After 276 days in orbit, China's reusable experimental spacecraft landed at its planned site at the Jiuquan Satellite Launch Center in Northwest China's Gansu Province on Monday, and Chinese space watchers said it was a milestone in China's efforts to develop a fully reusable space transportation system.

The success is an important breakthrough in China's research on reusable spacecraft technologies, which will provide more convenient and affordable round trips for the peaceful use of space, the Xinhua News Agency said on Monday.

China launched a reusable experimental spacecraft using its Long March-2F carrier rocket from the Jiuquan Satellite Launch Center on August 5, 2022, to test reusable technologies and in-orbit service technologies to support the peaceful use of space, Xinhua previously reported. 

China tested the reusable experimental spacecraft in September 2020, and the spacecraft returned to the planned landing site after two days in orbit. The spacecraft was also launched with a Long March-2F carrier rocket.

Chinese authorities have disclosed few details about the craft's technology. So far, no images of the spacecraft or footage of its launch or landing have been disclosed.

Many space lovers compared it to the US Air Force's X-37B, an autonomous Boeing space plane that can remain in orbit for long periods before returning to Earth on its own, saying that the technology used in the experiment is "too advanced to show" on social media. 

The reusable spacecraft's technology has evidently matured, considering how much longer it can stay in orbit, Chinese space watchers noted on Sunday. 

Song Zhongping, a space expert and TV commentator, told the Global Times on Monday that the reusability of such spacecraft would drastically reduce costs. More importantly, the longer orbiting time means that the spacecraft can perform more complicated missions such as changing trajectory in near-Earth orbit and sending various payloads into orbit. 

Judging from the rocket used in the launch, the Long March-2F carrier rocket - which is one of China's most advanced and reliable rockets for manned space flights - the reusable spacecraft may be used in future manned missions, said another space observer, speaking on condition of anonymity.

Given the payload launching capability of the Long March-2F, the spacecraft could weigh around 8 tons, which is very similar to the launch mass for the X-37B at around 5 tons, so that speculation about their similarity is not groundless, the observer said.

The latest test of the reusable spacecraft was the longest, most complicated, and most challenging so far. The record of 276 days in orbit even beat the first mission of the US' X-37B in 2010, which remained in orbit for 224 days, Huang Zhicheng, a space industry expert, told the Global Times on Monday. 

At the China Space Conference opening event to mark the April 24 Space of China this year in Hefei, East China's Anhui Province, Wang Wei, an academician of the Chinese Academy of Sciences and director of the research and development department of the state-owned aerospace giant China Aerospace Science and Technology Corp, listed the 10 major scientific and technology problems for the 2023 space development.

Among them is the health monitoring of the power system for the reusable vehicle and an assessment of its service life.

Huang said that one of the many advantages of the reusable spacecraft is that it can land horizontally, which is more comfortable and safer for the crew. Once it's mature, such technology could be applied to suborbital or space tourism, and even super fast point-to-point transportation for people or cargo on Earth.

The US often hypes and smears China's technology breakthroughs as militarization because it sees all breakthroughs by non-Western countries as a threat. This is meant to maintain a technology monopoly, which is unfair to other countries and serves US hegemony, Chinese experts said.

In order to prevent any impact caused by Japan's dumping of Fukushima nuclear-contaminated wastewater into the Pacific Ocean, more countries, especially those in the Asia-Pacific like China, Thailand and Russia, are taking actions, including strengthened testing of aquatic products imported from Japan, while more people and organizations from South Korea and the Pacific Island countries are voicing their opposition and concern over Tokyo's decision.

Although the US said it is "satisfied" with Japan's act, which has caused fury and concern worldwide, the US is in fact the country that has seen the greatest reduction in imports of Japanese seafood and rice wine, media reported, citing data published by Japan's Ministry of Agriculture, Forestry and Fisheries.

Experts said this has exposed the hypocrisy of Washington to trade permission for Japan's dumping for Tokyo's loyalty to serve its geopolitical strategy, and that the administration of US President Joe Biden cares less about the safety and environmental protection that seriously matters to the US people. The US priority is geopolitics and US hegemony, as its poor handling of the Hawaii wildfires and hypocritical response to Japan's nuclear-contaminated water dumping plan are hard evidence of this.

