Ramjet Fusion: Our Ticket to Proxima b

An artist’s impression of tidally locked Proxima Centauri b.

Okay, so Proxima b is likely not as habitable as we’d once hoped it would be. Tidally locked with one side of its face to its host star, Proxima b is likely a scorching desert world on one half of the planet, and a frozen ice brick on the other. Though its mass and composition may resemble that of the Earth, its surface conditions would most definitely speak otherwise. Proxima b is, by human standards, uninhabitable.

Why should that deter us from exploring it? There is still so much to be learned from extrasolar planets and the formation thereof. Maybe not by means of a manned mission, but suppose an interstellar probe with equipment to study the extrasolar body; and not just Proxima b, but the entire Alpha Centauri system.

When Voyager 1 first took pictures of Jupiter’s moons and Saturn’s rings with breathtaking resolution, it opened our eyes to these new and wonderful worlds. They no longer were viewed as little white specs on a black backdrop or fuzzy ears circling a giant ball. They became real worlds with real features and ever changing surfaces. These detailed images and the close proximity data from the interplanetary probe also provided astronomers with brand new insight on celestial tectonics, planetary composition, and survivability of the moons and outer planets.

Voyager 1’s view of Io, Jupiter’s closest large moon.

In the same way, an interstellar mission to the Centauri system would be a stunning first class ticket to the secrets of planetary formation, extrasolar habitability, and the potential for life elsewhere in our galaxy.

Fast-forward to the year 2080. Humanity has well explored the solar system, including the far reaches of the Trans-Neptunian objects and the Kuiper belt. But our domestic exploration has reached a limit. We can learn no more from our own solar system. It’s time to explore the vast net of other star systems dissimilar to our own in order to advance our understanding of the cosmos further. First stop: The Centauri system.

Map of the closest stars to the sun.

Its a close target, and fantastic for a test trial, well known to have a stable planetary system and 3 independent stars to study. But Proxima Centauri is no stones throw away from the solar system, in terms of Earthly metrics. 4.22 light years is a long, long distance. Fortunately, the engineers of the future have constructed a propulsion system to get us there.

It’s called ramjet fusion, and the idea is fairly simple in principle. We know that empty space isn’t really empty. Interstellar dust fogs up the milky way with up to a million particles per square meter in the densest regions near stars. Most of this dust is actually molecular hydrogen, the same fuel that is used to keep stars ignited for billions of years. Using an electromagnetic field, it is possible to harness these molecular hydrogen atoms into a confined space, where they can then be fused with other hydrogen atoms. This would release a tremendous amount of energy which, when channeled, can provide a great deal of thrust.

Concept blueprints of a Bussard Ramjet.

It has obvious advantages over conventional propulsion at first glance. First of all, fusion reactions are six million times more energy efficient than chemical rockets, meaning you would need less fuel. And speaking of fuel, a ramjet fusion engine would have no need to carry the excess fuel with them to accelerate and decelerate to the target destination, as it collects its fuel as it moves through space.

Now remember, its 2080, so technology has become advanced enough to the point to allow for nuclear fusion to be a regular practice, even though today it has yet to churn out a positive energy yield. Technology has also excelled itself so far, that an effective interstellar probe need not be very large. In fact, the limiting factor is the engine and collection system for the hydrogen itself. So lets run the numbers.

Let’s say our vessel has a mass of 6 million kg, about the mass of the Eiffel tower, and we want our trip to be relatively fast (please excuse the pun). We will accelerate the craft to 50% of the speed of light. This will leave the mission transit time close to 10 years, not too bad for an interstellar mission. We start with kinetic energy:

Ke = 0.5*m*v²

Ke = 6.75e22 J

But we also need to decelerate back to a stop for approach:

Ke_total = 1.35e23 J

The proton-proton fusion chain.

