In 2017, when APSS-2 started, it was estimated that there were less than 2,000 active satellites in orbit around Earth. By 2030, studies suggest there may be over 100,000. As these breakdown they will constantly need to be replaced and even once replaced, will continue to drift lifelessly. At the lowest orbits, satellites will take years to re-enter Earth’s atmosphere. At higher altitudes, they will effectively never de-orbit.
This leads to a rapidly accelerating accumulation of what is termed ‘space junk’. Defunct hardware that still orbits the Earth, each piece a potential collision waiting to happen. As a result, governments around the world have introduced policy aiming to reduce space junk. An example is New Zealand’s Outer Space and High Altitude Activities Act which requires the submission of an orbital debris mitigation plan before spacecraft can be approved for launch.
The most common approach to debris mitigation is to design satellites such that they break apart and burn-up during re-entry into Earth’s atmosphere. For some spacecraft, this means operating in a low altitude orbit and allowing the satellite to de-orbit naturally. For some spacecraft, this means deliberately decreasing altitude at the end of the mission, for example through the use of thrusters. However, active de-orbit mechanisms are complex and require significant space, mass, and power – all of which are critical limitations on a spacecraft.
An electrodynamic tether is one such de-orbit mechanism which uses electricity to generate a force that slows the spacecraft down. The primary mission of APSS-2 is to pursue and test the miniaturization of electrodynamic tether technology. If the technology can be successfully miniaturized, then satellite designers can more easily justify the inclusion of an electrodynamic tether in future missions. As a result, more satellites will have the capability of actively deorbiting and the growth of space junk will be reduced.
At the spacecraft, a thin wire is heated to a high temperature. This causes a phenomenon known as thermionic emission in which negatively charged electrons are released from the wire. A surrounding enclosure is also negatively charged and is used to accelerate the electrons away from the spacecraft and into the space plasma. Meanwhile, a high voltage is put across the electrodynamic tether, using the now negatively charged space plasma as an electrical ground in order to complete the circuit. With a complete circuit, current is able to flow through the tether.
When any current flows through a conductive material, a magnetic field is generated around that material. The result of this is that when active, a magnetic field forms around the tether. This interacts with the Earth’s magnetic field resulting in a push and pull that slows down the spacecraft and results in the orbital altitude decreasing.
The force created by an electrodynamic tether is proportional to the surface area of the tether. Obviously, miniaturising the entire tether subsystem requires reducing the size of the tether. The challenge therefore becomes reducing the size of the subsystem while maximising the amount of tether inside. The final payload design had a volume of 10x10x2.6cm. Contained within this was over 50m of carefully packed tether. The payload included a novel deployment mechanism designed to release the tether in a precise fashion so as to minimise rotation as well as a custom brake design in order to minimise shock forces on the spacecraft as the tether reaches its full length.
The launch of APSS-2 is planned for 2025!
Contributors
Ajay Mistry
Alex Brown
Amelia Cordwell
Anastasiya Kiddle
Andrew Battley
Anthony Atia
Arthur Zhang
Arvin Lim
Ben Parnell
Benjamin Wood
Connor Mattson
Daneka Boogaard
David Ma
Denholm Orr
Dominic Kay
Douglas Lindsay
Drishti Murara
Eleanor van Bysterveldt
Filip Kus
Hamish Varney
Harry Duncan
Harry Lawton
Harry McLoughlin
Henissa Tong
Henry Flint
Ishan Rama
Ivy Doak
James Banyard
James Brown
James Shore
Jannik Wittgen
Jayti Pattni
Jenni Gordon
Jerry Sun
Jesse Stayte
Jiaming Cen
Jonathan Goldsmith
Jonathan Mace
Jose Miguel Torres
Kaemon Ah Kuoi-Simich
Kallista Pijnaker
Kenny Yu
Keshav Krishna
Lara Collier
Lee Violet Ong
Liam Devaroyan
Louis Lin
Lucia Quirke
Madeleine Beagley
Marianne Fernandes
Michael Boyer
Mikhael Sayat
Morgan Davies
Oliver Wright
Parie Malhotra
Rory Allen
Ross Porter
Ryan Gilbertson
Ryan Stickland
Saif Charania
Satnam Bhatt
Sean Reeves
Shawn Zhao
Hari Thatithuri
Stella Li
Taiji Endo
Tomas Antunes
Tony Kamberi
Victoria Skeggs