The DART Mission: A Test to Change an Object’s Orbit

By Betty Robinson,

Over 4.5 billion years ago, the solar system was filled with billions of pieces of rocky debris. Over time, this debris coalesced into the solar system components we know today: the planets, moons, asteroids, comets, and other objects.

Today, there is still debris in space. While the pieces of rocky debris are nowhere near as numerous as they were billions of years ago, there are still asteroids and other rocky bits that approach Earth. These objects are called near-Earth objects, and the larger ones (140 metres in diameter) are definitely of concern. They are fast-moving, and when they hit another object, they could cause much harm. One such event 60 million years ago wiped out the dinosaurs! Scientists are looking for ways to protect Earth from these near-Earth objects.

There are four main ways to protect Earth from near-Earth objects under study at the moment:

      1. For a predicted small impact, clear the area of people.
      2. For predicted larger impacts, try to gradually change the object’s orbit with small nudges so it doesn’t hit Earth at all.
      3. Try to quickly change the object’s orbit by giving it a huge nudge so it doesn’t hit Earth at all.
      4. More or less blow it up.

A NASA mission is underway right now to test the nudging option. The mission is called DART, or the Double Asteroid Redirection Test. Launched on November 24, 2021, DART is the first demonstration of trying to change the orbit of a celestial object. DART is headed for an asteroid called Didymos. This asteroid has a moon called Dimorphos, about 1.2 kilometres away.

Didymos is Greek for “twin,” and Dimorphos is Greek for “two forms.” This refers to the change in the orbit of Dimorphos before and after DART collides with it.

Dimorphos is about 160 metres in diameter. For scale, the height of the Skylon Tower in Niagara Falls, Ontario, is 158 metres. And the diameter of Didymos is about 780 metres, about the height of five stacked Skylon Towers.

DART will crash into Dimorphos in the fall of 2022. A camera onboard DART will photograph the collision, as will a tiny satellite, or CubeSat. DART will release the CubeSat 10 days before the impact so that it will be trailing DART and can watch the whole thing. The collision is predicted to increase the speed of Dimorphos around Didymos, which will shorten the time it takes to orbit Didymos by up to 10 minutes. So the orbit will change. And it is possible that the energy from the collision could make Dimorphos unstable and start to tumble. Scientists haven’t yet determined the precise shape and composition of Dimorphos, but it could be a pile of rubble, like Didymos may be.


Artist’s conception of DART and the CubeSat approaching Dimorphos. DART will hit Dimorphos head-on at a speed of about 23,760 kilometres per hour. Credit: NASA/Johns Hopkins Applied Physics Lab (Note: The drawing is not to scale. DART is about the size of a small car.)

As a follow-up, in 2024 the European Space Agency (ESA) will launch a spacecraft called Hera. (Hera is the Greek goddess of marriage.) Hera will study the Didymos–Dimorphos system to see the effects of the DART collision. Hera’s mission will also include CubeSats, which will do the close-up observations of Dimorphos.

The ESA and NASA missions reinforce the need for, and importance of, international co-operation to help protect our planet.

 The results of this demonstration will show us if we can, in fact, divert an asteroid that is on a collision course with Earth and do what the dinosaurs could not: avert a species-ending disaster.


Progress Toward an Invisibility Cloak?

By Betty Robinson,

With starship cloaking technology and Harry Potter’s invisibility cloak, the ability to hide in plain sight has been fiction. But different technologies are being developed to turn science fiction into reality. Some of the reasons why we would want to do this are just because it’s fun and for military advantages. There are some potential medical applications—for instance, getting the right cloaking device could allow surgeons to see through their hands and work on something hard to reach or see. And no doubt there will be uses no one has thought of yet.

One of the technologies is at the nanoscale and involves materials science. Another demonstrates the effect with optical lenses. Whatever the technology, it comes down to light. If light reflected from an object doesn’t reach our eyes, then we can’t see the object.

The cloaking device I’m demonstrating here is called the Rochester Cloak, developed at the University of Rochester by John Howell and Joseph Choi. The great guidelines that I used can be found here: This device uses lenses to manipulate light so that it does not interact with the object to be cloaked—light doesn’t hit the object, light can’t reflect off the object to our eyes, so we can’t see it. But we can still see the background.

