Guide to Space Archives

May 9, 2013March 16, 2017 by Fraser Cain

How Long Does it Take to Get to Mars?

This article originally appeared in Universe Today in July, 2012, but it’s been updated with a related video.

The planet Mars is one of the brightest objects in the night sky, easily visible with the unaided eye as a bright red star. Every two years or so, Mars and Earth reach their closest point, called “opposition”, when Mars can be as close as 55,000,000 km from Earth. And every two years, space agencies take advantage of this orbital alignment to send spacecraft to the Red Planet. How long does it take to get to Mars?

The total journey time from Earth to Mars takes between 150-300 days depending on the speed of the launch, the alignment of Earth and Mars, and the length of the journey the spacecraft takes to reach its target. It really just depends on how much fuel you’re willing to burn to get there. More fuel, shorter travel time.

History of Going to Mars:

The first spacecraft ever to make the journey from Earth to Mars was NASA’s Mariner 4, which launched on November 28, 1964 and arrived at Mars July 14, 1965, successfully taking a series of 21 photographs. Mariner 4’s total flight time was 228 days.

The next successful mission to Mars was Mariner 6, which blasted off on February 25, 1969 and reached the planet on July 31, 1969; a flight time of only 156 days. The successful Mariner 7 only required 131 days to make the journey.

*The NASA team threw in every bit of data they could to model the Mars Curiosity landing. Credit: NASA*

Mariner 9, the first spacecraft to successfully go into orbit around Mars launched on May 30, 1971, and arrived November 13, 1971 for a duration of 167 days. This is the same pattern that has held up for more almost 50 years of Mars exploration: approximately 150-300 days.

Here are some more examples:

Viking 1 (1976) – 335 days
Viking 2 (1976) – 360 days
Mars Reconnaissance Orbiter (2006) – 210 days
Phoenix Lander (2008) – 295 days
Curiosity Lander (2012) – 253 days

Why Does it Take So Long?:

A top-down image of the orbits of Earth and Mars. Image: NASA — *A top-down image of the orbits of Earth and Mars. Credit: NASA*

When you consider the fact that Mars is only 55 million km away, and the spacecraft are travelling in excess of 20,000 km/hour, you would expect the spacecraft to make the journey in about 115 days, but it takes much longer. This is because both Earth and Mars are orbiting around the Sun. You can’t point directly at Mars and start firing your rockets, because by the time you got there, Mars would have already moved. Instead, spacecraft launched from Earth need to be pointed at where Mars is going to be.

The other constraint is fuel. Again, if you had an unlimited amount of fuel, you’d point your spacecraft at Mars, fire your rockets to the halfway point of the journey, then turn around and decelerate for the last half of the journey. You could cut your travel time down to a fraction of the current rate – but you would need an impossible amount of fuel.

How to Get to Mars with the Least Amount of Fuel:

The primary concern of engineers is how to get a spacecraft to Mars, on the least amount of fuel. Robots don’t really care about the hostile environment of space, so it makes sense to decrease the launch costs of the rocket as much as possible.

NASA engineers use a method of travel called a Hohmann Transfer Orbit – or a Minimum Energy Transfer Orbit – to send a spacecraft from Earth to Mars with the least amount of fuel possible. The technique was first proposed by Walter Hohmann who published the first description of the maneuver in 1925.

Instead of pointing your rocket directly at Mars, you boost the orbit of your spacecraft so that it’s following a larger orbit around the Sun than the Earth. Eventually that orbit will intersect the orbit of Mars – at the exact moment that Mars is there too.

If you need to launch with less fuel, you just take longer to raise your orbit, and increase the journey to Mars.

Other Ideas to Decrease the Travel Time to Mars:

Although it requires some patience to wait for a spacecraft to travel 250 days to reach Mars, we might want a completely different propulsion method if we’re sending humans. Space is a hostile place, and the radiation of interplanetary space might pose a longterm health risk to human astronauts. The background cosmic rays inflict a constant barrage of cancer-inducing radiation, but there’s a bigger risk of massive solar storms, which could kill unprotected astronauts in a few hours. If you can decrease the travel time, you reduce the amount of time astronauts are getting pelted with radiation, and minimize the amount of supplies they need to carry for a return journey.

Go Nuclear:
One idea is nuclear rockets, which heat up a working fluid – like hydrogen – to intense temperatures in a nuclear reactor, and then blast it out a rocket nozzle at high velocities to create thrust. Because nuclear fuels are far more energy dense than chemical rockets, you could get a higher thrust velocity with less fuel. It’s proposed that a nuclear rocket could decrease the travel time down to about 7 months

Go Magnetic:
Another proposal is a technology called the Variable Specific Impulse Magnetoplasma Rocket (or VASIMR). This is an electromagnetic thruster which uses radio waves to ionize and heat a propellant. This creates an ionized gas called plasma which can be magnetically thrust out the back of the spacecraft at high velocities. Former astronaut Franklin Chang-Diaz is pioneering the development of this technology, and a prototype is expected to be installed on the International Space Station to help it maintain its altitude above Earth. In a mission to Mars, a VASIMR rocket could reduce the travel time down to 5 months.

Go Antimatter:
Perhaps one of the most extreme proposals would be to use an antimatter rocket. Created in particle accelerators, antimatter is the most dense fuel you could possibly use. When atoms of matter meet atoms of antimatter, they transform into pure energy, as predicted by Albert Einstein’s famous equation: E = mc². Just 10 milligrams of antimatter would be needed to propel a human mission to Mars in only 45 days. But then, producing even that minuscule amount of antimatter would cost about $250 million.

