Why are radar images of comets shaded only on one side?

Why are radar images of comets shaded only on one side?

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NASA published a picture of a comet the other day. The image shows the comet being lit from above. See NASA's picture below.

However, since it is a radar image, I would have assumed to get a shading on all sides and grazing angles, like velvet or scanning electron microscopy. Or alternatively to have the sides facing the radar dish being shaded, and the edges being dim. For the velvety look, compare to this picture from Wikipedia:

So why is the comet's image shaded only from above?

There is a very good blog article here that explains this effect but basically the Doppler shifting of the radar return means that certain parts of the asteroid light up more than others in the observing wavelengths from Earth. Its better explained in the article>>

Taken from the article "How radio telescopes get "images" of asteroids" Posted by Emily Lakdawalla:

As an asteroid rotates, some parts of it are moving toward us, while other parts are moving away. As the broadcast radio wavefronts hit the part of the asteroid that is moving toward us, the asteroid smacks into each wavefront faster than it would if it were not rotating. The speed of the wavefronts does not change, because the speed of light is constant, so the wavefronts end up being packed closer together. This is a Doppler shift. The asteroid has taken the broadcast wavelength and reflected it at a shorter wavelength from the parts of the asteroid that are rotating toward us. On the other side of the asteroid, which is rotating away, the opposite thing happens; each arriving wavefront smacks into the asteroid a little later than it would if the asteroid were not rotating, so the reflected waves are spread farther apart.


Deep (fake) impact

Every two years, asteroid experts from across the globe come together to pretend an asteroid impact is imminent. During these week-long impact scenarios, participants don’t know how the situation will evolve know from one day to the next but must make plans based on the daily updates they are given.

For only the second time in the conference’s history, ESA will be live tweeting the hypothetical impact scenario – so you’ll find out the ‘news’ as the experts do. What will they do? What would you do?

This year’s asteroid – ‘2021 PDC’

Potential impact hemisphere for 2021 PDC

— An asteroid was discovered on 19 April 2021 and has been given the name “2021 PDC” by the IAU’s Minor Planet Center.

— The day after 2021 PDC is discovered, NASA and ESA ‘impact monitoring systems’ identify several future dates when this asteroid could potentially impact the Earth. Both agree the most likely potential impact is on 20 October 2021 – just 6 months away.

— The probability of that impact is about 1 in 2500. With only two days of tracking on this object, no better estimate of impact probability can be made.

— Very little is known about the physical properties of 2021 PDC. Its size, in particular, is highly uncertain. Its ‘apparent magnitude’ suggests an asteroid of about 120 meters in size. But it could range from 35 – 700 metres!

— Astronomers continue to track the asteroid every night after discovery, and the impact probability steadily increases. As of April 26, 2021, the first day of the 2021 Planetary Defense Conference, the probability of impact has climbed to about 5%. The rest of the scenario will be played out at the conference.

DAY 1: Meet the asteroid – what’s the risk?

Day 1 of the Planetary Defense Conference and we looked in a little more detail at hypothetical asteroid 2021 PDC, as well as determining some initial ideas of the impact effects, if it were to strike.

Chance of impact: 5%

At present, due to limited observations of the asteroid, the uncertainty in 2021 PDC’s path is high.

The red dots show possible positions of 2021 PDC on October 20, 2021, computed using just one week of tracking data since discovery. 5% of the red dots currently intersect Earth, giving a 1 in 20 chance of impact. As more observations are made, the uncertainty region will shrink.

Object size: highly uncertain.

Accounting for current uncertainties, fictional asteroid 2021 PDC could be as large as 700 m or as small as 35 m.

Potential impact zone: 2/3 of Earth’s surface

Based on current orbit knowledge, 2021 PDC could impact anywhere within a region that covers 2/3 of the Earth surface, shaded here in red/purple.

Effect of potential impact:

The worst case scenario for 2021 PDC is it measures 700 m in size. Such an asteroid striking Earth would have catastrophic results, although it is beneath the 1 km threshold for a possible global catastrophe. Best case? At 35 m the asteroid, if it strikes, would cause a major airbust if it broke up in the atmosphere up to devastation on a local scale.

If we take an average size of

150 m, what kind of damage could 2021 PDC do? Anywhere from 0 – 86 million people could be affected. Damage from the blast is the predominant hazard, while heat damage and tsunamis are possible, although less likely and severe.

Potential damage sizes, severities, and locations remain very uncertain.

The primary hazard is the airburst/impact causing blast ‘overpressure’, resulting in minor structural damage to potentially unsurvivable levels
The expected area of damage would have a radius ot 0–500 km,

90 km on average. However, the chance of impact is still slim, meaning there is a 97% chance of no damage, with small chances that an impact could affect thousands to millions of people.

Day 2: New observations confirm *fictional *asteroid will impact

In our hypothetical scenario, one week has past and we are now on 2 May, 2021. New observations confirm that fictional asteroid 2021 PDC will strike Earth in six months. Regions at risk include Europe and Northern Africa. What will the international community do next?

Impact probability: 100 %

Date of impact: 20 October 2021, 17:13 UTC +/- 82 s

Object size: still highly uncertain

Accounting for current uncertainties, the asteroid could still be as large as 700 m or as small as 35 m.

Impact region/location:

Day 2 of the Planetary Defense Conference and our knowledge of the asteroid’s path has improved. Unfortunately, the *fictional* asteroid will strike somewhere in the red region.

Somewhere within a large region covering much of Europe and extending into northern Africa. Countries most at risk include Denmark, Germany, Czech Republic, Austria, Slovakia, Hungary, Slovenia, Croatia, Serbia, Montenegro and Albania. The region extends on the north to Norway and Sweden, on the west, to England, France and Italy, on the east, to countries including Lithuania, Poland, Ukraine, Romania and Bulgaria, and on the south, to Greece and Egypt.

Size of Damage Area Around Impact Site: Highly uncertain

Depending on the object size, severe damage from the airblast could extend anywhere from “Minimal” (a few kilometers) to “Local” (tens of kilometers) to “Regional” (hundreds of kilometers).

