What is the Weather Like on Mercury?

Weather on Mercury

With the dawning of the Space Age in the 1950s, human beings were no longer confined to studying the Solar planets and other astronomical bodies with Earth-based instruments alone. Instead crewed missions have gone into orbit and to the Moon while robotic missions have traveled to every corner of the Solar System. And in the process, we have learned some interesting things about the planets, planetoids, and asteroids in our Solar neighborhood.

For example, we have learned that all the Solar planets have their own particular patterns and cycles. For instance, even though Mercury is an airless body, it does have a tenuous exosphere and experiences seasons of a sort. And while it is known for being extremely hot, it also experiences extremes of cold, to the point that ice can exist on its surface. While it is by no means what we are used to here on Earth, Mercury still experiences a kind of “weather”.

Mercury’s Atmosphere:

As noted, Mercury has no atmosphere to speak of, owing to its small size and extremes in temperature. However, it does have a tenuous and variable exosphere that is made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10-14 bar (one-quadrillionth of Earth’s atmospheric pressure).

The Fast Imaging Plasma Spectrometer on board MESSENGER has found that the solar wind is able to bear down on Mercury enough to blast particles from its surface into its wispy atmosphere. Shannon Kohlitz, Media Academica, LLC

It is believed this exosphere was formed from particles captured from the Sun (i.e solar wind) as well as volcanic outgassing and debris kicked into orbit by micrometeorite impacts. In any case, Mercury’s lack of a viable atmosphere is the reason why it is unable to retain heat from the Sun, which leads to extreme variations between night and day for the rocky planet.

Orbital Resonance:

Mercury’s temperature variations are also attributed to its orbital eccentricity of 0.2056, which is the most extreme of any planet in the Solar System. Essentially, its distance from the Sun ranges from 46 million km (29 million mi) at its closest (perihelion) to 70 million km (43 million mi) at its farthest (aphelion). As a result, the side facing the Sun reaches temperatures of up to 700 K (427° C), the side in shadow dips down to 100 K (-173° C).

With an average rotational speed of 10.892 km/h (6.768 mph), Mercury also takes 58.646 days to complete a single rotation. This means that Mercury has a spin-orbit resonance of 3:2, where it completes three rotations on its axis for every two rotations completed around the Sun. This does not, however, mean that three days last the same as two years on Mercury.

The Orbit of Mercury during the year 2006. Credit: Wikipedia Commons/Eurocommuter

In fact, its high eccentricity and slow rotation mean that it takes 176 Earth days for the Sun to return to the same place in the sky (aka. a solar day). In short, a single day on Mercury is twice as long as a single year! Mercury also has the lowest axial tilt of any planet in the Solar System – approximately 0.027 degrees compared to Jupiter’s 3.1 degrees (the second smallest).

Polar Ice:

This low tilt means that the polar regions are constantly in shadow, which leads to another interesting feature about Mercury. Yes, despite how hot its Sun-facing side can become, the existence of water ice and even organic molecules have been confirmed on Mercury’s surface. But this only true at the poles, where the floors of deep craters are never exposed to direct sunlight, and temperatures within them therefore remain below the planetary average.

These icy regions are believed to contain about 1014–1015 kg (1 to 10 billion metric tons, 1.1 to 11 billion US tons) of frozen water, and may be covered by a layer of regolith that inhibits sublimation. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by the impacts of comets.

Mercury transit
The Big Bear Solar Observatory Captures a high-res image of this week’s transit of Mercury across the face of the Sun. Image credit: NJIT/BBSO

When one talks about the “weather” on Mercury, they are generally confined to talking about variations between the Sun-facing side and the night side. Over the course of two years, that weather will remain scorching hot or freezing cold. In that respect, we could say that a single season on Mercury lasts a full four years, and includes a “Midnight Sun” that lasts two years, and a “Polar Night” that lasts the same.

Between its rapid and very eccentric orbit, its slow rotation, and its strange diurnal and annual patterns, Mercury is a very extreme planet with a very extreme environment. It only makes sense that its weather would be similarly extreme. Hey, there’s a reason nobody lives there, at least not yet

We have written many interesting articles about the weather on other planets here at Universe Today. Here’s What is the Weather like on Venus?, What is the Weather Like on Mars?, What is the Weather Like on Jupiter?, What is the Weather Like on Saturn?, What is the Weather Like on Uranus?, and What is the Weather Like on Neptune?