The Japanese government said on Saturday that "no detectable amount of tritium was found in the first fish samples taken in waters near the Fukushima nuclear plant." The fish samples, "a gurnard and olive flounder," were collected Friday within five kilometers of the dumping outlet of the Fukushima Daiichi complex, the Fisheries Agency of Japan said on its website.

This information release just caused more suspicion among the public, as many netizens on Chinese social media networks asked "why the Japanese government only took two fish to sample." They doubted the credibility of the detection and research results that are provided by the Japanese government.

According to South Korean media Hankyoreh on Friday, South Korean experts on nuclear energy are concerned that after public attention toward the dumping declines, the Japanese government will dump the nuclear-contaminated wastewater more recklessly, as Japan has consistently denied South Korea and other relevant countries' requests to directly collect samples.

More countries take actions

After Japan started dumping nuclear-contaminated wastewater from the crippled Fukushima Daiichi nuclear power plant into the sea on Thursday, more and more Asia-Pacific countries have taken actions in response. Apart from China's decision to suspend imports of all aquatic products from Japan, Thailand and Russia vowed to tighten control and strengthen testing of aquatic product imports from Japan.

The Food and Drug Administration of Thailand said if the seafood imported from Japan exceeds the standard of radiation and other relevant indexes, the country is ready to order a recall and suspend imports, according to Thai media PPTV.

According to Singapore media zaobao.com, as Thailand is located in the area with high risks caused by Japan's dumping of nuclear-contaminated water, the Thai governmental departments of fisheries, food and drug administration, nuclear energy and other relevant agencies have had an emergency meeting, where they decided to strengthen supervision measures. Thai officials said that Thai customs and relevant departments that conduct inspections of imported seafood are being prepared.

Russia's quarantine agency has tightened quality control of seafood imported from Japan after the dumping of nuclear-contaminated wastewater started, the Xinhua News Agency reported on Friday.

In a news release issued on Thursday, the Federal Service for Veterinary and Phytosanitary Supervision of Russia said that "Russia shares the concerns of many countries regarding the impact of nuclear-contaminated wastewater discharge on food safety, thus the agency introduces a regime of enhanced control on radiological parameters of fish and seafood imported from Japan."

"If an excess of radioactive material is detected, restrictive measures will be taken in relation to the supply of such goods from Japan," the Russian agency said.

Analysts said the Japanese government has put in a lot of resources to buy "support" or "understanding" from other countries and regions, as it understands that some countries are concerned or opposed to Japan's irresponsible decision, especially countries and regions in the Asia-Pacific. However, even if some countries did not express open oppositions, they are taking serious actions to handle the impacts.

Rising concerns

In some other countries, concern and opposition against Japan's actions have increased, despite their governments remaining silent or exhibiting tolerance toward Tokyo.

In the Pacific Island Country of Fiji, the Suva Fish Market Association came out strongly on Thursday and stated that it does not agree with the dumping of the Fukushima nuclear-contaminated wastewater into the Pacific Ocean, and they are also concerned over the Fijian government's attitude that "the discharged water is safe," according to Fiji media fijivillage.com.

Samu Maraiwai, president of the association, said the nuclear-contaminated wastewater to be dumped into the Pacific Ocean poses "a risk of massive destruction to our marine ecosystem and our source of livelihood." Maraiwai said the nuclear-contaminated waste will be toxic to a certain level and it will affect the marine ecosystem including fish, seaweeds, corals and other sources of livelihood.

According to South Korean media Yonhap News, about 50,000 people rallied in Seoul on Saturday to "protest Japan's release of radioactive water from the crippled Fukushima nuclear power plant," with the participation of some 90 civic groups which have formed a coalition to protest the dumping of nuclear-contaminated water and members of four opposition parties, including the main opposition Democratic Party (DP).

Occupying four traffic lanes, the protesters chanted slogans and held up signs reading, "Retract disposal of Fukushima contaminated water," and "Denounce the Yoon Suk Yeol administration."

"Japan has crossed a line that shouldn't be crossed," DP leader Lee Jae-myung said from a platform installed for the rally. "Discharging nuclear contaminated water is a declaration of war against nations bordering the Pacific Ocean.

"Japan should apologize to the Republic of Korea, which is its nearest country and is suffering the most damage," he noted. Lee also accused South Korean President Yoon Suk-yeol of justifying and supporting the water dumping when Japan was hesitant out of concern about its neighbors.

