A comely cometary coincidence / New camera to record cargo ship’s fiery reentry

In this happy alignment, perfectly composed and exposed by Italian amateur astronomer Rolando Ligustri, Comet Jacques pairs up with IC 405, the Flaming Star Nebula on July 26. The comet will be visible in binoculars now until the moon returns to brighten the sky around August 8. Credit: Rolando Ligustri

A stunning photo! It’s comet C/2014 E2 Jacques, tail as straight as a Q-tip, forming a cosmic question mark with the glowing cloud of hydrogen gas called the Flaming Star Nebula. Two tails stand out. The one reaching beyond the frame is made of carbon monoxide gas fluorescing in the sun’s ultraviolet light. To the left of the bright head a meeker dust tail shines by reflected sunlight.

This close-up photo taken July 25 reveals that the glowing gas tail (right) is made of multiple streamers. Heat from the sun vaporizes ices which stream back to form a comet’s tails. Credit: Damian Peach

The nebula’s 1,500 light years away in the direction of the constellation Auriga the Charioteer, while Jacques plies the solar system just 112 million miles from Earth. Discovered by a group of Brazilian amateur astronomers last March, a study of its orbit hinted it might wax bright enough to see with binoculars after making its closest approach to the sun in late May.

That’s exactly what happened, and you can see it right now – assuming you’re willing to rise at 4 a.m. – low in the northeastern sky just before the start of morning twilight. I caught it in 8×40 and 10×50 binoculars Saturday from home. No tail stood out but the comet’s head looked like a small, fuzzy spot compared to the sharp points of nearby stars. Through a telescope I saw a dense, bright cotton ball and hint of a tail.

Follow Jacques in a small telescope or binoculars in its travels across Auriga into Perseus during the next two weeks. Comet positions are shown for 4 a.m. CDT every 5 days. Stars to magnitude +8.0. Click to enlarge. Source: Chris Marriott’s SkyMap

Comet Jacques glows at magnitude 6.5 and will remain about that bright through early August. Because the comet’s moving up and away from the sun, it’s getting higher in the east and easier to see with each passing morning.

If you need another reason to arise so early, the International Space Station will light your path all this week and next. Head over to Heavens-Above and click on the ISS link to get times for passes over your city. Simultaneous evening passes begin on or around August 2.

The last of the European Space Agency’s five automated space freighters, ATV-5, is being prepared for launch to the ISS on Tuesday, July 29. Named “Georges Lemaître” in honor of the Belgian astronomer who first proposed the idea of the Big Bang, the ship will ferry six tons of supplies including lots of drinking water and food to the astronauts. If there’s an opportunity to see it ‘chase’ the space station, I’ll provide an update.

Artist’s view of ATV-5’s destructive reentry into Earth’s atmosphere over the Pacific Ocean. A special camera will record the scene from inside. Copyright: ESA–D. Ducros

ATV-5 is the last of the European cargo ships and will burn up like the others during atmospheric reentry once its mission is complete. But this one ends with a twist. The fiery burn-up and disintegration will be recorded from the inside by a unique infrared camera. Before the camera becomes toast, it will transmit the images to a ‘black box’ called the Reentry SatCom, a spherical capsule protected by a heatshield. The SatCom will relay the data to a nearby Iridium satellite and from there back to mission control. Can’t wait to see that video!

Tomorrow’s new moon foretells October’s solar eclipse

Tomorrow July 26, 2014, the invisible new moon will pass a few degrees south of the sun in the daytime sky. Stellarium

New moons aren’t much to look at. You can’t even see them most months of the year. That’s true for tomorrow’s new moon which will invisibly accompany the sun in its journey across the sky.

New moons occur about once a month when the moon passes between the sun and Earth. We can’t see them for two reasons: first, no sunshine touches the Earth-facing side when the moon lies in the same direction as the sun. It’s completely dark. From our perspective, the out-of-view lunar farside gets all the sunlight. Second, since the moon is nearly in line with the sun, it’s utterly lost in the glare of daylight.

