Figure 4 Impact sequences of DebrisSat. Credit: Arnold Engineering Development Complex/Air Force.
Fragment Collection, Processing, and Future Measurement Plan. After the impact of DebrisLV, all intact foam panels, broken foam pieces, loose fragments, and dust were carefully collected, documented, and placed in bags or plastic containers for shipping to a storage facility. The same process was repeated after the DebriSat impact. Simple, ballpark estimates based on the current NASA Breakup Model indicate the number of 2 mm (and larger) fragments from DebriSat and DebrisLV are approximately 85,000 and 35,000, respectively.
The effort to process the foam panels and to extract fragments is a very time consuming task. X-ray scanning of the foam panels and broken foam pieces is underway. This will help identify the locations of embedded fragments with sizes of 2 mm and larger, speeding the extraction of those fragments. Once the fragments are available for measurements, each piece’s three orthogonal dimensions and mass will be measured and recorded in a database, along with a description of its material constituents and shape classification. The data will be processed and analyzed to improve the satellite breakup model for collisions.
Representative fragments also will be selected for 3-D scanning measurements to obtain cross-sectional area and volume data, yielding area-to-mass ratio, volume, and density distributions. Additional representative fragments will be selected and subjected to laboratory radar and optical measurements to improve the radar debris size estimation model, to develop an optical debris size estimation model, and to bridge the interpretation of ground-based radar and optical observations of debris populations in Earth orbit.
NASA scientists continue to develop and upgrade orbital debris models to describe and characterize the current and future debris environment. Engineering models, such as ORDEM 3.0, can be used for debris impact risk assessments for spacecraft and satellites, including the International Space Station. Whereas, evolutionary models, such as LEGEND, are designed to predict the future debris environment. They are reliable tools to study how the future debris environment reacts to various mitigation practices.
The latest version of the NASA Orbital Debris Engineering Model, ORDEM 3.0, has been released. The model is appropriate for those engineering solutions requiring knowledge and estimates of the orbital debris environment (debris spatial density, flux, etc.). ORDEM 3.0 can also be used as a benchmark for ground-based debris measurements and observations.
ORDEM 3.0 incorporates significant improvements over its predecessor, ORDEM 2000, which was released in 2001. For the first time, ORDEM includes uncertainties in the flux estimates. The model includes material density classes. It has also been extended to describe the orbital debris environment from low Earth orbit past geosynchronous orbit (100 to 40,000 km altitude).
Incorporated in ORDEM 3.0 is a large set of observational data (both in-situ and ground-based) that reflects the current debris environment. This data covers the object size range from 10 µm to 1 m. Analytical techniques (such as maximum likelihood estimation and Bayesian statistics) are employed to determine the orbit populations used to calculate population fluxes and their uncertainties.
The model output lists fluxes of debris in half-decade size bins by distinct material characteristics (i.e., intact objects, high-, medium-, or low-material density objects, and NaK droplets) either by direction and velocity for an encompassing ‘igloo’ (for spacecraft) or by range bins (for a sensor beam on the Earth’s surface), depending on the user’s chosen operational mode.
The program graphical user interface (GUI), executable, data files, and an ORDEM 3.0 User’s Guide are included in the package. ORDEM 3.0 has been subjected to extensive verification and validation. Currently, ORDEM 3.0 runs on Windows XP and higher computers.
Orbital debris protection involves conducting hypervelocity impact tests to assess the risk presented by orbital debris to operating spacecraft and developing new materials and new designs to provide better protection from orbital debris with less weight penalty. The data from this work provides the link between the environment defined by the models and the risk presented by that environment to operating spacecraft. Based on this research, recommendations are then presented on design and operations procedures to reduce the risk as required. This data also helps in the analysis and interpretation of impact features on returned spacecraft surfaces. The primary facility for this research is the Hypervelocity Impact Technology Facility (HITF) at the Johnson Space Center in Houston, although there are other facilities at JSC, New Mexico, and various DOD laboratories.
