astronomy

High Earth orbit

High Earth orbit is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand High Earth orbit rather than just read about it. In short: A high Earth orbit is a geocentric orbit with an apogee farther than that of the geosynchronous orbit, which is 35,786 km (22,236 mi) away from Earth. In this article, the non-standard abbreviation of HEO is used for high Earth orbit.

Key takeaways

High Earth orbit belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect High Earth orbit to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of High Earth orbit from memory before moving on to harder problems.

Reference excerpt

A high Earth orbit is a geocentric orbit with an apogee farther than that of the geosynchronous orbit, which is 35,786 km (22,236 mi) away from Earth. In this article, the non-standard abbreviation of HEO is used for high Earth orbit. The development of HEO technology has had a significant impact on space exploration and has paved the way for future missions to deep space. The ability to place satellites in HEO has allowed scientists to make groundbreaking discoveries in astronomy and Earth science, while also enabling global communication and navigation systems. The Moon's Hill sphere is entered at a distance to the Moon of 60,000 km (37,000 mi), lunar orbits until a distance of 690 km (430 mi) are unstable due to Earth's gravitational reach. Near-rectilinear halo orbits around the Moon are within these distances to the Moon, occupying cislunar space. Earth's hill sphere extends to a distance of 1,471,400 km (914,300 mi), encompassing halo orbits, orbits around the Sun-Earth Lagrange points, with orbits increasingly being heliocentric, co-orbiting with Earth the Sun before orbits go deeper into interplanetary space.

Common types of high Earth orbits

Satellites in High Earth orbits are primarily used for communication, navigation, scientific research, and military applications. One of the main benefits of HEO is that it provides a nearly unobstructed view of the Earth and deep space. This makes it an ideal location for astronomical observations and Earth monitoring. In addition, satellites in HEO can provide a continuous coverage of the Earth's surface, making it very useful for communication and navigation purposes. A variety of satellites, such as TESS, have been placed in HEO. There are four main reasons that most satellite are placed in lower orbits. First, a HEO can take a month or more per orbit. This is because HEOs are very large orbits and move at only 3000 m/s (11,000 km/h, 7000 mph). Meanwhile, a LEO (low Earth orbit) can take less than 90 minutes. So, for satellites that need to orbit quickly, HEO is not a good fit. Second, HEOs take far more energy to place a satellite into than LEOs. To place a satellite into HEO takes nearly as much energy as to place it into a heliocentric orbit. For example, an expended Falcon 9 can carry 22,500 kg (50,000 lbs) to LEO. However, it can only carry around 4500 kg (10,000 lbs) to HEO. This means that it costs 5 times more to place a payload in HEO versus placing it in LEO. Third, HEOs are extremely far from Earth. This means that there is a constant communication delay when sending signals to and from the satellite. This is actually because the signals can only travel at the speed of light. This means that it can take around 0.1 to 4.5 seconds in delay time each way. This makes it useless for internet, and hard to use for other things as well. The fourth reason is radiation. HEO is outside of the magnetic field of Earth. This means that there is far more radiation in HEO. As a result, spacecraft in HEO require specialized equipment and shielding to protect them from radiation. As a result, only satellites that require the unique characteristics of HEO use this orbit. A special case of a high Earth orbit is the highly elliptical orbit where altitude at perigee may reach as low as 2,000 km (1,200 mi).

Examples of satellites in high Earth orbit

Illustrations

Worked examples

Example 1 — a first encounter with High Earth orbit

Start with the simplest possible case. Write down what High Earth orbit claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to High Earth orbit before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about High Earth orbit ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of High Earth orbit

In research: High Earth orbit appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses High Earth orbit in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: High Earth orbit is common in secondary-school and first-year university syllabi. It links to neighbouring topics Earth orbits, so understanding it makes those chapters shorter.
In everyday life: Look for High Earth orbit outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study High Earth orbit in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what High Earth orbit means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain High Earth orbit out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is High Earth orbit in simple terms?

Why does High Earth orbit matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study High Earth orbit?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on High Earth orbit.