Going Interstellar

It's the tenth anniversary of a modern sci-fi classic, and a chance to talk about the real science of getting to the stars.

Christopher Nolan’s science fiction epic Interstellar was released ten years ago this year, and it has been back in cinemas for the anniversary. If you haven’t seen this film I can highly recommend it.

Interstellar is a visual masterpiece, an emotionally compelling story, and inspirational in its message of never giving up and striving for the future. It tells it’s story against a background of (mostly) credible science, and was written in collaboration with the physicist Kip Thorne.

One point that irks scientists and engineers about the film though, aside from what is depicted inside a black hole, is the Ranger spacecraft the crew use to go to and from the surface of planets. It is not only a single-stage-to-orbit spacecraft, impressive in itself, but it can also move around the gravity well of a supermassive black hole.

We can posit that they have some kind of amazing futuristic technology that enables this performance - but this produces a plot hold. Why didn’t they just use this propulsion technology to evacuate the Earth in the first place? Why the need to employ gravity manipulation?

Interestingly, there is a real life technology that neatly solves both these issues. A propulsion technology of phenomenal performance but dependent on a crippling rare fuel source that would preclude its use for mass transport.

Fusion Rocket

There have been many proposals through the years to find increased rocket efficiency, measuring in specific impulse, by going beyond simple chemical reactions. The closest to achieving flight was the NERVA program in the 1970s which would have provided an upper stage, initially intended to replace the S-IVB liquid hydrogen/liquid oxygen upper stage of the Saturn V, which would have doubled the specific impulse. It worked by simply pushing liquid hydrogen through the core of a nuclear reactor and then out of a nozzle. Whilst certainly interesting for interplanetary travel in our solar system, the performance increase is not enough for what we see on screen. For that, likely fusion is involved.

Around the same time as NERVA was being tested, the British Interplanetary Society was conducting a study called Daedalus for an interstellar mission. This required a leap in propulsion performance and their chosen method was inertial confinement fusion - lasers are fired on a fuel pellet from multiple directions to compress it violently enough to trigger a fusion reaction, and the magnetically channelled products of this provide thrust.

Render of an ICF engine firing. Credit: The Bastards That Killed Our Space Game

This concept was intended to send a probe to Barnard’s star within a century, as that at the time was thought to be the best nearby candidate for an exoplanet. The issue with this is that we don’t yet have the ability to produce an inertial confinement fusion drive.

But this is where antimatter comes in. Instead of heating the surface of the pellet with a laser, the concept is to cover the pellet with Uranium 238, and bombard the surface with antiprotons. These particles do not need to be accelerated significantly - merely on contact they will cause a violent fusion of the Uranium, releasing enough energy to compress the Deuterium core of the pellet for fusion.

This concept is called Antimatter Catalysed Microfusion (ACMF), although that is something of a misnomer because the antimatter is consumed in the process. Compared to the specific impulse of about 400s for the best chemical engines, and 900s for the best nuclear thermal engines, an ACMF engine described here can produce over 8000s of specfic impulse at a relatively high thrust, compared to ion engines, of 80kN. This does depend on a large canopy to catch and utilise the fission products, but if a more compact ‘nozzle’ could be made, this might suit a vehicle like Ranger.

At this specific impulse, only about 11% of the mass of the spacecraft need be propellant to get from the surface to orbit around an Earth-like planet in a single stage.

Another variant of this technology, Antimatter Initiated Microfusion (AIM) in contrast uses fuel already in a plasma state, inside a Penning trap, into which the antiprotons are injected. Compared to ACMF, the proposed technology is lower thrust but higher specific impulse, so it wouldn’t be suitable for the spacecraft shown in Interstellar. But in the real world it has been suggested as a means of reaching the Oort cloud in reasonable time.

These technologies don’t quite solve the Interstellar plot hole though - because whilst the elegantly allow repeated single-stage to orbit flight, they don’t quite cover the need to move around near the black hole, where orbital speeds can be a significant fraction of the speed of light - as evidenced by the significant time dilation on the first planet they visit.

So how about more performance? What of pure antimatter?

The Last Propulsion Technology

The annihilation of a proton with an antiproton produces a shower of short lived particles, including charged pions, before they decay into gamma radiation and neutrinos. As these are expelled from a the reaction, a magnetic field can be used to push against them generating thrust. Incredible speeds are possible through this method.

A design for such a vehicle has been proposed by Charles Pellegrino - Project Valkyrie

This vehicle minimises mass by putting the propulsion at the front, and dragging the payload behind. This makes the structure a lot light, and means it can be made much longer without onerous mass requirements. Given that the antimatter reaction produces an intense gamma ray shine, and this drops in accordance with the inverse square law, this is a very mass efficient way of protecting the crew. In addition, a small tungsten shield is located close to the engine and the shadow it casts is large enough to encompass the crew compartment. There are more details of the design in the link above, including how it deals with the interstellar matter impinging on its front at relativistic speeds.

This paper goes through the calculations for the performance of an antimatter rocket but I shall summarise here - 22% of the mass lost in the annihilation ends up as massive particles, of which about half are charged and can be accelerated. Using a formula taking into account special relativity, this gives an effective exhaust velocity for this sort of rocket of 58% the speed of light. The rocket equation works slightly differently at these velocities - it looks like this:

If the weight saving measures above allow m0/m1 of 4, then it is possible for such a rocket to accelerate up to 0.38c and back to rest again. At such a speed it would take a mere 11 years to reach the nearest star. If the spacecraft were accelerated on its outbound journey by a laser, and only had to decelerate, it could make the trip at 0.67c and get there in a mere 6.5 years. These are the kinds of speeds, going back to the start, which would be required to really solve the plot hole in Interstellar, given the proximity of at least one of the planets to the black hole.

With this sort of technology in the real world, the stars would be open to us. But it would require huge quantities of antimatter - hundred tonne blocks of it, frozen to extremely cold temperatures.

The First Steps

CERN in Switzerland has been producing small quantities of antimatter for many years now, in a particle accelerator they literally refer to as the antimatter factory. They currently underway experiment underway called BASE, which seeks to determine if there is any difference in the magnetic moments of protons and antiprotons, in order to understand why the universe is dominated by matter rather than being a mixture of matter and antimatter.

In order to do this, they have developed technology for storage of antiprotons for as long as a year, and now have created a project called BASE-STEP to master the long distance transportation of these particles. They have recently transported 70 unbounded protons in a Penning trap across the site in a truck, proving the concept in preparation to do the same with antiprotons. The goal is to be able to deliver the particles generated at CERN to any lab which has the best equipment to study them.

A magnetic trap being loaded for transport. Credit: CERN

Contrary to what you might have read in a Dan Brown novel, CERN does not have a space program. Their only use for handling antimatter is to study it for scientific research. But this may still pave the way for broader applications of antimatter in propulsion technology.

We are quite a way off any of the propulsion methods mentioned above - think of BASE-STEP as the first experimental transistor at Bell Labs in 1947, compared to the gigantic GPU clusters we now use to train the latest AI models. The scale isn’t there yet, but the essence of the technology can be seen.

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