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Why do some planets have moons? A physics expert explains why Earth has only one moon while other planets have hundreds

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theconversation.com – Nicole Granucci, Instructor of Physics, Quinnipiac – 2024-06-24 07:18:47

Some planets, such as Saturn, have more than a hundred moons, while others, such as Venus, have none.

NASA/JPL-Caltech/Space Science Institute via AP

Nicole Granucci, Quinnipiac University

Curious Kids is a for of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.

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Why do some planets have moons and some don’t? – Siddharth, age 6,


On Earth, you can look up at night and see the Moon shining bright from hundreds of thousands of miles away. But if you went to Venus, that wouldn’t be the case. Not every planet has a moon – so why do some planets have several moons, while others have none?

I’m a physics instructor who has followed the current theories that describe why some planets have moons and some don’t.

First, a moon is called a natural satellite. Astronomers refer to satellites as objects in space that orbit larger bodies. Since a moon isn’t human-made, it’s a natural satellite.

Currently, there are two main theories for why some planets have moons. Moons are either gravitationally captured if they are within what’s called a planet’s Hill sphere radius, or they’re formed along with a solar system.

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The Hill sphere radius

Objects exert a gravitational force of attraction on other nearby objects. The larger the object is, the greater the force of attraction.

This gravitational force is the reason we all stay grounded to Earth instead of floating away.

The solar system is dominated by the Sun’s large gravitational force, which keeps all of the planets in orbit. The Sun is the most massive object in our solar system, which means it has the most gravitational influence on objects such as planets.

In order for a satellite to orbit a planet, it has to be close enough for the planet to exert enough force to keep it in orbit. The minimum distance for a planet to keep a satellite in orbit is called the Hill sphere radius.

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The Hill sphere radius is based on the mass of both the larger object and the smaller object. The Moon orbiting Earth is a good example of how the Hill sphere radius works. The Earth orbits around the Sun, but the Moon is close enough to Earth that Earth’s gravitational pull captures it. The moon orbits around the Earth, rather than the Sun, because it is within Earth’s Hill sphere radius.

A diagram showing Earth, with a long radius around it and a circle representing the Moon within that radius, and Mercury, with a short radius around it.

Earth has a larger Hill sphere radius than Mercury.

Nicole Granucci

Small planets like Mercury and Venus have a tiny Hill sphere radius, since they can’t exert a large gravitational pull. Any potential moons would likely get pulled in by the Sun instead.

Many scientists are still looking to see whether these planets may have had small moons in the past. Back during the formation of the solar system, they may have had moons that got knocked away by collisions with other objects.

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Mars has two moons, Phobos and Deimos. Scientists still debate whether these came from asteroids that passed close into Mars’ Hill sphere radius and got captured by the planet, or if they were formed at the same time as the solar system. More evidence supports the first theory, because Mars is close to the asteroid belt.

Jupiter, Saturn, Uranus and Neptune have larger Hill sphere radii, because they are much larger than Earth, Mars, Mercury and Venus and they’re farther from the Sun. Their gravitational pulls can attract and keep more natural satellites such as moons in orbit. For example, Jupiter has 95 moons, while Saturn has 146.

Moons forming with a solar system

Another theory suggests that some moons formed at the same time as their solar system.

Solar start out with a big disk of gas rotating around a sun. As the gas rotates around the sun, it condenses into planets and moons that rotate around them. The planets and moons then all rotate in the same direction.

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This animation shows how the planets in our solar system formed. The dark rings in the disk represent the formation of the planets and moons. Eventually, the gas condenses into planets, natural satellites and asteroids.

But only a few moons in our solar system were likely created this way. Scientists predict that Jupiter’s and Saturn’s inner moons formed during the emergence of our solar system because they’re so old. The rest of the moons in our solar system, Jupiter’s and Saturn’s outer moons, were probably gravitationally captured by their planets.

Earth’s Moon is special because it likely formed in a different way. Scientists believe that long ago, a large, Mars-sized object collided with the Earth. During that collision, a big chunk flew off the Earth and into its orbit and became the Moon.