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’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49:  Mercury.

Sources:

How Long is Day on Mercury?

Mercury is one of the most unusual planets in our Solar System, at least by the standards of us privileged Earthlings. Despite being the closest planet to our Sun, it is not the hottest (that honor goes to Venus). And because of its virtually non-existence atmosphere and slow rotation, temperatures on its surface range from being extremely hot to extremely cold.

Equally unusual is the diurnal cycle on Mercury – i.e. the cycle of day and night. A single year lasts only 88 days on Mercury, but thanks again to its slow rotation, a day lasts twice as long! That means that if you could stand on the surface of Mercury, it would take a staggering 176 Earth days for the Sun to rise, set and rise again to the same place in the sky just once!

Distance and Orbital Period:

Mercury is the closest planet to our Sun, but it also has the most eccentric orbit (0.2056) of any of the Solar Planets. This means that while its average distance (semi-major axis) from the Sun is 57,909,050 km (35,983,015 mi) or 0.387 AUs, this ranges considerably – from 46,001,200 km (2,8583,820 mi) at perihelion (closet) to 69,816,900 km (43,382,210  mi) at aphelion (farthest).

A timelapse of Mercury transiting across the face of the Sun. Credit: NASA

Because of this proximity, Mercury has a rapid orbital period, which varies depending on where it is in its orbit. Naturally, it moves fastest when it is at its closest to the Sun, and slowest when it is farthest. On average, its orbital velocity is 47.362 km/s (29.43 mi/s), which means it takes only 88 days to complete a single orbit of the Sun.

Astronomers used to suspect that Mercury was tidally locked to the Sun, meaning that it always showed the same face to the Sun – similar to how the Moon is tidally locked to the Earth. But radar-Doppler measurements obtained in 1965 demonstrated that Mercury is actually rotating very slowly compared to the Sun.

Sidereal vs. Solar Day:

Based on data obtained by these radar measurements, Mercury is now known to be in 3:2 orbital resonance with the Sun. This means that the planet completes three rotations on its axis for every two orbits it makes around the Sun. At it’s current rotational velocity – 3.026 m/s, or 10.892 km/h (6.77 mph) – it takes Mercury 58.646 days to complete a single rotation on its axis.

While this might lead some to conclude that a single day on Mercury is about 58 Earth days – thus making the length of a day and year correspond to the same 3:2 ratio – this would be inaccurate. Due to its rapid orbital velocity and slow sidereal rotation, a Solar Day on Mercury (the time it takes for the Sun to return to the same place in the sky) is actually 176 days.

In that respect, the ratio of days to years on Mercury is actually 1:2. The only places that are exempt to this day and night cycle are the polar regions. The cratered northern polar region, for example, exists in a state of perpetual shadow. Temperatures in these craters are also cool enough that significant concentrations of water ice can exist in stable form.

For over 20 years, scientists believed that radar-bright images from Mercury’s northern polar regions might indicate the presence of water ice there. In November of 2012, NASA’s MESSENGER probe examined the northern polar region using its neutron spectrometer and laser altimeter and confirmed the presence of both water ice and organic molecules.

View of Mercury’s north pole. based on MESSENGER probe data, showing polar deposits of water ice. Credit: NASA/JHUAPL/Carnegie Institute of Science/NAIC/Arecibo Observatory

Yes, as if Mercury weren’t strange enough, it turns out that a single day on Mercury lasts as long as two years! Just another oddity for a planet that likes to keep things really hot, really cold, and is really eccentric.

We’ve written many articles about Mercury for Universe Today. Here’s How Long is Day on the Other Planets?, Which Planet has the Longest Day?, How Long is a Day on Venus?, How Long is a Day on Earth?, How Long is a Day on the Moon?, How Long is a Day on Mars?, How Long is a Day on Jupiter?, How Long is a Day on Saturn?, How Long is a Day on Uranus?, How Long is a Day on Neptune?, and How Long is a Day on Pluto?

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

We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.

