Formed in 2009, the Archive Team (not to be confused with the archive.org Archive-It Team) is a rogue archivist collective dedicated to saving copies of rapidly dying or deleted websites for the sake of history and digital heritage. The group is 100% composed of volunteers and interested parties, and has expanded into a large amount of related projects for saving online and digital history.
Download PDF: Deorbit_Dec2018.pdf
Introduction
IIt 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 LEO (LEO) altitudes1. As a result of all the launches into space, 94% are considered to be space debris, and 64% of those are fragments with a collective mass of 7,500 metric tons1.
The Figure below is a representation of the debris around Earth2. 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 years post-mission, or 30 years after launch if unable to be stored in a graveyard orbit3.
Small spacecraft are typically launched into LEO as it is a more accessible and less expensive orbit to reach. There are lots 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 altitudes 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 uncertainties in atmospheric density, shown below2,4. As the majority of those spacecraft are unable to be parked in a graveyard orbit because that requires additional 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. There is no intention of mentioning certain companies and omitting others based on their technologies.
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 on 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 mission5. 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 | ||
|---|---|---|
| Product | Manufacturer | TRL Status |
| Drag-Net | MMA Design | 9 |
| RODEO | Composite Technology Development, Inc. | 7 |
| AEOLUS | Clyde Space | 7 |
| Terminator Tape | Tethers Unlimited, Inc. | 7 |
| Drag Sail | UTIAS-SFL | 9 |
| Exo-Brake | NASA | 9 |
| ROC FALL | ROCCOR | 6 |
Solar Sails
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 in 2011, from the minisatellite FAStsat HSV into a 650 km altitude and 72° inclined orbit, and demonstrated the deorbit capability of a low mass, high surface area sail5. 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).
Recent CubeSats have used NASA’s Exo-Brake Parachute for mission deorbiting. An Exo-Brake increases the spacecraft’s drag once the tension-based, flexible braking device that resembles a cross-parachute is deployed from the rear. The Exo-Brake development is funded by the Entry Systems Modeling project within the NASA Space Technology Mission Directorate’s Game Changing Development program. Four Technology Education Satellite (TechEdSAt) 3U CubeSat missions have used 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. TechEdSat-6 and EcMASat, 3U and 6U form factors respectively, both are also equipped with the Exo-Brake module, but have not yet been activated.
Deployables
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. It was successfully deployed on suborbital RocketSat-8 August, 20137.
AAC-Clyde 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 is supported by boom-springs that open the sail to generate aerodynamic drag against the upper atmosphere8.
Electromagnetic Tethers
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 mission9. 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, but the CubeSat is currently still in orbit and the terminator tape has not yet been activated10 and the terminator tape has not yet been activated.
Summary
Small spacecraft deorbit systems have been shown to be quite effective in meeting 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 use of drag sails for deorbiting in LEO within the 25 year post-mission requirement. EcAMSat and TechEdSat-6 will hopefully successfully demonstrate their deorbiting systems soon. Terminator Tape currently being flown on Aerocube-V CubeSat is another deorbit option that uses electromagnetic tethers.
For Feedback solicitation, please email: arc-sst-soa@mail.nasa.gov. Please include a business email so someone may contact you further.