I just set up the device according to the instructions in the above pdf; there is even a suggested shopping list and suggested retailer (; an American company, but they ship to Canada). The focal lengths of the lenses are key to the Rochester Cloak, so you have to know their values—the focal lengths of the first two lenses must meet, and the same for the other two lenses. And you have to know the focal lengths to work out the distances between the lenses. But the pdf above spells it all out very clearly.

You set up four convex lenses, with two different focal lengths, according to a formula provided in the pdf. The lenses with the shorter focal length (thicker lenses) are in the middle; the lenses with the longer focal length (thinner lenses) are on the ends. And you put a background of some kind at the end of the setup. It’s good to use something with a grid, like graph paper, so that you can see there is no distortion of the background.

Pockets, or cloaked regions, are formed where the light coming through the lens doesn’t interact with anything placed in it. In the illustration below, these are the areas with red squiggles. So if you place an object in the cloaked region, the light doesn’t interact with it, and we just see the background. No interaction of light with the object, no light reaching our eyes, no object to see.

The blue lines are rays of light refracting through the lenses. The rays cross where the two focal points meet. The areas with the red squiggles are the cloaked areas. The light coming through the lenses does not pass through these areas so we can’t see an object placed in the cloaked areas. The cloaked areas are above and below the centre line. Objects placed in the centre line do not get cloaked.



Migrating Monarchs

By Betty Robinson,

Around this time every year, monarch butterflies in Canada and the eastern United States begin their journey southward to spend the winter in Mexico, where they will arrive in November. Monarchs on the west side of Canada and the United States migrate to California. Monarchs are the only butterflies known to make a north-south migration.

A beautiful monarch butterfly. Photo by Sid Modsell, Creative Commons

Monarch migration is a remarkable feat for many reasons.

Every year, there are four generations of monarchs. Non-migrating adult monarchs only live for about two to six weeks. But the last generation of the year—the generation that migrates to Mexico, spends the winter, and starts heading north again—will have been alive for about eight months!

So, how do the monarchs alive now know they are supposed to migrate, and not other monarchs born earlier in the year? In the monarchs alive now, genes get expressed by a combination of triggers (not one main trigger): an internal clock in the monarch, Earth’s magnetic field, and the position of the Sun. As a result of the expressed genes, the migrating generation of monarchs needs less oxygen. Further, the monarchs stop mating until it’s time to head back north in the late winter/early spring. At that time, they start mating and laying eggs in milkweed plants on their way back. The new monarchs will continue to head north, and the cycle continues.

How can they fly around 5,000 kilometres? They’re so small—they weigh less than a gram. A comparison of monarchs migrating from different areas, as well as monarchs that don’t migrate at all, showed that over time, the butterflies have adapted to typical conditions: the monarchs that do the long-distance migration from Canada to Mexico have larger bodies than the non-migrating monarchs that live in Puerto Rico, Hawaii, and other tropical regions. The migrating monarchs also have larger bodies and more angular forewings.

And the monarchs that live in western North America have bodies and wings more suited to gliding.

Another interesting feature of migrating monarchs is their path: the monarchs heading to Mexico follow a few defined, separate paths, but the paths all unite near southern/central Texas. And when the monarchs head north again, they follow the same route: one path until southern/central Texas, then they diverge and follow the paths that took them south in the fall. The generations of monarchs heading north didn’t migrate south, yet they know which path to take.

When the monarchs arrive in Mexico, they return to the same dozen or so spots in oyamel forests on the hillsides of mountains that are about 2 to 4 kilometres above sea level, in Michoacan. The coolish temperature there (0 to 15 degrees C) and the humidity allow them to survive without losing too much energy. They need to conserve energy because they don’t feed a lot while in Mexico; they live on their stored energy. So migrating to a spot that allows them to conserve energy is ideal, because they need the energy to begin their northward migration.

You can take part in monarch migration by visiting Journey North, at Help track their migration by reporting your monarch sightings.

Where Did Earth’s Water Come From?

By Betty Robinson,

Our solar system formed around 4.6 billion years ago, and evidence suggests that abundant liquid water appeared about 4.3 billion years ago, so pretty quickly.

There have been many theories on how solar systems form. And as our discoveries of more and more planets around other stars continue, the theories must be modified—sometimes the distances from the star of the new planets don’t fit with our models.

The prevailing theory links formation of the solar system bodies (e.g., planets, dwarf planets, asteroids) with star formation, in our case, the formation of the Sun. According to this theory, the Sun formed within a huge, messy sphere of gas and dust grains. Everything in space spins, so the sphere kept spinning and eventually gravity caused most of the gas and dust grains to coalesce into a pancake-like disc with the Sun at the centre. Over time and through countless collisions of gas and dust particles, bodies formed and became the planets.

