Going straight up is hard. It takes a lot of energy. For those of us who enjoy hiking, cycling or running, hills are the bane of our existence. The hills sap us of our strength and speed, and they require more effort than we often want to expend.

Rockets are the ultimate definition of vertical ascent: They go up, fast. They need lots of raw power, and they need it immediately. Interestingly, though, the modern incarnation of reusable rocketry has come back to the same basic fuel as the human body uses: hydrocarbons. Granted, SpaceX and Blue Origin’s massive rockets are not using sugar, carbs or fats, but they are using the simplest hydrocarbon, methane: a single carbon atom with four hydrogen atoms around it, CH₄.

As a physical chemist, I get to explore how molecules produce and absorb energy. I have seen how various chemicals have different benefits and drawbacks for a variety of energy applications.

Orchestrating the pros and cons is like how different plays in football accomplish the same goal of getting the ball down the field but do so with distinct approaches. None are perfect, and some are more spectacular than others.

A different type of fuel

The use of methane as a component of rocket fuel is different from what was used during Apollo or in the earlier crewed space rockets, and even in the space shuttle main engines. In all those rockets, hydrogen gas was the primary fuel.

In the simplest terms, hydrogen in the form of H₂ reacts with oxygen – O₂, the same stuff you breathe – to produce water and a copious amount of energy. Hydrogen itself is light, and this reaction is incredibly efficient. The power-to-weight ratio of such fuel is astronomical, and it moves mass off the surface of our planet really well and quickly.

However, H₂ is no panacea and has arguably more drawbacks than benefits. Being so small, the hydrogen molecules can actually seep through the walls of most fuel tanks. Preventing them from doing so requires special materials – expensive materials.

To combat this problem, the hydrogen is liquified. But to do so, it must first be cooled to temperatures that would freeze the feathers off a penguin: minus 400 degrees Fahrenheit (minus 250 Celsius). Again, this process is expensive.

Then, it takes awhile to fill the tanks on the rocket that hold the liquified hydrogen. You have to do this slowly to keep the liquified form from clogging and fouling up the fuel lines – and that’s also expensive.

To combat these issues, SpaceX and Blue Origin have opted for methane instead of liquified hydrogen in their Starship and New Glenn rockets. While methane is still typically liquified and must be cold to do so, cooling it to minus 260 F (minus 162 degrees C) is much less expensive than minus 400, as would be needed for H₂.

The methane molecules are also much larger than the hydrogen molecules, measuring more than twice as far across from their furthest points. Hence, it doesn’t weasel its way through the storage tanks and fuel lines like H₂ does. As a result, methane can be transported and filled into tanks much easier and faster. Then, since methane isn’t as leak-prone and stores better, the whole rocket ship itself can actually be reused; that makes the entire launch process cheaper overall.

Methane: For rockets, but not just for rockets

So methane is cheaper and affords reusability, but is it somehow safer than liquid hydrogen?

Well, on May 28, 2026, the folks at Blue Origin found out how explosive methane can be. While the cause has yet to be reported, somehow the methane in the tanks ignited, resulting in an epic explosion seen dozens of miles away from the launchpad.

Yes, a methane-fueled rocket can fling astronauts into the sky in ways that seem magical, but if anything goes wrong, methane still goes boom in a very destructive way.

The reaction of hydrocarbons is actually the same type of explosion that fuels automobiles. The difference is that the explosion in a car engine cylinder drives the motion of a piston. In cars, the explosion of octane, a hydrocarbon eight times the size of methane, is directed with a purpose, creating what physical chemists like me call work. Heat is simply the same thing as work, but it just goes in random directions and does not accomplish some desired task.

Kicking a football through the uprights is like work, while missing the ball is like heat. Hence, methane in a rocket directed through a nozzle does work to send the craft into the sky. If undirected, the hydrocarbon reaction produces heat in the form of an explosion that sets back years of planning for Jeff Bezos.

This type of accident is relatively common in rocketry, as SpaceX has had its fair share of explosions. Getting all the right pieces to sync is a challenge, but decades of successful spaceflight indicate that this a surmountable issue.

The reaction of hydrocarbons with oxygen in car engines or even rockets is, in a way, the same chemistry as what human bodies do in metabolism. Some hydrocarbons like sugar or a carbohydrate – but not the methane of rockets or the octane of cars – reacts with the oxygen that you breathe to produce carbon dioxide and water. Your body just performs this reaction slowly and in each cell, all over, warming you up.

The rocket, however, is powered by a reaction between methane and oxygen in a single point at the nozzle. The energy is concentrated and directed together to fling the ship into space – unless, of course, it explodes in an uncontrolled fireball.

Ryan C. Fortenberry receives funding from NASA.

Virtually every living thing on Earth, from Patagonian penguins to newborn human babies, has been touched by the synthetic chemicals known as per- and polyfluoroalkyl substances, or PFAS. In fact, you would be hard pressed to find a sample of human blood, tissue or breast milk without detectable levels of at least one type of PFAS.

Making matters worse, researchers are continually uncovering links between human exposure to PFAS and poor health outcomes, including a weakened immune system, a heightened risk of kidney and testicular cancer, and pregnancy complications, including preeclampsia and reduced birth weight. The levels of some PFAS considered safe in U.S. drinking water are decreasing. Despite this, The Trump administration is in the process of revoking and possibly rewriting proposed regulations for all but PFOA and PFOS, two of the most commonly used PFAS until the early 2000s. U.S. maximum contaminant level goals for PFOA and PFOS are 0 parts per trillion – meaning there are no levels the U.S. Environmental Protection Agency considers safe.