The moon seesaws 5 degrees north and south of Earth’s orbit during its monthly cycle because its orbit is tilted with respect to Earth’s. Only when the moon crosses the plane of Earth’s orbit at the same time as a new moon do we see a solar eclipse. Illustration: Bob King

We normally have to wait two days after new moon – when the moon’s orbital motion carries it to the left (east) of the sun – to see it as a thin crescent at dusk.

Most of the time the moon passes north or south of the sun at new phase because its orbit is tilted 5 degrees with respect to Earth’s. But 2.4 times a year on average, new moon coincides with the time the moon’s seesawing path slices through the plane of Earth’s orbit. For a brief time during that crossing, all three bodies are aligned and happy earthlings witness a solar eclipse.

If the alignment is imprecise, the moon blocks only a part of the sun, giving us a partial solar eclipse.  If dead-on, we see a rarer total solar eclipse.

View of the partial solar eclipse across the Upper Midwest a half hour before sunset on October 23. By coincidence, Venus will be near conjunction at the same time and only a couple moon diameters north of the pair. Seeing the planet in a telescope will still be challenging because of daylight glare.  Stellarium

On October 23 this year, the lineup at new moon will be a good if imperfect one with a maximum of 81% of the sun covered. The partial eclipse will be visible across much of North America; from the eastern half of the U.S. and Canada the event will occur near sunset, adding a touch of drama to the scene.

I wrote earlier that we can’t see a new moon. That’s only partly true. We mostly pay attention to the sun’s changing shape during solar eclipses, but the dark, curving bite working its way slowly across the sun’s disk is none other than the new moon seen in silhouette.

Rosetta’s comet shows off new bumps, bruises and bright collar

New views of Comet 67P/C-G show more surface features including a bright collar. The dark band in the middle and right images is the shadow cast by one part of the comet on its other half during rotation. Each picture is separated by 2 hours. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Pictures taken July 20 of comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft show cool new stuff. They include several tantalizing depressions in both lobes of the comet and a bright collar where the two dissimilar halves meet.

3-D model of the comet’s shape based on July 14 images. Credits:ESA/Rosetta/MPS for OSIRIS Team MPS

What makes the comet’s ‘neck’ bright might help us understand how 67P/C-G got its strange shape. It could be a region of freshly exposed ice, differences in the composition of the material where the two lobes meet or a change in elevation in the landscape. More detailed studies including examination with a spectrograph to nail down its composition will have to wait until August 6th and beyond, when Rosetta parks itself in orbit.

Look closely and you’ll also see several depressions. One of the largest is a shallow bowl at the top of the smaller lobe. No one knows if these are impact craters or some kind of collapse pits created where ice below the surface vaporized away during the comet’s regular swings past the sun every 6.5 years.

More and more observations of the 67P/C-G are being made every day. Everything from determining its mass and volume, which will be used to arrive at the comet’s density, to measuring the rate at which gas and dust boils off the icy nucleus.

The animation above covers one full 12.4 hour rotation of the nucleus. The next batch of pictures is expected on July 31 with the first high resolution images arriving on August 6.


Chandra’s X-ray eyes behold catastrophe

To mark the 15th anniversary of NASA’s Chandra X-ray Observatory, four newly processed images of supernova remnants illustrate Chandra’s ability to explore high-energy processes in the cosmos. See end of article for detailed explanations of each. Click to enlarge. NASA/CXC/SAO

15 years ago to the day, NASA’s Chandra X-ray Observatory opened its eyes to the high-energy universe. It was launched aboard the space shuttle Columbia and entered a long elliptical orbit that takes it more than a third of the distance to the moon before returning to its closest approach to Earth of 9,942 miles. This specially tailored path keeps it above the Van Allen radiation belts – which would interfere with its X-ray vision – more than 85% of the time.

Chandra’s long elliptical orbit around the Earth keeps it away from the Van Allen belts and allows the telescope to study an object up to 55 hours without interference. NASA

Chandra, named for Indian-American astrophysicist Subrahmanyan Chandrasekhar who did groundbreaking work on white dwarf stars, is specially designed to detect X-rays emitted by hot and energetic objects in the universe. What we feel as heat – infrared light – is low-energy radiation. Planets, comets and asteroids warmed by the sun emit infrared as surely as our own bodies do.