An artist’s conception of the swirling gas and dust during the formation of the solar system. Credit: NASA/JPL-Caltech

Having planets form from gas and dust grains doesn’t scream water. So how did our planet come to be so water-rich? For a long time, the major theory was that Earth’s water came as a result of collisions of comets with Earth. Comets are basically dirty snowballs; they contain different types of ice, but about 80% is water ice. And we know that Earth has been hit by all kinds of debris—including comets and asteroids—throughout its 4.6 billion-year history.

A more recent theory suggests that asteroid collisions with Earth may have been responsible for the abundance of water on our planet. Through analysis of different asteroids, scientists discovered that a certain class of asteroid, called carbonaceous chondrites, has levels of hydrogen and nitrogen isotopes that are similar to those found in ocean water on Earth. (Isotopes are variations of atoms.) And carbonaceous chondrites are among the oldest types of asteroids in the solar system.

But now there is a new theory for where the water came from. The theory is from several researchers from around the world, including Michel Fich, of the University of Waterloo.

Professor Fich and colleagues used data from the Herschel Space Observatory to study regions in space where stars are forming. Turns out that water molecules form in regions like this. The hot water molecules get lost during star formation—they get pushed out into space. But the cooler water molecules, both cool water and ice, stay behind. The icy molecules form layers around the dust grains. Then, as outlined in the formation of the solar system above, innumerable collisions cause all the particles to coalesce and eventually form planets and other bodies.

So, the water has been there since the beginning! In theory, new solar systems contain enough water that their planets could become habitable.

The European Space Agency’s Herschel Space Observatory studied star-forming regions areas in the infrared. Credit: ESA

Professor Fich and graduate student Mollie Conrad contributed to this research by studying Herschel data from the nebula called NGC 7129, which is a star-forming region in the constellation called Cepheus. Credit: SIRTF/NASA

What Happened to the Liquid Water on Mars?

By Betty Robinson,

The geology on Mars definitely shows evidence that Mars likely supported liquid water in the past. Scientists know that the early Martian atmosphere was thick, which allowed the planet to keep warm enough to allow liquid water. But scientists also know that once Mars lost its atmosphere, the liquid water took a while to disappear. Why? This question is called the Mars climate puzzle. (Of course, the bigger question is, was there life, because water on Earth means life, but that’s for another blog posting if any when any evidence comes in.)

The surface of Mars shows clear evidence of previously flowing water. Shown here are possible clay beds in West Ladon Valles Channels, Mars. Credit: NASA/JPL/University of Arizona

Liquid water on Mars was directly dependent on Mars’s early atmosphere and indirectly dependent on its early, global magnetic field.

About 4 billion years ago, Mars’s atmosphere was much thicker than it is now. And it contained much more carbon dioxide (and other gases). Carbon dioxide is a greenhouse gas, which helps warm the planet. A warm planet can support liquid water (as long as other necessary conditions are also supported).

Scientists theorize that our and Mars’s magnetic fields formed in the same way: Oversimplifying, our own magnetic field is generated as a result of the dynamo effect: the hot, liquid outer core moves around the hot, solid inner core. The movement is opposite in direction to the direction of Earth’s rotation. The movement generates a magnetic field.

Mars is smaller and less dense than Earth, so Mars cooled faster. There is still some question regarding the composition of Mars’s core. But many think that the core has cooled enough that it can no longer generate a magnetic field. So, Mars’s magnetic field disappeared about 4 billion years ago, give or take half a billion years.


Around that same time, the Sun was young, and the solar wind (made up of charged particles) was more intense. Without the magnetic field to deflect the charged particles and protect the planet, the solar wind started stripping the atmospheric particles away, and Mars’s atmosphere started to disappear into space.

So no magnetic field led to no atmosphere. No atmosphere led to no liquid water, eventually.

So, the atmosphere started to thin and disappear, but the water kept flowing and didn’t dry up right away. Why? Why didn’t all the water disappear when the atmosphere disappeared?

According to studies by two independent teams, the clue may be hydrogen. A young Mars was volcanically active, so volcanoes would have spewed a lot of hydrogen into the atmosphere.