Artist's concept of Antimatter propulsion system. Credit: NASA/MFSC — *Artist’s concept of Antimatter propulsion system. Credit: NASA/MFSC*

Future Missions to Mars:

Even though some incredible technologies have been proposed to shorten the travel time to Mars, engineers will be using the tried and true methods of following minimum energy transfer orbits using chemical rockets. NASA’s MAVEN mission will launch in 2013 using this technique, as well ESA’s ExoMars missions. It might be a few decades before other methods become common techniques.

Research further:
Information about Interplanetary Orbits – NASA
7 Minutes of Terror – The Challenge of Landing at Mars
NASA Proposal for a nuclear rocket engine
Hohmann Transfer Orbits – Iowa State University
Minimum Transfers and Interplanetary Orbits
New and Improved Antimatter Space Ship for Mars Missions – NASA
Astronomy Cast Episode 84: Getting Around the Solar System

This article originally appeared in Universe Today in July, 2012, but it’s been updated with a related video.

April 14, 2013October 16, 2016 by Fraser Cain

How Long Does it Take Sunlight to Reach the Earth?

Here’s a question… how long does it take sunlight to reach Earth? This sounds like a strange question, but think about it. Sunlight travels at the speed of light. Photons emitted from the surface of the Sun need to travel across the vacuum of space to reach our eyes.

The short answer is that it takes sunlight an average of 8 minutes and 20 seconds to travel from the Sun to the Earth.

If the Sun suddenly disappeared from the Universe (not that this could actually happen, don’t panic), it would take a little more than 8 minutes before you realized it was time to put on a sweater.

Here’s the math. We orbit the Sun at a distance of about 150 million km. Light moves at 300,000 kilometers/second. Divide these and you get 500 seconds, or 8 minutes and 20 seconds.

This is an average number. Remember, the Earth follows an elliptical orbit around the Sun, ranging from 147 million to 152 million km. At its closest point, sunlight only takes 490 seconds to reach Earth. And then at the most distant point, it takes 507 seconds for sunlight to make the journey.

But the story of light gets even more interesting, when you think about the journey light needs to make inside the Sun.

You probably know that photons are created by fusion reactions inside the Sun’s core. They start off as gamma radiation and then are emitted and absorbed countless times in the Sun’s radiative zone, wandering around inside the massive star before they finally reach the surface.

What you probably don’t know, is that these photons striking your eyeballs were ACTUALLY created tens of thousands of years ago and it took that long for them to be emitted by the sun.

Once they escaped the surface, it was only a short 8 minutes for those photons to cross the vast distance from the Sun to the Earth

As you look outward into space, you’re actually looking backwards in time.

The light you see from your computer is nanoseconds old. The light reflected from the surface of the Moon takes only a second to reach Earth. The Sun is more than 8 light-minutes away. And so, if the light from the nearest star (Alpha Centauri) takes more than 4 years to reach us, we’re seeing that star 4 years in the past.

There are galaxies millions of light-years away, which means the light we’re seeing left the surface of those stars millions of years ago. For example, the galaxy M109 is located about 83.5 million light-years away.

If aliens lived in those galaxies, and had strong enough telescopes, they would see the Earth as it looked in the past. They might even see dinosaurs walking on the surface.

We have written many articles about the Sun for Universe Today. Here’s an article about the color of the Sun, and here are some interesting facts about the Sun.

If you’d like more info on the Sun, check out NASA’s Solar System Exploration Guide on the Sun, and here’s a link to the SOHO mission homepage, which has the latest images from the Sun.

We’ve also recorded an episode of Astronomy Cast all about the Sun. Listen here, Episode 30: The Sun, Spots and All.

Source: NASA

October 22, 2012December 23, 2015 by Jason Major

Valles Marineris: The Grandest Canyon of All

A digital terrain model of a portion of Mars’ Valles Marineris, the largest canyon in the Solar System. Credit: ESA/DLR/FU Berlin (G. Neukum)

Anyone who’s visited the Grand Canyon in Arizona can attest to its beauty, magnificence and sheer sense of awe that comes upon approaching its rim, whether for the first time or hundred-and-first. “Grand” almost seems too inferior a title for such an enormous geological feature — yet there’s a canyon much, much bigger stretching across the surface of Mars, one that could easily swallow all of our Grand Canyon within one of its side gullies.

The image above, released online for the first time today by ESA, is a digital terrain model of a portion of Mars’ Valles Marineris: our Solar System’s grandest canyon.
It’s easy to fall into hyperbole when describing Valles Marineris. Named for NASA’s Mariner 9 spacecraft, which became the first spacecraft to orbit Mars on November 14, 1971, the canyon is over 4000 km long, 200 km wide, and 10 km deep (2,480 x 125 x 6 miles) — that’s five times deeper than the Grand Canyon and long enough to stretch across the entire contiguous United States! It’s a rift unparalleled on any other world in the Solar System.

Valles Marineris is thought to be the result of the formation of the nearby Tharsis volcanic region, home to Olympus Mons, the Solar System’s largest volcano. As the region swelled with magma billions of years ago the planet’s crust stretched and split, collapsing into a vast, deep canyon.

Much later, landslides and flowing water would help erode the canyon’s steep walls and carve out meandering side channels.

The 45-degree view above was was made from data acquired during 20 individual orbits of ESA’s Mars Express. It is presented in near-true color with four times vertical exaggeration (to increase relief contrast.) Download a high-res JPEG version here.

The largest portion of the canyon seen crossing left to right is known as Melas Chasma. Candor Chasma is the connecting trough to the north, and Hebes Chasma is in the far top left.

Below is a video released by JPL in 2006 showing a virtual fly-through of Valles Marineris, shown as if you were on a Grand Canyon-style helicopter sightseeing tour (that is, if helicopters could even work in the thin Martian air!)

Hopefully someday we’ll be seeing actual videos taken above Valles Marineris and photos captured from its rim… perhaps even by human explorers! (Please exit through the gift shop.)

