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Interstellar Travel: the Birth of Fusion Drives

Welcome back to our series on Interstellar Travel, where we examine concepts that have been proposed since the dawn of the Space Age. In our previous installment, we looked at the earliest concepts to emerge from the Space Age. These reflected the nature of the times, where two superpowers were locked in a constant state of competition and advances in rocketry paralleled the development of nuclear weapons.

During this period, NASA and the Soviet Union also built and tested nuclear reactors that would generate heat or electricity for an advanced propulsion system. Their proposed systems fell into two categories: those that harnessed the power of shockwaves produced by nuclear explosions – Nuclear Pulse Propulsion (NPP) – and those that used nuclear reactors to heat propellant or generate electricity for an ion engine – Nuclear Thermal Propulsion (NTP), and Nuclear Electric Propulsion (NEC).

Around the same time, advances in thermonuclear weapons (aka. hydrogen bombs) also led to proposals for fusion propulsion concepts. These ambitious projects offered the prospect of making interstellar journeys within a human’s lifetime. Not unlike nuclear concepts such as Project Orion, they also came with monumental challenges, not the least of which was the expense involved in building them. Here’s a look at some of the more well-known ideas that came from this era.

Fusion Power (1960 – 2023)

During the 1960s, investigation into fusion propulsion broke off from previous work on NTP systems. Similar to nuclear propulsion, fusion concepts can also be divided into categories: Magnetic Confinement Fusion (MCF) and Inertial Confinement Fusion (ICF). Whereas MCF involves propellant being heated in a plasma to extreme temperatures (over 100 million °C; 212 million °F), ICF relies on powerful lasers to rapidly compress tiny pellets of deuterium, helium-3, and/or tritium (D, ³He, T) to create ultra-hot plasma.

With more advanced reactor configurations, velocities of 3,500 km/s to 7,800 km/s were reportedly possible. However, a spacecraft with such drive systems would still take 364 to 163 years to reach Proxima b, respectively. According to various estimates, a fusion system would be capable of producing a specific impulse (Isp) of between 10,000 and 1,000,000 seconds, translated to an exhaust velocity of 98,067 m/s and 9,806,650 m/s. At this speed, a fusion-powered spacecraft could reach Proxima Centauri in about 130 years.

However, concepts emerged during this period that promised even faster transits, including the extensive work of noted physicist Robert Bussard. In 1955, he joined the Los Alamos Laboratory’s Nuclear Propulsion Division, where he worked on Project Rover, the first U.S. attempt at creating an NTP engine. In 1958, he and fellow nuclear physicist researcher R.D. DeLauer published their first treatise on the subject, titled “Nuclear Rocket Propulsion.”

In this, and the later treatise, “Fundamentals of Nuclear Flight” (released in 1967), they explained the advantages ship’s equipped with nuclear reactors had over chemical rockets.

Famed physicist Freeman Dyson, the chief scientist for Project Orion until its cancellation in 1963, also explored how fusion power could be harnessed for interstellar travel. In 1968, he penned an article in Physics Today titled “Interstellar transport.” Similar to the Orion concept, Dyson addressed the possibility of using thermonuclear warheads to generate acceleration, rather than fission devices.

Similarly, he acknowledged that “controlled fusion reactors” could be used instead of bombs, if they could be produced cheaply (which he doubted). According to Dyson’s calculations, the “energy density of thermonuclear fuel makes mission velocities in the range 10³-10⁴ km/sec reasonable.” At speeds of 10,000 to 100,000 km/s, a fusion-powered spacecraft could reach Proxima b in less than 13 years.

Bussard Ramjet

Then, in 1960, Bussard released what would become his most influential paper, “Galactic Matter and Interstellar Flight,” in which he proposed the concept for an interstellar ramjet. The proposed spacecraft, named the Bussard Ramjet in his honor, called for a scoop generating a funnel-shaped magnetic field at the ship’s front. This field would allow the spacecraft to scoop neutral hydrogen from the surrounding medium as it traveled.