Radio waves, some infrared and visible light penetrate the atmosphere and make it to the ground. Shorter wavelength light from energetic UV to gamma rays are stopped by the atmosphere. A good thing.

As we move to light of shorter wavelengths, energy content rises. Visible light is more energetic than infrared, UV light more so (it can give us a painful sunburn) and X-rays very much more so. To spew X-rays, something very powerful must be happening in space like a supernova explosion or matter heated to incandescence as it disappears down a black hole.

Earth’s atmosphere acts to filter out dangerous much of the more energetic particles and light waves careening around the cosmos, the reason Chandra had to be pitched into the vacuum of space to use its X-ray specs.

Chandra has observed objects ranging from the closest planets and comets to the most distant known quasars. It has imaged the remains of exploded stars, or supernova remnants, observed the region around the supermassive black hole at the center of the Milky Way, and discovered black holes across the universe.

To celebrate the anniversary, NASA released  four newly processed pictures of supernova remnants, the dusty, gassy leftovers of stars blown to smithereens. Let’s take a look at each in turn:

Chandra view of the Crab Nebula expansion in just 7 months

* Crab Nebula: At its center is a city-sized, extremely compact, rapidly rotating neutron star left after the original sun went supernova in 1054 A.D. Also called a pulsar, the star spews zillions of high-speed particles that plow into the expanding debris field to create a ghostly X-ray nebula.

* G292.0+1.8:  One of only three supernova remnants in the Milky Way known to contain large amounts of oxygen. These oxygen-rich supernovas are of great interest to astronomers because they are one of the primary sources of elements heavier than hydrogen and helium that are necessary to form planets and people. The image shows a rapidly expanding debris field that contains, along with oxygen (yellow and orange), other elements such as magnesium (green) and silicon and sulfur (blue) that were forged in the star before it exploded.

* Tycho’s remnant: The supernova that created the remnant was first noticed by Danish astronomer Tycho Brahe in 1572 as a brand new star in the constellation Cassiopeia. The supersonic expansion of the exploded star produced a shock wave moving outward into the surrounding interstellar gas, and another, reverse shock wave moving back into the expanding stellar debris. Heated to millions of degrees, the gas and debris produce copious X-rays.

* 3C58: 3C58 is the remnant of a supernova observed in the year 1181 AD by Chinese and Japanese astronomers. It contains a rapidly spinning neutron star surrounded by a thick ring of X-ray emission. The pulsar also has produced jets of X-rays blasting away from it to both the left and right, and extending trillions of miles. These jets are responsible for creating the elaborate web of loops and swirls.

As a kid, we used to joke about wishing we had X-ray vision. Now we really do.

How to find the center of the Milky Way … and what lurks there

Want to know where the center of our galaxy is? Face south around nightfall in late July and find the Teapot of Sagittarius about ‘two fists’ to the left of bright Antares in Scorpius. The core is a blank bit of sky just above the spout near the 4.5 magnitude star 3 Sagittarii.  Stellarium

Ever stared straight at the heart of the Milky Way galaxy? Give it a try this coming week. With dark skies and no moon, the time is right.

Artist’s view of the 4 million mass black hole at the center of the Milky Way. The hole measures about 28 million miles in diameter. Credit: NASA

Notice I didn’t say into the heart. No human eyes can penetrate the veil of interstellar dust that cloaks the galactic central point 26,000 light years away in the direction of the constellation Sagittarius. Only X-ray, gamma ray and radio telescopes can ‘part the way’ and expose the galaxy’s dark secret which astronomers call Sagittarius A*.

There, at the center of it all, lies a black hole with a mass of 4 million suns. The innermost 3.2 light years centered on the black hole swarms with thousands of aged stars and about 100 fresh-born ones, some in very tight orbits about the hole. Gas clouds abound, and there’s at least another smaller black hole nearby.

NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, captured these first, focused views of the supermassive black hole at the heart of our galaxy in high-energy X-ray light. Known as Sagittarius A* (A star), the bright flare formed when Sgr A* was consuming and heating matter. The background image, taken in infrared light, shows its location. Credit: NASA

Occasionally the central black hole flares to life when a random asteroid, gas cloud or stray star passes too close and gets ripped to pieces before disappearing down the gullet of the beast. Heated by friction, the material sends out every type of light from visible to X-rays and gamma rays. But no one can see all the excitement because it’s hidden by light years of dust grains. To the eye, the center looks nondescript and static, but nothing could be further from the truth.

The Milky Way is beautiful to gaze at this time of year. Take a drive to the country and park your car where the sky is dark and open to the south. At nightfall, you’ll see a fiery-hued star a few fists up from the southern horizon at nightfall. That’s Antares in Scorpius. Now shift your gaze two fists to the left or east and see if you can spot the outline of the Teapot. Once you’ve found it, galactic center lies just above the spout.

Closeup of the Spout showing a couple bright star clusters and the Lagoon Nebula, a rich star-forming region. Credit: Bob King

Though the center remains hidden, large chunks of the Milky Way hover like clouds against the black sky. Every puffy piece is comprised of billions of distant stars the light of which blends together to form a misty haze. Here and there are smaller knots. These are individual gas clouds called nebulae and bright star clusters. A pair of 40-50mm binoculars will show many of these wonders and countless fainter stars plainly. If we could magically remove the dust between us and the galactic center, the rich intensity of stars in the Sagittarius direction would be bright enough to cast shadows at night.

Take it all in. Let your eyes follow the arc from the southern horizon clear up across the eastern sky and back down to the northeastern horizon. We live here – can you believe it?

Moon nestles in Hyades then departs for Venus

The crescent moon slips in front of the Hyades star cluster only a degree from Aldebaran tomorrow morning. Don’t miss the other bright star cluster, the Pleiades, just above. Look low in the northeastern sky about an hour before sunrise to catch the scene. Stellarium

That old devil moon’s up to its old tricks again. Tomorrow morning, early risers will see it tucked inside the V-shaped face of Taurus the Bull. Better known as the Hyades star cluster, look for the crescent to pass just 1° north of the bright star Aldebaran. A pair of binoculars will enhance the view by pulling in more stars and revealing details in the spooky, earth-lit moon. Sunlight illuminates the lunar crescent, but the remainder is light reflecting off Earth out to the moon and back again.

The crescent is lit by the sun while the remainder glows dimly from twice-reflected light called earthshine. Credit: Bob King

To the eye, ‘earthlight’ looks smoky gray and nearly featureless though binoculars will show the lunar seas and larger craters. The quality of the light mimics a lunar eclipse but instead of red we see the pale blue glow of sunlight reflecting back from our planet’s oceans.

At 153 light years, the Hyades is the nearest star cluster to our solar system, one of the reasons you can see it without a telescope. Aldebaran appears to be a full-fledged cluster member, but it’s a ruse. The bright, ruddy star lies much closer to us along the same line of sight.

Venus and a very thin crescent moon on July 24 about 45 minutes before sunrise low in the northeast. Stellarium

The Hyades were born in a dense cloud of interstellar dust and gas 625 million years ago around the time underwater life flourished in the late Precambrian era. When you gaze at the cluster tomorrow, the light that touches your retinas left the Hyades the same time Abraham Lincoln took office.

The moon moves on toward Venus after vacationing in the Hyades, passing south of the planet on Thursday morning. It will be extremely thin that morning and should make a pretty sight for anyone looking low in the northeastern sky 45 minutes before sunrise.

Shhh! Don’t wake the sun

Contrast these views of the nearly spotless sun on July 16-17, 2014 with a picture taken about two weeks earlier (below). Credit: Giorgio Rizzarelli

Who doesn’t enjoy a nap on a lazy summer afternoon? That’s what the sun’s been up to past few days. Instead of a steady parade of sunspots, it put its pencils away and went to sleep. For a time on July 17 not a singe magnetic blemish marred the entire Earth-facing hemisphere. The last time that happened was nearly 3 years ago on Aug. 14, 2011.