Image source: ESA. Video by Eric M. De Jong and Phil Christiansen et. al, Arizona State University.

July 8, 2012December 23, 2015 by Fraser Cain

Why Planets Orbit the Sun

Why Do Planets Orbit the Sun — The Solar System

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In ancient times, astronomers thought that all celestial objects – the Sun, Moon, planets and stars – orbited around the Earth in a series of crystal spheres. But as modern science developed, astronomers were better able to understand our place in the cosmos. They discovered that all the planets, including the Earth, actually orbit around the Sun.

Not only did scientists discover that the simple fact that the planets orbit the Sun, they uncovered the underlying reasons for why. What chain of events led us to our current Solar System, with planets orbiting the Sun?

Astronomers Used to Think the Earth was the Center of the Solar System
Because we live on Earth, and we see objects passing across our view of the skies, it’s natural to assume that the Earth is the center of the Universe. In fact, this perspective – known as geocentrism – was the default for all ancient civilizations. The Sun, the Moon, the planets and the stars appeared to move around the Earth each day. And because the Earth itself didn’t seem to be moving, astronomers like Ptolemy assumed that Earth was the center of the Universe. In fact, they went so far as to create very detailed models for predicting the motions of objects with a high degree of accuracy, using this completely inaccurate model of the Solar System. The predictions made by Ptolemy were used to make astrological predictions for more than 1500 years, until a much better model came along.

Actually, the Sun is the Center of the Solar System
A new, more accurate model of the Solar System didn’t come around until the 16th century, when the Polish astronomer Nicolai Copernicus published his Universe-changing book: On the Revolutions of the Heavenly Bodies. Copernicus accurately reorganized the Solar System, putting the Sun at the center in a heliocentric model. And the Earth took its proper place, as just another planet orbiting the Sun – one of the 6 known to astronomers at the time.

Copernicus’ model helped answer two questions which had troubled astronomers for centuries: why the planets brighten and dim over the course of several months (because they’re getting closer and further away), and why the planets seem to reverse and move in a retrograde direction. Easily explained because of the changing positions of the Earth, planets and the background stars.

But Why Do They Orbit the Sun?
Once they could accurately describe the nature of the planetary motion in the Solar System, they were left with a more fundimental question: Why do the planets orbit the Sun? What sequence of events led to the current motions of the planets around the Sun?

To explain this, we need to look back 4.6 billion years ago, before there was even a Solar System. In our place instead, there was a massive cloud of hydrogen gas left over from the Big Bang. Some event, like a nearby supernova explosion triggered a gravitational collapse of the cloud, causing the hydrogen atoms to attach to one another through mutual gravity.

Each individual hydrogen atom had its own momentum, and so when the atoms collected together into larger and larger clumps of gas, the conservation of momentum across all the particles set these clumps of gas spinning. Imagine two spinning skydivers colliding with one another in mid-air; after their collision, they’ll have a new rotation speed and direction based on the addition of their original directions.

Eventually all of this hydrogen gas was collected together into a massive spinning ball of gas that continued to collapse under its own gravity. As it collapsed, it began to spin faster and faster, just like a figure skater pulling in her arms increases her rotation speed.

The spinning cloud of gas and dust flattened out because of the rotational force, with the Sun at the center, and then a pancake-shaped disc of material surrounding it. The planets formed out of this disk of material, collecting together particles of dust into larger and larger rocks until planet-sized objects had accumulated together.

The Planets are in Perfect Balance

The planets orbit the Sun because they’re left over from the formation of the Solar System. Their current motion depends on the gravitational attraction of the Sun at the center of the Solar System. In fact, they’re in perfect balance.

There are two opposing forces acting on the planets: gravity pulling them inward, and the inertia of their orbit driving them outwards. If gravity was dominant, the planets would spiral inward. If their inertia was dominant, the planets would spiral outward into deep space.

The planets are trying to fly out into deep space, but the gravity of the Sun is pulling them into a curved orbit.

Research further:
Cornell Astronomy
The Universe of Aristotle and Ptolemy
Copernical Model: A Sun-Centered Solar System
The Solar Nebula
On the Revolution of the Heavenly Bodies
The Copernican Revolution

April 29, 2012December 23, 2015 by Fraser Cain

Pictures of the Moon

The Moon is one of the most familiar and beautiful objects in the night sky (and daytime too!). Let’s take a look at some beautiful images of the Moon. Of course, since Universe Today is a space and astronomy website, all of these pictures of the Moon were taken by spacecraft, or people on board spacecraft.

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Here’s one of the most important pics of the Moon ever captured. That’s because you can see the whole of the Earth as well. This picture of the Moon is called Earthrise, and it was taken by NASA’s astronauts on board Apollo 8 just after it completed its lunar insertion orbit.

The Moon at its nearest and furthest points of its orbit. Image credit: NASA

The Moon follows an elliptical orbit as it travels around the Earth. At some points in its orbit, the Moon is closer to the Earth than others. This picture of the Moon from NASA’s Galileo spacecraft shows the difference in sizes that the Moon can get.

The Earth and Moon, seen from the shuttle Discovery. Image credit: NASA

This is a picture of the Moon, but it’s also a picture of the Earth, seen from space as well as the space shuttle Discovery. This image of the Moon was captured during a mission in 1998.

Here’s a side of the Moon that very few people have ever seen with their own eyes. This photo of the Moon shows its far side. The image was taken by NASA’s Galileo spacecraft as it was speeding out on its journey to Jupiter.

And finally, this isn’t a photograph, but it’s an artist’s illustration of what might have happened during the formation of the Moon. In this image of the Moon, a Mars-sized object is crashing into the Earth. After this, the spray of debris from the collision orbited the Earth and eventually collected together to form the Moon.

