sea," says the study's lead author, Jiayue Chen, from the Hereon Institute of Coastal Systems—Analysis and Modeling. Notably, about 52% of the total sediment redistribution occurs in the German Bight. "This highlights this region as particularly affected."

As a next step, the researchers plan to investigate how these changes specifically affect particularly sensitive coastal areas like the Wadden Sea, which relies on a continuous supply of sediment to compensate for rising sea levels. They are also examining how these effects influence the role of the ocean as a carbon sink.

"With an improved understanding of sediment distribution and carbon storage in the North Sea, we can assess long-term risks to coastal stability, navigational safety in shipping, and the functioning of ecosystems in the German Bight," says Chen. "Our findings provide a valuable foundation for the sustainable expansion of offshore wind energy and help decision-makers in politics, business, and industry to plan new wind farms in an environmentally friendly way." 

Provided by Helmholtz Association of German Research Centres  

by Torsten Fischer, Helmholtz Association of German Research Centres

edited by Lisa Lock, reviewed by Robert Egan 

Source: North Sea wind farms may be reshaping sediment flows by 1.5 million tons a year