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Space News Update — November 19, 2019 —

Contents

In the News

Story 1:

First Parker Solar Probe Science Data Released to Public

Story 2:

NASA's Mars 2020 Will Hunt for Microscopic Fossils

Story 3:

Magnetized Plasma Jets Likely Mechanism for Heating Sun’s Corona

Departments

The Night Sky

ISS Sighting Opportunities

NASA-TV Highlights

Space Calendar

Food for Thought

Space Image of the Week

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1. First Parker Solar Probe Science Data Released to Public

Data from the Wide-field Imager for Solar Probe (WISPR) instrument on board Parker Solar Probe captured during

the spacecraft’s first solar encounter in November 2018. Credit: NASA/Naval Research Laboratory/Parker Solar

Probe

On Nov. 12, 2019, NASA’s Parker Solar Probe team released scientific data collected during the spacecraft’s

first two solar orbits to the general public.

Data can be accessed through the NASA Space Physics Data Facility, the Solar Data Analysis Center, the APL

Parker Solar Probe Gateway, and the Science Operation Centers of the four science investigation teams (the

University of California, Berkeley; Princeton University; Harvard-Smithsonian Center for Astrophysics; and

Naval Research Laboratory.) The newly released data, in the form of data files and graphical displays, is

available for interested public users to manipulate, analyze, and plot in any way they choose.

The released encounter data encompasses measurements made during the first two solar encounters,

spanning the time between Oct. 31 and Nov. 12, 2018, and March 30 and April 19, 2019, when the spacecraft

was within 0.25 AU of the Sun, as well as data collected at farther distances. One AU, or astronomical unit, is

about 93 million miles, the average distance between the Sun and Earth.

Science teams led by principal investigators from partner institutions have been busy poring over the wealth of

information collected by Parker Solar Probe in preparation for the mission’s first science results, to be released

later this year. The four instrument suites onboard – FIELDS, ISʘIS, SWEAP, and WISPR – have been

observing the characteristics of the solar wind (fields, waves, flows, and particles) in the immediate

environment surrounding the Sun, called the corona.

“Parker Solar Probe is crossing new frontiers of space exploration, giving us so much new information about

the Sun,” said Parker Solar Probe Project Scientist Nour E. Raouafi, from the Johns Hopkins Applied Physics

Laboratory, or APL, in Laurel, Maryland, which manages the mission for NASA. “Releasing this data to the

public will allow them not only to contribute to the success of the mission along with the scientific community,

but also to raise the opportunity for new discoveries to the next level.”

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With three of 24 planned solar orbits under its belt, Parker Solar Probe will continue to get closer to the Sun in

the coming years, eventually swooping to within 4 million miles of the Sun’s surface, facing heat and radiation

like no spacecraft before it. The mission seeks to provide new data on solar activity and how the solar corona

works, which contributes significantly to our ability to forecast major space weather events that impact life on

Earth. The mission launched in 2018 and is slated to perform its primary science mission until 2025.

Source: NASA Return to Contents

Parker Solar Probe location as of Nov. 17,

2019 http://parkersolarprobe.jhuapl.ed

u/The-Mission/index.php#Where-Is-PSP

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2. NASA's Mars 2020 Will Hunt for Microscopic Fossils

Lighter colors represent higher elevation in this image of Jezero Crater on Mars, the landing site for NASA's Mars 2020 mission. The

oval indicates the landing ellipse, where the rover will be touching down on Mars. Credits: NASA/JPL-Caltech/MSSS/JHU-APL/ESA

Scientists with NASA's Mars 2020 rover have discovered what may be one of the best places to look for signs of

ancient life in Jezero Crater, where the rover will land on Feb. 18, 2021.

A paper published today in the journal Icarus identifies distinct deposits of minerals called carbonates along the

inner rim of Jezero, the site of a lake more than 3.5 billion years ago. On Earth, carbonates help form structures

that are hardy enough to survive in fossil form for billions of years, including seashells, coral and some stromatolites

— rocks formed on this planet by ancient microbial life along ancient shorelines, where sunlight and water were

plentiful.

The possibility of stromatolite-like structures existing on Mars is why the concentration of carbonates tracing

Jezero's shoreline like a bathtub ring makes the area a prime scientific hunting ground.