Moon Landing Photos

Moon Landing Pictures — Apollo moon landing sites

For all you conspiracy buffs out there, here’s evidence that the Moon landings really happened. Here are some pictures of the lunar surface taken by NASA’s Lunar Reconnaissance Orbiter showing the location of all the lunar landings. The pictures are so high resolution, you can see the shadows of the lander and even the astronaut footprints.

This is a portrait of astronaut and scientist Harrison H. Schmitt standing beside the US flag on the Moon. While most astronauts were test pilots, Schmitt was an actual geologist. It was incredibly useful to have a scientist studying the lunar rocks and soil, searching for evidence.

This is astronaut Alan Bean standing on the surface of the Moon. He’s holding a special container that has lunar soil in it. This picture was taken in the vicinity of Sharp Crater.

Here’s a classic picture of Buzz Aldrin’s footprint on the Moon; he was the second person to set foot on the Moon. Because there’s no weather on the Moon, this footprint should remain here for millions of years.

This is a photo of Buzz Aldrin climbing down outside the Apollo 11 capsule, becoming the second person to set foot on the surface of the Moon. This picture was taken by Neil Armstrong, the first person on the Moon.

Full Moon Pictures

This is a stunning photo of full moon taken by the astronauts onboard the International Space Station during the Expedition 10 mission. The moon is the only natural satellite of the planet Earth.

This breathtaking photo moon and the earth’s atmosphere was taken from the International Space Station by an Expedition 10 crew member in October 2004. Expedition 10 crew members, Leroy Chiao and Salizhan Sharipov relieved the two Expedition 9 crew members, Mike Fincke and Gennady Padalka.

Here’s another amazing picture of the moon in full view. This image was taken by the Expedition 12 crew members onboard the International Space Station on February 12, 2006.

Full Moon with Earth's Horizon and Airglow Visible at Left

This is an Expedition 14 picture of the full moon taken on December 4, 2006. The moon is the brightest object visible in the earth’s sky after the sun.

Here’s a nice photo of the earth’s moon generated from the 18 images captured by the Galileo spacecraft on December 7, 1992 on its way to Jupiter. The Moon is the only natural satellite of the earth. The moon’s surface, as seen on the image is composed of many impact craters.

New Moon Pictures

Almost New Moon with Venus. Image credit: Voobie

This is an image of the Moon when it was almost a new moon. The bright star in the picture isn’t a star at all but the planet Venus. This photo was taken by Voobie.

This is an image of a double conjunction, where the Moon was close in the sky to two planets, Jupiter and Venus.

New Moon with airplane. Image credit: Stefan Seip

Amateur astronomer Stefan Seip caught this amazing photograph of a passenger airplane passing in front of an almost perfect New Moon.

New moon with Venus. Image credit: James W. Young

Another great image of a new moon. This time the Moon is only 37 hours old. This picture was taken by James W. Young from the Table Mountain Observatory.

April 1, 2012December 23, 2015 by Fraser Cain

Characteristics of Mercury

MESSENGER's first image from Mercury orbit, with the bright Debussy crater visible at upper right. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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Quick Mercury Stats
Mass: 0.3302 x 10²⁴ kg
Volume: 6.083 x 10¹⁰ km³
Average radius: 2439.7 km
Average diameter: 4879.4 km
Density: 5.427 g/cm³
Escape velocity: 4.3 km/s
Surface gravity: 3.7 m/s²
Visual magnitude: -0.42
Natural satellites: 0
Rings? – No
Semimajor axis: 57,910,000 km
Orbit period: 87.969 days
Perihelion: 46,000,000 km
Aphelion: 69,820,000 km
Mean orbital velocity: 47.87 km/s
Maximum orbital velocity: 58.98 km/s
Minimum orbital velocity: 38.86 km/s
Orbit inclination: 7.00°
Orbit eccentricity: 0.2056
Sidereal rotation period: 1407.6 hours
Length of day: 4222.6 hours
Discovery: Known since prehistoric times
Minimum distance from Earth: 77,300,000 km
Maximum distance from Earth: 221,900,000 km
Maximum apparent diameter from Earth: 13 arc seconds
Minimum apparent diameter from Earth: 4.5 arc seconds
Maximum visual magnitude: -1.9

Size of Mercury
How big is Mercury? Mercury is the smallest planet in the Solar System by surface area, volume, and equatorial diameter. Surprisingly, it is also one of the most dense. It gained its ‘smallest’ title after Pluto was demoted. That is why older material refers to Mercury as the second smallest planet. The aforementioned are the three criteria that we will use to show the size of Mercury in relation to Earth.

Some scientists think that Mercury is actually shrinking. The liquid core of the planet occupies about 42% of the planet’s volume. The spin of the planet allows a small portion of the core to cool. This cooling and shrinking is thought to be evidenced by the fracturing of the planet’s surface.

The surface of Mercury is heavily cratered, much like the Moon, and the continued presence of those craters indicates that the planet has not been geologically active for billions of years. That knowledge is based on partial mapping of the planet(55%). It is unlikely to change even after NASA’s MESSENGER spacecraft maps the entire surface. The planet was most likely bombarded heavily by asteroids and comets during the Late Heavy Bombardment about 3.8 billion years ago. Some regions would have been filled by magma eruptions from within the planet. These created smooth plains similar to those found on the Moon. As the planet cooled and contracted cracks and ridges formed. These features can be seen on top of other features, which is a clear indication that they are more recent. Volcanic eruptions ceased on Mercury about 700-800 million years ago when the planet’s mantle had contracted enough to prevent lava flow.

This WAC image showing a never-before-imaged area of Mercury’s surface was taken from an altitude of ~450 km (280 miles) above Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Diameter of Mercury (and the Radius)
The diameter of Mercury is 4,879.4 km.