What are our options for a space mission?
The Space Missions Planning and Advisory Group (SMPAG) – an international forum of space agencies – is considering the feasibility of space missions as a coordinated international response to 2021 PDC – the main issues are: time is limited and we do not have a clear idea of the size of the asteroid.

Because of the very short time until the fictional impact, our options are limited. Most of the options described in day 1’s Twitter poll are most effective when used to nudge the asteroid gently, resulting in a notable change of direction that builds up over time.

However, the force required to shift fictional asteroid 2021 PDC off a collision course with Earth is so large it risks breaking up the asteroid – perhaps creating multiple large fragments that could impact Earth.

Current options available are to send a reconnaissance mission (to get more info on the imagined asteroid), and/or send a mission with a nuclear explosive device of 4.5 million tonnes – the deliverable yield of a high-speed intercept mission.

However, various international laws rule out use of nuclear weapons in space. So, what will the international community do?

Day 3: Mission impossible

It’s day three of the Planetary Defense Conference and there are new – not-so-positive – developments playing out in the fictional impact scenario. We now jump ahead two months to 30 June, less than four months until imaginary asteroid 2021 PDC is due to strike Earth. New space-based infrared measurements have improved our understanding of the asteroid’s impact effects.

The fictional impact is expected to occur somewhere within an area of central Europe roughly 800 km long by 250 km wide. Countries at risk include Germany, Czech Republic, Austria, Slovenia and Croatia.

There is a 99% chance the impact will be located within the large shaded region, an 87% chance it will occur within the middle contour, and 40% inside the central dark red region. Future predicted impact regions will be smaller, and they will nest within the current large shaded region.

So what about the size of the imaginary asteroid? New measurements by the NEOWISE satellite indicate 2021 PDC cannot be as large as previously thought possible. The new size range is anywhere from 30 – 500 m.

Taking an average size for the asteroid of 136 m – what kind of damage could we expect? Anywhere from 0 – 6.6 million people could be affected. The primary hazards are the airburst and impact, which would damage a region up to 250 km

The following image shows the region of potential damage risk, which is much larger than the region in the previous image because serious damage could extend for tens or even hundreds of kilometers around the impact point. This potential damage risk region is about 1400 kilometers long by 700 kilometers wide.

So … what about the possibillity of a space mission to deal with the asteroid? The Space Missions Planning and Advisory Group (SMPAG) has concluded that no space missions can be launched to fictional asteroid 2021 PDC in time to deflect or disrupt it.

In our fictional impact scenario, the date is now 14 October 2021, six days before imaginary asteroid 2021 PDC impacts Earth. The asteroid is currently 6.3 million km away, heading for Earth at a speed of 10.7 km/s.

Chance of impact: 100%

Size of object: New radar images show the size of 2021 PDC to be 105 m +/- 10%

Impact velocty: 15.2 km/s

Impact location: A region about 23 km across, centered near the borders of three countries – Germany, Czech Republich and Austria. The impact location can be predicted to within 23 km, and time to within one second.

The shaded regions in this image show where the impact is most likely to occur. There is a 99% chance the impact will be located within the outer contour, 87% inside the middle contour, and 40% inside the central dark red region.

The following image shows the region of potential damage risk, which is much larger than the region in the previous image because serious damage could extend for up to a hundred kilometers or so from the impact point. In the highest impact-energy case, the region for serious potential damage risk is about 300 km across, as indicated by the
shaded region the extent of serious damage for the average case, indicated by the line contours, is about 150 km across.

These Images Expose the Dark Side of the Solar System

I f you want to understand the flamboyant family of objects that make up our solar system—from puny, sputtering comets to tremendous, ringed planets—you could start by immersing yourself in the technical terms that fill the scientific literature. Oblateness. Grabens. Magnetosphere. Volatiles. By all means, take the plunge if you are so inclined. It is quite rewarding. But if your goal is to develop a more intuitive feel for your place within this colorful community around the sun, you can start the speed course with a single word taken from the world of art: chiaroscuro.

Renaissance artists coined the term to describe a novel aesthetic defined by extreme contrasts between the bright and dim parts of a painting. Chiaroscuro (“light-dark” in Italian) gave the canvas an expansive, three-dimensional feel and a sense of emotional mystery. To the artists who embraced it, including Leonardo, Rembrandt, and Vermeer, this fresh approach also established a sharp break from the hard, flat style of medieval art. To skywatchers, though, chiaroscuro was just an extremely belated recognition of a natural truth that their predecessors had discovered millennia earlier. The workings of the heavens are expressed through the contrasting interplay of light and dark.

The most gorgeous eclipses in the solar system are never visible from Earth.

Although light might seem to be the dominant element in the sky, darkness often contains the most powerful lessons. Darkness defined the most awesome and most feared of astrological events, a total eclipse of the sun, and inspired some of the greatest advances in the history of science. Chinese astrologers began producing written records of eclipses going back to at least 2000 B.C. By the sixth century B.C., the Babylonians had developed a sophisticated calendar that allowed them to predict eclipses with remarkable accuracy.

In modern times, solar eclipses led to the discovery of the element helium and violent eruptions on the sun in 1919, the shadow of a solar eclipse enabled researchers for the first time to validate Albert Einstein’s general theory of relativity. Even today, chiaroscuro remains a powerful tool of discovery and comprehension. Even today, chiaroscuro remains a powerful tool of discovery and comprehension. All across the solar system, darkness exposes the secret locations that light only obscures: the remarkable places where moons are ripped apart, comets are born, and alien life may be swimming about in an ice-cloaked ocean.

NASA / JPL-Caltech/Malin Space Science Systems / Texas A&M Univ.

A Blotted Sun on Mars

Solar eclipses are not unique to Earth. They are possible on any world that has a moon aligned with the sun. Mars has two such moons, Phobos and Deimos. Both are tiny—Phobos, the larger, is 14 miles in diameter, scarcely bigger than Manhattan Island—but they also circle extremely close to the planet. As a result, Phobos appears large enough to blot out a large fraction of the sun as seen from the surface of Mars. Phobos also completes an orbit in just 7 hours and 39 minutes, so its dark shadow is constantly sweeping across the Martian landscape.

