NASA plans to send crewed missions to Mars over the next decade – but the 140 million-mile (225 million-kilometer) journey to the red planet could take several months to years round trip.

This relatively long transit time is a result of the use of traditional chemical rocket fuel. An alternative technology to the chemically propelled rockets the agency develops now is called nuclear thermal propulsion, which uses nuclear fission and could one day power a rocket that makes the trip in just half the time.

Nuclear fission involves harvesting the incredible amount of energy released when an atom is split by a neutron. This reaction is known as a fission reaction. Fission technology is well established in power generation and nuclear-powered submarines, and its application to drive or power a rocket could one day give NASA a faster, more powerful alternative to chemically driven rockets.

NASA and the Defense Advanced Research Projects Agency are jointly developing NTP technology. They plan to deploy and demonstrate the capabilities of a prototype system in space in 2027 – potentially making it one of the first of its kind to be built and operated by the U.S.

Nuclear thermal propulsion could also one day power maneuverable space platforms that would protect American satellites in and beyond Earth’s orbit. But the technology is still in development.

I am an associate professor of nuclear engineering at the Georgia Institute of Technology whose research group builds models and simulations to improve and optimize designs for nuclear thermal propulsion systems. My hope and passion is to assist in designing the nuclear thermal propulsion engine that will take a crewed mission to Mars.

Nuclear versus chemical propulsion

Conventional chemical propulsion systems use a chemical reaction involving a light propellant, such as hydrogen, and an oxidizer. When mixed together, these two ignite, which results in propellant exiting the nozzle very quickly to propel the rocket.

These systems do not require any sort of ignition system, so they’re reliable. But these rockets must carry oxygen with them into space, which can weigh them down. Unlike chemical propulsion systems, nuclear thermal propulsion systems rely on nuclear fission reactions to heat the propellant that is then expelled from the nozzle to create the driving force or thrust.

In many fission reactions, researchers send a neutron toward a lighter isotope of uranium, uranium-235. The uranium absorbs the neutron, creating uranium-236. The uranium-236 then splits into two fragments – the fission products – and the reaction emits some assorted particles.

More than 400 nuclear power reactors in operation around the world currently use nuclear fission technology. The majority of these nuclear power reactors in operation are light water reactors. These fission reactors use water to slow down the neutrons and to absorb and transfer heat. The water can create steam directly in the core or in a steam generator, which drives a turbine to produce electricity.

Nuclear thermal propulsion systems operate in a similar way, but they use a different nuclear fuel that has more uranium-235. They also operate at a much higher temperature, which makes them extremely powerful and compact. Nuclear thermal propulsion systems have about 10 times more power density than a traditional light water reactor.

Nuclear propulsion could have a leg up on chemical propulsion for a few reasons.

Nuclear propulsion would expel propellant from the engine’s nozzle very quickly, generating high thrust. This high thrust allows the rocket to accelerate faster.

These systems also have a high specific impulse. Specific impulse measures how efficiently the propellant is used to generate thrust. Nuclear thermal propulsion systems have roughly twice the specific impulse of chemical rockets, which means they could cut the travel time by a factor of 2.

Nuclear thermal propulsion history

For decades, the U.S. government has funded the development of nuclear thermal propulsion technology. Between 1955 and 1973, programs at NASA, General Electric and Argonne National Laboratories produced and ground-tested 20 nuclear thermal propulsion engines.

But these pre-1973 designs relied on highly enriched uranium fuel. This fuel is no longer used because of its proliferation dangers, or dangers that have to do with the spread of nuclear material and technology.

The Global Threat Reduction Initiative, launched by the Department of Energy and National Nuclear Security Administration, aims to convert many of the research reactors employing highly enriched uranium fuel to high-assay, low-enriched uranium, or HALEU, fuel.

High-assay, low- enriched uranium fuel has less material capable of undergoing a fission reaction, compared with highly enriched uranium fuel. So, the rockets needs to have more HALEU fuel loaded on, which makes the engine heavier. To solve this issue, researchers are looking into special materials that would use fuel more efficiently in these reactors.

NASA and the DARPA’s Demonstration Rocket for Agile Cislunar Operations, or DRACO, program intends to use this high-assay, low-enriched uranium fuel in its nuclear thermal propulsion engine. The program plans to launch its rocket in 2027.

As part of the DRACO program, the aerospace company Lockheed Martin has partnered with BWX Technologies to develop the reactor and fuel designs.

The nuclear thermal propulsion engines in development by these groups will need to comply with specific performance and safety standards. They’ll need to have a core that can operate for the duration of the mission and perform the necessary maneuvers for a fast trip to Mars.

Ideally, the engine should be able to produce high specific impulse, while also satisfying the high thrust and low engine mass requirements.

Ongoing research

Before engineers can design an engine that satisfies all these standards, they need to start with models and simulations. These models help researchers, such as those in my group, understand how the engine would handle starting up and shutting down. These are operations that require quick, massive temperature and pressure changes.

The nuclear thermal propulsion engine will differ from all existing fission power systems, so engineers will need to build software tools that work with this new engine.

My group designs and analyzes nuclear thermal propulsion reactors using models. We model these complex reactor systems to see how things such as temperature changes may affect the reactor and the rocket’s safety. But simulating these effects can take a lot of expensive computing power.

We’ve been working to develop new computational tools that model how these reactors act while they’re starting up and operated without using as much computing power.

My colleagues and I hope this research can one day help develop models that could autonomously control the rocket.

Dan Kotlyar, Associate Professor of Nuclear and Radiological Engineering, Georgia Institute of Technology

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Homelessness is a rare issue in American politics that does not cut neatly along party or ideological lines. It can be hard to predict who will support or oppose measures to expand affordable housing and services for people without homes.