Need some way to compare that to something more familiar? The diameter of Mercury is only 38% the Earth’s diameter. In other words, you could put almost 3 Mercurys side to side to match the diameter of Earth.

In fact, there are two moons in the Solar System which actually have a larger diameter than Mercury. The largest moon in the Solar System is Jupiter’s moon Ganymede, with a diameter of 5,268 km and the second largest moon is Saturn’s moon Titan, with a diameter of 5,152 km.

The Earth’s moon is only 3,474 km, so Mercury isn’t much bigger.

If you want to calculate the radius of Mercury, you need to divide the diameter of Mercury in half. While the diameter is 4,879.4 km, the radius of Mercury is only 2,439.7 km.

Diameter of Mercury in kilometers: 4,879.4 km
Diameter of Mercury in miles: 3,031.9 miles
Radius of Mercury in kilometers: 2,439.7 km
Radius of Mercury in miles: 1,516.0 miles

Circumference of Mercury
The circumference of Mercury is 15,329 km. In other words, if Mercury’s equator was perfectly flat, and you could drive around it in your car, your odomotor would add 15,329 km from the trip.

Most planets are oblate spheroids, so their equatorial circumference is larger than their pole to pole. The more rapidly they spin, the more the planet flattens out, so the distance from the center of the planet to its poles is shorter than the distance from the center to the equator. But Mercury rotates so slowly that its circumference is the same no matter where you measure it.

You can calculate the circumference of Mercury all by yourself, using the classic mathematical formulae to get the circumference of a circle.

Circumference = 2 x pi x radius

We know the radius of Mercury is 2,439.7 km. So if you put these numbers in: 2 x 3.1415926 x 2439.7, you get 15,329 km.

Circumference of Mercury in kilometers: 15,329 km
Circumference of Mercury in miles: 9,525 miles

Volume of Mercury
The volume of Mercury is 6.083 x 10¹⁰km³. That seems to be a huge number on the face of it, but Mercury is the smallest planet in the Solar System by volume (since the demotion of Pluto). It is even smaller than some of the moons in our Solar System. The Mercurian volume is only 5.4% of Earth’s and the Sun has 240.5 million times the volume of Mercury.

Over 40% of Mercury’s volume is occupied by its core, 42% to be exact. The core is about 3,600 km in diameter. That makes Mercury the second most dense planet amongst our eight. The core is molten and mainly consists of iron. The molten core is able to produce a magnetic field which helps to deflect the solar wind. The magnetic field and slight gravity of the planet allow it to hold onto a tenuous atmosphere.

It is thought that Mercury was at one time a larger planet and; therefore, had a higher volume. There is one theory to explain its current size that many scientists accept on several levels. The theory explains Mercury’s density and the high percentage of core material. The theory states that Mercury originally had a metal-silicate ratio similar to common meteorites, as is typical of rocky matter in our Solar System. At that time, the planet is thought to have had a mass approximately 2.25 times its current mass, but, early in the Solar System’s history, it was struck by a planetesimal that was about 1/6 its mass and several hundred kilometers in diameter. The impact would have stripped away much of the original crust and mantle, leaving the core as a large percentage of the planet and greatly reducing the planet’s volume as well.

Volume of Mercury in cubic kilometers: 6.083 x 10¹⁰km³

Mass of Mercury
The mass of Mercury is only 5.5% of the Earth’s; the actual value is 3.30 x 10²³ kg. Since Mercury is the smallest planet in the Solar System, you would expect this relatively small mass. On the other hand, Mercury is the second most dense planet in our Solar System (after Earth). Given its size, the density comes largely from its core, estimated at almost half the planet’s volume.

The planet’s mass is comprised of materials that are 70% metallic and 30% silicate. There are several theories to explain why the planet is so dense and the abundance of metallic material. The most widely held theory holds that the high core percentage is the result of an impact. In this theory the planet originally had a metal-silicate ratio similar to the chondrite meteorites common in the Universe and around 2.25 times its current mass. Early in the history of our Solar System, Mercury was struck by a planetesimal sized impactor that was about 1/6 of its hypothesized mass and hundreds of km in diameter. An impact of that magnitude would strip away much of the crust and mantle, leaving behind a large core. Scientists believe that a similar incident created our moon. An additional theory says that the planet formed before the Sun’s energy had stabilized. The planet would have had much more mass in this theory as well, but the temperatures created by the protosun would have been as high as 10,000 K and the majority of the surface rock could have been vaporized. The rock vapor could have then been carried away by the solar wind.

Mass of Mercury in kg: 0.3302 x 10²⁴ kg
Mass of Mercury in pounds: 7.2796639 x 10²³ pounds
Mass of Mercury in tonnes: 3.30200 x 10²⁰ tonnes
Mass of Mercury in tons: 3.63983195 x 10²⁰

Artist's concept of MESSENGER in orbit around Mercury. Courtesy of NASA

Gravity on Mercury
Gravity on Mercury is 38% of the gravity here on Earth. A man weighing 980 Newtons on Earth (about 220 pounds), would only weigh about 372 Newtons (83.6 pounds) landing on the planet’s surface. Mercury is only slightly bigger than our moon, so you might expect its gravity to be similar to the Moon’s at 16% of Earth’s. The big difference Mercury’s higher density – it’s the second densest planet in the Solar System. In fact, if Mercury were the same size as Earth, it would be even more dense than our own planet.

It’s important to clarify the difference between mass and weight. Mass measures how much stuff something contains. So if you have 100 kg of mass on Earth, you will have the same amount on Mars, or intergalactic space. Weight, however, is the force of gravity you feel. While bathroom scales measure pounds or kilograms, they should really be measuring newtons, which is a measure of weight.

