OrbitalHub

Credits: NASA

Let us see how the areas mentioned in the previous Sustainability in LEO post are covered at national level in the United States.

The United States has implemented a space traffic management program in the form of the Joint Space Operations Center (JSpOC) of the U.S. Strategic Command at Vandenberg Air Force Base in California.

JSpOC conducts periodic conjunction assessments for all NASA programs and projects that operate maneuverable spacecraft in low Earth orbits (LEO) or in geosynchronous orbits (GEO). Depending on the mission, the conjunction assessments can be performed up to three times daily. If JSpOC identifies an object that is expected to come in the proximity of a NASA spacecraft, and the collision risk is high enough (for manned missions the minimal value accepted is 1 in 10,000, while for robotic missions the threshold is 1 in 1,000), a conjunction assessment alert message is sent to the mission control in order to have collision avoidance maneuver commands sent to the spacecraft. The alert messages contain the predicted time and distance at closest approach, as well as the uncertainty associated with the prediction.

The control of the creation of space debris is addressed by orbital debris mitigation standard practices in four major areas: normal operations, accidental explosions, safe flight profile and operational configuration, and post-mission disposal of space structures. There are also NASA standards and processes that aim at limiting the generation of orbital debris.

The commonly-adopted mitigation methods, which focus on minimization of space debris creation, will not preserve the near-Earth environment for the future generations. As a matter of fact, the debris population increase will be worse than predicted by LEGEND-generated models due to ongoing launch activities and unexpected (but possible) major breakups. Here is where active space debris environment remediation comes into play.

The active space debris environment remediation is mainly concerned with the removal of large objects from orbit. Such large objects are defunct spacecraft (i.e. communication satellites that exceeded their operational life), upper stages of launch vehicles, and other mission-related objects. The removal of large objects from orbit is known as Active Debris Removal (ADR). Several innovative concepts are under study. Among them, tethers used for momentum exchange or electro-dynamic drag force, aerodynamic drag, solar sails, and auxiliary propulsion units. LEGEND studies have revealed that ADR is a viable control method as long as an effective removal selection criterion based on mass and collision probability is used, and there are at least five objects removed from orbit every year. The electrodynamic tethers seem to lead the competition so far, as they have a low mass requirement and can remove spent or dysfunctional spacecraft from low Earth orbit rapidly and safely.

Re-entry in the Earth’s atmosphere of space mission related objects is an important aspect to be considered in this context. Even though no casualties or injuries have been reported so far being caused by components of re-entering spacecraft, fragments from space hardware pose a risk to human life and property on the ground. One big concern is caused by the fact that the point of impact from uncontrolled re-entries cannot be calculated exactly. The uncertainties are due to a large number of parameters that affect the trajectory and the heat of ablation of objects re-entering the atmosphere.

Credits: NASA

Space debris mitigation measures address issues in two major areas: protection from space debris and reduction of the space debris population growth.

Protection methods vary depending on the size of the space debris. Space debris fragments smaller than 0.1 mm in diameter do not have enough energy to penetrate protection panels. Even spacecraft thermal blankets and structural panels can offer protection from such small kinetic impactors.

However, larger debris fragments pose a more serious threat and there are few active measures that can be adopted to minimize the possibility or the consequences of impacts.

First, avoiding space debris is the method that always proves successful, so flying at altitudes and inclinations where space debris density is low should always be considered. Second, orienting sensitive surfaces away from or mounting a bumper on the leading edge can offer more protection for the spacecraft. Multilayered bumpers cause the fragmentation of the space debris and also prevent ejected material from dispersing from the point of impact. Bumpers add mass and volume to the spacecraft and this is why few space missions can exercise this option.

A promising new method for protection from orbital debris impact is shielding with metallic foams. Extensive research and experimental impact campaigns have proven that metallic open-cell foams provide improved protection against hypervelocity impacts with almost no ejecta generated at impact, while offering comparable mechanical and thermal performance to honeycomb structures, which are currently used for shielding. The fact that no ejecta are generated is of great importance, as fragments generated at impact can add to the already existing debris population in Earth orbit.

At this point, it appears that the best protection method is avoiding the creation of space debris.