The collected hydrogen would then be funneled into a magnetic confinement chamber, where it would be compressed until thermonuclear fusion occurred. The energy and elementary particles released in the process would then be channeled through nozzles to generate thrust. As Bussard described it:

[B]y abandoning the interstellar rocket entirely, turning to the concept of an interstellar vehicle which does not carry any of the nuclear fuel or propellant mass needed for propulsion, but makes use of the matter spread diffusely throughout our galaxy for these purposes. By rough analogy with its atmospheric counterpart, we call this an interstellar ramjet.

The most obvious advantage, as Bussard noted, was that this spacecraft concept did not require any propellant to be transported along, thus reducing the size and mass requirements of the ship considerably. In addition, as the ship continued to travel, its velocity would increase to the point that it would eventually reach velocities of up to 4% the speed of light. Unfortunately, the concept was limited by the problem of drag, which would accumulate as the ship continued to accelerate.

The ship’s potential velocity was also limited to the amount of hydrogen it could scoop up. Bussard’s calculations were based on then-estimates of hydrogen in the interstellar medium (ISM), which have since been revised and shown to be much lower than previously thought.

Project Daedalus

Between 1973 and 1978, the British Interplanetary Society (BIS) conducted a feasibility study for an ICF spacecraft known as Project Daedalus. Among the study’s criteria, the spacecraft had to rely on existing or near-future technology to reach its destination within a human lifetime. The proposed spacecraft would be powered by an open-cycle fusion engine and consisted of a two-stage uncrewed vehicle with an initial mass of 54,000 tonnes, including 50,000 tonnes of fuel and 500 tonnes of scientific payload (~59,525, 55,115, and 550 U.S. tons).

The first stage, the larger of the two, would operate for 2.05 years and accelerate the spacecraft to 7.1% the speed of light (0.071 c). This stage would then be jettisoned, at which point the second stage would ignite its engine and accelerate the spacecraft up to about 12% of light speed (0.12 c) over the course of 1.8 years. The second-stage engine would then be shut down, and the ship would enter into a 46-year cruise period.

According to the Project’s estimates, the spacecraft could achieve an exhaust velocity of 10,000 km/s, allowing it to reach Barnard’s Star within 50 years. Adjusted for Proxima Centauri, the spacecraft could make a transit in 36 years. However, the project also identified numerous issues that made it unfeasible using then-current technology. Most of these issues are still unresolved today.

In 2009, members of the British Interplanetary Society (BIS) and the Tau Zero Foundation (TZF) founded Icarus Interstellar in the hopes of revitalizing the project. Like the previous study, Project Icarus envisioned a smaller spacecraft that also relied on ICF and could be realized with existing or near-future technology. In 2013, a design competition was launched that produced multiple concepts (including Icarus Firefly). However, the project was unofficially ended in 2019 when a review determined it had failed to create a workable, updated starship design.

Artist’s rendering of the Project Daedalus spacecraft, with a Saturn V provided for scale. Credit: Adrian Mann/British Interplanetary Society

Enzmann Starship

In keeping with the philosophy that interstellar spacecraft should be extremely fast, or designed for the long haul (Generation Ships), another fusion concept to emerge during the Cold War was the Enzmann Starship. The idea was the brainchild of MIT Professor and Raytheon Corporation member Dr. Robert Enzmann, who proposed it in 1964 while working in space-mission designs. This fusion-powered interstellar spacecraft consisted of a mirror-finished sphere measuring 305 m (1,000 ft) in diameter and a cylindrical habitat/propulsion section connected to it.

The sphere would contain frozen pellets of D, which would be fused to create pulses of thrust from 6 to 12 engines, achieving a small fraction of the speed of flight. Long-time space advocate G. Harry Stine popularized the concept in an article, “A Program for Star Flight,” that appeared in the Analog Science Fact & Science Fiction magazine in 1973. He described a ﬂeet of Enzmann starships traveling to the nearest stars in a century or so, carrying up to 2,000 passengers each.