Take your current weight in either pounds or kilograms and then multiply it by 0.38 with a calculator. For example, if you weigh 150 pounds, you’d weigh 57 pounds on Mercury. If you weigh 68 kilograms on the bathroom scale, your weight on Mercury would be 25.8 kg.

You can also turn this number around to figure out how much stronger you would be. For example, how high you could jump, or how much weight you could lift. The current world record for the high jump is 2.43 meters. Divide 2.43 by 0.38, and you get the world’s high jump record if it were done on Mercury. In this case, it would be 6.4 meters.

In order to escape the gravity of Mercury, you would need to be traveling 4.3 kilometers/second, or about 15,480 kilometers per hour. Compare this to Earth, where the escape velocity of our planet is 11.2 kilometers per second. If you compare the ratio between our two planets, you get 38%.

Surface gravity of Mercury: 3.7 m/s²
Escape velocity of Mercury: 4.3 kilometers/second

Density of Mercury
The density of Mercury is the second highest in the Solar System. Earth is the only planet that is more dense. It is 5.427 g/cm³ compared to Earth’s 5.515 g/cm³. If gravitational compression were to be removed from the equation, Mercury would be more dense. The high density of the planet is attributed to its large percentage of core. The core constitutes 42% of Mercury’s overall volume.

Mercury is a terrestrial planet like Earth, one of only four in our Solar System. Mercury is about 70% metallic material and 30% silicates. Add the density of Mercury and scientists can infer details of its internal structure. While the Earth’s high density mainly results from gravitational compression at the core, Mercury is much smaller and is not so tightly compressed internally. These facts have allowed NASA scientists and others to surmise that its core must be large and contain overwhelming amounts of iron. Planetary geologists estimate that the planet’s molten core accounts for about 42% of its volume. On Earth that percentage is 17.

That leaves a silicate mantle that is only 500–700 km thick. Data from Mariner 10 led scientists to believe that the crust is even thinner, at a mere 100–300 km. This surrounds a core that has a higher iron content than any other planet in the Solar System. So, what caused this disproportionate amount of core material? Most scientists accept the theory that Mercury had a metal-silicate ratio similar to common chondrite meteorites several billion years ago. They also believe that it had a mass of about 2.25 times its current; however, Mercury may have been impacted by a planetesimal 1/6 that mass and hundreds of km in diameter. The impact would have stripped away much of the original crust and mantle, leaving the core as a major percentage of the planet.

While scientists have a few facts about the density of Mercury, there is still more to be discovered. Mariner 10 send back a great deal information, but was only able to study about 44% of the planet’s surface. The MESSENGER mission is filling in some of the blanks as you are reading this article and the BepiColumbo mission will go even farther in extending our knowledge of the planet. Soon, there mat be more than theories to explain the high density of the planet.

Density of Mercury in grams per cubic centimeter: 5.427 g/cm³

Axis of Mercury
Like all of the planets in the Solar System, the axis of Mercury is tilted away from the plane of the ecliptic. In this case, Mercury’s axial tilt is 2.11 degrees.

What exactly is a planet’s axial tilt? First imagine that the Sun is a ball in the middle of a flat disk, like a record or a CD. The planets orbit around the Sun within this disk (more or less). That disk is known as the plane of the ecliptic. Each planet is also spinning on its axis as it’s orbiting around the Sun. If planet was spinning perfectly straight up and down, so that a line running through the north and south poles of the planet was perfectly parallel with the Sun’s poles, the planet would have a 0-degree axial tilt. Of course, none of the planets are like this.

So if you drew a line between Mercury’s north and south poles and compared it to an imaginary line if the Mercury had no axial tilt at all, that angle would measure 2.11 degrees. You might be surprised to know that this Mercury tilt is actually the smallest of all the planets in the Solar System. For example, the Earth’s tilt is 23.4 degrees. And Uranus is actually flipped completely over on its axis, and rotates with an axial tilt of 97.8 degrees.

Here on Earth, the axial tilt of our planet causes the seasons. When it’s summer in the northern hemisphere, the Earth’s north pole is angled towards the Sun. and then in the winter, the north pole is angled away. We get more sunlight in the summer so it’s warmer, and less in the winter.

Mercury barely experiences any seasons at all. This is because it has almost no axial tilt. Of course, it also doesn’t have much of an atmosphere to hold the Sun’s heat. Whichever side is facing the Sun is heated to 700 degrees Kelvin, and the side facing away drops to less than 100 Kelvin.

Axial tilt of Mercury: 2.11°

References:
NASA StarChild: Mercury
Wikipedia
NASA: Mercury
European Space Agency
NASA: Mercury Exploration
NASA Solar System Exploration
JAXA: Mercury Quantities
NASA MESSENGER Mission
European Space Agency
NASA Solar System Exploration: Mercury

March 13, 2012December 23, 2015 by Fraser Cain

Does The Sun Rotate?

A mosaic of 4 images taken of the Sun on Nov. 13, 2011. Credit: Leonard Mercer.

The rotation of the Sun is kind of hard to pin down. That’s because a day on the Sun depends on which part of the Sun you’re talking about. Confused yet? It kept astronomers puzzled for years too. Let’s look at how the rotation of the Sun changes.

A spot on the equator of the Sun takes 24.47 days to rotate around the Sun and return to the same position. Astronomers call this sidereal rotation period, which is different from the synodic period – the amount of time it takes for a spot on the Sun to rotate back to face the Earth. But the Sun’s rotation rate decreases as you approach the poles, so it can actually take 38 days for regions around the poles to rotate once.