Sources:

NASA Planetary Science Trio Honored as ‘Best of What’s New’ in 2011- Curiosity/Dawn/MESSENGER

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A trio of NASA’s Planetary Science mission’s – Mars Science Laboratory (MSL), Dawn and MESSENGER – has been honored by Popular Science magazine and selected as ‘Best of What’s New’ in innovation in 2011 in the aviation and space category.

The Curiosity Mars Science Laboratory was just launched to the Red Planet on Saturday, Nov. 26 and will search for signs of life while traversing around layered terrain at Gale Crater. Dawn just arrived in orbit around Asteroid Vesta in July 2011. MESSENGER achieved orbit around Planet Mercury in March 2011.

Several of the top mission scientists and engineers provided exclusive comments about the Popular Science recognitions to Universe Today – below.

“Of course we are all very pleased by this selection,” Prof. Chris Russell, Dawn Principal Investigator, of UCLA, told Universe Today.


Dawn is the first mission ever to specifically investigate the main Asteroid Belt between Mars and Jupiter and will orbit both Vesta and Ceres – a feat enabled solely thanks to the revolutionary ion propulsion system.

“At the same time I must admit we are also not humble about it. Dawn is truly an amazing mission. A low cost mission, using NASA’s advanced technology to enormous scientific advantage. It is really, really a great mission,” Russell told me.

Vesta is the second most massive asteroid and Dawn’s discoveries of a surprisingly dichotomous and battered world has vastly exceeded the team’s expectations.

Asteroid Vesta from Dawn - Exquisite Clarity from a formerly Fuzzy Blob
NASA's Dawn spacecraft obtained this image of the giant asteroid Vesta with its framing camera on July 24, 2011. It was taken from a distance of about 3,200 miles (5,200 kilometers). Before Dawn, Vesta was just a fuzzy blob in the most powerful telescopes. Dawn entered orbit around Vesta on July 15, and will spend a year orbiting the body before firing up the ion propulsion system to break orbit and speed to Ceres, the largest Asteroid. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

“Dawn is NASA at its best: ambitious, exciting, innovative, and productive,” Dr. Marc Rayman, Dawn’s Chief Engineer from the Jet Propulsion Lab (JPL), Pasadena, Calif., told Universe Today.

“This interplanetary spaceship is exploring uncharted worlds. I’m delighted Popular Science recognizes what a marvelous undertaking this is.”

JPL manages both Dawn and Mars Science Laboratory for NASA’s Science Mission Directorate in Washington, D.C.

Dawn is an international science mission. The partners include the German Aerospace Center (DLR), the Max Planck Institute for Solar System Research, the Italian Space Agency and the Italian National Astrophysical Institute.

“Very cool!”, John Grotzinger, the Mars Science Laboratory Project Scientist of the California Institute of Technology, told Universe Today.

“MSL packs the most bang for the buck yet sent to Mars.”

Last View of Curiosity Mars Science Laboratory Rover - inside the Cleanroom at KSC.
Curiosity just before Encapsulation for 8 month long interplanetary Martian Journey and touchdown inside Gale Crater. Credit: Ken Kremer

Curiosity is using an unprecedented precision landing system to touch down inside the 154 km (96 miile) wide Gale Crater on Aug. 6, 2012. The crater exhibits exposures of phyllosilicates and other minerals that may have preserved evidence of ancient or extant Martian life and is dominated by a towering mountain.

“10 instruments all aimed at a mountain higher than any in the lower 48 states, whose stratigraphic layering records the major breakpoints in the history of Mars’ environments over likely hundreds of millions of years, including those that may have been habitable for life.”

“It’s like a trip down the Grand Canyon 150 years ago, with the same sense of adventure, but with a lot of high tech equipment,” Grotzinger told me.

MSL also has an international team of over 250 science investigators and instruments spread across the US, Europe and Russia.

Curiosity Mars Science Laboratory rover soars to Mars atop an Atlas V rocket on Nov. 26 at 10:02 a.m. EST from Cape Canaveral, Florida. Credit: Ken Kremer

MESSENGER is the first probe to orbit Mercury and the one year primary mission was recently extended by NASA.

Sean Solomon, of the Carnegie Institution of Washington, leads the MESSENGER mission as principal investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft for NASA.