US hypocrisy

Defying mounting oppositions and contrary to its self-styled image as a global leader in environmental protection, the US Department of State endorsed Tokyo's controversial dumping of the Fukushima nuclear-contaminated water into the ocean, saying the US is "satisfied with Japan's safe, transparent, and science-based process."

Endorsing Japan's selfish and irresponsible practice that harms the global environment and the health of human beings in the planet, the US has trampled on its own image as a leader in global environmental protection, exposing its selfishness and hypocrisy of prioritizing its geopolitical interests above the long-term well-being of people around the world, experts said.

As the US publicly backs the Japanese government's plan to dump nuclear-contaminated water into the ocean, media reports quoting data from the Ministry of Agriculture, Forestry and Fisheries of Japan revealed that the US was making the biggest moves in decreasing imports of agricultural and aquatic products from Japan in the first half of 2023.

Data showed that the US is the country that reduced imports of Japanese agriculture, forestry and fishery products the most in the first half of the year, with imports down by 8.3 billion yen ($57 million). The main production areas of the three kinds of products are all in areas affected by the dumping of nuclear-contaminated water, according to media reports.

Though well aware of the risks brought about by the nuclear-contaminated water to surrounding oceans and the entire global environment, the US has succumbed to its political interests and is going against its image as a leader of environmental protection activism, Lü Chao, an expert on Korean Peninsula issues at the Liaoning Academy of Social Sciences, told the Global Times on Saturday.

"It is laughable to see the hypocrisy and double standards held by the US by covering up Japan's extremely selfish and irresponsible wrongdoing," Lü said.

"Acts and deeds from the US show that it purely considers Japan's act of harming the global environment and human beings of all countries from a geopolitical perspective. The US's real intention is to tie Japan to its chariot of geopolitical games," Li Haidong, a professor at the China Foreign Affairs University, told the Global Times on Saturday.

Without the indulgence of the US, the Japanese decision to dump nuclear- contaminated water would not have been so arbitrary. Japan opened a Pandora's box on Thursday with the strong backing of the US, allowing crisis to plague other countries and the international community, Li noted.

Debris from a cosmic explosion bumped into a neighboring star, a new study reports, suggesting that the surviving star might be responsible for its partner’s demise.

The explosion, known as a type 1a supernova, was discovered in 2012. It went off in a galaxy about 50 million light-years away in the constellation Virgo. Astronomers quickly noticed more blue light coming from the supernova than expected. The excess light probably came from gas that was compressed and heated as the shock wave ran into another star, Howie Marion, an astronomer at the University of Texas at Austin, and colleagues report online March 22 in the Astrophysical Journal. It’s the first strong evidence that some normal type 1a supernovas have orbiting companions.

Astronomers suspect that a type 1a supernova is the detonation of a white dwarf, the dense core left behind after some stars die. What pulls the trigger is up for debate. Two white dwarfs could spiral together and explode. Or one white dwarf could siphon gas off of a companion star until the white dwarf could no longer support its own weight, triggering a destructive regurgitation. Seeing glowing gas from the shock wave slamming into a companion supports the idea that some white dwarfs eat until they explode.

Last year, researchers reported similar observations from another supernova (SN: 6/27/15, p. 9), but that explosion was just one one-thousandth as bright as a typical type 1a. It might not be representative of all type 1a supernovas, which are frequently used as distance markers that help measure the expansion of the universe.

Taking a combo of HIV drugs can make unprotected sex a whole lot safer.

Antiretroviral therapy cut HIV transmission between partners to zero, researchers report July 12 in JAMA.

That doesn’t mean there’s no risk, says infectious disease researcher Alison Rodger of University College London. But for heterosexual couples with an HIV-positive member who is on therapy and has low levels of virus in the blood, “the risk is extremely low — likely negligible,” she says. That may also be true for homosexual couples, Rodger says, but her team needs more data to say for sure.
Antiretroviral therapy curbs the amount of HIV circulating in the bloodstream. Scientists knew that HIV-positive people taking this therapy were less infectious than normal, but no one had nailed down their risk of spreading the virus through condom-free, penetrative sex.

Rodger and colleagues analyzed data from 1,166 couples enrolled in an observational study to assess HIV transmission risks. All couples had reported having unprotected sex, and one member of each couple was HIV-positive and on therapy. Researchers tested the negative partner for HIV every six to 12 months.

Among 888 couples eligible for follow-up, researchers didn’t find a single case of partner-to-partner HIV transmission for about one and a half years, despite frequent unprotected sex.