Meanwhile, thousands of PFAS have not been studied and have no regulation or oversight. In many cases, there is no monitoring data on their presence in consumer products, water and food.

As an expert in chemical pollution, I have studied a wide range of synthetic and natural chemicals that can have harmful health effects for humans and wildlife. A major focus of my current research is tracing PFAS from their initial source – including consumer products, contaminated food and water, and the air – to their resulting fingerprint in an organism’s blood and tissues.

By following the journey of how PFAS move into the bodies of living things – including people – scientists like me are working to improve safety recommendations and usage guidelines for these chemicals. First, though, we need to understand how these complex chemical mixtures are transformed as they accumulate in the body.

What are PFAS?

PFAS are a large class of organic chemicals – meaning molecules that contain carbon atoms – that have fluorine atoms added to them. This fluorination allows PFAS to aggregate on surfaces in ways that are desirable for many applications.

For example, PFAS are used in nonstick cookware, food packaging, cosmetics, textiles and even toilet paper, among many other commercial and industrial products. They’re also heavily used in semiconductor manufacturing and lithium-ion batteries.

PFAS are commonly called forever chemicals because of their astonishing persistence – due to the strong chemical bonds between carbon and fluorine, they don’t break down easily. This durability is desirable for manufacturers, as materials made with PFAS can function for a long time without degrading.

However, persistence becomes problematic when PFAS leach or evaporate out of products and into the surrounding environment. PFAS can remain in drinking water sources and in sediment for decades to centuries.

If dissolved in water or released into the air, PFAS can also travel long distances from their point of origin, ending up in remote locations. For example, PFAS initially released from industrial regions can end up in the blood of white sharks in the Atlantic Ocean or in Arctic environments.

PFAS fingerprints

What happens when PFAS are absorbed and accumulate in the body?

When someone is exposed to PFAS, it leaves a unique pattern of chemical contamination – what researchers call a PFAS fingerprint – in their blood. Studying these PFAS fingerprints enables scientists to learn about sources of PFAS exposure and how they differ among people who live in different places, have different jobs and use different products, among other factors.

But to be able to use these PFAS fingerprints, researchers first need to understand how specific exposures contribute to someone’s PFAS fingerprint over time. The composition of this fingerprint is different from the mixture of chemicals someone was initially exposed to, as some PFAS accumulate in blood to a greater extent than others. Without understanding how a PFAS mixture is distorted and changed in the body, it’s very difficult to know what sources were major contributors to a person’s lifelong PFAS exposure.

For example, firefighters and military service members use aqueous film-forming foams that contain hundreds of poorly studied PFAS. These are soapy, sudsy materials that form a film over fire and starve it of oxygen. They’re commonly used in emergencies, such as airplane crashes, train wrecks, vehicle fires or any other fire involving fuels.

Many firefighters and first responders who have used these foams are now grappling with serious health problems, including cancer, and many have wondered whether PFAS contributed to their illness.

A clearer understanding of the PFAS fingerprint that would be expected in someone’s blood after years of using these foams could help determine whether they are a unique source of the PFAS accumulating in their blood.

PFAS in the body

Fingerprints at the scene of a crime are often a major clue leading detectives to the perpetrator. When it comes to identifying sources of PFAS contaminating human bodies, however, researchers like me aren’t always so lucky.

For one, PFAS are typically present at low concentrations in the environment but can build up to higher levels in the body. For example, people drinking water containing PFOS will typically have levels 50 to 100 times higher in their blood than were measured in the water. This is because the body’s rate of PFOS uptake exceeds its rate of excretion.

But not all PFAS will increase in blood to the same degree. PFAS that are more likely to bind to biological components, such as proteins and fats, will more readily accumulate in the body. As the mixture of chemicals in drinking water, for example, continues to accumulate in the body, these types of more bioaccumulative PFAS, such as PFOS, will make up a higher proportion of the fingerprint than other types. This distortion complicates my and other scientists’ job, since we need to be able to predict how much each PFAS accumulates in the body to estimate how these chemicals will change in the body.

On top of predicting which PFAS will accumulate in the body and which will be excreted, researchers also have to contend with a person’s metabolism, the process by which chemicals – including some PFAS – are biologically transformed by the body.

Although the chemical structure of PFAS may change in the body, the resulting chemical is usually still a PFAS: a highly fluorinated molecule. After entering the body, many types of PFAS used in different products can be transformed over days to years, while the highly fluorinated backbone of the molecule remains intact. By these processes, many different PFAS eventually transform into just a few highly persistent PFAS. For example, many distinct PFAS containing a PFOS backbone can ultimately change to PFOS in the body.

Once these distinct PFAS have all become the same common chemical, it may be impossible to identify how a person was initially exposed.

Protecting yourself from PFAS

Despite all the complexities of PFAS research, researchers are making progress toward better understanding how these thousands of chemicals accumulate and transform in the body. Studying real products that contain complex PFAS mixtures can help researchers get closer to finding biomarkers that can pinpoint a PFAS source in a person’s blood.

The most effective way to protect human health would be to cease the use of PFAS entirely in all but the most essential of products. Until then, consumers can look to resources such as those from the Green Science Policy Institute and Environmental Working Group to help them avoid PFAS in products they use.

There are also a number of commercial laboratories that offer drinking water and blood testing for some common PFAS. But it’s important to remember that these tests don’t capture the whole picture of your PFAS fingerprint. Scientists like me are still hard at work capturing many more PFAS that have been overlooked.

Carrie McDonough receives funding from the Toxic Exposure Research Program (TERP) and NIH.