“Planetary has 3 missions there… Dawn, MESSENGER, and MSL,” Jim Green proudly said to Universe Today regarding the Popular Science magazine awards. Green is the director, Planetary Science Division, NASA Headquarters, Washington

“Three out of 10 [awards] is a tremendous recognition of the fact that each one of our planetary missions goes to a different environment and takes on new and unique measurements providing us new discoveries and constantly changes how we view nature, ourselves, and our place in the universe.”

The First Solar Day
After its first Mercury solar day (176 Earth days) in orbit, MESSENGER has nearly completed two of its main global imaging campaigns: a monochrome map at 250 m/pixel and an eight-color, 1-km/pixel color map. Apart from small gaps, which will be filled in during the next solar day, these global maps now provide uniform lighting conditions ideal for assessing the form of Mercury’s surface features as well as the color and compositional variations across the planet. The orthographic views seen here, centered at 75° E longitude, are each mosaics of thousands of individual images. At right, images taken through the wide-angle camera filters at 1000, 750, and 430 nm wavelength are displayed in red, green, and blue, respectively.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Read more about the Popular Science citations and awards here
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Read continuing features about Curiosity, Dawn and MESSENGER by Ken Kremer starting here:

Curiosity Mars Rover Launch Gallery – Photos and Videos
Curiosity Majestically Blasts off on ‘Mars Trek’ to ascertain ‘Are We Alone?
Dawn Discovers Surprise 2nd Giant South Pole Impact Basin at Strikingly Dichotomous Vesta
Amazing New View of the Mt. Everest of Vesta
MESSENGER Unveiling Mercurys Hidden Secrets

MESSENGER Unveiling Mercurys Hidden Secrets

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NASA’s MESSENGER probe to Mercury, the scorched, innermost planet of our solar system, is sending back so much startling and revolutionary data and crystal clear images that the results are forcing scientists to toss out previously cherished theories and formulate new ones even as the results continues to pour in. And the mission has barely begun to explore Mercury’s inner secrets, exterior surface and atmospheric environment.

MESSENGER became the first spacecraft ever to orbit planet Mercury on March 18, 2011 and has just completed the first quarter of its planned one year long mission – that’s the equivalent of one Mercury year.

MESSENGER has collected a treasure trove of new data from the seven instruments onboard yielding a scientific bonanza; these include extensive global imagery, measurements of the planet’s surface chemical composition, topographic evidence for significant amounts of water ice, magnetic field and interactions with the solar wind, reported the science team at a press conference at NASA Headquarters.

Schematic illustration of the operation of MESSENGER's X-ray Spectrometer (XRS). When X-rays emitted from the Sun’s corona strike the planet, they can induce X-ray fluorescence from atoms at the surface. Detection of these fluorescent X-rays by the XRS allows determination of the surface chemical composition. Credit: NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

“We are delighted to share the findings of the first 25% of our year long mission,” said MESSENGER principal investigator Sean Solomon of the Carnegie Institution of Washington at a press briefing for reporters. “We receive new data back almost every day.”

“MESSENGER has snapped over 20,000 images to date,” said Solomon, at up to 10 meters per pixel. The probe has also taken over two million laser-ranging topographic observations, discovered vast volcanic plains, measured the abundances of many key elements and confirmed that bursts of energetic particles in Mercury’s magnetosphere result from the interaction of the planets magnetic field with the solar wind.

“We are assembling a global overview of the nature and workings of Mercury for the first time.”

“We had many ideas about Mercury that were incomplete or ill-formed, from earlier flyby data,” explained Solomon. “Many of our older theories are being cast aside into the dust bin as new observations from new orbital data lead to new insights. Our primary mission has another three Mercury years to run, and we can expect more surprises as our solar system’s innermost planet reveals its long-held secrets.”

Magnetic field lines differ at Mercury's north and south poles As a result of the north-south asymmetry in Mercury's internal magnetic field, the geometry of magnetic field lines is different in Mercury's north and south polar regions. In particular, the magnetic "polar cap" where field lines are open to the interplanetary medium is much larger near the south pole. This geometry implies that the south polar region is much more exposed than in the north to charged particles heated and accelerated by solar wind–magnetosphere interactions. The impact of those charged particles onto Mercury's surface contributes both to the generation of the planet's tenuous atmosphere and to the "space weathering" of surface materials, both of which should have a north-south asymmetry given the different magnetic field configurations at the two poles. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

NASA’s Mariner 10 was the only previous robotic probe to explore Mercury, during three flyby’s back in the mid-1970’s early in the space age.