San Francisco, for example – one of the most progressive U.S. cities – has adopted numerous policies that make it easy for opponents to slow or block proposed housing projects. In contrast, churches of many denominations across the U.S. have challenged local zoning ordinances by providing food and shelter to people without homes, even when city laws and codes ban sleeping or eating in areas where the churches are located.

The Supreme Court’s 2024 ruling in Grants Pass v. Johnson allows cities to penalize individuals for sleeping in public spaces even when no shelter is available. It overturned the 9th U.S. Circuit Court of Appeals’ previous decision that anti-camping ordinances violated the Eighth Amendment’s prohibition against cruel and unusual punishment.

I am a researcher specializing in homelessness, and signed an amicus brief submitted by 57 social scientists in the Grants Pass case, supporting plaintiffs who sued on behalf of homeless people living in the Oregon city of Grants Pass. In my view, the outcome of the court’s ruling is both predictable and deeply troubling. Many U.S. cities now are moving aggressively to clear homeless encampments, often without providing sufficient shelter or support to the people they are displacing.

Cities take action

In California, Gov. Gavin Newsom signed an executive order in July that exemplifies this shift by calling for cities to “humanely remove encampments from public spaces.” This approach, which prioritizes clearing visible homelessness over addressing a systemic lack of housing options, often leads to forced displacement that makes people without housing more likely to be arrested and experience increased instability and trauma.

Newsom’s order opened the door for more punitive actions across the state. The San Joaquin County Board of Supervisors is considering revisions to a local camping ordinance that would ban sleeping in a tent, sleeping bag or vehicle for over 60 minutes, and would forbid people from sleeping within 300 feet of a public location where they had slept in the past 24 hours.

The city of Fresno recently banned public camping at any time and in any location, regardless of whether shelter is available. The new law bans sleeping or camping at any entrance to public or private property along a public sidewalk.

It also prohibits sitting, lying down, sleeping or camping on “sensitive use” properties, including schools, child care facilities, parks, libraries, government buildings, warming or cooling centers and existing homeless shelters. Violations are punishable by up to a year in jail, a fine of up to $1,000, or both.

Other jurisdictions are following California’s lead. Grand Rapids, Michigan, has enacted new laws to criminalize activities associated with homelessness, such as loitering and storing unattended personal property. In Illinois, a government lobbying association drafted a model camping ban that enforces fines for initial violations and stricter penalties, including possible jail time, for repeat offenses. Several Illinois cities have adopted the ordinance.

Ironically, Grants Pass has not been able to clear its homeless encampments because of a law Oregon enacted in 2021. This measure allows local governments to enact restrictions on sleeping on public property, such as time, place and manner, as long as they are “objectively reasonable.” It requires communities to consider local ordinances in the context of available shelter services and public space.

This approach, which strikes a balance between public concerns and the needs of people who are homeless, prevents the kind of punitive measures that the Supreme Court ruling now permits elsewhere.

The Housing First approach

Many Americans are frustrated by the homelessness crisis. In their view, cities have made little progress on this issue despite substantial investments.

However, research overwhelmingly shows that criminalizing homelessness perpetuates the problem. It creates a cycle of arrest, incarceration and release, without addressing root causes, such as economic inequality, inadequate mental health and addiction services and a lack of affordable housing. People without housing are at risk of early death from violent injuries, substance abuse or preventable diseases.

In my view, supportive Housing First approaches are more effective than punitive bans. Housing First is a strategy that quickly provides permanent housing to people experiencing homelessness, without requiring them to be sober, employed or in treatment for mental health disorders.

This approach recognizes stable housing as a basic human right and a foundation for addressing other challenges that people without homes often face. By meeting their immediate need for housing, it helps people recover from the stress of being homeless and leads to better long-term results. Research shows that Housing First programs are more effective and cost-efficient than requiring treatment for issues such as addiction as a condition for housing.

Critics say that Housing First is expensive and that providing housing without mandatory support services leads to inefficient use of funds. Some studies highlight challenges in ensuring that services match individual needs. Another critique calls Housing First a “one-size-fits-all” solution that may not adequately address the homeless population’s diverse needs.

Rental access and assistance

In 2024, the federal government awarded US$3.16 billion to communities nationwide through the Department of Housing and Urban Development’s Continuum of Care program, the largest investment toward ending homelessness in U.S. history.

This HUD program provides funding and support to local communities to coordinate efforts aimed at ending homelessness, such as providing rapid rehousing and support services to homeless people. This is a crisis-response strategy designed to minimize trauma associated with living on the street by moving people into housing as quickly as possible.

Making a serious and lasting dent in this problem will require scaling up proven solutions, such as rental assistance and access to affordable rental housing. A study published by HUD in 2016 found that giving homeless families permanent housing subsidies, such as housing choice vouchers, was the most effective way to ensure long-term housing stability.

Housing choice vouchers cover most of a family’s rent costs, leaving families to pay about 30% of their income on housing, with no time limit as long as participants follow program rules. The HUD study found that compared with other short-term programs, this approach improved participants’ mental health, stabilized families, supported child development and reduced the likelihood of participants becoming homeless again.

Homeless encampments raise legitimate public concerns about health and safety, including the welfare of people living in the camps. But clearing them and banning public camping won’t solve homelessness. As I see it, providing permanent housing subsidies, expanding access to affordable housing and implementing Housing First approaches, paired with supportive services, is a more effective and humane approach.

Deyanira Nevárez Martínez, Assistant Professor of Urban and Regional Planning, Michigan State University

This article is republished from The Conversation under a Creative Commons license. Read the original article.