The Sun’s rotation is seen by observing sunspots. All sunspots move across the face of the Sun. This motion is part of the general rotation of the Sun on its axis. Observations also indicate that the Sun does not rotate as a solid body, but it spins differentially. That means that it rotates faster at the equator of the Sun and slower at its poles. The gas giants Jupiter and Saturn also have differential rotation.

And so, astronomers have decided to measure the rotation rate of the Sun from an arbitrary position of 26° from the equator; approximately the point where we see most of the sunspots. At this point, it takes 25.38 days to rotate and return to the same spot in space.

Astronomers also know that the interior of the Sun rotated differently than the surface. The inner regions, the core and the radiative zone, rotate together like a solid body. And then the outer layers, the convective zone and photosphere, rotate at a different speed.

The Sun and the entire solar system orbits around the center of the Milky Way galaxy. The average velocity of the solar system is 828,000 km/hr. At that rate it will take about 230 million years to make one complete orbit around the galaxy. The Milky Way is a spiral galaxy. It is believed that it consists of a central bulge, 4 major arms, and several shorter arm segments. The Sun and the rest of our solar system is located near the Orion arm, between two major arms, Perseus and Sagittarius. The diameter of the Milky Way is about 100,000 light years and the Sun is located about 28,000 light-years from the Galactic Center. It has been suggested fairly recently that ours is actually a barred spiral galaxy. That means that instead of a bulge of gas and stars at the center, there is probably a bar of stars crossing the central bulge.

So when someone asks you what the rotation of the Sun is, ask them which part.

Here’s an article from Universe Today about the Sun’s magnetic field flip, and here’s an article about how there were no sunspots on the surface of the Sun.

Here’s more information on the topic from Windows on the Universe, and here’s an article from NASA.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

March 13, 2012December 23, 2015 by Fraser Cain

Who Discovered Mercury?

Mercury's limb. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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Mercury is one of the 5 planets visible with the unaided eye. Even thousands of years ago, ancient astronomers knew that the 5 wanderers were different from the other stars in the sky. The 5 planets visible with the unaided eye are Mercury, Venus, Mars, Jupiter and Saturn. They gave them distinct names, and charted their positions with incredible accuracy. It’s impossible to say “when was Mercury discovered”, since that would have been before recorded history.

But when did astronomers realize that Mercury was a planet? That happened with Copernicus developed his model of a Sun-centered Solar System, published in 1543. With the Sun at the center of the Solar System, and not the Earth, it meant that both the Earth and Mercury were planets. This discovery was confirmed when Galileo first turned his telescope on the planets and realized they matched predictions made by Copernicus. Unfortunately, Galileo’s telescope wasn’t powerful enough to reveal a disk for Mercury, but it did show how Venus went through phases like the Moon.

This model was backed up by Galileo, who pointed his first rudimentary telescope at Mercury in the 17th century. Unfortunately his telescope wasn’t powerful enough to see Mercury go through phases like he saw with Venus.

Because it’s so small and close to the Sun, Mercury was difficult to observe with ground-based telescopes. More powerful telescopes only revealed a small grey disk; they didn’t have the resolution to display features on the planet’s surface, like craters or lava fields.

It wasn’t until the early 1960s when radio astronomers started bouncing signals off the surface of Mercury that more information was finally known about the planet. These signals revealed that Mercury’s day length is about 59 days. Even more detailed observations have been made with the Arecibo telescope, mapping surface features down to a resolution of 5 km.

The most detailed observations of Mercury have come from the exploration from spacecraft sent from Earth. NASA’s Mariner 10 spacecraft swept past Mercury in 1974, capturing images from an altitude of just 327 km. It eventually mapped about half of the planet in unprecedented detail, revealing that the planet looked very similar to the Earth’s moon, with many impact craters and ancient lava fields.

If you’re wondering who discovered the element mercury, nobody knows that either. The element has been known for thousands of years, and was used by the ancient Chinese. Liquid mercury was found in Egyptian tombs closed up almost 4,000 years ago.

We have written many articles about Mercury for Universe Today. Here’s an article about new mysteries unveiled on Mercury, and the possibility that Mercury could cause an interplanetary smash-up.

Want more information on Mercury? Here’s a link to NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Mission Page.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.

References:
NASA Cosmic Distance Scales
NASA Solar System Exploration: Mariner 10

March 12, 2012December 23, 2015 by Fraser Cain

Atmosphere of Mercury

A High-resolution Look over Mercury's Northern Horizon. Credit: MESSENGER team

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When you look at an image of Mercury, it looks like a dry, airless world. But you might be surprised to know that Mercury does have an atmosphere. Not the kind of atmosphere that we have here on Earth, or even the thin atmosphere that surrounds Mars. But Mercury’s atmosphere is currently being studied by scientists, and the newly arrived MESSENGER spacecraft.

Mercury’s original atmosphere dissipated shortly after the planet formed 4.6 billion years ago with the rest of the Solar System. This was because of Mercury’s lower gravity, and because it’s so close to the Sun and receives the constant buffeting from its solar wind. Its current atmosphere is almost negligible.

What is Mercury’s atmosphere made of? It has a tenuous atmosphere made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor. Astronomers think this current atmosphere is constantly being replenished by a variety of sources: particles of the Sun’s solar wind, volcanic outgassing, radioactive decay of elements on Mercury’s surface and the dust and debris kicked up by micrometeorites constantly buffeting its surface. Without these sources of replenishment, Mercury’s atmosphere would be carried away by the the solar wind relatively quickly.

Mercury atmospheric composition:

Oxygen 42%
Sodium 29%
Hydrogen 22%
Helium 6%
Potassium 0.5%
With trace amounts of the following:
Argon, Carbon dioxide, Water, Nitrogen, Xenon, Krypton, Neon, Calcium, Magnesium

In 2008, NASA’s MESSENGER spacecraft discovered water vapor in Mercury’s atmosphere. It’s thought that this water is created when hydrogen and oxygen atoms meet in the atmosphere.