Ten groups including three visible with the naked eye dot the sun on July 8, 2014. Credit: NASA

The solar blank stare lasted but a day; by the 18th two small groups emerged. Today three little spot clusters have emerged but again, they’re on the small side.

I think the reason the sun looks so stark is that only two weeks ago nearly a dozen sunspot regions freckled the disk including three visible with the naked eye with a safe solar filter.

These ups and downs aren’t unusual unless this downturn continues for weeks. Expect more bubbles of magnetic energy to rise from beneath the glaring surface of the sun called the photosphere and spawn fresh groups soon. Because we now have eyes on the farside of the sun courtesy of the dual STEREO solar probes, we know the complete story. There are at least seven spotted regions in hiding there today.

Sunspot numbers are plotted for the last three solar cycles through the present. The double peak of the current cycle is shown. Credit: NASA

Sunspots and flares peak approximately every 11 years. We’re still riding the roller coaster near the top of the arc after the most recent solar maximum in late 2013. Some maxima are strong, others weak. The current max – Cycle 24 – is the weakest since Cycle 14 in February of 1906 and one of the wimpiest on record. Occasionally a cycle will have two peaks like the current one. The first peak occurred in Feb. 2012 and the second just this past June. What makes Cycle 24 even more unusual is that the second peak is higher than the first – the first time this has ever been recorded. Like people, every maximum has a personality of its own.

Doug Bieseker of the NOAA Space Weather Prediction Center has analyzed historical records of solar activity and he finds that most large events such as strong flares and significant geomagnetic storms typically occur in the declining phase of solar cycles—even weak ones, so don’t give up hope for some great auroral displays ahead.

A coronal mass ejection blew off on the farside of the sun early this morning July 20. It appears to envelop Jupiter, but the planet is 490 million miles in the background. SOHO uses an occulting disk to block the brilliant sun. Credit: NASA/ESA

The sun’s got a buddy this week – Jupiter! We can’t see the planet from the ground because it’s swamped by solar glare, but the Solar and Heliospheric Observatory (SOHO) has a great view from space. Watch the sun approach from the right and pass the planet over the next few days. After the 24th, Jupiter will move into the morning sky.

Let’s face it, comets are just weird

Comet 67P/C-G photographed from a distance of about 7,500 miles (12,000 km) on July 14 the European Space Agency’s Rosetta spacecraft. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Mesmerizing. The recent video of Comet 67P/Churyumov-Gerasimenko tumbling end over end looks like a boot booted into space. 36 images were used to create the brief time lapse which makes it look like as if the comet’s spinning rapidly. Its actual rotation period is 12.4 hours. Still, the extremely irregular shape of the 67P/C-G poses new challenges for the Rosetta team as they contemplate how to safely set down the Philae lander on such an irregularly shaped body come November 11.

Questions abound on how 67P/C-G got its peculiar shape. Most familiar solar system bodies like the planets and many of their moons are spherical or nearly so. Gravity’s the sculptor here. If an object’s about 240 miles (385 km) or larger in diameter, self-gravity will pull everything to the center and collapse the body into a sphere. Small objects like comets and most asteroids just don’t have enough material to ‘go spherical’. Comet 67P/C-G is only a few miles across, so it’s free to assume a variety of shapes.

These are all the comets we’ve seen up close so far by sending spacecraft there. All are small and most non-spherical. Credit: NASA/ESA

This all reminds me of a famous anecdote about Fritz Zwicky, a brilliant but prickly Swiss astronomer who worked most of his life at Caltech. He pioneered the use of supernovae as ‘yardsticks’ to measure distances to faraway galaxies and was the first to propose the existence of dark matter. Zwicky didn’t get along with everyone at Mt. Wilson Observatory, calling the astronomers there “Spherical bastards”. Why? “Because they’re bastards no matter how you look at them.”

