There has been rapid growth in space flight in the past decade as the price for building and launching a small spacecraft has become relatively inexpensive for commercial space programs, government space agencies and universities. It has been estimated that as a result of space flight, there has been an accumulation of space debris consisting more than 700,000 particles with a diameter 1-10 cm and over 20,000 pieces with diameters >10 cm in orbit between Geostationary (GEO) and Low Earth Orbit (LEO) altitudes . Figure 12.1 is a representation of the debris around Earth . 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 and they have mandated either a lifetime requirement for all spacecraft or storage in a graveyard orbit. The lifetime requirement is 25 year post-mission or 30 year after launch if unable to be stored in a graveyard orbit .
Small spacecraft are typically launched into LEO as it is a more accessible and less expensive orbit to obtain: there is high availability to LEO through all commercial launch providers; the close proximity to Earth reduces mass and power requirements the communication system; it can employ a relatively small propulsion system; and the radiation environment is relatively benign. Small spacecraft that are 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 , . 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.
State of the Art
Since deorbit systems are still in their infancy, there are few high TRL 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. For example, a steered drag sail relies on a functioning attitude system post mission for control. This can be challenging for small spacecraft, as this demand “increases complexity and cannibalizes precious mass and volume” . 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 .
In contrast, passive deorbit methods require no further active control after deployment. Therefore, the state of the art section will focus on passive deorbit mechanisms. Table 12.1 displays current state of the art technology for passive deorbit systems.
|RODEO||Composite Technology Development, Inc.||TRL 7|
|AEOLDOS||Clyde Space||TRL 7/8|
|Terminator Tape||Tethers Unlimited, Inc.||TRL 8/9|
|Deorbit Sail||Surrey Satellite Technology Ltd.||TRL 9|
Several small spacecraft missions have been developed and launched to demonstrate passive deorbit technologies using a drag sail or boom, such as NanoSail-D2 and CanX-7. NanoSail-D2, deployed from FASTSAT in late January 2011 into a 650 km altitude 72° inclination orbit, demonstrated deorbit capability of a large low mass high surface area sail . 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, plans to deployed a drag sail developed and tested at University of Toronto Institute for Aerospace Studies Space Flight Laboratory (UTIAS-SFL) (shown in Figure 12.3).
DeorbitSail is a 3U cubesat built at University of Surrey that will demonstrate a deorbiting technique using a 25 m2 solar sail contained in four booms and occupying 1U volume. DeorbitSail launched July 2015 into a 600 km Sun Synchronous Orbit (SSO) and has a mission lifetime of 180 days before planned reentry . Figure 12.4 is a labelled diagram of the Deorbitsail concept .
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 12.5. It was successfully deployed on suborbital RocketSat-8 on 13 August 2013 .
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 . Figure 12.6 is a representation of the AEOLDOS membrane after deployment . 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 . Currently on orbit with Aerocube-V cubesats, the terminator tape module is expected to activate at the end of 2015 and three more cubesat Terminator Tape modules are manifested for flight in 2016 .
Small spacecraft deorbit systems are relatively immature but are necessary to meet space debris mitigation 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, DeorbitSail and CanX-7 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. Terminator Tape is another deorbit option that uses electromagnetic tethers that is currently being flown on Aerocube-V cubesat.