This animation from NASA shows a simulation of how our Moon was formed during the collision.

Scientists guess that the Moon formed this way because they’ve found a type of rock called basalt in soil on the Moon’s surface. The Moon’s basalt looks the same as basalt found inside the Earth.

Ultimately, the question of why some planets have moons is still widely debated, but factors such as a planet’s size, gravitational pull, Hill sphere radius and how its solar system formed may play a role.

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Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the where you .

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.The Conversation

Nicole Granucci, Instructor of Physics, Quinnipiac University

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CubeSats, the tiniest of satellites, are changing the way we explore the solar system

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theconversation.com – Mustafa Aksoy, Assistant Professor of Electrical & Computer Engineering, at Albany, State University of New York – 2024-09-27 07:32:30

Mustafa Aksoy, University at Albany, State University of New York

Most CubeSats weigh less than a bowling ball, and some are small enough to hold in your hand. But the impact these instruments are having on exploration is gigantic. CubeSats – miniature, agile and cheap satellites – are revolutionizing how scientists study the cosmos.

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A standard-size CubeSat is tiny, about 4 pounds (roughly 2 kilograms). Some are larger, maybe four times the standard size, but others are no more than a pound.

As a professor of electrical and computer engineering who works with new space technologies, I can tell you that CubeSats are a simpler and far less costly way to reach other worlds.

Rather than carry many instruments with a vast array of purposes, these Lilliputian-size satellites typically focus on a single, specific scientific goal – whether discovering exoplanets or measuring the size of an asteroid. They are affordable throughout the space community, even to small startup, private companies and university laboratories.

Tiny satellites, big advantages

CubeSats’ advantages over larger satellites are significant. CubeSats are cheaper to develop and test. The savings of time and money means more frequent and diverse missions along with less risk. That alone increases the pace of discovery and space exploration.

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CubeSats don’t travel under their own power. Instead, they hitch a ride; they become part of the payload of a larger spacecraft. Stuffed into containers, they’re ejected into space by a spring mechanism attached to their dispensers. Once in space, they power on. CubeSats usually conclude their missions by burning up as they enter the atmosphere after their orbits slowly decay.

Case in point: A team of at Brown University built a CubeSat in under 18 months for less than US$10,000. The satellite, about the size of a loaf of bread and developed to study the growing problem of space debris, was deployed off a SpaceX rocket in May 2022.

A CubeSat can go from whiteboard to space in less than a year.

Smaller size, single purpose

Sending a satellite into space is nothing new, of course. The Soviet Union launched Sputnik 1 into Earth orbit back in 1957. Today, about 10,000 active satellites are out there, and nearly all are engaged in communications, navigation, military defense, tech or Earth studies. Only a few – less than 3% – are exploring space.

That is now changing. Satellites large and small are rapidly becoming the backbone of space research. These spacecrafts can now travel long distances to study planets and , places where human explorations or robot landings are costly, risky or simply impossible with the current technology.

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But the cost of building and launching traditional satellites is considerable. NASA’s lunar reconnaissance orbiter, launched in 2009, is roughly the size of a minivan and cost close to $600 million. The Mars reconnaissance orbiter, with a wingspan the length of a school bus, cost more than $700 million. The European Space Agency’s solar orbiter, a 4,000-pound (1,800-kilogram) probe designed to study the Sun, cost $1.5 . And the Europa Clipper – the length of a basketball court and to launch in October 2024 to the Jupiter moon Europa – will ultimately cost $5 billion.

These satellites, relatively large and stunningly complex, are vulnerable to potential failures, a not uncommon occurrence. In the blink of an eye, years of work and hundreds of millions of dollars could be lost in space.

Two scientists wearing masks, gloves, head coverings and white clean suits work on an instrument in a laboratory.
NASA scientists prep the ASTERIA spacecraft for its April 2017 launch.
NASA/JPL-Caltech

Exploring the Moon, Mars and the Milky Way

Because they are so small, CubeSats can be released in large numbers in a single launch, further reducing costs. Deploying them in batches – known as constellations – means multiple devices can make observations of the same phenomena.

