It has been estimated that as a result of increased space flight, there has been an accumulation of orbital debris consisting of more than 750,000 particles with a diameter 1-10 cm and over 29,000 pieces with diameters >10 cm in orbit between Geostationary (GEO) and Low Earth Orbit (LEO) altitudes . As a result of all the launches into space, 94% are estimated to be considered space debris, and 64% of those are fragments, a collective mass of 7500 metric tons 1.
Figure 12.1 is a representation of the debris around Earth 2. The objective of the NASA Orbital Debris Program along with the Inter-Agency Space Debris Coordination Committee (IADC) is to limit the creation of space debris. They have mandated that all spacecraft either deorbit within a given amount of time or be placed into a graveyard orbit for safe storage. The lifetime requirement is 25 year post-mission or 30 year after launch if unable to be stored in a graveyard orbit 3.
Small spacecraft are typically launched into LEO as it is a more accessible and less expensive orbit to reach. There is high availability of rideshare opportunities to LEO through several commercial launch providers. The close proximity to Earth can relax spacecraft mass, power and propulsive constraints. Additionally, the radiation environment in LEO is relatively benign for altitude below 1000 km. Small spacecraft launched at or around ISS altitude (400 km) naturally decay in well under 25 years. However at orbit altitudes beyond 600 km, it can no longer be guaranteed that a small spacecraft will naturally decay in 25 years due to uncertainty of atmospheric density, as seen in Figure 12.2 2,4. As the majority of those spacecraft are unable to be parked in a graveyard orbit due to required excess propellant to increase their altitude, the only option for small spacecraft in lower orbits is to deorbit.
The author would like to highlight that the presented tables are not intended to be exhaustive but to provide an overview of current state of the art technologies and their development status for this small spacecraft subsystem.
State of the Art
Since deorbit systems are still in their infancy, there are only a few high TRL (TRL≥7) devices guaranteed to satisfy the 25 year requirement. Deorbit techniques can be either passive or active, although the primary focus has been in the design of passive methods. Active deorbiting requires attitude control and surplus propellant post mission, such as a steered drag sail that relies on a functioning attitude control system for pointing the sail. Propulsive devices have also been examined for deorbiting techniques (please refer to Propulsion Chapter for this capability), however this approach is still considered risky. Even if enough excess propellant was carried for an active decay approach and adequate attitude control capability post mission was assured, this method requires continuous operation until reentry is met, making it inconvenient and costly for a small spacecraft mission 5. Overall, active deorbiting methods are still considered challenging for small spacecraft, as this demand increases design complexity and uses valuable mass and volume.
In contrast, passive deorbit methods require no further active control after deployment. Therefore, this chapter will focus on passive deorbit mechanisms only. Table 12.1 displays current state of the art technology for passive deorbit systems.
|Table 12-1: Passive Deorbit Systems|
|RODEO||Composite Technology Development, Inc.||7|
|Terminator Tape||Tethers Unlimited, Inc.||7|
Figure 12.3: CanX-7 deployed drag sail representation. Image Courtesy of Bonin et al. (2013).
Several small spacecraft missions have been developed and launched to demonstrate passive (uncontrolled) deorbit technologies using a drag sail or boom, such as NanoSail-D2 and CanX-7. NanoSail-D2 was deployed from minisatellite FAStsat HSV in late January 2011 into a 650 km altitude and 72° inclined orbit and demonstrated the deorbit capability of a low mass high surface area sail 5. The 3U spacecraft, developed at NASA Marshall Space Flight Center, reentered Earth’s atmosphere in September 2011. CanX-7, still in orbit at an initial 800 km SSO, deployed a drag sail in May 2017. The sail was developed and tested at University of Toronto Institute for Aerospace Studies Space Flight Laboratory (UTIAS-SFL) (shown in Figure 12.3).
Recent CubeSats have utilized NASA’s Exo-Brake Parachute for mission deorbiting, see Figure 12‑4. An Exo-Brake increase the spacecraft’s drag once the tension-based, flexible braking device that resembles a cross-parachute deployed from the rear. The Exo-Brake development is funded by Entry Systems Modeling project within NASA’s Space Technology Mission Directorate’s Game Changing Development program. Four TechEdSat (Technology Education Satellite) 3U CubeSat missions have utilized several versions of the Exo-Brake module. The latest two of the four TechEdSat spacecraft are TechEdSat-5 and TechEdSat-6; TechEdSat-5 was deployed from the ISS March 2017 and demonstrated this deorbiting capability after 144 days in orbit 6. TechEdSat-5 orbited at 400 km altitude when the Exo-Brake was enabled, see figure 4. TechEdSat-6 and EcMASat, 3U and 6U form factors respectively, both are also equipped with the Exo-Brake module, but have not been yet activated.
Composite Technology Development, Inc. has developed the Roll-Out DeOrbiting device (RODEO) that consists of a lightweight film attached to a simple, ultra-lightweight, roll-out composite boom structure, see Figure 12‑6. It was successfully deployed on suborbital RocketSat-8 on 13 August 2013 7
Clyde Space collaborated with the University of Glasglow to construct the Aerodynamic End-of-Life Deorbit system for cubesats (AEOLDOS), where a lightweight, foldable “aerobrake” made from a membrane supported by boom-springs that open the sail to generate aerodynamic drag against the upper atmosphere 8. Figure 12.6 is a representation of the AEOLDOS membrane after deployment 9.
In addition to drag sails, an electromagnetic tether has also been shown to be an effective deorbit method. An electromagnetic tether uses a conductive tether to generate an electromagnetic force as the tether system moves relative to Earth’s magnetic field. Tethers Unlimited developed Terminator Tape that uses a burn-wire release mechanism to actuate the ejection of the Terminator’s cover, deploying a 30 m long conductive tape (electromagnetic tether) at the conclusion of the small spacecraft mission 10. There are currently two modules: one sized for 180 kg ESPA class spacecraft and the other sized for CubeSat form factors, called nanoTerminator Tape. Reach from Tethers Unlimited show that orbit raising and lowering is most effective in low to moderate inclinations (>70deg). Terminator tape has heritage on Aerocube-V which launched in 2015, and is currently still in orbit 11and the terminator tape has not yet been activated.
Small spacecraft deorbit systems have been shown to be quite effective in meeting the mandated lifetime requirements. As most small spacecraft are unable to relocate to a graveyard orbit due to propulsion limitations, deorbit system development has focused on passive devices. NanoSail-D2, CanX-7, TechEdSat-3, TechEdSat-4, and TechEdSAt-5 are all CubeSat platforms that have successfully demonstrated the utilization of drag sails for deorbiting in Low Earth Orbit within the 25 year post mission requirement. EcAMSat and TechEdSat-6 will hopefully soon demonstrate their successful deorbiting systems. Terminator Tape is another deorbit option that uses electromagnetic tethers that is currently being flown on Aerocube-V CubeSat.