Does Mercury shrink every day?

We’re Sending Another Probe to Mercury to Work Out Why The Planet’s Shrinking

Mercury shot by Messenger, enhanced to show rock types. Credit: NASA/JPL

The European Space Agency (ESA) and Japan Aerospace Exploration Agency (JAXA) have unveiled the probe they’re sending to study Mercury in 2018, tasked with figuring out why the smallest planet in the Solar System appears to be shrinking.

The BepiColombo spacecraft is also going to be tasked with looking for water ice at Mercury’s poles and in its volcanoes, which should give us more clues about the composition and evolution of the planet.

«Mercury is the least explored of the rocky planets, but not because it is uninteresting,» says the head of the ESA, Alvaro Giménez Cañete. «It’s because it’s difficult – difficult to get there, even more difficult to work there.»

Mercury lies some 77 million kilometres (or 48 million miles) away from Earth, but that’s not as much of a problem for scientists as the planet’s very thin atmosphere, which makes slowing down a probe hurtling through space very difficult.

To put the brakes on as much as possible before it gets to Mercury, BepiColombo will go through a total of nine fly-bys – one around Earth, two around Venus, and six around Mercury – to use up some of its energy.

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Rendering of BepiColombo in flight. Credit: ESA/ATG medialab

By the time the probe settles down into orbit, sometime in 2025, it will have covered a distance equivalent to going around the Solar System 18.5 times.

Once in position, BepiColombo will need to survive through the extreme temperatures of Mercury, which can range from -170 degrees Celsius (-280 degrees Fahrenheit) during the night to 430 degrees Celsius (800 degrees Fahrenheit) during the day.

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«It’s like operating a spacecraft in a pizza oven,» says BepiColombo project manager Ulrich Reininghaus of the ESA.

The spacecraft is actually going to split up into two probes when in orbit, one made by the ESA and one made by JAXA.

Then the real scientific work can begin, building on data collected by the two previous probes to visit the planet: Mariner 10, which arrived in 1974, and Messenger, which arrived in 2008 and has given us our best pictures of the planet.

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BepiColombo ready to go. Credit: ESA–C. Carreau, CC BY-SA 3.0 IGO

Measurements of the surface suggest Mercury is tectonically active and shrinking still today as its core cools – and scientists want to know why.

They also want to look for evidence of water ice hidden away in the shadowy craters and volcanoes of Mercury, protected from the fierce solar glare and heat. If BepiColombo can take chemical measurements from the ice, it might give us a better idea of how Mercury was formed in the first place.

Mercury is also special because it sits so deep in the Sun’s gravitational field. Scientists want to use BepiColombo to test Einstein’s general theory of relativity with up to 100 times more accuracy than they can here on Earth.

Another mystery is why Mercury has a large iron core topped with a thin layer of silicate rocks. The hypotheses that the Sun eroded some of its outer layers, or that they were knocked off by a collision with another planet, don’t match up with the surface scans that Messenger took.

By using more accurate instruments and taking a closer look at Mercury, the experts at ESA and JAXA are hoping to get answers to some of these questions, and we can’t wait to see what it’s going to find.

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If you want to see the ESA briefing in full it’s available below:

Question Why is Pluto no longer a planet?

The International Astronomical Union (IAU) downgraded the status of Pluto to that of a dwarf planet because it did not meet the three criteria the IAU uses to define a full-sized planet. Essentially Pluto meets all the criteria except one—it “has not cleared its neighboring region of other objects.”

In August 2006 the International Astronomical Union (IAU) downgraded the status of Pluto to that of “dwarf planet.” This means that from now on only the rocky worlds of the inner Solar System and the gas giants of the outer system will be designated as planets. The “inner Solar System” is the region of space that is smaller than the radius of Jupiter’s orbit around the sun. It contains the asteroid belt as well as the terrestrial planets, Mercury, Venus, Earth, and Mars. The “gas giants” of course are Jupiter, Saturn, Neptune, and Uranus. So now we have eight planets instead of the nine we used to have.

What is a Dwarf Planet?

A “dwarf planet,” as defined by the IAU, is a celestial body in direct orbit of the Sun that is massive enough that its shape is controlled by gravitational forces rather than mechanical forces (and is thus ellipsoid in shape), but has not cleared its neighboring region of other objects.

So, the three criteria of the IAU for a full-sized planet are:

It is in orbit around the Sun.
It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
It has “cleared the neighborhood” around its orbit.

Pluto meets only two of these criteria, losing out on the third. In all the billions of years it has lived there, it has not managed to clear its neighborhood. You may wonder what that means, “not clearing its neighboring region of other objects?” Sounds like a minesweeper in space! This means that the planet has become gravitationally dominant — there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence, in its vicinity in space.

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So any large body that does not meet these criteria is now classed as a “dwarf planet,” and that includes Pluto, which shares its orbital neighborhood with Kuiper belt objects such as the plutinos.

History of Pluto

The object formerly known as the planet Pluto was discovered on February 18, 1930 at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh, with contributions from William H. Pickering. This period in astronomy was one of intense planet hunting, and Pickering was a prolific planet predictor.

In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed “Planet X.” By 1909, Lowell and Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted the search until his death in 1916, to no avail. Unknown to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but they were not recognized for what they were. Lowell was not the first to unknowingly photograph Pluto. There are sixteen known pre-discoveries, with the oldest being made by the Yerkes Observatory on August 20, 1909.

The search for Planet X did not resume until 1929, when the job was handed to Clyde Tombaugh, a 23-year-old Kansan who had just arrived at the Lowell Observatory. Tombaugh’s task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.

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The discovery made headlines across the globe. The Lowell Observatory, which had the right to name the new object, received over 1,000 suggestions from all over the world; the name Pluto was proposed by Venetia Burney, an eleven-year-old schoolgirl in Oxford, England. Venetia was interested in classical mythology as well as astronomy, and considered the name for the god of the underworld appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford’s Bodleian Library. Madan passed the name to Professor Herbert Hall Turner, who then cabled it to colleagues in the United States. Pluto officially became Pluto on March 24, 1930. The name was announced on May 1, 1930, and Venetia received five pounds (£5) as a reward.

Published: 11/19/2019. Last Updated: 3/2/2023. Author: Science Reference Section, Library of Congress

Related Websites

International Astronomical Union (IAU): Pluto and the Developing Landscape of our Solar System External — A discussion about Pluto from IAU that includes a history, references to how a planet is defined and a link to the report on the final resolution. Also included are questions and answers about Planets, Dwarf Planets, and Small Solar System Bodies.
The Girl Who Named a Planet External — This is an article about Venetia (Burney) Phair, the girl who named the planet Pluto.
NOVA: The Pluto Files External — Watch the PBS program which features Neil deGrasse Tyson exploring the rise and fall of America’s favorite planet.
Solar System Exploration: Pluto — NASA provides an abundance of information about Pluto such as facts, images, headline news, and a video.