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Credits: Ronald C. Wittmann

 

There are numerous examples of successful implementation of mitigation measures, but also some not so successful, and even failures. There are two cases that I will mention, one from each camp.

 

Cosmos 954 was a Soviet Radar Ocean Reconnaissance Satellite (RORSAT) powered by an onboard nuclear reactor. At the time, the Russian designers were not able to find an alternative for the power system due to the power requirements of the payload carried by the spacecraft, which was a powerful radar. A post-mission mitigation method that involved parking the nuclear reactor on a higher orbit (with an estimated lifetime of hundreds of years) was adopted.

 

 

It seems that not enough effort was put into designing a reliable solution for the post-mission disposal method of the nuclear reactor. Besides the inherent low reliability associated with hardware in developmental phases, the quality assurance practices at that time were most likely affected by the conditions of the Cold War. In both camps, the concerns regarding the environment were ignored in favor of the military and political goals.

 

In 1978, COSMOS 954 failed to separate its nuclear reactor core and boost it into the post-mission parking orbit as planned. The reactor remained onboard the satellite and eventually re-entered into the Earth atmosphere and crashed near the Great Slave Lake in Canada’s Northwest Territories. The radioactive fuel was spread over a 124,000 km2 area. The recovery teams retrieved 12 large pieces of the reactor, which comprised only 1% of the reactor fuel. All of these pieces displayed lethal levels of radioactivity.

 

To highlight how dangerous and how serious the use of nuclear power sources for space mission is, consider these figures: at present, there are 32 defunct nuclear reactors in orbit around the Earth. There are also 13 reactor fuel cores and at least 8 radio-thermal generators (RTGs). The total mass of RTG nuclear fuel in orbit is in the order of 150 kg. The total mass of Uranium-235 reactor fuel in orbit is in the order of 1,000 kg.

 

 

RADARSAT-1 is an Earth observation satellite developed in Canada. Equipped with a powerful synthetic aperture radar (SAR) instrument, RADARSAT-1 monitors environmental changes and the planet’s natural resources. Well beyond the planned five-year lifetime, the satellite continues to provide images of the Earth for both scientific and commercial applications.

 

Following the guidelines of the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) document entitled Guidelines for Space Debris Mitigation, and implementing mitigation measures required for the space hardware manufacturers in Canada, the Canadian Space Agency has prepared post-mission disposal plans for its remote sensing satellite RADARSAT-1. As a prerequisite to the end of mission procedures, the energy stored in the propellant tanks, the wheels, and the batteries of the satellite will be removed, as suggested in the COPUOS guidelines. Also, the remaining fuel will be used to lower the orbit in addition to orienting the satellite so that drag is maximized. These measures will aim to reduce the orbit life span of the satellite to the lowest possible.

 

 

Simulations performed using NASA’s long-term debris environment evolutionary model (LEO-to-GEO Environment Debris model or LEGEND) or ESA’s debris environment long-term analysis tool (DELTA) have shown that even if new launches are not conducted, the existing population of orbital debris will continue to increase. This increase in number is caused by collisions between the objects already orbiting the Earth at the present time. Following the Iridium/Cosmos collision in 2009, the U.S. Air Force has issued hundreds of notifications to Russia and China regarding potential crashes between their satellites and other objects in orbit.

 

Even if we are contemplating grim future developments like the one mentioned above, international initiatives do not seem to gain enough momentum. NASA (National Aeronautics and Space Administration) and DARPA (Defense Advanced Research Projects Agency) were the sponsors of the first International Conference on Orbital Debris Removal, which was held in Chantilly, Virginia, December 8-10, 2009. The conclusions of the conference included the observation that:

 

“No evident consensus or conclusions were reached at the conference. Removing existing, non-cooperative objects from Earth orbit is an extremely difficult and likely expensive task. Although some of the techniques for removal discussed at the Conference have the potential of being developed into technically feasible systems, each concept seems to currently suffer from either a lack of development and testing or economic viability.”

