Sling On: Scientist Sees Future In Space Tethers
A Q&A with space tether pioneer Rob Hoyt.
Robert Hoyt dreams that one day the International Space Station (ISS) won’t need fuel to stay in orbit.
“When you consider that launching one kilogram into orbit costs about $20,000 and that the [International Space] Station needs something on the order of ten tons of propellant per year, that can add up to hundreds of millions or even billions of dollars over the lifetime of the station,” Mr. Hoyt said.
Hoyt is the CEO and chief scientist for Tethers Unlimited, Inc., a research firm located near Seattle, Wash. His company is working to craft a system of space tethers to propel the ISS away from the Earth without burning any chemicals.
“What we are trying to design is sort of like an orbital tugboat,” Mr. Hoyt said.
That tugboat works because when an electrically charged wire passes through the Earth's magnetic field, it generates energy. Hoyt wants to control that energy, feeding it through a series of extremely tough, thin wires attached to the ISS. The design is kind of like a long, sparse net, Hoyt said. Multiple tethers placed far apart mean the system is engineered to withstand inevitable tears from a space tether's mortal enemy -- debris or micrometeor impacts.
This week, Hoyt’s company received a Small Business Innovation Grant from NASA to continue their tether design work for the ISS. I spoke with Mr. Hoyt about his designs and the history of space tether experimentation. What follows is an edited and condensed version of our conversation. (Fuller version in the audio.)
What is a space tether?
A space tether is basically a long wire or string deployed from a spacecraft that can be used to move spacecraft around in orbit without burning up propellent. If the string has high strength you can tie two spacecraft together, enabling one spacecraft to fling another into a different orbit. If the tether contains a conductive wire, then you can drive currents along the wire and those currents will interact with the Earth’s magnetic field, producing forces you can use those to raise or lower the orbit of the spacecraft.
Electrodynamic propulsion …
We are working on several different uses of electrodynamic propulsion. One of them is called the Terminator Tether, which uses a wire or a conductive tape deployed from a spacecraft to drag it down from orbit after it’s completed its mission. The idea is to reduce the buildup of space debris.
That seems very marketable. Has it been tested in space?
We haven’t yet gotten to demonstrate it in space. We started developing that about 14 years ago and at that point we were probably about 14 or 15 years ahead of the market. There wasn’t a real requirement for deorbit at that time. But over the past few years, NASA and other organizations have started to take stronger steps to make sure we don’t contribute to the growth of space debris. Space programs are starting to be required to take care of “end of mission” disposal. So we are seeing renewed interest in the concept, but it’s an unconventional technology and the space industry tends to be very leery of new technologies because it costs so much to get stuff up into orbit.
On Wednesday (July 17), your company received a SBIR grant from NASA to continue work on design plans for a tether system on the International Space Station. What is it you are trying to do?
What we are trying to design is sort of like an orbital tugboat. It would be another small spacecraft that would be deployed from the space station and remain connected to it by a long tether or cable. That tugboat would use the cable to generate propulsion forces, driving currents along the tether to push against the earth’s magnetic field. [Trivia - this repulsion works due to the Lorentz Force]
Where would you get the power to drive the current?
We’d probably use solar panels on the orbital tugboat to drive the current along the tether. There might be power available on the space station but we are not going to rely on that.
Do you have an elevator pitch for NASA about why we should do this?
Right now, NASA has to rely on the Russian and European Space Agency to launch several fuel tankers up to the International Space Station every year to provide it the propellent it needs to stay in orbit. As a result, a significant amount of NASA’s budget is being used to subsidize this instead of being spent on funding work in the United States.
We are hoping to develop a way of providing the propulsion the station needs to stay in orbit without continually burning up propellent. Then we can reduce the cost that NASA needs to bear to maintain the International Space Station.
How much money could this potentially save?
It could save huge amounts of money. When you consider that launching one kilogram into orbit costs about $20,000 and that the station needs something on the order of ten tonnes of propellant per year - that can add up to hundreds of millions or even billions of dollars over the lifetime of the station.
In terms of what Tethers Unlimited is presenting to NASA - how far along in the process are you with them? And how soon, if everything went well, could this become part of the International Space Station?
What we are working on now is a preliminary design and feasibility study. I have to make it clear that NASA upper management hasn’t made any decision to use this on the station or even fund a test fight. What we are trying to do now is develop a credible design and show that it would be affordable to do a demonstration flight on another test vehicle. We need to show that the technology works. And then if it does work, hopefully within a few years we could put the system in place on the International Space Station. We are hoping to put together a design for a flight experiment that we could carry out probably about three or three-and-a-half years from now.
Tell me about some other tether experiments that paved the way for your current work.
In 1993 NASA flew an experiment called the Plasma Motor Generator Experiment, which deployed a half-kilometer long conducting electrodynamic tether. The experiment demonstrated that you could flow currents up and down the tether and make electrical connection to the conducting plasma that is in the earth’s ionosphere.
Unfortunately, most people only heard about an experiment that flew twice on the Space Shuttle and ran into issues both times. [Here’s video from the second Space Shuttle mission (STS-75), which was widely circulated by UFO enthusiasts.]
Beyond that there have been a number of other tether experiments that flew and were highly successful. The most recent one was an experiment called T-Rex that the Japanese Space Agency flew on a suborbital rocket about a year or year and a half ago. That demonstrated deployment of a conducting tape and that they could make electrical contact with the ionosphere and flow current through it. So the basic physics of electrodynamic tethers have been proven. We’ve shown that it can work, but what we still need to do is show that we can produce enough thrust to controllably move a spacecraft around or keep a large system like the International Space Station in orbit.
Do you think there has been a stigma attached to the technology since the Space Shuttle incident that you mentioned? It just didn’t work and it was kind of a high profile case.
Yeah. Unfortunately, most people have not heard of the many tether experiments that worked perfectly, they’ve only heard about two on the shuttle that didn’t go perfectly. So we do constantly have to address the common perception that tethers are problematic. But if you look at the early history of the the development of rocket systems, most of those early systems blew up or went off course.
But the organizations developing them kept investing in them and were able to get them to where they are highly reliable. Although they do still occasionally have incidents. Tether technology is still in its infancy. We’ve had a number of successes, we’ve had some missteps. But with further investment and hard work we think we can get it to the point where it can be a reliable propulsion system.
Robert Hoyt is CEO and Chief Scientist of Tethers Unlimited, Inc., located in Bothell, Wash.