The crowding of small satellites in orbit was seen as a potential issue as early as 2001 when three individuals, Yakovlev, Kulik, and Agapov, wrote a paper for a European conference on space debris. Graphs using NASA data showed the increasing favorability for lighter weight satellites. They concluded that if the number of small satellites continued to increase, space debris in those regions could become a problem. Any object in Earth orbit can pose as a threat, but space debris can be looked at as objects that simply clutter space and serve no purpose. Here, we define space debris as rocket bodies, defunct satellites, and any other human-made piece of satellite in space. Of the aforementioned categories, 18,747 objects (greater than 10 cm) exist: 4,614 are payloads (active and defunct) and 14,133 objects being rocket bodies and debris. For objects between 1 cm and 10 cm in diameter, NASA estimates approximately 750,000 in orbit. For objects less than 1 cm in diameter, NASA believes the number is around 166 million.
The biggest threat when it comes to debris types and sizes are particles less than 10 cm in diameter.
Because they travel at speeds averaging 10km/sec, even the smallest particles can result in catastrophic damage. The energy of an aluminum sphere 1 cm in diameter is comparable to a 400-lb safe traveling at 60 mph. Particles as small as 1 cm are able to damage spacecraft shielding, and particles 10 cm and less become difficult or impossible to track with current technology on Earth. Thus, there are a significant number of objects that we cannot track but can cause substantial spacecraft damage. In 2007, N.L Johnson, chief scientist for the Orbital Debris Office at NASA, said, “By far the greatest source of debris now in orbit about the planet has been the often-violent break-up of spacecraft and rocket bodies.” Interestingly, four years later, Shenyan Chen, faculty member in the School of Astronautics at the Beijing University of Aeronautics and Astronautics (BUAA), tells us that the majority of debris stems from collisions. The increase in debris from collisions will surely continue to be the primary cause for space debris in the future, but the larger amount of collisions seen by Chen in 2011 was not a result of CubeSat launches. At that time, less than 100 CubeSats had been launched since 2000. The increased debris from collisions was attributed to the debris created from large satellite collisions such as the one involving the Iridium-33 and Kosmos-2251 satellites in 2009. Additionally, in January of 2007 a Chinese spacecraft, Fenyun-1C was intentionally self-destructed leaving an enormous amount of debris expanding in different orbits. Six years later, a Russian satellite called, “BLITS,” was impacted by a debris fragment from a piece resulting from the Chinese satellite’s debris. The debris field continues to expand around the Earth to this day, and for the Chinese satellite’s debris, calculations show that if the breakup occurred at 1,000 km, two-thirds or more of the debris greater than 1 cm in size would stay in orbit for more than a decade. If the breakup occurred at 800 km, nearly 35% of the debris would stay in orbit for more than a decade.
A data table from a study done by Darren McKnight shows a high positive correlation between debris-related anomalies above 700 km and the destruction of the Chinese satellite using DMPS and SOARS satellite data. Issues arise when debris are created in high altitudes such as 700 km and above due to what is known as “collisional cascading.” Collisional cascading happens when debris cause collisions resulting in more debris, which cause further collisions and so on. It tends to happen most often in the higher altitudes due to high debris population and less air drag. This phenomenon amplified the danger of collisions at this altitude and lends justification for further regulation in altitude restrictions on CubeSats.
The generally accepted maximum orbital lifetime for CubeSats is 25 years, meaning it should take no more than 25 years for the CubeSat to fully decay from its orbit and burn up. In order to accomplish this, the maximum orbital height a CubeSat should be deployed is about 600 km. However, this is only a guideline and is not strictly enforced. Nevertheless, the vast majority of CubeSats in orbit do follow this guideline presently. Looking at data from 2015, the orbit region with the largest percentage of CubeSats compared to all other objects was between 425 km and 550 km; that percentage is a mere 14.3%. CubeSats are a minuscule factor when considering the dangers of a collision. On the other hand, companies such as OneWeb and SpaceX have plans for extremely large satellite constellations consisting of several thousand satellites (SpaceX plans 12,000). As companies begin launching massive satellite constellations, it is crucial for CubeSat launchers to stay within the 600km boundary as long as they continue being produced without maneuverability devices. It is unlikely CubeSats will add propellant for maneuvering, as that would defeat the purpose of a low-cost entry into space. Fortunately, there is a clear trend for the most recently launched CubeSats to begin their orbital lifetime at lower altitudes, between 500 km and 600 km.
One way to begin dealing with the increased debris in LEO (low Earth orbit) has been proposed by Gheorghe and Yuchnovicz. If regions are split and assigned individual hazard values, appropriate action can be taken for satellites in those regions or planning to be placed in those regions. Expanding upon the zonal idea, it may be wise to place in orbit satellites with low life expectancies or satellites with less experienced developers in very low orbits (below 300 km) to reduce the probability of any catastrophic collisions. For mitigation techniques, active debris removal technology (ADR) has been discussed extensively, but the improvement of post mission disposal (PMD) tactics is an area where focus should be directed if debris is to be reduced in the future. Examples of PMD techniques are the use of an inflatable balloon or sails that could be used for de-orbiting. These are especially attractive for those higher altitudes (above 600 km)where solar radiation pressure (SRP) is not as high and decay times are longer. Armored shielding and onboard propulsion have been mentioned as a mitigation techniques in the past, but both are quite impractical in terms of the added weight and cost respectively. Essentially the Kessler syndrome, or cascade effect as mentioned earlier, proves that active debris removal (ADR) is not feasible as a sole focus for remediation.
It has been suggested that the orbital debris problem is an argument for large, multi-purpose, satellites rather than smaller, shorter-lived satellites or constellations. In other words, many believe that fewer satellites will reduce the number of collisions, and as a result, the amount of debris. However, as the aforementioned evidence has shown, this is an incorrect argument for several reasons. The ratio of active satellites to debris particles greater than 1 cm in LEO is around 1 to 550. Realizing the particles 1–10 cm in size are damaging and untraceable, if the number of active satellites were increased, the change in the number of objects in orbit that pose a threat would be negligible. Furthermore, the collisional cross section of CubeSats is smaller, therefore they are less likely to collide with other satellites. The fact that small satellites usually do not include deployable arrays also contributes to the lower probability of collision. Also operational CubeSats are typically at low altitudes where the debris cloud decays and re-enters over months or a few years as opposed to single large satellites at high latitudes where the debris cloud decays over 100’s or 1000’s of years! On the same note, it is a good idea for CubeSats to avoid being deployed with large satellites in the same orbit.
Overall, the biggest threat of space debris in Earth orbit comes not solely from the number of spacecraft, but from the altitudes in which the satellites are deployed as well.
With CubeSats being around for a mere two decades, a serious threat pertaining to space debris is not yet present. They represent a very small fraction of objects 10 cm and larger (approximately 1%). Nonetheless, a serious space debris threat is quite plausible as the number of launched small satellites increases each year. Even without the addition of more satellites in orbit, an increase in collisions has been estimated through computer simulations. Unfortunately, the situation may have just become increasingly complicated with the FCC’s approval for 1,500 of SpaceX’s satellites to be positioned in the 550km orbit. It will not be surprising to see more emphasis on tracking and collision avoidance for CubeSats in the future.