The long-term projections of the space debris environment generated by models like LEGEND or DELTA have proven that only active methods can maintain a stable environment of artificial objects in Earth orbit. It is quite obvious that unchanged operational practices or even an immediate stop to launch and release activities will not prevent the collisions between already existing space hardware. Scenarios like the one proposed by NASA scientist Donald J. Kessler in 1978 are very likely to occur. In the scenario proposed by Kessler, which is also known as the Kessler Syndrome, space exploration and the use of satellites in proximity of Earth will become unfeasible due to an exponential growth of the debris population caused by collisional cascading. The exponential growth is due to the fact that the material ejected during a hypervelocity impact becomes space debris itself. Laboratory experiments have shown that as a result of such an impact, 1 kg of aluminum can form several hundred thousand 1 mm sized particles.

Failure to address the potential uncontrollable growth of the space debris population will lead to major restrictions on the ability to exploit space. There is a debate in the scientific community over whether or not critical density has already been reached in certain orbital regions and if we are beyond the point where we can address the growth of the debris population.

There are various classifications of the space debris mitigation methods. For example, two broad categories might include, on one hand, measures that restrict the generation of space debris in the near future, such as limiting the production of mission-related objects and the avoidance of breakups, and, on the other hand, measures that restrict their generation for the long term, which include post-mission disposal methods and active measures to remove space debris from protected regions.

There are three major areas that could allow space faring nations to maintain a stable debris environment: space traffic management, control the creation of space debris, and active space debris environment remediation.

Credits: NASA

As mentioned in a previous post, only a small fraction of the existing space debris population is detectable and tracked by ground systems. A smaller fraction is catalogued by special programs and/or departments of national space agencies. This is where statistics comes into play. Numerous models have been created in order to assess present collision risks associated with certain orbits and to predict future evolution of the debris environment around Earth.

The National Aeronautics and Space Administration (NASA) has developed two categories of applications for modeling of space debris environment and risk analysis. The first category, based on evolutionary models such as NASA’s long term debris environment evolutionary model (LEO-to-GEO Environment Debris model or LEGEND), are designed to predict the evolution of the debris environment.

These models cover the near-Earth space between 200 km and 50,000 km, provide space debris characteristics for a debris population consisting of particles as small as 1 mm, and have a typical projection period of 100 years. The second category, which consists of engineering models like ORDEM2000, is used for debris impact risk assessment for spacecraft and satellites, and also as benchmarks for ground-based debris measurements and observations.

The European Space Agency (ESA) has a different set of tools used for modeling the space debris environment and assessing risk associated with collisions in Earth orbit. The DISCOS database (the Database and Information System Characterizing Objects in Space) consolidates the knowledge on all known objects tracked since Sputnik-1, and it is recognized as a reliable and dependable source of information on space objects in Earth orbit. MASTER (Meteoroid and Space Debris Terrestrial Environment Reference) is the agency’s most prominent debris risk assessment tool, which uses statistical methods to determine the impact flux information from all recorded historic debris generation events. ESA also uses DELTA (Debris Environment Long-Term Analysis) to conduct analysis of the effectiveness of debris mitigation measures on the stability of the debris population. Such analysis can cover 100 to 200 year time spans.

Credits: NASA

As the primary cause of concern from space debris is physical damage upon impact, extensive efforts have been made for their detection. There are several detection methods, and they are grouped into two classes: active and passive.

Radar sensors fall into the first class, and radio interferometers and optical sensors in the second. One important element that has to be considered is the accuracy of the method used for detection.

The motion of an object in Earth orbit is completely determined if the so-called orbital elements are known. In theory, the orbital elements of a satellite can be calculated from only one observation. In practice, due to inherent observation errors, there is more than one observation needed to attain the precision required for orbital surveillance and prediction. Some 100-200 observations are required during the first days of orbit, 20-50 observations per day to update established orbits, and 200-300 observations per day to confirm and locate reentry in the case of decaying orbits.