Project Longshot

In what was another attempt to realize ICF, the US Naval Academy and NASA pursued the Longshot design-study from 1987 to 1988. It was designed to be built at Space Station Freedom – the planned U.S. space station that gave way to the International Space Station (ISS) – and with existing technology in mind. The spacecraft would be uncrewed and would rely on a fission reactor to power a system of lasers used to trigger fusion in an inertial confinement chamber, similar to Daedalus.

Longshot would have a mass of 396 tonnes (436.5 tons) at the start of the mission, including 264 tonnes of helium-3/deuterium propellant. The main propulsion section would be discarded once the spacecraft’s propellant was spent, at which point the remaining spacecraft would have a mass of about 30 tonnes (33 tons). Based on the project’s calculations, a trip to Alpha Centauri would take about a century. Adjusted for Proxima Centauri, this would work out to about 97 years.

Once again, research into advanced propulsion produced some very interesting proposals during the Space Age, Cold War, and after. Some of the ideas produced in this period are still with us today, waiting for the day when they will be realizable. The same challenges remain, of course: cost and the need for technology to mature. In the meantime, several other ideas have since emerged that offer possible solutions and alternatives. We’ll take a look at some of these in the coming installments.

Further Reading:
– Galactic Matter and Interstellar Flight, Bussard, R. Astronautica Acta (1960), vol. 6, sec. 4
–Interstellar transport, Dyson, F. Physics Today (1968)
– Project Daedalus – The British Interplanetary Society (BIS)
– The Enzmann Starship: History and Engineering Appraisal, Crowl, A., et al. Journal of the British Interplanetary Society, vol. 65, pg. 185-199
– Project Icarus – Icarus Interstellar
– Project Longshot – NASA Technical Reports Server (NTRS)

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California firefighters help battle raging Washington wildfires

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Santa Monica Fire Deputy Chief Tom Clemo has barely slept since Saturday. He’s spent the last three days based in a high school in Spokane helping manage the response to devastating wildfires sweeping across eastern Washington.

Three blazes ignited in the Spokane area Saturday while the region was under a “particularly dangerous situation” alert — the same extreme fire weather warning Los Angeles County was under when the Palisades and Eaton fires began.

Fueled by ample dry vegetation after a snow-poor winter, the Spokane blazes had, by Monday, destroyed more than 700 structures and forced some 67,000 residents to evacuate.

Clemo serves as incident commander for California Interagency Management Team 7, a federally coordinated group trained to help lead the response to wildfires across the nation.

“At 3 o’clock [Saturday] we grabbed all 123 members of the team, put them on the road, drove north to Spokane and immediately moved our operations personnel into the field to assist with the management in the initial attack,” he said.

Clemo’s team is largely made up of firefighting personnel based in Southern California and the Central Coast, including members from Santa Monica, Los Angeles, Santa Barbara and Ventura. The California Department of Forestry and Fire Protection has dispatched about 150 personnel and 20 engines to Washington, while individual California fire departments are also loaning crews.

The Los Angeles County Fire Department, for example, sent an eight-person camp team and a three-person dozer crew to the Spokane wildfires on Monday, according to a department spokesperson.

The three fires burning around Spokane are already on track to become some of the most destructive blazes in the state’s history. The fires had collectively charred some 8,000 acres Monday afternoon with 0% containment.

“What we have been witness to and are managing now is a wind-driven brush fire that resulted in an urban conflagration in neighborhoods similar to [those in the] Palisades and Eaton [fires],” Clemo said. “They’re small in footprint but enormous in damage.”

Fortunately, lessons from Los Angeles County’ 2025 fire season have been applied in Washington, Clemo said.

For example, many firefighting resources were strategically pre-positioned once the National Weather Service issued a particularly dangerous situation warning for Saturday, he said.

This is the highest-level fire warning, signifying that the elements are in place for a perfect storm — with strong, gusty winds, low humidity and dry fuels capable of creating a raging wildfire.

Local law enforcement agencies also worked with great urgency to evacuate residents, Clemo added.

“The sheriff evacuated 60,000 people out of a portion of Spokane in a matter of two hours without injury or accident,” he said. “That’s unbelievable.”

