Space

Earth's atmosphere extends much farther than previously thought

New analysis of data captured in the mid 1990s by the SOHO observatory has shown that the outer reaches of the Earth's atmosphere extends twice as far as the Moon's orbit
New analysis of data captured in the mid 1990s by the SOHO observatory has shown that the outer reaches of the Earth's atmosphere extends twice as far as the Moon's orbit
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The geochorona as seen by Apollo 16
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The geochorona as seen by Apollo 16
Artist’s impression of the ESA/NASA Solar and Heliospheric Observatory (SOHO)
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Artist’s impression of the ESA/NASA Solar and Heliospheric Observatory (SOHO)
The geocorona as measured by the SWAN instrument on board the ESA/NASA Solar and Heliospheric Observatory, SOHO
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The geocorona as measured by the SWAN instrument on board the ESA/NASA Solar and Heliospheric Observatory, SOHO
Where Earth’s atmosphere merges into outer space, there is a cloud of hydrogen atoms called the geocorona
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Where Earth’s atmosphere merges into outer space, there is a cloud of hydrogen atoms called the geocorona
New analysis of data captured in the mid 1990s by the SOHO observatory has shown that the outer reaches of the Earth's atmosphere extends twice as far as the Moon's orbit
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New analysis of data captured in the mid 1990s by the SOHO observatory has shown that the outer reaches of the Earth's atmosphere extends twice as far as the Moon's orbit

Most people think that the Earth's atmosphere stops a bit over 62 miles (100 km) from the surface, but a new study based on observations made over two decades ago by the joint US-European Solar and Heliospheric Observatory (SOHO) satellite shows that it actually extends as far 391,000 miles (630,000 km) or 50 times the Earth's diameter. This makes the Moon a very high altitude aircraft.

It's often very hard to figure out where one thing ends and another begins in space. On Earth, such limits seem very obvious. The sea ends at the shore, the ground ends at the peak of the tallest mountain, and the atmosphere ends where it becomes a vacuum – at least, that's the idea.

Where Earth’s atmosphere merges into outer space, there is a cloud of hydrogen atoms called the geocorona
Where Earth’s atmosphere merges into outer space, there is a cloud of hydrogen atoms called the geocorona

The thing is, when one moves into the realm of outer space, it's often less about where one thing ends and another begins and more where one set of forces ceases to dominate over the other. Case in point is the Earth's atmosphere. Being made up of gas, it tends to expand to fill whatever space its in, and space is one great, big infinite void. The Earth's gravity keeps most of the air around our planet where it should be, but the outer layers of the atmosphere don't stop so much as peter out.

This is the reason why satellites fall out of orbit. Hundreds of miles up, it is to all intents and purposes a hard vacuum, but there are still enough air molecules to generate drag on a satellite, slowing it down until it plunges into the lower layers and burns up. But how far out does the atmosphere actually go?

Launched on December 2, 1995 atop an Atlas IIAS launcher from Cape Canaveral Air Force Station in Florida, SOHO is parked in the first Lagrange point (L1) 930,000 miles (1.5 million km) from Earth where it has carried out studies of the Sun and the solar winds, and will continue to do so until at least 2020. From this vantage point, the observatory's Solar Wind Anisotropie (SWAN) instrument is able to measure the presence of hydrogen by looking at the Lyman-alpha line in the solar spectrum. And what works for the Sun, works for Earth.

The geochorona as seen by Apollo 16
The geochorona as seen by Apollo 16

By turning SWAN on the Earth at the right times of the year, SOHO was able to detect hydrogen atoms from the atmosphere and measure how far out they extend into what space scientists call the geocorona. While the existence of the geocorona is well known – the telescope set up by the Apollo 16 astronauts on the Moon even photographed it – no one was sure how far out it reaches until now.

By looking at data collected by SOHO in the mid 1990s, scientists from Russia's Space Research Institute and elsewhere were able to work out the extent and density of the geocorona. What they found was that sunlight on the day side of the Earth compresses the hydrogen until it reaches a density of 70 atoms per cubic cm at an altitude of 37,000 miles (60,000 km), and on the night side it can expand out until it has a density of only 0.2 atoms per cubic cm at the distance of the Moon's orbit.

Artist’s impression of the ESA/NASA Solar and Heliospheric Observatory (SOHO)
Artist’s impression of the ESA/NASA Solar and Heliospheric Observatory (SOHO)

According to the study leader Igor Baliukin, the geocorona is so tenuous that it poses no hazard to astronauts or spacecraft. There are too few atoms and even the ultraviolet radiation associated to the geocorona is too low to be of any consequence. However, it is enough to interfere with more sensitive astronomical observations working in the ultraviolet band of the spectrum.

Jean-Loup Bertaux, former principal investigator of SWAN, also points out that if the geocorona is typical of Earthlike planets, then this new knowledge could be useful when looking for signs of a water on worlds outside our Solar System.

Source: ESA

7 comments
SimonClarke
Whilst this study is true it would also be true to say that the Moon has an atmosphere. The total atmosphere on the Moon is around 25 tonnes, as calculates by NASA. 25 tonnes spread out over the surface of the Moon is a miniscule amount that, for any practical purposes, is not worth taking into account. NASA's space lab's life was shortened considerable because the sun warmed the earths atmosphere more than was expected. It was positioned at a height of only a couple of hundred miles. The ISS is at around 400 miles where there is negligible effects of the earths atmosphere. So again, while this study is presenting true facts, they are really not worth anything.
DavidB
First, Mr. Clark, there are no circumstances in which facts have no worth. Science is about finding what's true, even if it's just for the sake of knowledge itself. Some knowledge lies unused for years before suddenly contributing to a significant breakthrough. Second, "true facts" is redundant and thus, one might say, "really not worth anything," because what isn't true isn't a fact, and what is true is a fact.
Username
It would help to clarify discussions if there was a concise definition of what constitutes an atmosphere.
Douglas Bennett Rogers
The backside of the Earth must be a "hydrogen shadow" since the interstellar medium has several hydrogen atoms per cubic centimeter.
Thinker
Ok. If space is a vacuum, as stated in the article, does that mean like a vacuum cleaner. If it is, when a rocket penetrates earth’s atmosphere, punches a hole in it why doesn’t it suck out our atmosphere? What effects do the continual penetration of rockets have on the earths atmosphere? ... wondering.
Robert in Vancouver
I very much agree with DavidB. All factual knowledge is or will be useful. I'm glad that real and honest researchers never say "the science is settled" and "you are a denier" if you ask anymore questions. Real and honest researchers never stop probing, digging, calculating, thinking, and asking questions. For example, unlike the theory of Man Made Global Warming, the theory of gravity is still being studied, tested, and debated.
Daishi
It is useful knowledge. The research has implications in our ability to try to terraform mars. There is concern that the atmosphere on mars if we were to improve it would get blown aware by solar winds and solar flares making the task of adding more atmosphere impossible. The fact that very thin atmosphere at 391,000 miles hasn't been blown away yet by solar winds gives some hope that improving the atmosphere of mars may not be impossible.