NASA’s nuclear-powered Opportunity rover observed one of the Phobos eclipses on Aug. 20, 2013. The timing of these events helps planetary scientists to monitor the movements of the Martian moons and to predict their fate. Phobos is steadily spiraling inward toward Mars the latest calculations indicate that it will be ripped apart by the planet’s gravity in less than 50 million years. Its remains will then be smeared out into a system of rings, like the rings of Saturn but smaller and darker.

The lumpy shape of Phobos is also evident from its irregular silhouette against the sun. Clearly it is not a moon like ours, and for now nobody is sure how it got there. One idea is that Phobos and Deimos are wayward asteroids that were captured by Mars. Another is that they formed from debris blasted off of the planet during a huge, ancient impact. Phobos might even be a kind of celestial Phoenix, born from the remains of an earlier moon that got ripped apart into rings that then reassembled. In 2024, the Japanese Space Agency will launch a mission called MMX (Martian Moons Exploration) to visit Phobos, sample its surface, and fill in the details of this shadow moon.

NASA / JPL-Caltech / Space Science Institute

Eclipsed by a Ringed Planet

The most gorgeous eclipses in the solar system are never visible from Earth, but one of them was captured in pointillist detail by the Cassini spacecraft that circled Saturn from 2004 to 2017. On Oct. 17, 2012, during the probe’s 174 th orbit, it passed directly behind the planet and plunged into its shadow. All was not truly dark, however. Light from the eclipsed sun can be seen streaming around the edges of Saturn, which allows researchers to study the structure of the planet’s thick, windy hydrogen-helium atmosphere.

The greatest visual drama comes from the planet’s rings, viewed from a unique perspective. Nothing that you see here is illuminated directly by the sun rather, you are seeing sunshine scattering off the icy chunks that comprise the rings. The colors indicate the structure and composition of those chunks, while the brightness indicates their average size evidently, they range in scale from dust specks to floating icebergs the size of a small house. The unusual palette is partially due to this unfamiliar type of illumination, and partly due to the way the image was composed: Cassini’s imaging camera took three pictures in infrared, red, and violet light, which were then combined to simulate a color view.

Popping out of the blackness at lower left are two faint, intriguing specks of light. They are Tethys and Enceladus, two of Saturn’s 82 moons. Enceladus is an extraordinary little world, harboring a deep, warm ocean beneath its ice. Astrobiologists now consider it one of the most likely places to search for alien life in the solar system. That effort is being aided by another play of light and shadow. More on that shortly.

NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

Eclipsed by a Dwarf Planet

Seeing a Saturnian eclipse is a rare thing. Seeing an eclipse of the sun by Pluto is a once-in-a-lifetime event. It happened on July 14, 2015, when NASA’s New Horizons spacecraft flew behind dwarf planet Pluto on its way out of the solar system, and there are no plans for it to happen ever again.

For a long time, Pluto was believed to be an airless, inert world. New Horizons proved otherwise. If Pluto were a bare ball of ice, it would look like a black disk, nearly invisible. The rich blue rim tells a very different story. Despite being smaller than Earth’s moon, and despite temperatures hovering around -390 degrees Fahrenheit, Pluto has a complex atmosphere composed of nitrogen laced with methane and carbon monoxide and filled with an unexpected blue haze. Look closely and you can see that the haze is divided into dozens of layers. Scientists working with the New Horizon data believe the haze is composed of photochemical smog (not unlike L.A. on a bad day), including organic compounds like ethylene and acetylene. The particles are so fine that they scatter mostly blue light, the same reason that our skies are blue. Why they follow such a complicated, layered structure, nobody knows.

Look closer still and you will see streaks of darkness cutting through the haze. These are the shadows of mountains on Pluto—craggy peaks that are composed of deep-frozen water ice and capped with nitrogen glaciers. The shadows resemble the lines of darkness, known as crepuscular rays, that you’ll often see around clouds lit from behind by the sun. Only in this case, the darkness provides our first-ever measure of the topography of Pluto’s frozen terrain.

ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Eclipsed by an Erupting Comet

Anything that passes in front of the sun can create an eclipse. It doesn’t have to be a planet or a moon. A comet will do the trick, too, as seen in this image of the comet known as 67P/Churyumov–Gerasimenko, or just “67P” to its friends. From 2014 to 2016, the European Space Agency’s Rosetta mission studied 67P up close, dropped a small lander on its surface, and obtained by far the best portraits ever taken of the cold, solid nucleus of a comet.

On March 29, 2016, Rosetta captured this view of Comet 67P from a distance of 510 miles. The comet itself is minuscule, a mere 2.5 miles wide across its longest dimension. Strictly speaking, this isn’t a true eclipse the sun is slightly off to the side rather than directly behind the comet. But in this arrangement, the comet is almost entirely backlit, with its night side facing us, set against a starry backdrop. You can see a trickle of sunshine falling on the top of the comet. What’s really interesting, though, are the fuzzy jets and streamers surrounding the comet, which are especially prominent when illuminated like this from behind.

Comet 67P spent most of its lifetime—billions of years—in the dim gloom of the Kuiper Belt, a region of the outer solar system that extends well beyond Pluto. At some point, it got disturbed and fell inward toward the sun. Then in 1959 it had the misfortune to pass close to Jupiter, which pushed it even closer. Now its frozen gases regularly vaporize under solar heat, releasing clouds of dust that make the jets and streamers visible. What you see here, never seen before, is the first stage of the process that gives comets their tails. The comet itself is about as black as a lump of coal the vast trail of gas and dust it leaves behind is what catches the sunlight and makes comets appear to glow beautifully in our skies.

NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

Silhouette in the Kuiper Belt

Sometimes astronomers can’t make sense of what they’re looking at until they can see a shadow. That was the case with the enigmatic object known as Arrokoth. It resides more than 4 billion miles from the sun in the Kuiper Belt (the same region where Comet 67P originated) and was the second destination for New Horizons after its encounter with Pluto and its moons.