One study was in 2018 by a team led by Paul Godin, who at the time was a York University Postdoctoral Fellow. He is currently a senior technologist at the University of Waterloo. Dr. Godin used an instrument at the Canadian Light Source*, Saskatoon, to test a theory based on collision-induced absorption. Molecules have their own absorption properties. Sometimes, two molecules in the atmosphere collide and produce a supermolecule. The supermolecule has its own absorption properties. The theory is that enough of these supermolecules in the thinning Martian atmosphere could have absorbed enough heat to keep the planet warm in order that the liquid water could remain for a while. So maybe the atmosphere didn’t have to be super thick if there were lots of these supermolecules to absorb heat and keep Mars warm.

The team used an instrument called a White cell to bounce light around within a gas, to measure the gas’s absorption properties. They found that supermolecules made of carbon dioxide and hydrogen are weak but could be strong enough to make a difference.

A second team used data obtained by Curiosity in Gale Crater. The full results are reported in a paper in PGR: Planets: Navarro-González et al., published in 2018. In this study, Navarro-González et al. found that rock samples analyzed by Curiosity contained fixed nitrogen**. On Earth, bacteria fix nitrogen. But there are no plants on Mars, so physical processes with a lot of kinetic energy, such as lightning and shock waves from asteroid impacts, likely fixed the nitrogen in the rocks. However, this fixing process can only happen in an atmosphere that is loaded with hydrogen.

In other samples analyzed by Curiosity—younger samples—they found that the amount of nitrogen decreased significantly, suggesting that once the hydrogen was gone, the planet could no longer support liquid water.

So maybe hydrogen is the key to understanding why liquid water remained on Mars after the planet lost its atmosphere.

* Canadian Light Source: The Canadian Light Source is Canada’s only synchrotron. It is located in Saskatoon, Saskatchewan ( A synchrotron accelerates electrons to near the speed of light. As the electrons accelerate while changing direction (they move in a circle), they emit a bright light. The light is used to study various samples of materials at the molecular level. The facility has other instruments, such as the White cell mentioned in this piece.

** Nitrogen fixation: Nitrogen fixation is the process of combining nitrogen from the air with another element or elements to form a different form of nitrogen, such as ammonia (NH3). Most nitrogen fixation is done by bacteria, but ultraviolet light and other physical processes such as lightning can also fix nitrogen.

The bacteria incorporate nitrogen from the air into compounds that they can use. Credit: Nefronus – Own work, CC BY-SA 4.0,

The James Webb Space Telescope and Lagrangian Points

By Betty Robinson,

The James Webb Space Telescope. Credit: NASA

The James Webb Space Telescope (JWST) is currently scheduled for launch on October 31, 2021. JWST is an infrared telescope that will carry on from Hubble. It will detect the infrared, or heat, signals from distant objects in space. JWST will operate at L2. L2 is a Lagrangian point.

Lagrangian points are locations in space associated with one small and two large bodies in a system, for example, the Sun, Earth, and a satellite. At a Lagrangian point, the gravitational attraction of the two large bodies equals the centripetal force required for the smaller object to be stationary relative to the other two bodies. So a satellite (or other object) at a Lagrangian point stays in the same spot relative to the two large bodies.

A two-body system has five Lagrangian points. The Swiss mathematician Leonhard Euler predicted the existence of L1, L2, and L3 around 1750. French astronomer Joseph Louis de Lagrange predicted the existence of the other two, L4 and L5, in 1772.

The points L1 and L2 are the same distance from Earth, about 1.5 million kilometres.

The Lagrangian points are useful for space exploration. As you can see in the diagram, L1 is between the Sun and Earth (the Moon is included with Earth). Satellites at L1 can continuously observe the Sun, so a number of Sun-observing satellites are positioned here, for example, the Solar and Heliospheric Observatory (SOHO). At L2, satellites can continuously observe deep space, and they are far enough away from Earth’s magnetosphere to avoid interference but close to enough to be able to communicate with Earth. JWST will be at L2. The WMAP observatory is located at L2, and Planck is currently there. (WMAP is the Wilkinson Microwave Anisotropy Probe used to study cosmology, and the Planck space probe also studied cosmology. Both are currently inactive.)

There are no uses for L3 at the moment because it is always behind the Sun for us. However, there have been suggestions for observations of the Sun at this point. For example, a satellite that monitors evolving sunspots could provide valuable advance notice before the sunspots rotated to the Earth side, about 7 days later (assuming there are some communications satellites to support it).

