LEO, MEO or GEO? Diversifying orbits is not a one-size-fits-all mission (Part 1 of 3)

Published July 18, 2023
By Lisa Sodders, SSC Public Affairs

EL SEGUNDO, Calif. -- All About Low Earth Orbit (LEO)
It may come as no surprise that most of the man-made objects in space can be found in Low Earth Orbit, also known as LEO. LEO is the orbital range closest to Earth, which also means it’s the easiest orbit to reach in terms of energy and rocket power. Satellites that orbit up to 1,200 miles above earth are in LEO. They include the International Space Station, the Hubble Telescope and some 4,000 Space X Starlink satellites, to name a few.

“LEO is used a lot for communications and imaging,” notes Kerstyn Auman, a space situational awareness analyst at the Aerospace Corporation. “One of the pros of this orbit is you have a really low latency (compared to other orbits) – how much time it takes to send out a signal and/or get one back. So when you’re thinking of things like voice communications or surfing the internet, you want it to be almost instantaneous.”

Because they’re so close to Earth, satellites in LEO also don’t need as much signal power to transmit and they can be much smaller in size, some as small as a Rubik’s cube (about 4” by 4” by 4”) and as light as three pounds. The trade-off is that you need a lot of them to sustain continuous coverage of a given area of earth. That’s because satellites in LEO don’t stay in one place for very long: they complete 16 orbits around the earth every 24 hours – about one orbit every 90 minutes. As a result, it can take hundreds or thousands of satellites in a LEO constellation to achieve continuous global coverage. But because the satellites are small, they are more cost efficient to produce and modern “ride shares” can launch multiple satellites on one rocket.

Once in LEO, satellites must travel at a rate of approximately 17,000 miles per hour to maintain the balance between momentum and gravity that is needed to keep it on course. If momentum lags, gravity can pull the satellite off-course.

Satellites in LEO are also subject to greater atmospheric drag. Like running against a strong wind, atmospheric drag can decrease satellite speed. As a result, satellites in LEO must burn fuel frequently versus other orbits to maintain their position or risk burning up as their orbit decays and they reenter Earth’s atmosphere.

“Drag is the biggest perturbing force that basically alters where you would expect your satellite to be,” Auman says. “It can be a lot more difficult to predict your future location in LEO because you have drag, and drag is affected by solar weather.”
Solar weather alters the composition of the earth’s atmosphere, which in turn alters the effects on a satellite in LEO.

“We have a general idea, but predicting that solar weather accurately is very difficult,” notes Auman. “Better measurements of a satellite position and velocity can improve the knowledge of where exactly a LEO satellite is, and thus where it is going. GPS satellites, with a vantage point from a higher orbit, can be used to track LEO satellites and improve location accuracy.”

Along with Medium Earth Orbit, LEO is the target destination for new missile warning, missile tracking and missile defense sensors currently under development at Space Systems Command. This is a significant departure from the current scheme, which utilizes very large satellites located 22,236 miles above the earth in Geostationary Orbit. In addition to spreading out risk by diversifying orbits, proliferated sensors operating closer to Earth will help to increase missile detection and tracking accuracy.

“By placing our sensors in varied orbits we gain multiple views of the same area and targets — enhancing our geometry with additional look angles and range,” says Col. Heather Bogstie, senior materiel leader for the Resilient Missile Warning, Tracking, and Defense Acquisition Delta at SSC.

The diversified orbital scheme for next generation missile warning, tracking and defense is based on a force design developed by the Space Warfighting and Analysis Center (SWAC) in direct response to emerging missile threats such as hypersonics which are more difficult to detect from GEO orbit.

“This design is based on improved sensor technology paired with an abundance of commercial space vehicles to give us affordable options to place more sensors closer to the targets we need to detect and track,” says Bogstie.

From LEO to GEO and orbits in between, Space systems Command is taking advantage of every lane in space to deliver more resilient space capabilities to combatant commands.

Test Your Orbital Knowledge: Choose the answer that doesn’t fit.
1. Low Earth Orbit is:

Up to 1,200 miles above Earth
Home to more than 5,000 satellites
The location of the James Webb Space Telescope

2. Compared to other orbits, satellites in Low Earth Orbit are generally:

Smaller and more proliferated
More cost efficient to produce and launch
Less fuel intensive to operate

3. A satellite in Low Earth Orbit:

Travels at approximately 17,000 miles per hour
Rotates the Earth every 90 minutes
Provides continues coverage over a given area

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Answers

C. The James Web Space Telescope orbits around the sun.
C. Satellites in LEO burn fuel more frequently to counter atmospheric drag.
C. Satellites in LEO do not provide continuous coverage because their position in relation to Earth is not stationary.