Mars 2020 is NASA's next-generation mission with a focus on astrobiology, or the study of life throughout the

universe. Equipped with a new suite of scientific instruments, it aims to build on the discoveries of NASA's Curiosity,

which found that parts of Mars could have supported microbial life billions of years ago. Mars 2020 will search for

actual signs of past microbial life, taking rock core samples that will be deposited in metal tubes on the Martian

surface. Future missions could return these samples to Earth for deeper study.

In addition to preserving signs of ancient life, carbonates can teach us more about how Mars transitioned from

having liquid water and a thicker atmosphere to being the freezing desert it is today. Carbonate minerals formed

from interactions between carbon dioxide and water, recording subtle changes in these interactions over time. In

that sense, they act as time capsules that scientists can study to learn when — and how — the Red Planet began

drying out.

Measuring 28 miles (45 kilometers) wide, Jezero Crater was also once home to an ancient river delta. The "arms" of

this delta can be seen reaching across the crater floor in images taken from space by satellite missions like NASA's

Mars Reconnaissance Orbiter. The orbiter's Compact Reconnaissance Imaging Spectrometer for Mars instrument, or

CRISM, helped produce colorful mineral maps of the "bathtub ring" detailed in the new paper.

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An animated flyover of the Martian surface explains why Mars’ Jezero Crater, a 28-mile-wide ancient lake-delta

system, is the best place for the Mars 2020 rover to find and collect promising samples for a possible future return

to Earth.

"CRISM spotted carbonates here years ago, but we only recently noticed how concentrated they are right where a

lakeshore would be," said the paper's lead author, Briony Horgan of Purdue University in West Lafayette, Indiana.

"We're going to encounter carbonate deposits in many locations throughout the mission, but the bathtub ring will

be one of the most exciting places to visit."

It isn't guaranteed that the shoreline carbonates were formed in the lake; they could have been deposited before

the lake was present. But their identification makes the site's western rim, called "the marginal carbonate-bearing

region," one of the richest troves of these minerals anywhere in the crater.

Color has been added to highlight minerals in this image of Jezero Crater on Mars, the landing site for NASA's Mars 2020

mission. The green color represents minerals called carbonates, which are especially good at preserving fossilized life on

Earth. Credits: NASA/JPL-Caltech/MSSS/JHU-APL/Purdue/USGS

The Mars 2020 team expects to explore both the crater floor and delta during the rover's two-year prime mission.

Horgan said the team hopes to reach the crater's rim and its carbonates near the end of that period.

"The possibility that the 'marginal carbonates' formed in the lake environment was one of the most exciting features

that led us to our Jezero landing site. Carbonate chemistry on an ancient lakeshore is a fantastic recipe for

preserving records of ancient life and climate," said Mars 2020 Deputy Project Scientist Ken Williford of NASA's Jet

Propulsion Laboratory in Pasadena, California. JPL leads the 2020 mission. "We're eager to get to the surface and

discover how these carbonates formed."

Jezero's former lake shoreline isn't the only place scientists are excited to visit. A new study in Geophysical

Research Letters points to a rich deposit of hydrated silica on the edge of the ancient river delta. Like carbonates,

this mineral excels at preserving signs of ancient life. If this location proves to be the bottom layer of the delta, it

will be an especially good place to look for buried microbial fossils.

The Mars 2020 rover will launch in July or August 2020 from Cape Canaveral, Florida.

Source: NASA Return to Contents

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3. Magnetized Plasma Jets Likely Mechanism for Heating Sun’s Corona

A multi-layered view of solar spicules: (from top to bottom) observations of the corona from NASA's Solar Dynamics

Observatory (SDO), followed by images from NJIT's Big Bear Solar Observatory of the chromosphere, the photosphere

and associated magnetic fields. The background is an image of the full solar disk from the NASA satellite. Credit: T.

Samanta, H. Tian, V. Yurchyshyn, H. Peter, W. Cao, A. Sterling, R. Erdélyi, K. Ahn, S. Feng, D. Utz, D. Banerjee, Y. Chen.

An international team of scientists, including three researchers from New Jersey Institute of Technology

(NJIT), has shed new light on one of the central mysteries of solar physics: how energy from the Sun is

transferred to the star’s upper atmosphere, heating it to 1 million degrees Fahrenheit and higher in some

regions, temperatures that are vastly hotter than the Sun's surface.