Points L1, L2, and L3 are unstable, and all satellites at these points must orbit the points in space (called halo orbits) and make course corrections to stay there. However, points L4 and L5 are stable. Because of their stability, debris such as dust and asteroids tend to collect at L4 and L5. (They don’t actually sit at the point; they librate around the point in space.)

Asteroids that settle at Lagrangian points are called Trojan asteroids. There are several thousand Trojan asteroids at L4 and L5 of the Sun–Jupiter system. Mars, Neptune, and some of Saturn’s moons have Trojan asteroids. There is only one known Trojan asteroid in the Sun–Earth system: asteroid 2010 TK7 was discovered at L4 in 2010 by astronomers using data from the space telescope WISE (Wide-field Infrared Survey Explorer).

The Magellanic Clouds

By Betty Robinson,

Our home galaxy, the Milky Way, has two satellite galaxies in orbit about it. They don’t look like the typical pinwheel, or spiral, images of galaxies we are familiar with. The satellite galaxies look like clouds, and they have come to be known as the Magellanic Clouds, named after the Portuguese explorer Ferdinand Magellan. While in the Southern Hemisphere during Magellan’s first trip around the world, from 1519 to 1522, he and the crew observed these celestial objects. However, the Indigenous peoples in the Southern Hemisphere had been observing them for thousands of years. They are known simply as the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). They can only be seen in the Southern Hemisphere, and you don’t need binoculars to see them! Scientists think that the SMC is actually orbiting the LMC.

The bright object to the left of the Small Magellanic Cloud is a globular star cluster in our own galaxy. The star cluster is called 47 Tucanae. Credit: ESO/S. Brunier – ESO, CC BY 4.0,

The Magellanic Clouds are among the closest galaxies to the Milky Way: The LMC is about 160,000 light years away from us, and the SMC is farther, about 190,000 years away. (The closest galaxy is the Sagittarius Dwarf Elliptical Galaxy, at 65,230 light years.) They are about 75,000 light years apart. Recall that a light year is the distance that light travels in one year, about 9.7 trillion kilometres. They are a lot smaller than the Milky Way, too. The Milky Way is about 20 times the diameter of the SMC, and about 10 times the diameter of the LMC.

This manipulated image shows where the Magellanic Clouds are in relation to the Milky Way. If we could look down on everything from above, this is what we would see. Credit: Nina McCurdy / Nick Risinger / NASA. Not to scale.

Because of their loosely defined shape and their size, the Magellanic Clouds are classified as irregular dwarf galaxies. The Milky Way is classified as a spiral galaxy. Another galaxy shape is elliptical. There are further subclassifications.

While the Magellanic Clouds are thought to have been formed at the same time as our own galaxy, about 14 billion years ago, they have only been in orbit around the Milky Way for about 1.5 billion years. The Milky Way’s gravity would have “captured” the two galaxies. In fact, for the clouds to orbit the Milky Way, it would take about 4 billion years. Since they have only been in orbit for 1.5 billion years, we might be seeing them after their initial capture. They may not have even made one complete orbit yet.

Further, there are gravitational, or tidal, forces at work, that result in a lot of tugging and pulling between the two clouds and between the clouds and the Milky Way. Scientists think the LMC-SMC system could be on a collision course with the Milky Way, although the collision won’t happen for another 2.4 billion years. The collision could disturb the supermassive black hole at the centre of our galaxy, causing it to consume more gas and other matter and increase in size. Stars closer to the black hole could get kicked out of the galaxy.

So, if something is classified as a galaxy, does it automatically have the same composition as all other galaxies? Nope. Apart from shape and size, the Magellanic Clouds differ in two more important ways from the Milky Way: the clouds have more hydrogen and helium than our home galaxy does, but less metal. There is a similarity, though: the Magellanic Clouds and the Milky Way both contain a range of very young stars to very old stars. This points to a long history of stellar formation.

In February 1987, Canadian astronomer Ian Shelton discovered a supernova in the LMC from the University of Toronto observatory in Chile. This is the closest supernova visible for study since the invention of the telescope. Astronomers continue to observe the supernova remnant with ground-based telescopes and telescopes in space.

Mercury, a Planet of Extremes and More

By Betty Robinson,

Mercury has been called a planet of extremes for many reasons. Of all the planets,

    • it’s the closest to the Sun.
    • it’s the smallest; in fact, it’s just a little larger than the Moon.
    • it’s the fastest; it only takes 88 Earth days to go around the Sun once.
    • it has the longest day: it takes 58.6 Earth days for Mercury to rotate once.
    • it has the smallest axis tilt, less than a degree; so it’s upright.
    • it has the most elliptical orbit. (See the table and diagram in Extra Information at the end.)
    • it has the highest orbital inclination. (See the table and diagram in Extra Information at the end.)