Okay, that’s a lot of joules. Let’s see how efficient our engine is. Hydrogen (Proton-proton) fusion requires 6 hydrogen atoms; 4 to be fused into two H2 molecules, and two more to fuse them into two He³ atoms. These He³ atoms are then fused together for an energy yield with a byproduct of He⁴ and two H atoms. Some mass is lost in the process, which has been converted directly to energy using E = mc². So here’s how much energy is generated with every reaction:

m_initial = 6*H

m_final = He⁴ + 2H

m_loss = 4H — He⁴ = 0.029158 amu = 4.842e-29 kg

And since mass is converted into energy:

E = mc²

E = 4.358e-12 J

So our engine produces that much energy every time 6 hydrogen are fused together for energy yield. So how much hydrogen do we need to meet our kinetic energy requirement:

# of reactions = 1.35e23/4.328e-12 = 3.12e34 reactions

# of H atoms = 6*3.12e34 = 3.67e35 atoms

Mass of hydrogen = 6.1e8 kg

This is the mass of hydrogen that needs to be collected for our ramjet to meet the energy requirement of the trip (it will need more, but we’ll see that in a minute). Okay, so now we need to find if interstellar space has enough hydrogen to propel this ship. Interstellar space averages about 500,000 atoms per square meter, about 70% of which are hydrogen atoms. Our spacecraft is traveling through space on a (relatively) straight line towards the Centauri system. If the electromagnetic field can pick up a radius of hydrogen along the path, of sorts, then we can calculate how big this radius needs to be in order for this interstellar voyage to succeed.

The volume of hydrogen the ramjet will “scoop” through space.

The ship only needs to be collecting fuel while its accelerating and decelerating, so it does not need the full 4.22 light years of fuel collecting. It will spend the remainder of the distance coasting at half the speed of light. Accelerating at 1*g (the acceleration you are being pulled down to Earth right now), you can not only have an efficient engine, but generate artificial gravity along the way. To accelerate to 0.5*c at 1*g would take 177 days; the same time it would also take to decelerate to the Centauri system. So the total thrust time is 354 days, almost a full year. During this time, the craft would have been under thrust for 4.6e15 meters, or 0.485 light years.

Since the craft is collecting molecular hydrogen along this 0.485 light year path, the electromagnetic “scoop” can be modeled as a cylinder, with h = 0.485 light years, and some radius “r” which the field is collecting. The volume of this cylinder is directly related to how much hydrogen can be collected, like so:

V = h*pi*r²

We know we need to collect 6.1e8 kg of hydrogen, from before, so lets solve for the radius of the field needed to collect this mass. The volume of interstellar space that holds 6.1e8 kg of hydrogen, assuming a density of 500,000 particles per square meter and 70% hydrogen by volume is:

V_need = # hydrogen atoms/density = 3.67e35/350000

V_need = 1.05e30 m³ of space

Setting our scoop volume to that gets us (0.485ly = 4.589e15 meters):

1.05e30 = 4.589e15*pi*r²

r = 8.53e6 meters, or 8534 kilometers

Earth’s magnetic field, powerful enough to manipulate the paths of charged particles.

Is this feasible? I mean, we’re talking about an electromagnetic field larger than the Earth’s radius! Well, how powerful is the Earth’s magnetic field? Earth’s magnetic field has a measured strength of about 0.4 Gauss, and it has the ability to manipulate the path of charged particles emanating from the sun through channels located above the poles. Earth’s magnetic field is also a couple orders of magnitude larger than ours. But remember, the year is 2080. These engineers and scientists have spent their life’s work on this project.

This ramjet fusion scoop isn’t without complications either. As you can see, such an immense electromagnetic field would need to be powered by some source. Also, the craft would need a power source to heat up the hydrogen atoms for fusion; you can’t just mingle two protons and have them explode into energy. Not to mention all of the other problems with interstellar travel such as blue shifted radiation and long term dust collision.

But the year is 2080, like I’ve been so keen at pointing out. The ramjet fusion engine may hold the most realistic key to exploring the local galaxy of stars and exoplanets. It could take probes to places man kind only dreamed of when Galileo first aimed his telescope at the moons of Jupiter. Who knows, it may even aid in the first manned interstellar mission. Proxima b may not be our first choice for extrasolar exploration, but it is certainly the most attainable. And whose to say that its not worth it?

Okay, maybe me. We should explore Tau Ceti instead.