Human eyes are capable of detecting a single photon — the tiniest possible speck of light — new research suggests.

The result, published July 19 in Nature Communications, may settle the debate on the ultimate limit of the sensitivity of the human visual system, a puzzle scientists have pondered for decades. Scientists are now anticipating possibilities for using the human eye to test quantum mechanics with single photons.

Researchers also found that the human eye is more sensitive to single photons shortly after it has seen another photon. This was “an unexpected phenomenon that we just discovered when we analyzed the data,” says physicist Alipasha Vaziri of Rockefeller University in New York City.
SUBSCRIBE
Previous experiments have indicated that humans can see blips of light made up of just a few photons. But there hasn’t been a surefire test of single photons, which are challenging to produce reliably. Vaziri and colleagues used a quantum optics technique called spontaneous parametric down-conversion. In this process, a high-energy photon converts into two low-energy photons inside of a crystal. One of the resulting photons is sent to someone’s eye, and one to a detector, which confirms that the photons were produced.

During the experiment, subjects watched for the dim flash of a photon, which arrived at one of two times, with both times indicated by a beep. Subjects then chose which beep they thought was associated with a photon, and how confident they were in their decision.

In 2,420 trials, participants fared just slightly better than chance overall. That seemingly unimpressive success rate is expected. Because most photons don’t make it all the way through the eye to the retina where they can be seen, in most trials, the subject wouldn’t be able to see a photon associated with either beep. But in trials where the participants indicated they were most certain of their choice, they were correct 60 percent of the time. Such a success rate would be unlikely if humans were unable to see photons — the chance of such a fluke is 0.1 percent.

“It’s not surprising that the correctness of the result might rely on the confidence,” says physicist Paul Kwiat of the University of Illinois at Urbana-Champaign, who was not involved with the research. The high-confidence trials may represent photons that made it through to the retina, Kwiat suggests.

Additionally, the data indicate that single photons may be able to prime the brain to detect more dim flashes that follow. When participants had seen another photon in the preceding 10 seconds, they had better luck picking out the photon.
Scientists hope to use the technique to test whether humans can directly observe quantum weirdness. Photons can be in two places at once, a state known as a quantum superposition. The technique could be adapted to send such quantum states to a subject’s eye. But, says Leonid Krivitsky, a physicist at the Agency for Science, Technology and Research in Singapore, “I’m pretty skeptical about this idea of observing quantumness in the brain.” The signals, he suggests, will have lost their quantum properties by the time they reach the brain.

Whether humans can see individual photons may seem to be a purely academic question. But, Vaziri says, “If you are somewhere outside of a city in nature and on a moonless night and you have only stars to navigate, on average the number of photons that get into your eye is approaching the single photon regime.” So, he says, having eyes sensitive enough to see single photons may have some evolutionary advantage.

How the seafloor quivers under an intense storm called a “weather bomb” could help reveal Earth’s innermost secrets.

Using a network of seismic sensors, researchers in Japan detected a rare type of deep-Earth tremor originating from a rapidly strengthening cyclone over the North Atlantic Ocean. Tracking how these newfound shakes ripple through the globe will help geoscientists map the materials that make up the planet’s depths, the researchers report August 26 in Science.

“We’re potentially getting a suite of new seismic source locations that can be used to investigate the interior of the Earth,” says Peter Bromirski, a geophysical oceanographer at the Scripps Institution of Oceanography in La Jolla, Calif., who wrote a commentary on the new research in the same issue of Science. “Further investigations will refine our understanding of how useful these particular waves will be.”
Tremors traveling through the ground speed up, slow down or change direction depending on the type of material they pass through. Carefully measuring these movements from earthquake waves has allowed scientists to gather clues about the structure and composition of Earth’s deepest layers.

Some regions — the middle of tectonic plates under the ocean, for instance — don’t see many earthquakes, though. Luckily, weather bombs can generate their own seismicity. Whipping winds can stir up towering ocean swells. When two opposing ocean swells collide, the meet-up can send a pressure pulse down to the ocean floor. The pulse thumps the seafloor, producing seismic waves that penetrate deep into the planet.
Scientists had previously detected only one type —called P waves —of these storm-generated seismic waves. P waves cause a material to compress and stretch like an accordion in the same direction that the wave travels. The other variety, called S waves, has proved more elusive. S waves formed by storms are typically weaker than P waves and cause material to ripple perpendicular to the wave’s path. The effect is similar to when one end of a garden hose is jerked up and down, producing waves that travel along the hose’s length.
Seismologists Kiwamu Nishida of the University of Tokyo and Ryota Takagi of Tohoku University in Sendai, Japan, hunted for the elusive S waves using a network of 202 seismic stations in Japan. Typically, the waves are lost within Earth’s natural seismic background noise. By combining and analyzing the data collected by the extra-sensitive seismometers, however, the researchers were able to tease out the S wave signals.