An important source of information about the debris environment is the study of impact pits on surfaces that have been exposed to space in Earth orbit. All spacecraft collide with very small orbital debris particles and meteoroids; consequently, spacecraft surfaces returned to Earth are found to have many small craters resulting from hypervelocity impacts. In most cases, these craters are too small to have any effect on the operation of the spacecraft. However, by examining them, important clues can be obtained on the sources of orbital debris, and the rate that it is changing.
Controlling the growth of the orbital debris population is a high priority for NASA, the United States, and the major space-faring nations of the world to preserve near-Earth space for future generations. Mitigation measures can take the form of curtailing or preventing the creation of new debris, designing satellites to withstand impacts by small debris, and implementing operational procedures such as using orbital regimes with less debris, adopting specific spacecraft attitudes, and even maneuvering to avoid collisions with debris.
In 1995 NASA was the first space agency in the world to issue a comprehensive set of orbital debris mitigation guidelines. Two years later, the U.S. Government developed a set of Orbital Debris Mitigation Standard Practices based on the NASA guidelines. Other countries and organizations, including Japan, France, Russia, and the European Space Agency (ESA), have followed suit with their own orbital debris mitigation guidelines. In 2002, after a multi-year effort, the Inter-Agency Space Debris Coordination Committee (IADC), comprised of the space agencies of 10 countries as well as ESA, adopted a consensus set of guidelines designed to mitigate the growth of the orbital debris population.
In February 2007, the Scientific and Technical Subcommittee (STSC) of the United Nations' Committee on the Peaceful Uses of Outer Space (COPUOS) completed a multi-year work plan with the adoption of a consensus set of space debris mitigation guidelines based on the IADC guidelines. The guidelines were accepted by the COPUOS in June 2007 and endorsed by the General Assembly of the United Nations in late 2007.
In the past, NASA’s ODPO has emphasized avoiding the creation of orbital debris through mitigation techniques. However, in 2005, a study by the ODPO that used the LEGEND model showed that even if no future launches occurred, collisions between existing satellites would increase the 10-cm and larger debris population faster than atmospheric drag would remove objects. The "No New Launches" scenario highlights the eventual need for remediation of the existing debris population (also known as active debris removal, or ADR). The potential for collisions to damage the space environment was underscored by the Chinese ASAT test in 2007 and the accidental collision between Cosmos 2251 and Iridium 33 in 2009 (ODQN 14-4, ODQN 14-2, ODQN 13-2, ODQN 13-1, ODQN 12-1, and ODQN 11-2). These two events increased the SSN tracked orbital population by almost one third.
If the goal of remediation is to reduce the risk to the current fleet of operational spacecraft, remediation techniques need to focus on removal of small sized (but still damaging) debris. If the goal is to control the long-term growth of the debris population, ADR techniques need to concentrate on the removal of large, massive objects such as intact rocket bodies and non-functional satellites. These massive objects are the long-term source of fragmentation debris from on-orbit explosions and collisions. Studies have indicated that the removal of as few as five of the highest risk objects (defined as mass times probability of collision) per year can stabilize the long-term low Earth orbit debris environment (ODQN 15-3, ODQN 15-2).
Any successful ADR concept must be technologically feasible, economically affordable, and politically acceptable to the international community. In addition, debris removal activities should also be accomplished in a manner that does not unduly increase hazards to people and property on Earth from reentering debris.
The June 2010 National Space Policy for the United States of America directs NASA and the Department of Defense to "Pursue research and development of technologies and techniques… to mitigate and remove on-orbit debris…" However, it should be noted that, currently, no U.S. government entity has been assigned the task of removing existing on-orbit debris. The June 2018 Space Policy Directive-3, the National Space Traffic Management Policy, also states that "The United States should pursue active debris removal as a necessary long- term approach to ensure the safety of flight operations in key orbital regimes. This effort should not detract from continuing to advance international protocols for debris mitigation associated with current programs."
Due to the increasing number of objects in space, NASA and the international aerospace community have adopted guidelines and assessment procedures to reduce the number of non-operational spacecraft and spent rocket upper stages orbiting the Earth. One method of postmission disposal is to allow the reentry of these spacecraft, either from natural orbital decay (uncontrolled) or controlled entry.