In addition, the size of the debris is an important factor that affects the accuracy of the detection methods, and this is why only a small fraction of the space debris population is detectable, and as a consequence, catalogued. For example, present equipment is capable of tracking only objects bigger than 5 cm in diameter in low Earth orbit (altitudes of 160-2,000 km), and bigger than 50 cm in diameter in geosynchronous orbit (altitudes of 35,000 km). Further, the characteristics of certain type of orbits can make detection very difficult. For example, the debris population generated on highly elliptical and high inclination orbits with perigees situated deep in the Southern Hemisphere, also known as Molniya orbits, is very difficult to track. The geographic location of the ground stations used for space debris tracking makes detection impossible.

For these reasons, out of an estimated debris population of 600,000 objects bigger than 1 cm in diameter, only 19,000 can be tracked as of today.

The measurement and detection methods mentioned above are all remote methods. In-situ measurements of the characteristics of the debris environment have been conducted as well. In April 1984, the Space Shuttle Challenger placed into low Earth orbit a NASA spacecraft carrying a number of experiments for the purpose of characterizing the low Earth orbit environment. The spacecraft, the Long Duration Exposure Facility (LDEF), was a twelve-sided cylindrical structure and three-axis stabilized in order to ensure an accurate environmental exposure, and was supposed to spend one full year in orbit. Before the planned retrieval, the Space Shuttle fleet was grounded as a result of the Challenger accident on January 28, 1986. Eventually, the exposed facility was returned to Earth by the Space Shuttle Columbia during a mission in January 1990. After the extended mission, the results of the onboard experiments facilitated to a greater extent the understanding of the interactions between artificial objects and the space debris environment in Earth orbit as numerous impact craters were found on the outer layers of the spacecraft and analyzed.

In-situ measurements of the characteristics of the space debris environment have also been conducted by the European Retrievable Carrier (EURECA) and the Space Flyer Unit (SFU).

Credits: NASA

Space debris, also known as orbital debris, consist of artificial objects in orbit around Earth that no longer serve any useful purpose.

Most of the space debris population consists of fragments resulted from explosions and collisions, but some are spent rocket stages and satellites that are no longer operational. Fragmentations occur during a satellite breakup or an anomalous event.

A satellite breakup is a destructive disassociation of a payload, rocket body, or structure. Most of the time, breakups generate ejecta with a wide range of velocities, and this affects the evolution of the particle cloud following the breakup. As the ejected particles spread out from the point of breakup with different initial conditions, some of them may remain well correlated for a long time, while others may disperse into dissimilar orbits. Satellite breakups are accidental, but there are exceptions when they are deliberate, as in the case of a space weapon test. An anomalous event is the unplanned separation of one or more detectable objects from the satellite. These separations happen at low velocities and the satellite remains intact. As an example, the separation caused by the deterioration of a thermal blanket or a protective shield is classified as an anomalous event. Clearly, a satellite breakup generates more debris than an anomalous event.

As the space debris are associated with spacecraft launches, the highest densities are found on the most popular altitudes and inclinations. While the altitudes are characteristic to mission types, the orbital inclinations correspond to the latitudes of the current launching facilities.
There are numerous characterizations of the space debris environment. A common method used for describing it is the spatial density of resident space objects, which is a representation of the effective number of spacecraft and other objects as a function of altitude. On such a representation there are several high-density regions that are evident: near 890 km due to Fengyun-1C event, around 780 km where the Iridium constellation of satellites resides, and the region around 1,400 km, inhabited by the GLOBALSTAR constellation. There are certain differences in the distribution in the low Earth orbit region (altitudes of 160-2,000 km) and the distribution in the geosynchronous orbit region (altitudes of 35,000 km). These are caused by the fact that high inclination orbits, characteristic to the LEO region, yield a greater collision rate because objects in these high inclination orbits can collide in the overlapping regions with other objects on complementary orbits, and also the GEO environment is characterized by lower collision velocities.

Another method of characterization is the population distribution by object type (e.g. spacecraft, rocket stage) and by source (e.g. United States, People’s Republic of China). For example, on August 1st, 2007, the U.S. Satellite Catalog presented the following Source vs. Type statistic for on-orbit objects.

Source vs. Type Accounting (on-orbit objects) / 1 August 2007 U.S. Satellite Catalog

It is interesting to see that debris is dominant among all sources, and they are mostly due to space activities of the United States, Commonwealth of Independent States, and People’s Republic of China.