For example, as part of the Artemis I mission in November 2022, NASA launched 10 CubeSats. The satellites are now to detect and map water on the Moon. These findings are crucial, not only for the upcoming Artemis missions but to the quest to sustain a permanent human presence on the lunar surface. The CubeSats cost $13 million.

The MarCO CubeSats – two of them – accompanied NASA’s Insight lander to Mars in 2018. They served as a real-time communications relay back to Earth during Insight’s entry, descent and landing on the Martian surface. As a bonus, they captured pictures of the planet with wide-angle cameras. They cost about $20 million.

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CubeSats have also studied nearby stars and exoplanets, which are worlds outside the solar system. In 2017, NASA’s Jet Propulsion Laboratory deployed ASTERIA, a CubeSat that observed 55 Cancri e, also known as Janssen, an exoplanet eight times larger than Earth, orbiting a star 41 light years away from us. In reconfirming the existence of that faraway world, ASTERIA became the smallest space instrument ever to detect an exoplanet.

Two more notable CubeSat space missions are on the way: HERA, scheduled to launch in October 2024, will deploy the European Space Agency’s first deep-space CubeSats to visit the Didymos asteroid system, which orbits between Mars and Jupiter in the asteroid belt.

And the M-Argo satellite, with a launch planned for 2025, will study the shape, mass and surface minerals of a soon-to-be-named asteroid. The size of a suitcase, M-Argo will be the smallest CubeSat to perform its own independent mission in interplanetary space.

The swift progress and substantial investments already made in CubeSat missions could make humans a multiplanetary species. But that journey will be a long one – and depends on the next generation of scientists to develop this dream.The Conversation

Mustafa AksoyUniversity at Albany, State University of New York

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Drug prices improved under Biden-Harris and Trump − but not for everyone, and not enough

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theconversation.com – C. Michael White, Distinguished Professor of Pharmacy Practice, University of Connecticut – 2024-09-26 07:29:23

Negotiations to reduce drug prices can sometimes shift costs onto consumers.

rudisill/iStock via Getty Images Plus

C. Michael White, University of Connecticut

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When it comes to drug pricing, the Trump and Biden-Harris administrations both have some very modest wins to tout.

As director of the Health Outcomes, Policy, and Evidence Synthesis group at the University of Connecticut School of Pharmacy, I teach and study about the ethics of prescription drug prices and the complexities of drug pricing nationally.

Delving into the presidential candidates’ successes on a number of drug-pricing policies, you’ll see a continuation of progress across the administrations. Neither the Trump administration nor the Biden-Harris administration, however, has done anything to truly lower drug prices for the majority of Americans.

$35 insulin

Insulin is a necessity for patients with diabetes. But from January 2014 to April 2019, the average price per unit went from US$0.22 to $0.34 before dropping back slightly by July 2023 to $0.29 per unit. Since dosing is weight-based, insulin costs for someone weighing 154 pounds would have risen from $231 to $357 a month from 2014 to 2019 and dropped to $305 a month by 2023. Price increases have led some patients to out their medications by taking less than the dose they need for good blood sugar control. One study estimated that over 25% of patients in an urban diabetes center were underusing their insulin.

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In July 2020, the Trump administration enacted a $35 cap on insulin copayments via executive order. In effect, it made participating Medicare Part D programs limit the price of just one of each type of insulin product to $35. For instance, if there were six short-acting insulin products on an insurance plan’s approved drug list, the insurer had to offer one vial form and one pen form at $35.

These price changes did not go into effect during Trump’s presidency. By 2022, only about 800,000 people – or around 11% of the more than 7.4 million people in the U.S. who use insulin to regulate their blood sugar – saw their prices reduced.

Person holding taking vial of insulin out of box

Millions of Americans need insulin to manage their diabetes.