Before New Horizons arrived on Dec. 31, 2018, we knew essentially nothing about this object it had been observed only as a dot in the dark by the Hubble Space Telescope. In the initial spacecraft images, Arrokoth seemed to resemble a 22-mile-tall snowman, with a smaller ball jammed onto a second, larger one. Then a day later, the spacecraft looked back on its target as it was speeding away, and the picture got more complicated.

Arrokoth showed up as two patches of emptiness, where its bulk blocked the light of the background stars, rimmed by two thin crescents of feeble sunlight (blurred here because of the long exposure). It became clear that the object was not a snowman so much as two pancakes stuck together at their edge. Nobody had ever seen anything like this before. Such a delicate structure could have formed only through an equally delicate, gentle process. The formation of the Earth was a violent process, marked by shuddering asteroid impacts and infernal, large-scale collisions. Arrokoth revealed that violence is not always the solution, however. In the outer fringes of the solar system, objects were so cold and slow-moving, apparently, that they could simply touch and stick together.

NASA / JPL-Caltech / Space Science Institute

Geysers in the Twilight

Enceladus is another fascinating body whose secrets emerge only if you don’t look at it too directly. The warm ocean that is hidden beneath its ice might well have remained hidden and unknown, if not for a crucial clue that literally leaked out: There are geysers of water that erupt from fissures in the south pole of this 310-mile-wide moon. The geysers are subtle features, invisible in direct images of the surface of Enceladus. But they pop out when seen in silhouette against empty space, especially if backlit by the sun.

In this Cassini image, taken on Nov. 30, 2010, multiple geysers are spouting off in a variety of directions. The shadow of Enceladus against the geysers helped scientists determine the location and geometry of the geysers watching how those shadows changed over the course of a season further refined the information. Chemical analysis of the material that is shooting out delivered several bits of exciting news. Along with the expected water and salts, the geysers contain organic compounds. Cassini’s instruments also detected the presence of silica, which can get mixed with water in undersea volcanoes.

Taken together, the evidence indicates Enceladus has both the raw ingredients for life and a potential energy source. Hydrothermal vents on Earth support rich ecosystems. If there is life on Enceladus, though, it may be hard to find. Even in its thinnest spots, the icy crust is probably at least a mile thick, meaning that anything alive must be swimming around in inky darkness.

Chabot, N.L., Shread, E.E., & Harmon, J.K. Investigating Mercury’s south polar deposits: Arecibo radar observations and high-resolution determination of illumination conditions. Journal of Geophysical Research: Planets (2018).

Cold Shadows on a Hot Planet

One of the most striking recent discoveries about the solar system is that there is water everywhere—just mostly not in plain sight. The ocean of Enceladus is one case in point (along with a similar buried ocean on Jupiter’s moon Europa, and very likely on several other moons as well). Even more surprising, perhaps, has been the discovery of water ice on Mercury, the planet closest to the sun. Daytime temperatures hover around 800 degrees Fahrenheit, which would seem to rule out any kind of water, much less ice. The secret, it turns out, is looking in the places where the sun doesn’t shine.

Around Mercury’s north and south poles, there are dozens of craters and basins that create permanent shadows. Since they receive no heat from the sun, and since Mercury has no atmosphere, these locations stay extremely cold, more than 300 degrees below zero degrees Fahrenheit. Ice can remain stable indefinitely at those temperatures, which would explain why radar waves bounced off those areas reflect back as if they have hit layers of ice.

Between 2011 and 2015, NASA’s MESSENGER spacecraft mapped the craters of perpetual darkness, and confirmed that they match up with the pattern of radar echoes. Topographic measurements from MESSENGER also show that the shadowed craters appear to contain thick, sloping deposits, which are probably accumulations of water ice. Similar deposits of ice seem to dot the poles of our moon, where they could provide useful resources for future lunar explorers.

NASA / JPL-Caltech / SwRI

Jupiter’s Ultraviolet Glow

Over the past few decades, the astronomical meaning of chiaroscuro has expanded as astronomers have developed tools not just for looking into the dark, but also for looking at forms of light that are invisible to the human eye. Familiar worlds have suddenly revealed unfamiliar faces as a result. Jupiter is well known for its iconic, colorful banded clouds and its great red spot. But through the ultraviolet eyes of the Juno spacecraft, the clouds vanish and the planet’s relentless aurora display bursts into view.

The blue oval shows the ring of auroras that continually surround Jupiter’s north pole. The glow happens when electrically charged particles from the sun get caught up in Jupiter’s magnetic field, then dumped into the atmosphere where they shed their energy in the form of ultraviolet rays.

Everything on Jupiter is supersized, and its auroras are no exception. The aurora ring is more than 20,000 miles wide, about three times the diameter of Earth. Jupiter’s magnetic field is so strong that it creates electric currents connected to its moons, hundreds of thousands away where those currents hit the planet, they created the dots in the image. And when charged particles crash into the atmosphere, they do so with up to 30 times as much energy as in the wimpy visible-light auroras on Earth.

JAXA / ISAS / DARTS / Damia Bouic

The second planet from the sun gets no respect the United States hasn’t sent a spacecraft to Venus in more than 30 years. Part of the problem is that it’s so boring to look at. The planet is covered with perpetual, unbroken clouds that reflect 75 percent of the light that hits them. That makes Venus bright and beautiful in Earth’s skies, but difficult to study in any meaningful way—at least, as long as you are limiting yourself to visible light.

Japan’s Akatsuki probe, currently in orbit around Venus, examines the planet in infrared radiation and sees a totally different world. Beneath its clouds, Venus has a thick carbon-dioxide atmosphere, 90 times as thick as Earth’s, that produces an extreme greenhouse effect, heating the surface to a searing 850 degrees Fahrenheit. At this temperature, the surface glows brightly in infrared, like the lamp on night-vision goggles. What Ataksuki sees is therefore like a photographic negative, with all the usual notions of bright and dark reversed.