 

 

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Credits: MSCI

 

The COMMStellation satellites will orbit the Earth on polar orbits 1,000 km above the surface of the Earth. The satellites will be deployed in six orbital planes, thirteen operational satellites per plane, plus one for redundancy. The deployment will be cost efficient, only six launches being required to deploy the whole constellation.

 

As oppose to Iridium, which is accessible from portable devices, the COMMStellation will be connected to terrestrial telecommunication networks through twenty ground stations located around the Earth. The required ground stations are less expensive than those used for communication with satellites on medium Earth orbits and geostationary orbits.

 

MSCI claims it has perfected the construction of microsatellites and the use of commercial-grade components for development of microsatellites. These factors have led to low manufacturing costs and improved schedules.

 

An alternative to COMMStellation is proposed by O3b Networks, located in St. John, Jersey, Channel Islands. The O3b Networks constellation satellites will provide broadband connectivity within forty-five degrees latitude north and south of the equator. The constellation will consist of eight satellites at 8,000 km above the surface of the Earth. There are a number of advantages in using the low Earth orbit polar microsatellites, as MSCI is proposing, over using equatorial medium Earth orbit satellites: the polar orbits provide full coverage of the terrestrial surface and microsatellite technology has less cost and increased reliability associated with it.

 

It is also worth mentioning a previous attempt at creating a constellation of low Earth orbit satellites to provide access to the Internet – Teledesic.

 

You can read more about COMMStellation on MSCI’s website.

 

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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

  US CIS France PRC India Japan ESA Other Total
Payloads 1,063 1,324 44 61 33 103 36 387 3,051
Rocket bodies 542 837 97 37 8 35 6 27 1,589
Mission related debris 779 507 92 62 1 36 12 5 1,494
Breakup debris 1,666 1,524 126 2,315 97 2 18 35 5,783
Anomalous debris 144 82 3 0 0 0 0 0 229
Totals 4,194 4,274 362 2,475 139 176 72 454 12,146

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.

 

 

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Credits: NASA

 

 

 

The Iridium satellite constellation, owned and operated by Iridium Communications Inc., is used to provide voice and data coverage over Earth’s entire surface.

 

Sixty-six satellites orbiting the Earth in low Earth orbit at 781 km altitude and 86.4 degrees inclination allow for pole-to-pole communication.

 

The Iridium modems deliver truly global communication capabilities. The solutions that incorporate the Iridium technology range from maritime voice terminals to vehicle tracking solutions. Most recently, we have seen them embedded in maritime robots like the iRobot seagliders collecting data in the Gulf of Mexico after the DeepWater Horizon accident.

 

 

Even if the majority of the applications are surface-to-surface communication, there have been attempts made to use the Iridium network for high-altitude communication. As such, weather balloons and sounding rockets have used the network to download data back to Earth.

 

A novel communication method for CUBESAT payloads using the Iridium network is proposed by Henric Boiardt and Christian Rodriguez from Florida International University for the PicoPanther payload, one of the entries in the Florida University Satellite competition.

 

The main challenges to overcome in order to adapt the Iridium technology to microsatellite communication in low Earth orbit are the miniaturization and the Doppler effects.

 

The CUBESAT standard was developed by California Polytechnic State University and Stanford. The standard specifies that one unit structure is a 10cm x 10cm x 10cm cube. This is quite a restrictive size-factor constraint.

 

The Doppler effects have to be considered due to the velocities at which the satellites operate in low Earth orbit. To minimize these effects, the microsatellite using the Iridium network to communicate with the ground station must have an orbit similar to the communication satellites in the network, which is a polar orbit. For the same reasons, Iridium itself supports inter-satellite links only between satellites orbiting in the same direction. Otherwise, the frequency shift due to orbital relative velocities would make communication unreliable.

 

If the proposed method is proven feasible, the Iridium network would definitely bring near-continuous communication to microsatellite technology.

 

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