Spencer Platt/Getty Images

In August 2022, the Biden-Harris administration signed the Inflation Reduction Act into law. This maintained the $35 insulin cap with the same stipulations but made the program mandatory for all Medicare Part D and Medicare Part B members. This expanded the number of people who could benefit from cheaper insulin to 3.3 million.

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This still doesn’t help a majority of diabetics. If you don’t have Medicare, the $35 reduction does not apply to you. Furthermore, pharmaceutical companies are not responsible for lowering insulin costs under these policies, but plans are on the hook for lowering copayments. Costs could be passed along to beneficiaries in future Medicare premiums.

Importing Canadian drugs

Americans pay nearly 2.6 times more for prescription drugs than people in other high-income countries. One way regulators have tried to reduce prices is to simply import drugs at the prices pharmaceutical companies charge those countries rather than those charged to U.S. consumers.

In July 2019, the Trump administration proposed importing drugs from Canada as a way to share Canadians’ lower drug costs with American consumers. He signed an executive order allowing the Food and Drug Administration to create the rules under which states could import the . When came into office, he left the executive order in place and the rulemaking continued.

Two pharmacists behind the counter, shelves of drugs behind them and an American and Canadian flag before them

Some Americans have traveled across borders for cheaper medications.

Jeff Haynes/AFP via Getty Images

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No under the Trump or Biden-Harris administrations has yet been able to successfully import a Canadian drug product. In January 2024, however, the Food and Drug Administration approved Florida’s plan to import Canadian drugs, the first state to receive the green light. Colorado, New Hampshire, New Mexico and Texas have applications pending as of September 2024.

Unfortunately, it is unlkely that Canada would allow their prescription drugs to be shipped in large quantities to American consumers, not without imposing high tariffs as a disincentive. That is because drug manufacturers could limit supplies to Canada and cause shortages if drugs are moved to the U.S. Manufacturers could also be less willing to negotiate lower prices for Canadians if that will U.S. profits.

Negotiating with the pharmaceutical industry

Be it prescription drugs or cars, both buyer and seller must agree on a price for a successful sale to occur. If the potential buyer is unwilling to walk away from negotiations, you will not get the seller’s best price. One reason U.S. drug prices are higher than other countries’ is because the is not a shrewd negotiator.

Negotiations that result in major reductions in drug prices frequently result from the drug manufacturer losing access to patients on a certain health plan or ending up in a higher drug tier that substantially raises a patient’s copay. However, if the buyer refuses the seller’s final offer, their members or citizens lose access to those drugs. While major private health plans and pharmacy benefit managers are able to directly negotiate drug prices with pharmaceutical manufacturers, often with substantial savings, Medicare was prevented from doing so by federal law until recently.

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In May 2018, the Trump administration released a so-called blueprint for reducing prescription drug prices that included negotiating Medicare prescription drug prices with the pharmaceutical industry. This plan wasn’t enacted during his term.

In August 2022, under the Biden-Harris administration, the Inflation Reduction Act enabled price negotiation and specified the number of drugs that negotiations could include in a year.

The Inflation Reduction Act allowed Medicare to negotiate drug prices for the first time.

The first negotiation between Medicare and the pharmaceutical industry took place over the summer of 2024, lowering costs for 10 Medicare Part D drugs, which include the blood thinner Xarelto and the drugs Farxiga and Jardiance, which treat Type 2 diabetes, heart failure and kidney disease. The resulting $1.5 in savings will be extended in 2026 to the approximately 8.8 million Medicare Part D patients who are taking these drugs. The prices for these drugs are still twice what they are in four other developed countries.

Prices will be negotiated for another 15 Medicare Part D drugs in 2027. Thereafter, drug negotiations could include Medicare Part D drugs, which you pick up from your pharmacy, and Medicare Part B drugs, which are administered or received from your doctor’s office.

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Another aspect of the Inflation Reduction Act is capping out-of-pocket expenses at $2,000. This won’t go into effect until 2025, however, and simply shifts costs above the cap onto taxpayers.