In this image, we’re looking at the night side of Venus. The illumination comes not from the sun but from the infrared energy of the planet and its lower atmosphere. Bright patches are areas where the upper clouds are thin, allowing the infrared to shine through dark patches are areas of thick, high-altitude clouds. Seen in this light, Venus is a wild, stormy, dynamic place. Venus is also very similar to Earth in size and composition, yet somehow it has turned into a hell planet. This planet could preview our own grim fate. It deserves a closer look.

Bonus: The Bright Side of the Moon

In all this talk about darkness in the solar system, you may have noticed that there’s no mention of the dark side of the moon. And for good reason: It’s not a real thing. Every part of it receives sunlight, except for those tiny pockets of shadowed craters near the poles. Even Pink Floyd knew this, gently reminding attentive listeners, “There is no dark side of the moon, really. Matter of fact, it’s all dark. The only thing that makes it look light is the sun.”

To get away from darkness and celebrate the light, check out this lovely animation created by NASA’s Science Visualization Studio. It offers a different kind of perceptual shift, showing what the phases of the moon would look like to someone standing above the far side—the side that people commonly describe as “dark.” Let the video play, and let the sunshine in.

Corey S. Powell enjoys exploring the outer possibilities of physics and astronomy. He writes the Out There blog and co-hosts the Science Rules podcast. @coreyspowell

Looking for a Second Earth in the Shadows

Some dark, clear nights, when the blazing stars cast shadows down on Mauna Kea, Hawaii, the astronomer Olivier Guyon steps away from his workbench and computer screens and walks outside the giant 8-meter Subaru Telescope to savor the heavens. Guyon. READ MORE

UA Researcher Captures Rare Radar Images of Comet 46P/Wirtanen

Comet P46/Wirtanen is seen here crossing a dark, moonless night sky on Dec. 17, with the Pleiades looming in the background. (left)

Although barely visible to the naked eye, Comet 46P/Wirtanen keeps some secrets so close that only radar can uncover them.

As the comet was making its close approach to Earth on Dec. 16, it was studied by a team of scientists led by Ellen Howell from the UA's Lunar and Planetary Laboratory. The team used Arecibo Observatory’s planetary radar, which is supported by NASA’s Near-Earth Object Observations program.

Studying the comet with radar provides a glimpse of its nucleus, the solid portion of the comet usually hidden inside a cloud of gas and dust that makes up the coma and tail. Radar images also allow for a precise determination of the comet’s orbit, allowing scientists to better predict how the gas and dust emission can alter the orbit.

Arecibo Observatory, a facility of the National Science Foundation operated by the University of Central Florida, is the only radar facility with the sensitivity to acquire images of Comet 46P/Wirtanen’s nucleus during its flyby. The Arecibo radar observations of Comet 46P/Wirtanen began Dec. 10 and continued through Dec. 18.

The radar images of the nucleus revealed an elongated, somewhat lumpy body that is much rougher than others that have been studied.

The new radar observations provided the first definitive measurements of Comet 46P/Wirtanen’s diameter, which is approximately 0.9 miles (1.4 km). Previous size estimates of the diameter were derived from the comet’s brightness, but radar provides a more direct measurement.

Howell’s team, which included scientists from the University of Central Florida and the Lunar and Planetary Institute, was also able to observe the comet’s large-grain coma, which is only detectable to radar. They discovered that it contains a significant population of particles, defined as those just under an inch (2 cm) and larger. This coma skirt, seen in some but not all comets observed with radar, is very extensive and asymmetric in this active comet.

“Radar observations give us images of the comet nucleus we can’t get any other way. This comet has a really rugged looking surface, which might be related to the large population of grains in its coma,” said Howell, a senior research scientist at the Lunar and Planetary Laboratory. “Every comet we study is unique. Radar images are important pieces of the puzzle.”

Howell’s team was also able to find some surprising differences between this and other comets of the same family.

Comet 46P/Wirtanen is one of a group of comets called Jupiter family comets, as their orbits are controlled by Jupiter’s gravity. Two other Jupiter family comets, 45P/Honda-Mrkos-Pajdusakova and 41P/Tuttle-Giacobini-Kresak, were also recently studied by radar in 2017.

Although the three comets have similar orbits and activity levels, the radar observations show that they are actually quite different, especially with regard to the large grains in the coma. Comet 46P/Wirtanen has a large population of large grains, 45P/Honda-Mrkos-Pajdusakova has a smaller population of these grains, but 41P/Tuttle-Giacobini-Kresak had none.

Comet 46P/Wirtanen made its closest approach of Earth at about 7.2 million miles (11.6 million km), or 30 Earth-Moon distances, at a speed of over 22 thousand miles per hour (10 km/sec) relative to Earth. Howell’s team collaborated with a larger UA research group, headed by Lunar and Planetary Laboratory professor Walter Harris, to observe the comet at many different wavelengths during the pass to characterize the gas and dust emanating from the nucleus that forms the coma.

Comets are remnants of the planet-forming process, and are part of a group of objects made of water, ice and rocky material that formed beyond Neptune. The study of these objects gives us an idea of how our solar system formed and evolved over time.

This comet is only the eighth imaged using radar in the last 30 years, as comets rarely come close enough to the Earth to get detailed images. In fact, although 46P/Wirtanen has an orbital period of about 5.44 years, it rarely passes this close to Earth. The next close approach by Comet 46P/Wirtanen will be in 2029, but during that approach the comet will be 10 times farther away from the Earth than it is now.

This flyby was the best known opportunity to image a comet with radar for the next 30 years.

Putting Philae to Work

Rosetta’s OSIRIS telephoto camera recorded the Philae lander after separation on November 12th.

The day began with Rosetta, the mission's "mother ship," maneuvering into position for the probe's release. Separation followed at 8:35 UT, and Philae began a 7-hour-long free fall toward its carefully selected landing site. (Initially designated simply "J," one among many candidates, the final site was christened Agilkia, for a small island in the Nile River.

Rosetta's camera captured the probe as it slowly drifted away. Philae also took images of the comet during the long descent, including one made public taken from an altitude of about 2 miles (3 km). The initial touchdown, at an estimated speed of just 2 miles per hour (1 meter per second), was unexpectedly soft — the craft's three shock-absorbing legs flexed only about 1½ inches (4 cm).