MESSENGER was launched in 2004 and the mission goal is to produce the first global scientific observations of Mercury and piece together the puzzle of how Mercury fits in with the origin and evolution of our solar system.

There was very little prior imaging coverage of Mercury’s northern polar region.

“We’ve now filled in many of the gaps,” said Messenger scientist Brett Denevi of Johns Hopkins University’s Applied Physics Laboratory (APL). “We now see large smooth plains that are thought to be volcanic in origin.”

“Now we’re seeing for the first time their full extent, which is around 4 million square kilometers (1.54 million square miles). That’s about half the size of the continental United States.”

MESSENGER is currently filling in coverage of Mercury’s north polar region, which was seen only partially during the Mariner 10 and MESSENGER flybys. Flyby images indicated that smooth plains were likely important in Mercury’s northernmost regions. MESSENGER's orbital images show that the plains are among the largest expanses of volcanic deposits on Mercury, with thicknesses of several kilometers in many places. The estimated extent of these plains is outlined in yellow. This mosaic is a combination of flyby and orbital coverage in a polar stereographic projection showing latitudes from 50° to 90° N. The longitude at the 6 o'clock position is 0°. Credit: NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

“We see all kinds of evidence for volcanism and tectonic deformation of the plains from orbit where we can look straight down,” added Denevi. “In the new images we see ghost craters from pre-existing impact craters that were later covered over by lava.’

Color images of the whole planet – with a resolution of about 1 kilometer per pixel – tell the researchers about the chemical composition and rock types on Mercury’s surface.

“We don’t know the composition yet.”

“We are very excited to study these huge volcanic deposits near the north pole with the implications for the evolution of Mercury’s crust and how it formed,” said Denevi.

“Targeted new high resolution imaging is helping us see landforms unlike anything we’ve seen before on Mercury or the moon.”

MESSENGER’s orbital images have been overlaid on an image from the second flyby shown in Image 1.2a. Even for previously imaged portions of the surface, orbital observations reveal a new level of detail. This region is part of the extensive northern plains, and evidence for a volcanic origin can now be seen. Several examples of “ghost” craters, preexisting craters that were buried by the emplacement of the plains, are seen near the center of the mosaic. Credit: NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Determining whether Mercury harbors caches of polar water ice is another one of the many questions the MESSENGER science team hopes to answer.

Two decades ago, Earth-based radar images showed deposits thought to consist of water ice near Mercury’s north and south poles. Researchers postulated a theory that these icy deposits are preserved on the cold, permanently shadowed floors of high-latitude impact craters, similar to those on Earth’s moon.

Early results from topographic measurements are promising.

“The very first scientific test of that hypothesis using Messenger data from orbit has passed with flying colors.”

“The area of possible polar water ice is quite a bit larger than on the moon,” said Solomon. “Its probably meters or more in depth based on radar measurements.”

“And we may have the irony that the planet closest to the sun may have more water ice at its poles than even our own moon.”

“Stay tuned. As this mission evolves, we will be relying on the geochemical and remote sensing instruments which take time to collect observations. The neutron and gamma ray spectrometers have the ability to tell us the identity of these icy materials,” said Solomon.

The same scene as that in Image 1.3a is shown after the application of a statistical method that highlights differences among the eight color filters, making variations in color and composition easier to discern. Credit: NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
This topographic contour map was constructed from the several MLA profiles (lines of white circles) that pass through and near the crater circled in Image 3.4. The color scale at right is in km, and north is at the 4 o’clock position. Calculations show that the topography of the crater is consistent with the prediction that the southernmost portion of the crater floor is in permanent shadow. Credit: NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
A cross-section of Mercury’s magnetosphere (in the noon-midnight plane, i.e., the plane containing the planet-Sun line and Mercury’s spin axis) provides context for the energetic electron events observed to date with the MESSENGER XRS and GRS high-purity germanium (HpGe) detectors. The Sun is toward the right; dark yellow lines indicate representative magnetic field lines. Blue and green lines trace the regions along MESSENGER's orbit from April 2 to April 10 during which energetic electrons were detected and MESSENGER's orbit was within ± 5° of the noon-midnight plane. The presence of events on the dayside, their lack in the southern hemisphere, and their frequency of occurrence at middle northern latitudes over all longitudes point to a more complex picture of magnetospheric activity than found at Earth. Credit: NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Alan Shepard and MESSENGER Stamps Unveiled at Kennedy Space Center Ceremony