The planet Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie


Mercury doesn’t have an atmosphere per se. There are atmospheric gases—oxygen, sodium, hydrogen, helium, and other gases—but they form a thin, inconsistent exosphere above the planet. An exosphere is the layer of atmosphere farthest from the surface of a planet. The space between the exosphere and Mercury’s surface is a vacuum.

Mercury is not volcanically active, there is no vegetation, no life, so how does Mercury keep adding gases to its exosphere? And Mercury’s gravity is low, so how can Mercury hold its exosphere in place?

Some impacts from meteoroids vaporize some of materials from the meteoroids themselves, as well some of Mercury’s surface rocks. The vaporized material goes off into the exosphere or off the planet. However, the major source of gases is through the interaction of Mercury’s magnetic field and the solar wind; the magnetic field also helps keep the tenuous exosphere in place. See below.

Magnetic Field

I first became interested in Mercury when I learned about its gravitational field. The strength of Mercury’s magnetic field is about 1% the strength of Earth’s magnetic field, so not strong at all. In addition, Mercury’s magnetic field is really offset: it’s three times stronger in the northern hemisphere than in the southern hemisphere. Earth’s magnetic field is more or less consistent in both hemispheres.

The solar wind is a thin plasma (charged particles), and it is more intense near the Sun than farther out. So, because Mercury is so close to the Sun, Mercury’s magnetic field interacts a fair amount with the fast-moving solar wind. Sometimes, as a result of this interaction, magnetic tornadoes form. These tornadoes aren’t like tornadoes on Earth: On Mercury, there is no dust, no air, no clouds, no weather. Magnetic tornadoes are invisible. They carry the solar wind plasma to the surface of the planet. When the charged particles in the plasma hit the surface, they cause atoms to fly off the surface into space and into the exosphere.

Double Sunrise/Sunset at Perihelion and Retrograde Motion of the Sun

As I read more about this planet, I became more intrigued. At one point in Mercury’s orbit around the Sun, the Sun undergoes retrograde motion: for an observer on Mercury’s equator about to experience daylight, the Sun rises, sets, and rises again. That point is when Mercury is closest to the Sun, or perihelion. (The point at which a planet is farthest from the Sun is called aphelion. See the diagram showing perihelion and aphelion at the end.) For an observer about to experience nighttime, the opposite happens. Check out these animations of this phenomenon: (double sunrise) (double sunset)

An observer at noon, at perihelion, would see the Sun stop, move eastward, then westward.

If you could be on Mercury’s surface, at the equator, at noon, at perihelion, then you could see the Sun in retrograde motion. The Sun’s real motion does not change. This is just a phenomenon that we would see if we were in the right place at the right time.

This phenomenon only happens on Mercury. Why does this happen?

To start, here’s a reminder of how planets move, thanks to Johannes Kepler (1571–1630). Kepler identified three laws of planetary motion. We just need the first two, but I have included the third law for completeness:

Law 1: Planets move in ellipses (not perfect circles), with the Sun at one focus; that is, the Sun is not the centre of the ellipse. (See the table and diagram at the end.)

Law 2 (Summarized): Planets move fastest at perihelion and slowest at aphelion.

Law 3: The time, T, it takes a planet to complete one orbit, squared, is proportional to the mean distance, r, from the Sun to the planet, cubed: T 2 µ r3.

Now, let’s look at Mercury: As per law 1, when a planet is at perihelion, it is moving faster than at any other time in its orbit. At perihelion, Mercury is moving about 1.5 times faster than it moves at aphelion. Note: Mercury’s speed around the Sun changes, but its rate of rotation does not change. Mercury’s rotation is pretty slow, about 58 Earth days to rotate once.

Also, since it only takes Mercury 88 Earth days to go around the Sun once, and since Mercury’s rotation is so slow, the Sun is in the Mercury sky for 176 Earth days every year (Mercury’s solar day). The 176 days is about three times longer than the time it takes for Mercury to rotate once on its axis.

Next, let’s look at the Sun from Mercury: As Mercury goes around the Sun, the Sun is in the sky for 176 days. During that time, the Sun moves, on average (remember how Mercury changes speed at different points), 2° west every Earth day. At aphelion, Mercury has slowed down, so the Sun’s movement appears to speed up a bit, to about 3° west every day.

