A tether covered in solar panels could boost the orbit of the International Space Station
The orbit of the International Space Station is slowly deteriorating. Although it may appear to be permanently stationary in the sky, the space laboratory it orbits is only about 400 kilometers above the planet. There may not be much atmosphere at this altitude. However, there is still some, and interaction with this gradually slows the station’s orbital speed, reducing its orbit, and eventually pulling it back to Earth. That is if we don’t do anything to stop it. Over the course of the station’s 25-year life, hundreds of tons of hydrazine rocket fuel were transferred to it to enable the rocket’s orbital maneuvers to maintain its orbit from deteriorating. But what if there was a better way – one that was self-powered, inexpensive, and didn’t require constant refueling?
A new paper by Giovanni Anis, a doctoral student at the University of Padua, and his team focuses on this concept. It uses a new idea called a bare photovoltaic rope (BPT), which is based on the old idea of a bare photovoltaic rope (EDT) but has some advantages due to the addition of solar panels along its length.
The basic idea behind BPT, and EDTs in general, is to extend a conductive boom into a magnetic field and use the natural magnetic forces in the environment to provide a driving force. Essentially, it deploys a giant conducting rod in a magnetic field and uses the force on the electric field created in that rod to transfer the force to where the rod is connected. It’s like the wind picking up an umbrella if the umbrella were a huge conducting rod and the wind was the planet’s natural magnetic field.
Electrodynamic cords are not a new concept. It was first introduced in 1968 by Giuseppe Colombo and Mario Grossi at the Center for Astrophysics at Harvard University. Several demonstration missions have already been flown, such as TSS-1R which launched aboard the Space Shuttle Atlantis in 1996 and successfully deployed a 10 km tether from the shuttle. Another experiment called a plasma drive generator was performed on the Russian Mir space station in 1999, which instead of using electromotive force to demonstrate the station’s orbital maintenance, generated power directly from the tether itself.
Engineers have long considered using EDT to perform station keeping duties on the International Space Station. However, a technical glitch made it impractical. To get the right kind of forces, the rope must be directed “down” toward the Earth or “up” away from the planet.
No matter which direction the rope is pointing, it will still need power to work. Without its magnetic field, generated by the electrical current running through it, it would act as an additional pull rather than a boost. Therefore, conventional EDT must be connected to the power system. However, if the EDT were deployed upwards on the ISS, this power system would block the approach paths of capsules attempting to dock with the station.
This requires an EDT facing downward so it can be connected to the ISS’s power system. While this works, according to a previous paper published by the authors, it is less than ideal as downward-guided tethers are typically used for deorbit maneuvers rather than orbit booster maneuvers.
Enter BBT. The main difference between it and a traditional EDT is that its surface is at least partially covered by solar panels. If there were enough of them, these solar panels could power the entire system, allowing the upward-facing BPT to operate without being tied to the ISS’s power grid and keeping approach paths clear for arriving spacecraft.
Mr Anis and his team considered different options in terms of length and coverage of the solar panels, regardless of the weight of the tether, as the difference in weight between the tether and the ISS itself was several orders of magnitude. They found they were able to counteract the relatively small force that causes an orbital drop of 2 kilometers per month from the ISS using a 15-kilometre-long tether that is approximately 97% covered with solar panels, on at least one side.
A 15-kilometre-long rope may seem absurdly long, but admittedly, if directed to the ground, it would cover a relatively large proportion of the total distance to the ground. However, it is within the realm of technological feasibility, especially since Atlantis deployed this 10-kilometre-long tether nearly 30 years ago.
To prove their point, the authors turned to a software package called FLEXSIM, which allowed them to simulate the orbital dynamics of the ISS associated with different BPT lengths. The ropes they chose were only 2.5cm wide, and the efficiency of the solar panels was only 4.23%, although this is likely influenced by the fact that they had to be small and flexible. With this length of solar panel, the system can provide 8.3 kilowatts of power to the entire tether, enough to boost the orbital path of the International Space Station.
There are some nuances about the effects of solar activity on the forces contributing to orbital boosting, but overall, the system seems to work, at least in theory. However, there has been a lot of discussion around the International Space Station recently about its end of life, which could come as early as 2031. So, even though there are still a few good years left on the station, it probably won’t benefit much from BPT. The system is as it was a few decades ago. However, it is possible that there will be an alternative in orbit one day, and it could benefit from such a system from the start, potentially saving hundreds of tons of fuel in orbit over its lifetime.
Learn more:
Anis et al – Characteristics of a bare photovoltaic tether for the International Space Station reboot
UT – Satellites equipped with the tether will be able to deorbit at the end of their lives
UT – A new satellite will attempt to maintain low Earth orbit without any propellant
UT – SpaceX unveils the augmented dragon that will deorbit the International Space Station
Lead Image:
Force diagram of the BPT tether system on the International Space Station.
Credit – Anis et al.
