Moving Asteroids with Dusty Plasma

Asteroid tug vs dust drive

About ten years ago I wrote a blog post where I outlined various ideas for deflecting asteroids as a means of protecting Earth from an extinction event. One idea that I outlined, but never gave the treatment I felt it deserved, was a proposal for an Ion Tether drive. Where an electromagnetic tether would be used to pull/eject ionized dust from the surface of an asteroid as a means of redirecting the asteroid. During my time in graduate school, I found a report that explored the physics of using the photoelectric effect to allow spacecraft to use voltage gradients to maneuver around an airless body. This renewed my interest in investigating the use of the ionized environment of asteroids to deflect asteroids and how efficient it would be compared to other approaches.

With current technology, there is no way to “stop” an asteroid big enough to cause a worldwide disaster. The kinds of asteroids that humanity is most worried about, are too fast and too massive to be “stopped”. Instead, scientists and engineers have been developing ways to change the path of an asteroid so that it won’t be on a collision course with Earth for the next few hundred years (we humans love kicking the can down the road). Changing the path of an asteroid can come in the form of subtle nudges over a period of years, or in the case of NASA’s recent DART mission, a single explosive impact.

On the subtler end of the spectrum for redirecting an asteroid we have the idea of a gravitational tug. Where we would place a satellite with an ion engine near an asteroid. The gentle pull of gravity between these two bodies would ever so slowly change the path of the asteroid, eventually eliminating the risk to Earth’s inhabitants for the foreseeable future. This is what I wanted to be able to compare against, at the time I lacked the experience/understanding to perform the appropriate math, well I’ve got the math. Time to take a look.

Wikipedia’s entry has a handy dandy reference on the Gravity Tractor entry, that helps us with our comparison. Basically, by starting with assumptions on how much we need to change the velocity of our asteroid, and a timeline for how long we have to achieve our goal we can calculate the parameters of our asteroid-deflecting spacecraft. As this document isn’t intended for professional publication I’m going to reuse the values from the entry. If you give a spacecraft about 10 years to nudge a billion-kilogram asteroid by 0.01 m/s. You would need an almost 20,000-kilogram spacecraft orbiting around for those full 10 years. That’s a non-trivial investment, but hey we are saving humanity.

How would a charged tether stack up? Let’s partially answer.

Ideally we could do a simple apples-to-apples comparison, what would it take for a tether to produce the same thrust on our target asteroid as the gravity tug. Well, like in so many engineering and science topics, it depends (gets annoying for many folks to hear that). The reason I say it depends, for a given average velocity of dust particles being ejected from the asteroid’s surface we will need a different amount of dust leaving the surface to produce our desired amount of thrust. Assuming our tether only needs to produce the same effective thrust as our gravity tug does, we get a handy dandy output that shows us that the faster we can throw the dust, the less mass we need to throw to achieve a given thrust. If our tether can launch dust at the same velocity as a person ambling (about 2.2 miles per hour, or 1 m/s) we would need to launch the equivalent mass of a full can of soda every second. 10x faster and we need 1/10th the mass. At dragster speeds (100 m/s) our tether would need to accelerate the equivalent mass of a single penny per second.

Cool the faster that we can launch stuff the less stuff we need to launch. Where things go from physics 102 to doctorate/research grant level (ok maybe graduate level, but I didn’t learn about this in my program*), is answering the following questions. How fast can we realistically accelerate small particles from the surface of an asteroid into deep space. What is the size distribution of small particles available to our tether engine? What percentage of mass of our asteroid is composed of appropriately sized particles?

Now relative dust mass and size distribution is not something that can be readily answered. What we can do is estimate a potential reasonable maximum dust velocity. Observations from Apollo 15 and 17 found that a faint cloud of ionized dust was found at altitudes over 100km. This means that dust particles had collected sufficient kinetic/electrostatic energy to either partially orbit or be lofted to these altitudes. If we assume that the electric charge of our dust gets the dust there in a single go we can estimate the “loft velocity” of the dust. Knowing the moon has a mass of 7.35*10^22 kg and a radius of 1738.1 km. We estimate that our levitating dust would have a potential energy of 145,000 J/kg and zero kinetic energy (not realistic, but a starting number for our estimations) which means that to launch to that altitude our dust would need an initial velocity of about 540 m/s, about the top speed of the F-35C.

Where we are back to asking more questions that need lots of research, is this 540 m/s really the best speed electrostatically accelerated dust particles can achieve?

A 2.25 meter rail gun achieved velocities of 1850 m/s. A longer electrostatic tether might not be able to achieve the same accelerating force, but with a longer distance to accelerate our dust particles, could we achieve similar velocities, better? (I’m honestly asking)

Ion engines achieve exhaust velocities between 10 and 40 km/s and need far less acceleration distance to achieve these values.

When it comes to using dusty plasma as a means of generating thrust there are too many unanswered questions, what’s cool is that there is an opportunity to develop means of propulsion that require no specific fuel to relocate an asteroid. “Just” add electric energy and a bit of patience and you can move a mountain of gold.

Now for the closing splash of cold(ish) water. An ion tether solution sounds cool, and if everything goes as planned you might use something like it to move an asteroid. Unfortunately reality might be more than this idea in its current form can handle. If time is off the essence, just punching an asteroid will impart far more kinetic energy and can be done in a proven fashion. Additionally, ionized particles have a nasty tendency to corrode the materials they interact with, a tether drive might be relatively mass efficient by using locally available fuel, so long as it experiences no corrosion/efficiency losses. Once you start to lose efficiency, it becomes a very hard numbers game.  I’m sorry I couldn’t offer a more conclusive analysis of this concept, but I do finish this article believing that this idea deserves review by engineers and physicists better able to provide robust insights. Maybe in 30 years dozens of asteroids will be gently pushed into new orbits to aid in our exploration of the solar system.

*it will also depend on when you read this article, if you’re in the 2050s there may have been enough asteroids landed on and probed that these details can be inferred by the asteroid’s orbit and mass, but in 2023, I don’t have easy access to that info.

 https://www.nasa.gov/centers/johnson/pdf/486013main_Stubbs.pdf

https://arc.aiaa.org/doi/10.2514/1.J061948 hey whadya know Newtons of thrust to the kW

 https://ntrs.nasa.gov/api/citations/20090010283/downloads/20090010283.pdf lunar dust and dusty plasma dynamics https://ntrs.nasa.gov/citations/20090010283

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/specific-impulse