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KENNEDY SPACE CENTER – 50 Years ago this week, Alan B. Shepard became the first American to be launched into space. Shepard blasted off on May 5, 1961 from Cape Canaveral, Florida. NASA and the US Postal Service honored Shepard’s historic achievement today (May 4) at an Official First-Day-of-Issue dedication ceremony at NASA’s Kennedy Space Center in Florida.

Alan Shepard was one of the seven Project Mercury astronauts – who will be collectively known for all eternity as – “The Original 7”.

The US Postal Service simultaneously released two new 44 cent Forever Stamps at today’s commemoration, which bookend two historic space achievements – Shepard’s inaugural manned spaceflight aboard the Mercury capsule and NASA’s unmanned MESSENGER mission which recently became the first probe from Earth to achieve orbit about the Planet Mercury.

Alan Shepard and MESSENGER First-Day-of-Issue Stamp dedication ceremony at NASA’s Kennedy Space Center on May 4, 2011. Alan Shepard is the only American astronaut to be honored with his image on a US postal stamp. Credit: Ken Kremer

Fellow Mercury Astronaut Scott Carpenter attended the ceremony and unveiled the stamps along with Steve Masse, United States Postal Service Vice President of Finance at the Rocket Garden at the KSC Visitor Complex.

Mercury Astronaut Scott Carpenter poses in front of a Mercury Atlas rocket at the Rocket Garden at KSC. Carpenter was propelled to space by the Atlas rocket as the 2nd American to orbit the Earth on May 24, 1962. Credit: Ken Kremer

“Today we celebrate the 50th anniversary of many, many important issues, among them is the first steps from the home planet that were taken by the family of man,” said Carpenter.

Although Shepards suborbital flight aboard the one man “Freedom 7” Mercury capsule lasted barely 15½ minutes, the flight ignited America’s Moon landing effort and propelled American Astronaut Neil Armstrong to become the first human to set foot on the moon on July 20, 1969 during the Apollo 11 mission – one of the crowning technological achievements of the 20th Century.

The success of “Freedom 7” emboldened President John F. Kennedy to declare that America “should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the Earth,” just three weeks later on May 20.

“That was largely a response to Alan’s success,” Carpenter told the crowd of assembled officials, journalists and visitors. Also on hand for the stamp dedication was Shepard’s daughter Laura Shepard Churchly; Charles Bolden, NASA Administrator and former shuttle astronaut; Bob Cabana, KSC Director and former shuttle astronaut; and Jim Adams, NASA deputy director, Planetary Science.

“A decision was made not to put 44 cents on the stamp, but it is forever,” Carpenter emphasized. “It is appropriate to the time we should honor and remember Alan B Shepard and Freedom 7.”

Alan Shepard display at the Kennedy Space Center Visitor Complex. Credit: Ken Kremer

Shepard’s tiny capsule measured just six feet by six feet, reached a maximum speed of 5,100 MPH, roughly eight times the speed of sound, and a zenith of 116 miles above the Earth. Freedom 7 was bolted atop a Redstone rocket that generated only 78,000 pounds of thrust, followed a ballistic arc and landed 300 miles down range in the ocean.

“These stamps, which will go out by the millions across this country, are a testament to the thousands of NASA men and women who shared dreams of human spaceflight and enlarging our knowledge of the universe,” said Bolden.
Shepard’s flight and MESSENGER both blasted off from launch pads quite close to one another at Cape Canaveral Air Force Station which is adjacent to the Kennedy Space Center.

