The recent advances in Low Earth Orbit (LEO) satellite technologies massively popularized the space industry. With so many new and exciting opportunities being explored by major corporations like SpaceX and Amazon, this post aims to provide 5 lesser-known facts about LEO satellites.
What is LEO?
A Low Earth Orbit (LEO) is an orbit around Earth, with an altitude of 200 kilometers (~124 miles) to 2000 kilometers (~1240 miles) above Earth’s surface. Compared to traditional satellites in the Geosynchronous Equatorial Orbits (GEO), which is located at 35786 kilometers (22,236 miles) altitude above Earth’s surface, LEO satellites are significantly closer to us.
Making use of LEO has only recently become a feasible option. LEO constellations require high numbers of satellites to operate, which has held back the industry from making use of LEO since recent times. Nowadays, both manufacturing cost and the size of satellites have gone down significantly; it is possible to manufacture small satellites (i.e., CubeSats) at high quantities within reasonable budgets, opening up all kinds of technological opportunities!
1. LEO allows satellites to be used in new areas!
The most popular use of LEO satellites is communication. You probably heard of Starlink, SpaceX’s LEO satellite constellation that provides internet access to users across the globe. Starlink remains the largest satellite constellation with almost 30000 operational satellites (with plans to expand even more), and its satellites are mainly communication satellites. Many other LEO constellations focus on communication aspects similar to Starlink, including examples like Amazon Kuiper and OneWeb.
The other popular use of LEO satellites is navigation. Since GPS opened up to public use in 1983, navigation has always been a major use case for satellites. LEO is also planned to contain constellations built to either augment existing non-LEO navigation systems like GPS, or to provide a self-sufficient navigation service. Some examples include Astrocast, GeeSat-1 and the upcoming Xona Space constellations.
Both communication and navigation are major application areas of satellite constellations, which has worked with non-LEO orbits before the possibility to use LEO became a reality, which is partly why we commonly think of one of these two applications when we think of what satellites do. However, the proximity to Earth’s surface allows LEO to be used for new applications, the two prime examples being Earth observation and weather observation.
Earth observation satellites typically photograph Earth and transmit the image back for use. Being stationed in LEO, it is possible to obtain resolutions that are simply not possible for higher orbits. For example, such satellites can be used to track the monthly differences between the melted ice in the polar regions. In a similar fashion, weather observation satellites are used to image clouds and measure the temperature and rainfall, and their main purpose is to help obtain a more accurate weather forecast. Examples include constellations such as BlackSky Global and Iceye.
2. CubeSats could be the future of LEO
Small satellites are often classified according to their weights; femto (<0.1 kilograms), pico (0.1-1 kilograms), nano (1-10 kilograms) and mini (100-1000 kilograms). A CubeSat, originally initiated by the CubeSat program at Stanford University in 1999, falls under the pico and nano categories, and they are developed with a cubic structure, aiming for ambitious technical specifications such as a mass of around ~1 kilograms, low power consumption, off-the-shelf commercial components, and less than 1000$ cost. The basic unit of a CubeSat is defined as “1U”, with dimensions of 10x10x10 centimeters (~4x4x4 inches). CubeSats vary from 1U to 16U; as an example, a CubeSat of 2U would have dimensions of 10x10x20 centimeters (~4x4x7.5 inches). They can be designed to operate in higher frequency bands (ISARA was able to operate a 3U CubeSat at Ka-Band for communication purposes successfully) and are typically put into orbit by deployers from the International Space Station (ISS) or launched as secondary payloads on a launch vehicle. Due to their small size, low cost, and operational compatibility with regards to many space missions requiring very high number of satellites, CubeSats provide an inherent benefit to utilize, especially in LEO, where lower power signals can reach Earth’s surface without much loss. Overall, as research continues and CubeSats become more capable, they could very likely take over LEO.
3. Satellites in LEO are the fastest
In LEO, objects experience a strong gravitational pull from Earth. While the exact numbers depend on altitude, an object needs to travel with a speed of around ~7.8 kilometers per second (~17500 miles per hour), horizontal and parallel to the Earth’s surface, to stay in orbit. At this speed, a complete orbit around the planet takes around ~90 minutes, which means LEO objects complete ~16 full rotations around the Earth every day. This includes objects like various LEO satellites, the Hubble Space Telescope, as well as the ISS.
At higher altitudes, the required speed to stay in orbit decreases. In GEO, the speed of objects is at around ~3 kilometer per second, which results in the objects matching Earth’s rotation.
4. The first LEO satellite
The very first satellite, Sputnik-1, was also the first LEO satellite. Its orbit had an apogee (farthest point from Earth) of ~940 kilometers (~584 miles) and a perigee (nearest point to Earth) of ~230 kilometers (~143 miles). It remained in orbit until January 4, 1958, before falling down and burning in Earth’s atmosphere.
Sputnik-1 was the first of three planned satellites that were part of the Sputnik program. It had a diameter of 58 centimeters (~22.8 inch) and it operated at 20.000 – 40.000 MHz frequency, emitting information including interior and outer temperature every 0.3 seconds. Due to its very straightforward design, it is also known as the simplest satellite.
5. Did you know that LEO altitudes have a high density of space junk in addition to the satellites?
Space debris are orbiting objects that present a major threat for space operations. It began to accumulate in Earth orbits since we started using satellites, and despite efforts to reduce it, remains a major concern in the industry. Currently, LEO orbits are crowded with space debris in addition to satellites, and in particular, the region below 1000 kilometer (~621 miles) is very dense. For relatively smaller objects (<1U in size), the most common way to address space debris threat is to shield the satellite, but for larger objects (>1U in size), collision avoidance is the preferred method, which requires very accurate information regarding the debris position and speed, and observation of the space debris is typically the key problem.
Space debris detection and observation is possible thanks to radio and optical telescope measurements we can gather from the orbit, which can detect, and even track space debris with high accuracy and reliability. For LEO, radar measurements are typically preferred since they are not affected by meteorological conditions. This observation can be both by satellites and telescopes orbiting the Earth, or from ground stations scattered around the globe. In Europe, there are around 50 different radio telescopes used to monitor space debris; for example, the European Space Agency collaborates with the Tracking and Imaging Radar (TIRA) system located in Germany, which is able to detect objects of diameters as low as 2 centimeters (~0.8 inches) at 1000 kilometers (~620 miles) altitude.
References
Muntoni G, Schirru L, Pisanu T, Montisci G, Valente G, Gaudiomonte F, Serra G, Urru E, Ortu P, Fanti A. Space Debris Detection in Low Earth Orbit with the Sardinia Radio Telescope. Electronics. 2017; 6(3):59. https://doi.org/10.3390/electronics6030059
N. Saeed, A. Elzanaty, H. Almorad, H. Dahrouj, T. Y. Al-Naffouri and M. -S. Alouini, "CubeSat Communications: Recent Advances and Future Challenges," in IEEE Communications Surveys & Tutorials, vol. 22, no. 3, pp. 1839-1862, thirdquarter 2020, doi: 10.1109/COMST.2020.2990499
NASA Space Science Data Coordinated Archive – Sputnik-1: https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1957-001B#:~:text=Description,of%20the%20former%20Soviet%20Union
NORAD GP Element Sets: https://celestrak.org/NORAD/elements/