Continuation of progress

It is often challenging to attribute policy successes to one administration versus another when assessing complex issues such as drug pricing. There were ideas initiated during the Trump administration that did not come to fruition until the Biden-Harris administration implemented and expanded on them.

For example, Medicare price negotiation, proposed in a Trump administration “blueprint,” was codified in law by President Biden, but the fruits of this policy will not be seen until the next administration. And regardless of who you attribute this success to, only a portion of people on Medicare will see any relief from high drug prices as a result.

Truly lowering the costs of prescription drugs would require identifying the maximum price the nation is willing to pay for , such as cost per quality adjusted life year at the federal, state and private payer levels, and being willing to walk away from negotiations if the price exceeds that level. This would not be a panacea, though, especially for patients with rare and ultrarare diseases, and would need to be eased in over time to avoid bankrupting the industry.The Conversation

C. Michael White, Distinguished Professor of Pharmacy Practice, University of Connecticut

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Fungal infections known as valley fever could spike this fall

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theconversation.com – Jennifer Head, Assistant Professor of Epidemiology, of Michigan – 2024-09-26 07:27:21

As the climate warms and landscapes become drier, researchers fear that valley fever could spread across other regions of the U.S.

Carolyn Van Houten/The Washington Post via Getty Images

Jennifer Head, University of Michigan; Alexandra K. Heaney, University of California, San Diego, and Simon Camponuri, University of California, Berkeley

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As the climate warms, the southwestern U.S. is increasingly experiencing weather whiplash as the region swings from drought to and back again. As a result, the public is hearing more about little-known infectious diseases, such as valley fever.

In May 2024, about 20,000 people attended a music festival in Buena Vista Lake, California. In the months that followed, at least 19 developed valley fever, and eight were hospitalized from their infection. This outbreak follows a dramatic increase of more than 800% in valley fever infections in California between 2000 and 2018.

In 2023, California reported the second-highest number of valley fever cases on record, with more than 9,000 cases reported statewide. And between April 2023 and March 2024, California provisionally reported 10,593 cases – 40% more than during the same period the prior year.

The Conversation U.S. asked Jennifer Head, Simon Camponuri and Alexandra Heaney – researchers specializing in the epidemiology of valley fever – to explain what valley fever is, and what might explain its rise in recent years.

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What is valley fever, and how do you get infected?

Valley fever is the common name for a disease called coccidioidomycosis, which is an infection caused by pathogenic fungi from the Coccidioides genus. The fungi are primarily found in arid soils of the southwestern United States, as well as parts of Central and South America.

When the fungus has access to moisture and nutrients, it grows long, branching fungal chains throughout the soil. When the soil dries out, these chains fragment to form fungal spores, which can be stirred up into the air when the soil is disturbed, such as by wind or digging. Airborne spores can then be inhaled and cause a respiratory infection.

Cases of valley fever are typically highest in California’s southern San Joaquin Valley and southern Arizona, but they have been increasing outside of these regions. Between 2000 and 2018, the incidence of valley fever cases increased fifteenfold in the northern San Joaquin Valley and eightfold along the Southern California coast. And between 2014 and 2018, incidence increased by more than eightfold along the central coast.

Because of these trends and the virulence of the pathogen that causes valley fever, it is listed as a priority pathogen by the World Health Organization. Historically, fungal infections have received very little attention and resources. By creating this list, the WHO is hoping to galvanize action surrounding listed pathogens, including getting more resources for research as well as the development of new treatments.

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Purple-stained image showing spores of the coccidioidomycosis fungus.

Coccidioides immitis, one of the two species of fungus that cause valley fever.

Smith Collection-Gado/Archive Photos via Getty Images

What are the symptoms, and what should people be looking for?

After inhaling fungal spores from the , Coccidioides initially infects the lungs, causing symptoms like mild to severe cough, fever, difficulty breathing, chest pain and tiredness. Valley fever symptoms can resemble other common respiratory infections, so it’s important for people to get checked by a doctor if they’ve experienced prolonged symptoms, particularly if they have been given antibiotics that they are not responding to.

