According to a new study published in the American Geophysical Union’s (AGU) Journal of Geophysical Research, the geocorona (the luminous part of the outermost region of the Earth’s atmosphere, the exosphere) reaches up to 630,000 kilometers (391,000 miles) away, or 50 times the diameter of our planet. For comparison, the average distance between Earth and the Moon is 384,400 km (238,855 miles). So, the outer edge of Earth’s atmosphere extends far beyond the moon.

It is often very difficult to know where one thing ends and another begins in space. The atmosphere of the Earth is an example. It is a composition of various gases, so it expands when the constituent molecules have more energy to move around and be free from the grip of other molecules – unless its temperature is absolute zero (zero Kelvin, or -459.67 °F, or -273.15 °C), which is physically impossible-to-reach. On the other hand, the gravity of the Earth keeps most of the air around our planet.

Moon flies through Earth’s atmosphere

Previously, the upper limit of the outermost region of the Earth’s atmosphere, the exosphere was thought to be around 200,000 km (124,000 mi) from Earth. Beyond that point, solar radiation pressure would override Earth’s gravity.

But, according to data from the SOHO spacecraft (Solar and Heliospheric Observatory) co-owned by the European Space Agency and NASA, the geocorona extends out to as much as 630,000 km (391,000 miles), scientists have found. That means, our satellite, the Moon flies through Earth’s atmosphere.

The atmosphere of Earth reaches beyond the Moon
Earth’s atmosphere stretches out to the Moon – and beyond. Image Credit: ESA

SOHO is located 1.5 million kilometers from the Earth at the Earth-Sun L1 point (see notes 1). It was launched on December 2, 1995, to study the Sun. Originally planned as a two-year mission, SOHO continues to operate after over 20 years in space.

By positioning an onboard instrument called SWAN (Solar Wind Anisotropies) towards Earth at the right time of the year, SOHO is able to detect the hydrogen atoms in the upper atmosphere and measure their extent in what space scientists call geocorona, the luminous part of the outermost region of the Earth’s atmosphere, the exosphere.

The first telescope on the moon, placed by Apollo 16 astronauts in 1972, captured an evocative image of the geocorona surrounding Earth and glowing brightly in ultraviolet light. They didn’t even know they were still inside it.

SOHO made these observations and recorded the data over two decades ago, between 1996 and 1998. The data had just been sitting in an archive, waiting for someone to analyze.

Scientists from Russia’s Space Research Institute just did that, analyzed the data obtained by the SOHO spacecraft and discovered that the geocorona extends far beyond than previously thought.

Researchers also found that the geocorona is denser on Earth’s dayside, because of compression from solar radiation, 70 atoms per cm3, at 60,000 kilometers above Earth’s surface, and thinning out to 0.2 atoms at lunar orbit (still effectively a vacuum). So the geocorona looks like a comet’s tail – which always points away from the Sun.

On the night side, the hydrogen density is higher due to solar radiation pressure – it kind of ends up looking a comet tail.

Igor Baliukin of Russia’s Space Research Institute, lead author of the study, says “On Earth, we would call it vacuum, so this extra source of hydrogen is not significant enough to facilitate space exploration”.

The geocorona is also important for the search for extraterrestrial life. For planets with hydrogen in their exospheres, water vapor is often seen closer to their surface. That is the case for Earth, Mars, and Venus.

Co-author of the study, Jean-Loup Bertaux says: “This is especially interesting when looking for planets with potential reservoirs of water beyond our solar system.”


1. Lagrangian Points

In celestial mechanics, the Lagrangian points are positions in an orbital configuration of two large bodies where a small object affected only by gravity can maintain a stable position relative to the two large bodies.

The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centripetal force required to orbit with them.

There are five such points, labeled L1 to L5, all in the orbital plane of the two large bodies. The first three are on the line connecting the two large bodies and the last two, L4 and L5, form an equilateral triangle with the two large bodies. The two latter points are stable, which implies that objects can orbit around them in a rotating coordinate system tied to the two large bodies.

Several planets have minor planets near their L4 and L5 points (trojans) with respect to the Sun, with Jupiter in particular, having more than a million of these. Artificial satellites have been placed at L1 and L2 with respect to the Sun and Earth, and Earth and the Moon for various purposes, and the Lagrangian points have been proposed for a variety of future uses in space exploration.

Lagrange points in the Sun–Earth system
Lagrange points in the Sun-Earth system (not to scale).


M. Özgür Nevres

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