Mercury Astronaut Scott Carpenter is applauded at tribute to Alan B. Shepard, first American in Space ceremony at the Rocket Garden at KSC on May 4, 2011. Credit: Ken Kremer

On Thursday May 5, watch for my on site coverage of NASA’s special ceremony marking the 50th Anniversary of Shepard’s milestone “Freedom 7” mission – and an interview with Scott Carpenter.

Shepard’s mission came barely three weeks after Cosmonaut Yuri Gagarin became the first human to orbit the Earth. The bold flights of these brave Cosmonauts and Astronauts – backed by a few insightful political leaders – began the Era of Human Spaceflight. As the shuttle program winds to a close, the future of US Human Spaceflight is very uncertain.

Read my related articles about Yuri Gagarin and the 50th Anniversary of Human Spaceflight:

Yuri Gagarin and Vostok 1 Photo Album – 50th Anniversary of Human Spaceflight
Countdown to Yuri’s Night and the 50th Anniversary of Human Spaceflight !
Stirring Video Tributes to Yuri Gagarin
Yuri Gagarin From the Earth to Mars Tribute

NASA Administrator and former shuttle astronaut Charles Bolden praises Alan Shepard at KSC stamp unveiling ceremony on May 4, 2011. Credit: Ken Kremer

Mercury Diagram

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Here’s a Mercury diagram, showing the interior of planet Mercury.

Mercury is the closest planet to the Sun, orbiting at an average distance of 57.9 million km from the Sun. It’s also the second densest planet in the Solar System, with an average distance of 5.427 grams per cubic centimeter. Based on this density, astronomers have some estimates about the interior structure of Mercury.

The center of Mercury is its metal core, similar to the Earth’s core. But in the case of Mercury, the core occupies 42% of the volume of Mercury, while the core of Earth is only 17%. And for some reason, the metal core of Mercury doesn’t create a magnetic field with the same intensity of Earth’s magnetic field. Mercury’s magnetosphere is only 1% as strong as Earth’s field.

Surrounding the core is Mercury’s mantle. This is a 500-700 km thick layer of rock, composed of silicates. And surrounding the mantle is Mercury’s crust. Based on observations made by Mariner 10 and Earth-based telescopes, astronomers think that Mercury’s crust is 100 – 300 kilometers thick. There are many large depressions in Mercury’s crust, and scientists think these formed as Mercury slowly cooled and contracted.

We’ve written many articles about Mercury for Universe Today. Here’s an article with a diagram of the Sun, and here’s a diagram of the Solar System.

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

We’ve also recorded an episode of Astronomy Cast about Mercury. Listen here, Episode 49: Mercury.

Length of Year on Mercury

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The length of year on Mercury is 87.969 days. In other words, it takes almost 88 Earth days for Mercury to complete one orbit around the Sun. Mercury completes just over 4 orbits for each year on Earth.

Mercury has the most eccentric of all the orbits of the planets. Its distance from the Sun varies between 46 million and 70 million kilometers. This means that the speed of its orbit varies dramatically depending on the point of its orbit. If you could stand on the surface of Mercury and watch the Sun, you would see the Sun rise in the morning go part way up in to the sky and then go backwards in the sky, and set again. And then it would rise again and this time it would go across the sky and set. Four days before the fastest point of its orbit around the Sun, Mercury’s orbital speed matches its rotational velocity so that the Sun appears to stop in the sky. Then it’s orbiting faster than it’s rotating for about 8 days and so the Sun appears to move backwards.

We’ve written several articles about the length of years for Universe Today. Here’s an article about the years of all the planets, and here’s an article about how long a year is on Mars.

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

We’ve also recorded several episodes of Astronomy Cast about Mercury. Listen here, Episode 49: Mercury.

Mercury Revolution

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In scientific terms, an orbital revolution is the amount of time it takes for one object to orbit completely around another. So a Mercury revolution is the amount of time it takes for Mercury to completely orbit the Sun and then come back to its initial position. Here on Earth, we call that a year.

Mercury’s revolution around the Sun takes 87.969 days. So, you could say that Mercury’s year lasts almost 88 days.