The washing-machine-size lander appears to have escaped damage. "We still do not fully understand what has happened," admitted Stephan Ulamec, lander manager at the DLR German Aerospace Center, during a post-landing briefing. But Philae's scientific payload was operating as planned, he says. "We have plenty of data."

Philae's ROLIS (Rosetta Lander Imaging System) looked down on the landing site ('Agilkia") during the lander's descent. At the time its altitude was about 2 miles (3 km).
ESA / ROLIS team

All this drama played out with the comet some 300 million miles (500 million km) from Earth. That's too far for Philae to communicate directly, so transmissions were relayed by Rosetta. Contact with the lander broke off soon after the lander finally settled down as Rosetta slipped below the horizon. But experiments continued working and storing data according to a preprogrammed sequence. An on-board battery will provide power only for about 64 hours of operation. After that, the 10 instruments aboard will draw electricity from a smaller, secondary battery that, with luck, will be recharged by solar cells mounted on Philae's exterior.

A fuller picture of what happened during the unorthodox landing has begun to emerge. Images relayed by the Comet Nucleus Infrared and Visible Analyzer (CIVA) show a surface far more rugged than expected or desired. The skewed perspective of some images suggests that Philae came to rest tipped up to one side, with one of its three legs suspended above the surface.

"It is difficult to know the angle of Philae," comments Phillipe Gaudon, Rosetta project leader at the French aerospace company CNES. "It is probably more than 30°." Adds CNES's Marc Kircher, "We are in a kind of cave — not a very flat area." Fortunately, whatever its final orientation, Philae ended up with its radio antenna pointed skyward.

Meanwhile, it appears that eight of the lander's 10 instruments are working. The other two — an alpha proton X-ray spectrometer (APXS) and the drilling system — have been disabled for now because both involve contact with the surface and engineers are worried about triggering shifts in the lander's apparently precarious orientation.

The mission team is weighing its options. Drilling to obtain a sample, essential for a planned assay of elemental isotopes and organic compounds in the icy surface, might dislodge the lander and cause it to shift in an uncontrolled way.

But time is running out: Philae's main battery has only a 64-hour store of electricity, and its solar-cell arrays are only getting 1½ hours of sunlight during each rotation of the comet — just 25% of the levels they would generate from a flat, open location. For now, the instruments are gathering as much data as they can. The situation will become clearer once the OSIRIS telephoto camera aboard Rosetta is able to pinpoint Philae's exact location.

NASA telescope studies quirky comet 45P

When comet 45P zipped past Earth early in 2017, researchers observing from NASA's Infrared Telescope Facility, or IRTF, in Hawai'i gave the long-time trekker a thorough astronomical checkup. The results help fill in crucial details about ices in Jupiter-family comets and reveal that quirky 45P doesn't quite match any comet studied so far.

Like a doctor recording vital signs, the team measured the levels of nine gases released from the icy nucleus into the comet's thin atmosphere, or coma. Several of these gases supply building blocks for amino acids, sugars and other biologically relevant molecules. Of particular interest were carbon monoxide and methane, which are so hard to detect in Jupiter-family comets that they've only been studied a few times before.

The gases all originate from the hodgepodge of ices, rock and dust that make up the nucleus. These native ices are thought to hold clues to the comet's history and how it has been aging.

"Comets retain a record of conditions from the early solar system, but astronomers think some comets might preserve that history more completely than others," said Michael DiSanti, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the new study in the Astronomical Journal.

The comet -- officially named 45P/Honda-Mrkos-Pajdu&scaronáková -- belongs to the Jupiter family of comets, frequent orbiters that loop around the Sun about every five to seven years. Much less is known about native ices in this group than in the long-haul comets from the Oort Cloud.

To identify native ices, astronomers look for chemical fingerprints in the infrared part of the spectrum, beyond visible light. DiSanti and colleagues conducted their studies using the iSHELL high-resolution spectrograph recently installed at IRTF on the summit of Maunakea. With iSHELL, researchers can observe many comets that used to be considered too faint.

The spectral range of the instrument makes it possible to detect many vaporized ices at once, which reduces the uncertainty when comparing the amounts of different ices. The instrument covers wavelengths starting at 1.1 micrometers in the near-infrared (the range of night-vision goggles) up to 5.3 micrometers in the mid-infrared region.

iSHELL also has high enough resolving power to separate infrared fingerprints that fall close together in wavelength. This is particularly necessary in the cases of carbon monoxide and methane, because their fingerprints in comets tend to overlap with the same molecules in Earth's atmosphere.

"The combination of iSHELL's high resolution and the ability to observe in the daytime at IRTF is ideal for studying comets, especially short-period comets," said John Rayner, director of the IRTF, which is managed for NASA by the University of Hawai'i.

While observing for two days in early January 2017 -- shortly after 45P's closest approach to the Sun -- the team made robust measurements of water, carbon monoxide, methane and six other native ices. For five ices, including carbon monoxide and methane, the researchers compared levels on the sun-drenched side of the comet to the shaded side. The findings helped fill in some gaps but also raised new questions.

The results reveal that 45P is running so low on frozen carbon monoxide, that it is officially considered depleted. By itself, this wouldn't be too surprising, because carbon monoxide escapes into space easily when the Sun warms a comet. But methane is almost as likely to escape, so an object lacking carbon monoxide should have little methane. 45P, however, is rich in methane and is one of the rare comets that contains more methane than carbon monoxide ice.

It's possible that the methane is trapped inside other ice, making it more likely to stick around. But the researchers think the carbon monoxide might have reacted with hydrogen to form methanol. The team found that 45P has a larger-than-average share of frozen methanol.

When this reaction took place is another question -- one that gets to the heart of comet science. If the methanol was produced on grains of primordial ice before 45P formed, then the comet has always been this way. On the other hand, the levels of carbon monoxide and methanol in the coma might have changed over time, especially because Jupiter-family comets spend more time near the Sun than Oort Cloud comets do.