In California and Arizona, an estimated one-third of community-acquired pneumonia cases – or pneumonia acquired outside of the hospital – are caused by valley fever. However, only a fraction of community-acquired pneumonia cases get tested for it, so it’s likely the number of valley fever cases is significantly higher. Among diagnosed cases, half experienced symptoms for two months or more before being diagnosed.

In 5% to 10% of cases, the fungus can spread from the lungs to other parts of the body, such as the central nervous system, liver and bones, causing meningitis or arthritis-like symptoms. These cases can be severe and possibly fatal.

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Antifungal treatment is available, and early diagnosis and treatment is critical for better outcomes.

A woman doctors a man with a bandage on his head.

Jose Epifanio Sanchez Trujeque of Lebec, Calif., spent four months in the hospital after contracting valley fever in 2023.

The Washington Post/Getty Images

What time of year should you be most concerned?

Valley fever cases can occur year-round, but in California, cases reported via surveillance tend to increase starting in August and September, peak in November and return to background levels in January and February.

Researchers believe that are likely exposed to the fungus in the summer and early fall months, typically one to three months prior to their diagnosis. This delay accounts for time between when patients are exposed, develop symptoms and are diagnosed with the disease. While cases peak in the fall on average, seasonal strength and timing varies regionally.

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Our research shows that this seasonal surge in the fall is especially strong wetter winters and that alternation between dry and wet conditions is associated with increased incidence in fall months.

Valley fever cases in California nearly doubled following wet winters that occurred one and two years after the 2007-2009 and 2012-2015 droughts.

In 2023, California experienced a similar transition, with an extreme drought occurring between 2020-2022 followed by heavy precipitation in the winter of 2022-2023.

This transition was followed by a near-record spike in cases in 2023. The state experienced another wet winter during the 2023-2024 wet season, furthering concern about continued high risk for valley fever in 2024.

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Our research team recently developed a model to forecast valley fever cases that will occur between April 2024 and March 2025 in California. We that the state is likely to see another spike in cases during the fall and winter of 2024, on par with the spike in 2023.

During high-risk periods, clinicians should consider valley fever as a potential diagnosis. This is especially true when evaluating a patient presenting with valley fever symptoms or a respiratory illness who lives in, works in or traveled to an endemic or emerging region.

We are currently working to characterize seasonal disease patterns in Arizona as well, which are different from California’s. This is likely because Arizona has two rainy seasons.

Are some people at greater risk than others?

Those who spend time or work outdoors in areas where valley fever is common, especially where they may be exposed to dirt and dust, are more likely to get it.

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While healthy people are still at risk of infection, certain factors can increase the likelihood of developing severe disease from valley fever. These include being an adult 60 years or older, diabetes, HIV or another that weakens the immune system, or being pregnant. People who are Black or Filipino also have been noted to have a higher risk of severe disease, which may relate to more exposure to the fungal spores, underlying health conditions, inequities in accessing care or other possible predispositions.

Dust billows as a farmer plows a dry field on a tractor.

People who work around dry, dusty conditions are at a higher risk of contracting valley fever.

David McNew/Getty Images News via Getty Images

How can you protect yourself from getting valley fever?

People who and work in the regions where the fungus is found should avoid exposure to dust as much as possible. When it is windy outside and the air is dusty, stay indoors and keep windows and doors closed.

When driving through a dusty area, limit vehicle speed, keep car windows closed and recirculate the air, if possible. When working outdoors, use dust suppression techniques, including wetting soil before digging to prevent stirring up dust, and installing fencing, windbreaks and vegetation where possible.

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For those who must directly stir up soil or be in dusty conditions, such as while doing construction or gardening work, consider using an N95 mask to limit dust inhalation.The Conversation

Jennifer Head, Assistant Professor of Epidemiology, University of Michigan; Alexandra K. Heaney, Assistant Professor in Climate and Health Epidemiology , University of California, San Diego, and Simon Camponuri, PhD Candidate in Environmental Health Sciences, University of California, Berkeley

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