The waves originated from a North Atlantic cyclone, the researchers found. That storm actually produced two types of S waves. SV waves shift material vertically relative to Earth’s surface and can form from P waves. SH waves shift material horizontally and their origins are more of a mystery. Those SH waves may form from complex interactions between the ocean and seafloor, Nishida says.

Combining measurements of P, SV and SH waves will “ultimately provide better maps of Earth’s mantle and maybe even the core,” says Keith Koper, a seismologist at the University of Utah in Salt Lake City. Koper and colleagues report similar observations of S waves generated in the Pacific Ocean and detected by a Chinese seismic network in the Sept. 1 Earth and Planetary Sciences Letters. “It’s nice to see someone else get similar results —it makes me feel more confident about what we observed,” Koper says.

A second kind of crow, native to Hawaii, joins the famous New Caledonian crows as proven natural tool-users.

Tested in big aviaries, Hawaiian crows (Corvus hawaiiensis) frequently picked up a little stick and deftly worked it around to nudge out hard-to-reach tidbits of meat that researchers had pushed into holes in a log, scientists report September 14 in Nature.

“A goosebump moment,” says study coauthor Christian Rutz of his first sight of Hawaiian crows tackling the test. Their nimble handling is “not some little fluke where a bird picks up a stick and pokes it in a hole,” he says. Anecdotes of such flukes abound, especially for crows. What’s rare are demonstrations that most able-bodied adults in a species show a capacity for tool use in chores important for life in the wild. Because Hawaiian crows are extinct in the wild, Rutz and his colleagues had the bittersweet ability to test literally all adult members of the species. Youngsters too developed tool skills on their own.

Rutz, of the University of St. Andrews in Scotland, has worked with New Caledonian crows, which routinely shape and wield food-snagging tools. These birds, like the Hawaiian crows, are native to remote tropical islands. So is the Galapagos woodpecker finch, one of the handful of other bird species proven expert in tool use. Remote islands may favor the evolution of such capacities, Rutz muses. There are no true woodpeckers to compete with birds for treats in crevices there. And few predators lurk to pounce on a bird distracted with its head practically in a hole.
GOOD STICKWORK A Hawaiian crow manipulates a twig in its beak to wiggle out a meaty tidbit hidden in a log. Crows dissatisfied with sticks that researchers set out for snagging food sometimes flew into the shrubbery and selected their own tools for the task.

Pain is contagious, at least for mice. After encountering bedding where mice in pain had slept, other mice became more sensitive to pain themselves. The experiment, described online October 19 in Science Advances, shows that pain can move from one animal to another — no injury or illness required.

The results “add to a growing body of research showing that animals communicate distress and are affected by the distress of others,” says neuroscientist Inbal Ben-Ami Bartal of the University of California, Berkeley.
Neuroscientist Andrey Ryabinin and colleagues didn’t set out to study pain transfer. But the researchers noticed something curious during their experiments on mice who were undergoing alcohol withdrawal. Mice in the throes of withdrawal have a higher sensitivity to pokes on the foot. And surprisingly, so did these mice’s perfectly healthy roommates that were in nearby cages. “We realized that there was some transfer of information about pain” from injured mouse to bystander, says Ryabinin, of Oregon Health & Science University in Portland.

When mice suffered from alcohol withdrawal, morphine withdrawal or an inflaming injection, they become more sensitive to a poke in the paw with a thin fiber — a touchy reaction that signals a decreased pain tolerance. Mice that had been housed in the same room with the mice in pain also grew more sensitive to the poke, Ryabinin and colleagues found. These bystander mice showed other signs of heightened pain sensitivity, such as quickly pulling their tails out of hot water and licking a paw after an irritating shot.

The results are compelling evidence for the social transmission of pain, says neuroscientist Christian Keysers of the Netherlands Institute for Neuroscience in Amsterdam.