Comet 8P/Tuttle, believed to be a contact binary, imaged by Arecibo radar Dec. 29, 2007-Jan. 5, 2008. Credit: Arecibo Observatory

ANYWAY … comets, being small icy bodies, come in a great variety of shapes from round to bowling pins to rubber duckies. Many ideas have been tossed around as to how 67P looks the way it does. I haven’t taken a poll but would suspect many astronomers would consider the comet a contact binary, two separate comets on convergent paths moving slowly enough that they melded together into one larger object.

Comet 67P/C-G and the Rosetta spacecraft to scale. The comet is about 4 km (2.5 miles) across. Credit: ESA

We also see contact binaries among the asteroids, but ice makes comets unique. Heat from the sun vaporizes that ice and carves away at the comet’s surface. Could eons of solar heating have shaped Churyumov-Gerasimenko? Comets are also fragile compared to most asteroids; some even crumble apart as they near the sun. It’s possible that vaporization of subsurface ices left parts of 67P in a weakened state which then crumbled away to sculpt its peculiar outline. Other possibilities include a near-catastrophic impact or gravitational stretching  during close encounters with Jupiter or Saturn.

Starting August 6 when Rosetta enters orbit around 67P, scientists will have more than a year to study it up close. Perhaps then we’ll get some more answers on its shapely origins. For instance, if we discover that each lobe of 67P has a different density or composition, the contact binary explanation would make a good fit. For now, let’s just say that comets’ weird shapes make them even more lovable.

Odd glows around sun may be caused by Canadian forest fires

A large, pale blue aureole or disk surrounds the sun this morning July 18 in a sky filled with high-altitude smoke from forest fires burning in Canada’s Northwest Territories. Wide-angle 15mm lens view. Credit: Bob King

It happens every summer. Forest fires in Canada pump out vast quantities of smoke which are carried by winds to the south and east. Arriving days later over the northern Great Plains and Midwest, the blue sky soon turns a pallid gray.

Smoke from forest fires near Faber Lake in the Northwest Territories streams south in this photo taken July 7, 2014 by NASA’s Terra satellite. Credit: NASA

The smoke spreads in subtle ripples and bands and dims sun and stars alike. Technically, the sky is clear, and that’s what you’ll hear from the weather service, but the smoky haze creates an overcast of its own. Sunlight is less intense, while the solar disk glows pale yellow-orange compared to its normal white-yellow. It may even disappear from view well before sunset, fading away in the fiery haze.

Wide-angle photo this morning showing the blue aureole and brownish outer ring around the sun. Could smoke particles be responsible for the appearance? Credit: Bob King

Early this morning, under faux clear skies, I noticed an unusual pale blue disk or aureole around the sun about four fists (40 degrees) wide. Beyond that lay a wide, darker ‘ring’ tinted a pale gray-brown. Forest fires release gobs of minute smoke particles and oil droplets into the atmosphere which, like the ash from volcanic eruptions, can occasionally color the sun or moon blue.

Patches and bands of smoke from forest fires are seen in this National Weather Service satellite photo taken this morning July 18, 2014. Credit:NASA

It works like this. Particles that are about 1 micron across (1/1000 of a millimeter) are the same size as the wavelength of red light. The sun pours out all colors of light, but when the red portion strikes the ash or smoke, it’s scattered about the sky. The shorter wavelength blue light isn’t affected and continues to pass directly to our eyes, coloring the sun a pale blue. In effect, the particles act like a blue filter.

Bishop’s Ring around the sun due to volcanic ash of the Eyjafjallajökull volcano on Iceland. Photographed from Leiden, the Netherlands on May 18, 2010. Credit: Marco Langbroek

I’ve seen no blue moons or suns yet, but I wonder if the blue aureole might be the result of smoke particles. It resembles a phenomenon called Bishop’s Ring seen around the sun during volcanic eruptions and created by ash and sulfur droplets. Notice though the ball of the sun remains red-orange, indicating that the smoke particles are not the right size to create a blue sun. At least not yet.