Two of those components are possible indicators of life as we know it: methane and water vapor(indirectly). Water or water ice is believed to be a necessary component for life. The presence of water vapor in the atmosphere of Mercury indicates that there is water or water ice somewhere on the planet. Evidence of water ice has been found at the poles where the bottoms of craters are never exposed to light. Sometimes, methane is a byproduct of waste from living organisms. The methane in Mercury’s atmosphere is believed to come from volcanism, geothermal processes, and hydrothermal activity. Methane is an unstable gas and requires a constant and very active source, because studies have shown that the methane is destroyed in less than on Earth year. It is thought that it originates from peroxides and perchlorates in the soil or that it condenses and evaporates seasonally from clathrates.

Despite how small the Mercurian atmosphere is, it has been broken down into four components by NASA scientists. Those components are the lower, middle, upper, and exosphere. The lower atmosphere is a warm region(around 210 K). It is warmed by the combination of airborne dust(1.5 micrometers in diameter) and heat radiated from the surface. This airborne dust gives the planet its ruddy brown appearance. The middle atmosphere contains a jetstream like Earth’s. The upper atmosphere is heated by the solar wind and the temperatures are much higher than at the surface. The higher temperatures separate the gases. The exosphere starts at about 200 km and has no clear end. It just tapers off into space. While that may sound like a lot of atmosphere separating the planet from the solar wind and ultraviolet radiation, it is not.

Helping Mercury hold on to its atmosphere is its magnetic field. While gravity helps hold the gases to the surface, the magnetic filed helps to deflect the solar wind around the planet, much like it does here on Earth. This deflection allows a smaller gravitational pull to hold some form of an atmosphere.

The atmosphere of Mercury is one of the most tenuous in the Solar System. The solar wind still blows much of it away, so sources on the planet are constantly replenishing it. Hopefully, the MESSENGER spacecraft will help to discover those sources and increase our knowledge of the innermost planet.

We have written many articles about Mercury’s atmosphere for Universe Today. Here’s an article about how magnetic tornadoes might regenerate Mercury’s atmosphere, and here’s an article about the climate of Mercury.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We have also recorded an entire episode of Astronomy Cast all about atmospheres. Listen here, Episode 151: Atmospheres.

References:
NASA: Atmosphere of Mercury
NASA Solar System Exploration
Wikipedia
Nature.com

March 12, 2012May 21, 2016 by Fraser Cain

The Rings of Neptune

Neptune's system of moons and rings visualized. Credit: SETI

Neptune is one of four planets in our Solar System with planetary rings. Neptune was not discovered until 1846 and its rings were only discovered definitively in 1989 by the Voyager 2 probe. Although the rings were not discovered until the late 1900’s, William Lassell who discovered Titan recorded that he had observed a ring. However, this was never confirmed. The first ring was actually discovered in 1968, but scientists were unable to determine if it was a complete ring. The Voyager’s evidence was the definitive proof for the existence of the rings.

Neptune has five rings: Galle, Le Verrier, Lassell, Arago, and Adams. Its rings were named after the astronomers who made an important discovery regarding the planet. The rings are composed of at least 20% dust with some of the rings containing as much as 70% dust; the rest of the material comprising the rings is small rocks. The planet’s rings are difficult to see because they are dark and vary in density and size. Astronomers think Neptune’s rings are young compared to the age of the planet, and that they were probably formed when one of Neptune’s moons was destroyed.

The Galle ring was named after Johann Gottfried Galle, the first person to see the planet using a telescope. It is the nearest of Neptune’s rings at 41,000–43,000 km. The La Verrier ring was named after the man who predicted Neptune’s position. Very narrow, this ring is only about 113 kilometers wide. The Lassell ring is the widest of Neptune’s rings. Named after William Lassell, it lies between 53,200 kilometers and 57,200 kilometers from Neptune, making it 4,000 kilometers wide. The Arago ring is 57,200 kilometers from the planet and less than 100 kilometers wide.

The outer ring, Adams, was named after John Couch Adams who is credited with the co-discovery of Neptune. Although the ring is narrow at only 35 kilometers wide, it is the most famous of the five due to its arcs. Adams’ arcs are areas where the material of the rings is grouped together in a clump. Although the Adams ring has five arcs, the three most famous ones are Liberty, Equality, and Fraternity. The arcs are the brightest parts of the rings and the first to be discovered. Scientists are unable to explain the existence of these arcs because according to the laws of motion they should distribute the material uniformly throughout the rings.

The rings of Neptune are very dark, and probably made of organic compounds that have been baked in the radiation of space. This is similar to the rings of Uranus, but very different to the icy rings around Saturn. They seem to contain a large quantity of micrometer-sized dust, similar in size to the particles in the rings of Jupiter.

It’s believed that the rings of Neptune are relatively young – much younger than the age of the Solar System, and much younger than the age of Uranus’ rings. They were probably created when one of Neptune’s inner moons got to close to the planet and was torn apart by gravity.

The innermost ring of Neptune orbits at a distance of 41,000 km from the planet, and extends to a width of 2,000 km. It’s named after Johann Gottfried Galle, the first person to see Neptune through a telescope. The next ring is the narrower LeVerrier ring, named after Neptune’s co-discoverer, Urbain Le Verrier. It’s only 113 km wide. Then comes the Lassell ring, the widest ring in the system at about 4,000 km. Then comes the Arago ring, and finally the very thin Adams ring, named after Neptune’s other co-discoverer.

We have written many articles about Neptune here at Universe Today. Here’s The Gas (and Ice) Giant Neptune, What is the Surface of Neptune Like?, 10 Interesting Facts About Neptune, and The Rings of Neptune.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.