"Comet scientists are like archaeologists, studying old samples to understand the past," said Boncho Bonev, an astronomer at American University and the second author on the paper. "We want to distinguish comets as they formed from the processing they might have experienced, like separating historical relics from later contamination."

The team is now on the case to figure out how typical their results might be among similar comets. 45P was the first of five such short-period comets that are available for study in 2017 and 2018. On the heels of 45P were comets 2P/Encke and 41P/Tuttle-Giacobini-Kresak. Due next summer and fall is 21P/Giacobini-Zinner, and later will come 46P/Wirtanen, which is expected to remain within 10 million miles (16 million kilometers) of Earth throughout most of December 2018.

"This research is groundbreaking," said Faith Vilas, the solar and planetary research program director at the National Science Foundation, or NSF, which helped support the study. "This broadens our knowledge of the mix of molecular species coexisting in the nuclei of Jovian-family comets, and the differences that exist after many trips around the Sun."

"We're excited to see this first publication from iSHELL, which was built through a partnership between NSF, the University of Hawai'i, and NASA," said Kelly Fast, IRTF program scientist at NASA Headquarters. "This is just the first of many iSHELL results to come."

Weighing space dust with radar

It is thought that over 1,000 kilograms of so-called interplanetary dust falls to Earth every day. This dust is essentially an untold number of small faint meteors, discarded remnants of asteroids and comets that pass by the Earth. Two ways to study faint meteors are radar and optical observations, each with advantages and limitations. Astronomers have combined specific observations with both methods, and can now use radar to make the kinds of observations that previously only optical telescopes could make.

Our solar system is a busy place -- in addition to the large bodies we are all familiar with exist an uncountably large number of rocky asteroids and icy comets. These mostly stay put in their orbits far from Earth but many also roam around the solar system. As they do, they shed some material due to collisions, deformations or heating. Due to this, the Earth is surrounded by small particles we call interplanetary dust. By investigating the size and composition of the interplanetary dust, astronomers can indirectly investigate the activity and makeup of the parent bodies.

"When in space, interplanetary dust is practically invisible. However, around 1,000 kilograms falls to Earth every day in the form of tiny meteors which appear as bright streaks in the night sky," said astronomer Ryou Ohsawa from the Institute of Astronomy at the University of Tokyo. "We can observe these with ground-based radar and optical instruments. Radar is useful as it can cover wide areas and gather vast readings, but optical telescopes can give more detailed information useful for our studies. So we set out to bridge this gap to boost our observational capacity."

Ground-based radar is very good at detecting the motion of meteors, but it does not reveal much information about the mass or composition of the meteors. Optical telescopes and sensors can infer those details based on the light given off by falling meteors due to interaction with the atmosphere. However, telescopes have a limited field of view and until recently lacked the sensitivity to see faint meteors at all. Ohsawa and his team wished to imbue radar observatories with the powers of optical ones. After a few years, they have finally succeeded.

"We thought that if you could observe enough meteors simultaneously with both radar and optical facilities, details of the meteors in the optical data may correspond to previously unseen patterns in the radar data too," said Ohsawa. "I am pleased to report this is in fact the case. We recorded hundreds of events over several years and have now gained the ability to read information about meteor mass from subtle signals in radar data."

In 2009, 2010 and 2018, the team used the Middle and Upper Atmosphere (MU) Radar facility, operated by Kyoto University and located in Shigaraki, Shiga Prefecture, and the Kiso Observatory, operated by the University of Tokyo, on the Nagano Prefecture side of Mount Ontake. They are 173 kilometers apart, which is important: the closer the facilities, the more accurately the data from them can be correlated. MU points directly upwards, but Kiso can be angled, so it was pointed 100 km above the site of MU. The team saw 228 meteors with both facilities and this was plenty to derive a statistically reliable relationship to connect radar and optical observations.

"Data analysis was laborious," said Ohsawa. "A sensitive instrument called the Tomo-e Gozen wide-field camera mounted to the Kiso telescope captured over a million images a night. This is too much for us to analyze manually so we developed software to automatically recognize faint meteors. From what we've learned here we hope to extend this project and begin using radar to investigate the composition of meteors. This could help astronomers explore comets and aspects of solar system evolution like never before."

Interstellar heavy metals

Another remarkable study published today in Nature shows that heavy metals are also present in the atmosphere of the interstellar comet 2I/Borisov. A team in Poland observed this object, the first alien comet to visit our Solar System, using the X-shooter spectrograph on ESO’s VLT when the comet flew by about a year and a half ago. They found that 2I/Borisov’s cold atmosphere contains gaseous nickel.

“At first we had a hard time believing that atomic nickel could really be present in 2I/Borisov that far from the Sun. It took numerous tests and checks before we could finally convince ourselves,” says study author Piotr Guzik from the Jagiellonian University in Poland. The finding is surprising because, before the two studies published today, gases with heavy metal atoms had only been observed in hot environments, such as in the atmospheres of ultra-hot exoplanets or evaporating comets that passed too close to the Sun. 2I/Borisov was observed when it was some 300 million kilometres away from the Sun, or about twice the Earth-Sun distance.

Studying interstellar bodies in detail is fundamental to science because they carry invaluable information about the alien planetary systems they originate from. “All of a sudden we understood that gaseous nickel is present in cometary atmospheres in other corners of the Galaxy,” says co-author Michał Drahus, also from the Jagiellonian University.

The Polish and Belgian studies show that 2I/Borisov and Solar System comets have even more in common than previously thought. “Now imagine that our Solar System’s comets have their true analogues in other planetary systems — how cool is that?,” Drahus concludes.

Meteorites, Comets, and Planets Titan

The complexity of Titan’s atmospheric chemistry began to be apparent when telescopic infrared spectroscopy detected a number of hydrocarbons in addition to the methane discovered in 1944 ( Danielson et al., 1973 Gillett, 1975 Kuiper, 1944 ). In 1981, Voyager observations determined that Titan’s atmosphere is primarily nitrogen with methane as a minor constituent. The surface temperature is ∼94 K and the surface pressure is high, ∼1.5 times the Earth’s ( Hanel et al., 1981 Lindal et al., 1983 Tyler et al., 1981 ). Complex photochemistry in the atmosphere produces a rich array of hydrocarbons ( Strobel, 1982 Yung et al., 1984 ), which have been identified in Voyager infrared spectra ( Hanel et al., 1981 Kunde et al., 1981 Lutz et al., 1981, 1983a, b Maguire et al., 1981 Samuelson et al., 1981 ). The currently identified atmospheric species are given in Table 2 .