Two major collision events in Earth’s orbit are mentioned in the scientific literature: the Chinese Fengyun-1C anti-satellite (ASAT) test in 2007, and the first accidental collision between two large intact satellites, Iridium 33 and Cosmos 2251, in 2009.

Fengyun-1C, a box shaped satellite weighing 950 kg, was launched in May 1999. The satellite was intercepted and destroyed at an altitude of 860 km on January 11, 2007, by a kinetic kill vehicle at a relative speed of approximately 12 km/s. The debris cloud formed as a result of the Chinese ASAT test represents the worst contamination of low Earth orbit in history. It was estimated that the impact generated more than 2,300 trackable objects, and more than 1,000,000 objects 1 mm in diameter or larger. More than half of the fragments created during the impact and identified by ground measurements have present orbits exceeding a mean altitude of 850 km, which means that they will be part of the debris population for decades. As a result of this test, an increase of the population of 69% has been observed. The Fengyun-1C ASAT test was not a first, though. The first artificial satellite used as a target in an ASAT test was Solwind (P78-1), a scientific spacecraft used for coronal research. Launched in February 1999, the P78-1 was experiencing automatic shutdown of the scientific payload due to degradation of the power systems, and it was destroyed by an ASM-135 ASAT suborbital rocket on September 13, 1985.

It is important to mention the details of the collision between the Iridium 33 and the Cosmos 2251 satellites. The event got a lot of media coverage and it seems that it was the catalyst of a number of new initiatives related to the space debris environment in the space industry. Iridium 33 was an operational communication satellite, one of the Iridium Constellation satellites. Cosmos 2251 was a Russian communication satellite, retired at the time of time of the collision. This was the first major collision of two satellites in Earth orbit.

From May 2003 to August 2007, there are twenty-one on-orbit fragmentations and seven anomalous events recorded. The historical total, recorded starting in October 1957, is 194 fragmentations and fifty-one anomalous events.

Credits: NASA

The adventure started on October 4, 1957, when the former Soviet Union successfully launched the first artificial satellite, Sputnik-1, using a rocket that was a modified Intercontinental Ballistic Missile (ICMB). Even if the political implications at that time were very important, as the launch ignited the Space Race within the Cold War, we can argue that the scientific accomplishments were more significant.

These accomplishments relied upon the theoretical work of scientists like Hermann Oberth and Konstantin Tsiolkovsky.

What followed this event, as mentioned above, was a race.

Explorer-1, the first American artificial satellite, was launched on January 31, 1958. Yuri Gagarin was the first human in outer space and the first to orbit the Earth on April 21, 1961. He was followed closely by Alan Shepard, who became the first American to travel into space onboard the Freedom-7 capsule, on May 5, 1961.

On August 19, 1964, the first geostationary communication satellite, Syncomm-3, was placed in orbit over the International Date Line. Syncomm-3 was used to relay the television coverage of the 1964 Summer Olympics in Tokyo, Japan, to the United States. The first to propose the concept of a communication satellite was Arthur C. Clarke, who in October 1945 published an article in the British magazine Wireless World that described the fundamental concepts behind the development of artificial satellites used to relay radio signals.

The first space station, Salyut-1, was launched on April 19, 1971. Even if the space station had a short operational life, as it re-entered the Earth atmosphere on October 11, 1971, it tested elements of the systems required on a space station and conducted scientific research and experiments. The construction of the first international research facility in Earth orbit, the International Space Station (ISS), began in 1998. The station is still under construction and it will be operational until at least 2015.

Where are we now, after 53 years of exploration of the space in the proximity of Earth? Since the launch of Sputnik on October 4, 1957, some 4,600 launches have orbited more than 6,000 satellites. All of these activities have created a cloud of orbiting particles around Earth. This new environment is referred to as space debris or orbital debris. Even if most of these particles are small in size (less than 1 cm), they are a source of great concern as the kinetic energies associated with impacts at orbital velocities, which are in the range 8-10 km/s or 28,800-36,000 km/h, are very high. It has been estimated that the total mass in orbit is 5,800 tons.