Pain’s contagion seemed to spread through the nose, further experiments revealed. After spending time with bedding used by mice in pain, healthy mice’s pain sensitivity went up. Some olfactory signals may have been transferred from the pained mouse onto the bedding before a mouse not experiencing pain showed up and began sniffing around. Ryabinin and colleagues are looking for compounds that might carry this pain signal mouse-to-mouse.

Implications for people are unknown. Humans’ olfactory skills fall short of other animals’, so it’s unclear whether odors can actually transmit information about pain, Ryabinin says.
While the data suggest that scent signals can carry the pain message, Keysers points out that other senses, such as hearing or vision, may be important too. Mice could see their compatriot in distress or hear pained squeaks. Still, the new paper fits with other work that shows “rodents exchange information about their states in many exciting and complex ways,” Keysers says.

A better understanding of the various ways animals can become more sensitive to pain may help explain more generally why pain comes and goes. The results suggest that sometimes, “there is no need for a specific injury for an animal to feel pain,” Ryabinin says. Instead, social factors or cues can influence pain perception. That idea may help explain the experience of some people who suffer from chronic pain, a condition that can begin mysteriously or persist long after an injury heals.

X-rays appear to be trickling away from Pluto, even though the dwarf planet has no obvious way of making the high-energy photons, a new study reports.

Given what researchers have learned about Pluto since the New Horizons spacecraft flew by in 2015 (SN: 8/8/15, p. 6), the discovery is surprising. For many planets and comets, X-rays are generated when the solar wind, a stream of charged particles from the sun, runs into neutral gas atoms or magnetic fields from these bodies. But the environment around Pluto isn’t conducive to producing X-rays: the dwarf planet has no measurable magnetic field, its atmosphere is very thin, and it’s losing that atmosphere at rates much lower than expected.
“We naively thought Pluto might be losing its atmosphere at the same rate as [some] comets,” says Carey Lisse, a planetary astronomer at the Applied Physics Laboratory in Laurel, Md. “We knew comets make X-rays, so we hoped that Pluto did, too.” Instead, interactions between the solar wind and a tenuous tail of methane gas hundreds of times longer than Pluto’s diameter might be the culprit, Lisse and colleagues suggest online October 25 on arXiv.org.

Lisse’s team used the Chandra X-ray telescope, once in 2014 and three more times in 2015, to look for Pluto X-rays. Chandra detected just seven photons streaming from Pluto in a total of about two days’ worth of observing time. Though the signal isn’t strong, that’s about six or seven more photons than expected based on New Horizons’ measurements of Pluto’s atmosphere and the solar wind.

“It’s a very puzzling finding,” says Konrad Dennerl, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. “I’m not fully convinced,” he adds. “It’s a very low signal.”

Lisse and collaborators note that the signal appears to follow Pluto across the sky. They detected X-ray photons on four separate occasions. The energy of the photons doesn’t appear to match that of the spurious X-ray noise that peppers the telescope, so the signal appears genuine. Still, Lisse and Dennerl are teaming up to get some Pluto time with another X-ray observatory, the European Space Agency’s XMM-Newton satellite.

“We understand that there’s a bit of skepticism,” Lisse says. “We’re going to do some follow-up with a totally different instrument to verify this.”

X-rays from Pluto aren’t just a quirky detail about this specific dwarf planet. If other bodies in the Kuiper belt, the ring of icy debris just past Neptune’s orbit, have atmospheres, then X-ray observations could help detect them.

An ocean of liquid water probably doesn’t lurk beneath the icy surface of Mimas, Saturn’s smallest major moon, new calculations suggest. In 2014, scientists had proposed the ocean to help explain an odd wobble in the moon’s orbit (SN: 11/15/14, p. 16).

Other ocean-harboring moons, such as Jupiter’s Europa and Saturn’s Enceladus, are crisscrossed by fractures opened by strong tides that cause their oceans to bulge outward. Mimas, though freckled with craters, lacks any such cracks.

Planetary scientist Alyssa Rhoden of Arizona State University in Tempe and colleagues calculated whether Mimas’ icy shell could withstand the stress of a subsurface ocean pushing outward. Taking into account the moon’s elongated orbit, the team estimates that a subsurface ocean would produce tidal stresses larger than those on crack-riddled Europa. Mimas therefore probably doesn’t have an ocean, the researchers conclude February 24 in the Journal of Geophysical Research: Planets.