Table 2 . Satellite composition summary

Planet satellitesSurface composition (including “condensed” trapped species)Atmospheric composition
IoSilicate, possibly ultramaficSO2, Sx, NaClSO2O, SO, S2, Na, K, NaCl
EuropaH2OH2O2, XySO4·NH2O, SO2, CO2, O2,O2Na, K, H
GanymedeH2OCO2, CH2, C≡N, H–S, XySO4, NH2O, SO2, O2, O3O2H
CallistoH2O, hydrated silicatesCO2, CH2, C≡N, H–S, XySO4·NH2O, SO2, O2O2?CO2
EnceladusH2OC–H,H2ON2, CH4
TitanH2O, hydrocarbonsCH4, C2H6N2 40 Ar, CH4, H2, C2H6, C2H2, C3H8, C2H4, C4H2, HCN, CO, CO2, H2O
IapetusH2O, dark material (?)CO2
MirandaH2O, dark material (?)
ArielH2O, dark material (?)
UmbrielH2O, dark material (?)
TitaniaH2O, dark material (?)
OberonH2O, dark material (?)
TritonH2O, CH4, N2CO2, CON2CH4, photochemical hydrocarbons

Optically the atmosphere is dominated by an opaque reddish aerosol haze produced by photochemical processes that masks the surface at visible wavelengths. As a result, little is known directly of the surface geology or composition. Presumably, as with Callisto and Ganymede, the crust and mantle are primarily water ice. Models of the atmospheric chemistry however suggest that the surface should receive a continual “rain” of hydrocarbon aerosols, some of which may be liquid under Titan surface conditions ( Lunine, 1993 Lunine et al., 1983 ).

The haze layers are penetrable by radar ( Muhleman et al., 1990 ) and by infrared images made in “windows” between the strong methane absorptions features in the atmospheric spectra ( Smith et al., 1996 ). These data show that the surface is variegated in radar scattering properties and in near-infrared albedo ( Griffith, 1993 Lorenz and Lunine, 1997 Smith et al., 2002 ), mitigating against a uniform global layer of liquid hydrocarbons. Clouds in the atmosphere have also been detected in infrared images ( Griffith et al., 2000 ). Analyses of the relative albedoes in different spectral windows suggest water ice exposed in some regions ( Coustenis et al., 1995 ). Recent studies confirm the presence of water ice and suggest that some areas may resemble Ganymede’s surface, with relatively high-albedo water ice exposed ( Griffith et al., 2003 ).

Data from the Cassini Orbiter and the Huygens Probe have now provided the first detailed views of the surface, as well as critical clues to the composition of the surface and atmosphere. A key finding in the early phases of the Cassini mission is the absence of obvious liquid lakes or seas of hydrocarbons. Instead images taken by the Huygens probe as it descended suggest a primarily solid surface, with evidence for extensive erosion in some regions by fluids. The low surface temperature (∼94 K) and the presence of methane in the atmosphere at near triple-point conditions, point to a “hydrological cycle” involving liquid methane, possibly mixed with other light hydrocarbons such as ethane ( Tomasko et al., 2005 ).

Clouds seen in ground-based data and in Cassini visible and near-infrared images also suggest that methane precipitation is possible at some times and places on Titan. Whether extensive amounts of liquid methane are or have been present on the surface as lakes remains an open question. The Huygens probe provided some clues to near-surface “moisture” when the mass spectrometer detected a strong signature of methane and ethane gases evolved from the surface following landing and subsequent heating of the instrument’s inlet port. These data have been interpreted as evidence for at least small amounts of liquid hydrocarbons in the near surface soils ( Niemann et al., 2005 ).

Atmospheric composition measurements are striking for the absence of expected primordial rare gases such as Xe, Kr, and 36 Ar. Only 40 Ar was detected by the Huygens mass spectrometer, suggesting a degree of outgassing from the interior. These results are interpreted as evidence that the ice and rock planetesimals which formed Titan were relatively warm, preventing the trapping of primordial gases in the water ice. A consequence of this scenario is the suggestion that the N2 in the atmosphere is also not primordial, but rather produced from NH3.

Cassini observations with visible and near-infrared imaging and radar have revealed an apparently youthful surface geologically. During the first few encounters with Titan, only two obvious impact structures were detected, although several other circular features may turn out to be of impact origin. In addition, the radar images have show that significant areas in the equatorial regions are covered with what appear to be longitudinal dune fields, suggesting major aeolian processes modifying the surface.

The general view of Titan from early Cassini/Huygens results suggests that it is a remarkably Earth-like place in many ways, at least with respect to geological and geochemical processes, with solid water ice “bedrock,” liquid methane “water,” and solid hydrocarbon “sands.”

Drake Bell shamed a crowd of concert-goers

In 2017, Drake Bell reportedly had a major public meltdown on stage during a concert with his band at the Northwest School of the Arts in Charlotte, N.C. According to TMZ, Bell opened his set with songs from his new album instead of his old hits, which made the crowd restless. The outlet noted that some members of the audience had raided a nearby ball pit and started firing balls onstage. It resulted in the singer picking up one of the balls and forcefully throwing it back at the crowd before stopping the concert midway through the Drake & Josh theme song to address their behavior.

"Stop the music right now!" Bell yelled. "Who the hell is throwing these things up on the stage, man? Stop it! It doesn't make you cool, man." He then threatened to end the concert early if they didn't behave accordingly. "We're trying to perform a show for you guys, you guys are throwing stuff at us. If you want us to go, we'll go. You're spoiling it for everybody, man," he told the crowd of high schoolers.

Although Bell's behavior was a little much here, it sounds like both sides were in the wrong.