A red sky sunset Friday evening July 18. Colors are enhanced from airborne smoke. Credit: Bob King

If you live where the sky is affected by the smoke of distant fires, keep an eye on the sun, moon and sky for unusual colors, disks and rings. We’d love to hear what you’re seeing.

Abundant high altitude dust on Mars scatters red light away from the sun, lending both the solar disk and sky near it a pale blue. Photo taken on May 19, 2005 by the Spirit Rover. Credit: NASA/ JPL-Caltech

My blue disk this morning also reminded me of the blue aureole around the rising sun on Mars taken by the Spirit Rover. Dust in the Martian atmosphere scatters red light like much like ash and fire smoke do on Earth. Blue sunrises and sunsets there are probably fairly common.

Can a boot print on the moon last a million years?

Buzz Aldrin first photographed a pristine patch of the lunar soil (left) before stepping onto it with his boot (right). The fine-grained consistency of the soil crisply records details in the tread. It’s estimated the impression will last 1 to 2 million years. Click to enlarge. Credit: NASA

One of my favorite pictures taken during the Apollo 11 mission to the moon 45 years ago was Buzz Aldrin’s famous boot print in the lunar soil. While it looks like he might have been doing it just for fun, pressing his boot into the fine, powdery soil had a purpose.

Aldrin and Neil Armstrong were asked to carefully observe and assess the properties of the regolith. Notice things like how deep their boots sank in the gritty stuff as well as how it affected their ability to walk about on the surface. Close up photos were taken, including 3D stereo images. Mission control didn’t leave a pebble unturned. It was all part of the mission’s Soil Mechanics Investigation.

After taking the first boot print photo, Aldrin moved closer to the little rock and took this second shot. The dusty, sandy pebbly soil is also known as the lunar ‘regolith’. Click to enlarge. Credit: NASA

The soil on the Moon is very fine-grained, with more than half of all grains being dust particles less than 0.1 millimeters across. It not only adhered to their boots in fine layers but provided good traction. Typically the astronauts boots sunk down only one-half to one inch (1.5-2.5 cm) into the lunar regolith.

In this view taken with a camera mounted on the Lunar Module (LEM), Buzz Aldrin takes the picture of his boot next to the rock seen in the earlier photo. The first boot print is just behind his foot. Credit: NASA

Even though the moon is airless, windless and essentially waterless, erosion happens. Bombardment by protons from the solar wind and micrometeorites (bits of interplanetary dust shed by comets and spalled from asteroids) never stops.

Earth’s atmosphere slows micrometeorites, allowing them to drift down gently to the surface. No so on the airless moon, where space grit grinds away mercilessly on the lunar rocks. Before the unmanned Surveyors landed, some astronomers thought that moon dust might be so deep it would swallow a spacecraft. Before Neil Armstrong made his historic ‘first step’, he first tested the ground to make sure it was firm.

More than 3.5 billion years of bombardment by micrometeorites have rounded the outlines of the lunar Apennine Mountains. The lunar module Apollo 15 ‘Falcon’ in the foreground. Click to visit NASA’s Apollo 11 image archive.  Credit: NASA

Their surfaces are riddled with countless ‘zap pits’ from micrometeorites that strike the surface at thousands of miles an hour. Slowly, inexorably the mountains are ground into more rounded forms. It’s estimated that micrometeorites churn the lunar soil once every 10 million years. Aldrin’s boot prints and for that matter, all the impressions in the dust left by the astronauts and their equipment, will remain in place for 1 to 2 million years. Incredibly long by human standards.

Extreme temperature differences between daytime highs and nighttime lows also must play a part in breaking apart rocks and furthering erosion. At the lunar equator, mean surface temperatures reach almost 260 ºF at noon and then drop to -279 ºF during the night. The moon also gets whacked by larger meteorites that send up plumes of dust that fill in crevices and soften sharp edges.

Sunday marks the 45th anniversary of the Apollo 11 lunar landing, when our toes touched a world other than Earth. While astronauts left countless impressions in the lunar dust, Aldrin’s single boot print has come to symbolize humanity’s first steps from the cradle of Earth into that big thing we call the universe. I hope we can return soon.