Now as Mercury gets closer to perihelion, it starts to speed up, but its slow rotation doesn’t change. The opposite to what happens at aphelion happens at perihelion: Mercury has sped up, so the Sun’s movement appears to slow. As Mercury moves close to perihelion, arrives at perihelion, and moves past perihelion, the Sun stops, then moves eastward instead of westward, and starts moving westward again. As Mercury continues to slow down after perihelion, the Sun’s apparent movement returns to about 2° west until Mercury reaches aphelion again, when the Sun speeds up to about 3° west every day.

So over a period of about 8 days before and after perihelion, an observer on the equator, at the terminator on Mercury, would see the Sun rise in the east, set, then rise again.

And throughout the entire orbit, the size of the Sun from Mercury appears to change: On average, the Sun appears to be over three times larger than what we see on Earth. So at perihelion, it would appear larger. At aphelion, it would appear smaller. Of course, the size of the Sun does not actually change.

Mercury’s Tail

There is some sodium in Mercury’s exosphere, but not that much. What makes the sodium mentionable is the fact that it absorbs and emits light quite easily, so it’s more visible than other elements. The solar wind is constantly interacting with Mercury’s magnetic field. The wind pushes gases, including sodium, from Mercury’s surface and exosphere away, and they form a tail behind the speeding planet. The ultraviolet light in sunlight causes the sodium to glow, or fluoresce, a yellowish colour, so under favourable conditions we can see a yellowish tail trailing behind Mercury.

Mercury’s tail. The sodium particles in the tail fluoresce, causing a yellowish glow. Credit: Andrea Alessandrini

Why Don’t We Send Probes to Mercury More Often?

Mercury is truly an intriguing planet. But so far, only two spacecraft have visited Mercury, both American: Mariner 10 (1973–1975) was the first spacecraft to study Mercury, and it only took 147 days to get there. However, Mariner 10 did not go into orbit around Mercury; it just did flybys. Messenger (2004–2015) was an outstanding mission. It took seven years to get to Mercury. The difference here is that Messenger didn’t just do flybys; it spent four years in orbit around Mercury.

The next mission to Mercury is the ESA spacecraft called BepiColombo. It was launched in October 2018 and will begin orbiting Mercury in December 2025, another seven-year mission. I’ll be following BepiColombo for sure.

An engineering model of BepiColombo, seen in the Science Museum, London. Photo by Randy Attwood

So why haven’t there been more spacecraft sent to Mercury, especially compared with missions to planets beyond Earth?

Mercury is tough to get to. Imagine that you are at the top of a high hill and you start running down the hill. Halfway there you decide to stop running. You will likely be unsuccessful and go tumbling head over heel down the rest of the hill. This is a rough comparison to what it’s like to send a spacecraft to Mercury. Mercury is so close to the Sun that a spacecraft en route to this tiny planet has to more or less head straight for the Sun. The closer it gets, the more the Sun’s gravity pulls it in. So spacecraft heading to Mercury with the purpose of going into orbit around it need to use gravity effects from Earth, Venus, and Mercury to slow down. This help is called a gravity assist. To do that takes longer.

And once the spacecraft gets there, it has to be able to endure the extreme heat and cold: Mercury’s daytime side can reach a temperature of 430 °C; the nighttime side can reach –180 °C. So spacecraft designers face design challenges to protect the spacecraft as well as the electronics in the instruments.








Extra Information


Eccentricity, e, is a measure of how elliptical a shape is. The closer e is to 0, then the closer the shape is to being a circle. In the table below, we can see that Venus’s orbit is the closest one to a circle, and Mercury’s orbit is the most elliptical.

Left: a circle with 0 eccentricity; middle: an ellipse with an eccentricity of 0.2; right: an ellipse with an eccentricity of 0.3. Not to scale.

Source: RASC Observer’s Handbook 2021

Perihelion and Aphelion

At perihelion, a planet is closest to the Sun. At aphelion, a planet is farthest from the Sun. Not to scale. Exaggerated to highlight differences.

Orbital Inclination

If we could view the planet orbits from the side, we could see how much they are tilted.

The orbital inclination is how much the orbit is tilted. Earth’s orbit is almost completely flat. But Mercury’s orbit is tilted a fair amount.

Source: RASC Observer’s Handbook 2021