One way to accelerate orbital decay is to lower the perigee altitude so that atmospheric drag will cause the spacecraft to enter the Earth’s atmosphere more rapidly. However, in such cases the surviving debris impact footprint cannot be guaranteed to avoid inhabited landmasses. Controlled entry normally is achieved by using more propellant with a larger propulsion system to cause the spacecraft to enter the atmosphere at a steeper flight path angle. The vehicle will then enter the atmosphere at a more precise latitude and longitude, and the debris footprint can be positioned over an uninhabited region, generally located in the ocean.
Spacecraft that reenter from either orbital decay or controlled entry usually break up at altitudes between 84-72 km due to aerodynamic forces causing the allowable structural loads to be exceeded. The nominal breakup altitude for spacecraft is considered to be 78 km. Large, sturdy, and dense satellites generally break up at lower altitudes. Solar arrays frequently break off the spacecraft parent body around 90-95 km because of the aerodynamic forces causing the allowable bending moment to be exceeded at the array/spacecraft attach point.
After spacecraft (or parent body) breakup, individual components, or fragments, will continue to lose altitude and receive aeroheating until they either demise or survive to impact the Earth. Spacecraft components that are made of low melting-point materials (e.g., aluminum) will generally demise at higher altitudes than objects that are made of materials with higher melting points (e.g., titanium, stainless steel, beryllium, carbon-carbon).
If an object is contained inside of a housing, the housing must demise before the internal object receives significant heating. Many objects have a very high melt temperature such that they do not demise, but some can be so light (e.g., tungsten shims) that they impact with a very low velocity. As a result, the kinetic energy at impact is sometimes under 15 J, a threshold below which the probability of human casualty is very low.
There are two NASA methods to compute the reentry survivability of spacecraft components. The Debris Assessment Software (DAS), is a conservative, easy-to-use tool; the Object Reentry Survival Analysis Tool (ORSAT) is a more accurate, higher fidelity model requiring operator expertise and training.
The Object Reentry Survival Analysis Tool (ORSAT) is the primary NASA computer code for predicting the reentry survivability of satellite and launch vehicle upper stage components entering from orbital decay or from controlled entry. The prediction of survivability is required in order to determine the risk to humans on the ground. According to NASA-STD 8719.14, Process for Limiting Orbital Debris, this risk, which is based on the predicted total debris casualty area, orbit inclination, and year of reentry, should be less than 1:10,000. Frequently, ORSAT is used for a higher-fidelity survivability analysis after the NASA Debris Assessment Software (DAS) has determined that a spacecraft is possibly non-compliant with the NASA Safety Standard.
The ORSAT code uses integrated trajectory, atmospheric, aerodynamic, aerothermodynamic, and thermal/ablation models to perform a complete satellite or launch vehicle upper stage component analysis in determining the impact risk. A three-degrees-of-freedom trajectory is used with the 1976 U.S. standard atmosphere, MSISe-90 atmosphere, or the GRAM-99 atmosphere to model various types of object shapes in either spinning or tumbling modes. Drag coefficients of these objects are considered from hypersonic to subsonic speeds to obtain the kinetic energy of objects at ground impact. Stagnation point continuum heating rates are obtained for spherical objects and are adjusted for other bodies and for rarefied flow regimes. Both lumped mass and 1-D heat conduction models may be used to compute the surface temperature. The object is considered to demise when its absorbed heat reaches the material heat of ablation.
Thermal properties for 80 materials are included in a database in ORSAT, with temperature-varying properties listed for thermal conductivity, specific heat, and surface emissivity. For objects that are on the threshold of demise or survival, parameters such as oxidation efficiency, initial temperature, surface emissivity, number of layers, dimensions, or breakup altitude may be varied in a single run to obtain the critical demise/survival point of a component. Good engineering judgment is applied in the parametric analysis to compute the best predicted total debris casualty area. The impact risk is then calculated to determine whether the satellite or upper stage is compliant with the NASA Standard 8719.14.