He attributes the fact that no fatalities have been reported in part to the seriousness with which residents and public safety personnel have handled evacuations.

The handling of evacuations emerged as a point of criticism in the aftermath of the Eaton and Palisades fires, which killed 31 people. This was particularly the case in western Altadena, where alerts came hours after the blaze started and almost all of the Eaton fire’s 19 deaths took place.

Calmer winds and cooler temperatures assisted fire crews in their battle against the Spokane blazes Monday, but temperature are anticipated to climb back into the 90s by Wednesday.

Around 1,000 firefighters were on the ground Monday, a number that is expected to double in the coming days as mutual-aid crews continue to arrive from out of state.

The Associated Press contributed to this report.

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The A.I. revolution is stalling because companies don’t want to admit that integrating the technology is expensive and slow and requires human effort.

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Charon’s Mountains Reveal Moon Once Spun 10x Faster

Pluto’s moon, Charon, is one of the most unexplored (and arguably underappreciated) planetary objects in the entire solar system. This is primarily because it’s only been visited once by NASA’s New Horizons spacecraft during its famous Pluto flyby in July 2015. Despite this quick encounter, New Horizons beamed back troves of data regarding Charon that scientists continue to pour over with the goal of gaining insight into Charon’s formation and evolution. This is because Charon is the largest moon compared to its parent body in the solar system, noted by it being half of Pluto’s diameter and one-eighth of Pluto’s mass.

Now, a team of scientists from the University of California, Los Angeles might be one step closer to piecing together Charon’s history, which could also contribute to the early histories of other outer solar system moons. Essentially, Charon is being used as a “testbed” for the formation and evolution of other outer solar system icy moons, with these findings being recently published in Nature Communications.

This is because Charon’s surface has been relatively undisturbed compared to other outer solar system moons, which have been blasted by resurfacing events like impact craters or internal heating. Additionally, Charon’s equator exhibits unique fault patterns, which scientists have hypothesized since the 1970s to be caused by Charon’s rotation initially starting fast but slowing down over time, also called despinning.

For the study, the researchers used a series of computer models to simulate Charon’s early history with the goal of confirming the despinning hypothesis or other factors that resulted in Charon’s present-day surface features. The researchers specifically focused on Oz Terra, which is a geologic region in the northern hemisphere of Charon featuring mountainous and fractured terrain that is vastly different from Charon’s much smoother southern hemisphere, Vulcan Planitia.

In the end, the researchers found that Charon potentially exhibited an initially ice shell thickness of about 30-36 kilometers (18-22 miles) while potentially having an initial rotation period of 14.3 hours. For context, Charon’s current rotation period is about 6.4 days (153.3 hours), which means Charon’s rotation in its early history was more than 10 times what it is today. This despinning could confirm the longstanding hypothesis that Charon’s notable geologic differences between the northern and southern hemispheres. Finally, the researchers note this despinning occurred prior to any cryovolcanism that might have erupted on Charon, indicating the despinning potentially occurred very early in Charon’s history.

The study notes, “Our work presents an approach for quantifying despinning-induced strain and stress on planetary bodies by adapting structural geology techniques developed for terrestrial settings. The distribution of tectonic provinces on Charon suggests that despinning was accompanied by global contraction, supporting a cold start for Charon. Although our work provides a plausible explanation for the tectonic patterns preserved in Charon’s northern highlands, further studies are needed for a more comprehensive understanding of the thermal-mechanical evolution of the crust.”

Discovered on June 22, 1978, at the United States Naval Observatory by American astronomer James W. Christy, it took only a few weeks for the International Astronomical Union to announce the discovery to the world. As noted, NASA’s New Horizons spacecraft is the only spacecraft to visit Charon during the historic Pluto flyby. However, those images and this study build on a growing body of knowledge about Pluto’s largest moon and could help scientists slowly piece together the formation and evolution of other icy outer solar system moons, including moons orbiting Jupiter, Saturn, Uranus, and Neptune.

What new insights into Charon and other solar system moons will researchers make in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!