But if you were standing on the surface of Mercury, you wouldn’t experience that many days. That’s because Mercury rotates on its axis very slowly, taking almost 59 days to rotate once. The strange thing is that if you were standing on the surface of Mercury, you would experience something very different. You would see the sun rise halfway, and then go back down again, and then rise up again before setting. The whole process would take about 2 of Mercury’s years.

Remember, the revolution of Mercury is how long the planet takes to travel around the Sun. The rotation of Mercury is how long it takes to turn once on its axis.

We’ve written many articles about Mercury for Universe Today. Here’s an article about the gravity on Mercury, and here’s an article about the composition of Mercury.

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

We’ve also recorded several episodes of Astronomy Cast about the Solar System. Listen here, Episode 49: Mercury.

How Long Does it Take Mercury to Orbit the Sun?

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Mercury is the closest planet to the Sun, and so it’s the fastest to orbit the Sun. In fact, Mercury only takes 88 days to orbit the Sun. In other words, Mercury’s orbit only takes 24% as long as Earth’s orbit.

If you were born on Mercury, you would have celebrated 4 times as many birthdays as you do on Earth. In other words, if you’re 10 here on Earth, you’d be 40 in Mercury years. Now that’s a possible way to grow up more quickly.

Mercury orbits the Sun at an average distance of only 57.9 million km. Compare this with Earth’s average orbital distance of 150 million km.

Unlike the other planets in the Solar System, Mercury doesn’t really experience any seasons. This is because Mercury has no atmosphere to trap heat from the Sun. Whichever side of Mercury is currently facing the Sun experience temperatures of up to 700 Kelvin. And then the side of the planet that’s in the shade dips down to only 100 Kelvin; that’s well below freezing. Even though Mercury is close, you would experience incredibly cold temperatures if you lived on the surface.

The orbit of Mercury was actually a great puzzle to astronomers until the 20th century. They couldn’t explain why the point of Mercury’s furthest orbit of the Sun was slowly drifting at a rate of 43 arcseconds per century. But this strange motion was finally explained perfectly by predictions made by Albert Einstein with his Theory of Relativity.

We have written many articles about Mercury for Universe Today. Here’s an article about Mercury giving up more secrets to the MESSENGER spacecraft, and here’s a massive mosaic image 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 just about Mercury. Listen here, Episode 49: Mercury.

Exploration of Mercury

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As one of the planets visible with the unaided eye, Mercury has been known before recorded history. But until the development of the telescope, the exploration of the Mercury was only unaided eye observations. Early cultures like the Mayans and ancient Greeks were diligent astronomers, and calculated the motions and positions of Mercury with tremendous accuracy.

But the exploration of Mercury really began with the invention of the telescope. Galileo Galilei was the first to turn his telescope on the 1st planet, seeing nothing more than a small disk. Galileo’s telescope wasn’t powerful enough to see that Mercury has phases, like the Moon and Venus. In 1631, Pierre Gassendi made the first observations of Mercury’s transit across the surface of the Sun, and further observations by Giovanni Zupi revealed its phases. This helped astronomers to conclude the Mercury orbited the Sun, and not the Earth.

Because Mercury is so small, and located so close to the Sun, astronomers weren’t able image features on its surface with any accuracy. It wasn’t until the 1960s, when Soviet scientists bounced radio signals off the surface of Mercury that astronomers got any sense of what its surface was like. These radio reflections also helped astronomers discover that Mercury’s day length is 59 days; almost as long as its year of 88 days.

But the best Mercury exploration happened when NASA’s Mariner 10 spacecraft first flew past Mercury in 1974. It revealed that Mercury’s surface is pockmarked with craters like the Earth’s moon. And like the Moon it has flat regions filled in with lava flows. After two additional flybys Mariner 10 ended up mapping only 45% of Mercury’s surface.

The next mission to explore Mercury was NASA’s MESSENGER spacecraft, launched on August 3, 2004. It made its first Mercury flyby on January 14, 2008, mapping more of Mercury’s surface. MESSENGER will eventually go into orbit around Mercury, mapping its surface in great detail and answering many unknown questions about Mercury and its history.

We have written many stories about Mercury here on Universe Today. Here’s an article about a the discovery that Mercury’s core is liquid. And how Mercury is actually less like the Moon than previously believed.

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

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 Solar System Exploration: Missions to Mercury
NASA: Planetary Science