With new images from NJIT’s Big Bear Solar Observatory (BBSO), the researchers have revealed in

groundbreaking, granular detail what appears to be a likely mechanism – jets of magnetized plasma known as

spicules that spurt like geysers from the Sun’s upper atmosphere into the corona.

In a paper published today in the journal Science, the team describes key features of jet-like spicules that are

in solar terms small-scale plasma structures, between 200 and 500 kilometers wide, that erupt continuously

across the Sun’s expanse. The researchers also, for the first time, show where and how the jets are generated

and the paths they travel, at speeds of around 100 kilometers per second in some cases, into the corona.

“Unprecedented high-resolution observations from BBSO’s Goode Solar Telescope clearly show that when

magnetic fields with opposite polarities reconnect in the Sun’s lower atmosphere these jets of plasma are

powerfully ejected,” said solar physicist Wenda Cao, BBSO’s director and an author of the paper.

He added, “This is the first time we’ve seen direct evidence of how spicules are generated. We have tracked

these dynamic features in the H-alpha spectral line down to their foot points, measured the magnetic fields at

their foot point, captured the migration of the emerging magnetic elements and verified their interaction with

existing magnetic fields of the opposite polarity.”

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Images captured in the extreme ultraviolet (EUV) spectrum by NASA’s Solar Dynamics Observatory spacecraft

were used to track the transportation of energy in the corona. These observations showed that it is also

common for spicules to be heated to typical coronal temperatures.

Invisible to the human eye except when it appears briefly as a fiery halo of plasma during a solar eclipse, the

corona remains a puzzle even to scientists who study it closely. Beginning 1,300 miles from the star’s surface

and extending millions more in every direction, it is more than a hundred times hotter than lower layers much

closer to the fusion reactor at the Sun’s core.

Solving what astrophysicists call one of the greatest challenges for solar modeling – determining the physical

mechanisms that heat the upper atmosphere – requires high-resolution images that were not available until

BBSO’s 1.6-meter telescope, the largest operating solar telescope in the world, began capturing images a

decade ago.

Scientists at Big Bear (below) have also captured the first high-resolution images, for example, of magnetic

fields and plasma flows originating deep below the Sun's surface, tracing the evolution of sunspots and

magnetic flux ropes through the chromosphere before their dramatic appearance in the corona as flaring

loops.

Cao says it took an international team with diverse expertise and equipment located on Earth and in space to

delve this deeply into the Sun’s fundamental physics. Cao developed the scientific instruments on Big Bear’s

telescope and oversaw their operation, while NJIT’s Vasyl Yurchyshyn generated the observations, processed

the data and advised on its use, and NJIT’s Kwangsu Ahn processed the vector magnetic fields data for

scientific usage. Tanmoy Samanta and Hui Tian from Peking University in China defined the novel discoveries

and wrote the manuscript; they are its first authors.

Scientists from the Max Planck Institute for Solar System Research in Germany, NASA’s Marshall Space Flight

Center, the University of Sheffield in the U.K., Eötvös University in Hungary, Kunming University of Science

and Technology in China, the University of Graz in Austria and Indian Institute of Astrophysics all played roles.

Whiskery plasma jets, known as spicules, on the sun appear as dark, threadlike structures in this image, acquired at the

Goode Solar Telescope in Big Bear, Calif. Credit: T. Samanta, GST & SDO

Source: New Jersey Institute of Technology/Astronomy Now Return to Contents

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The Night Sky

Friday, Nov. 22

• Venus and Jupiter have closed to just 2° apart low in the southwest in twilight, as shown here. On Saturday and

Sunday evenings they'll be even closer.

• Right after full darkness, Vega is the brightest star in the west. Its little constellation Lyra extends to its left.

Somewhat farther left, about a fist and a half at arm's length from Vega, is 3rd-magnitude Albireo, the beak of

Cygnus. This is one of the finest and most colorful double stars for small telescopes.

Saturday, Nov. 23

• In twilight this evening and tomorrow evening, Venus and Jupiter are just 1½° apart low in the southwest. Think

photo opportunity.

• Around 7 or 8 p.m. now, the Great Square of Pegasus stands in its level position very high toward the south. (It's

straight overhead if you're as far south as Miami.) Its right (western) side points very far down toward Fomalhaut.

Its eastern side points less directly toward Beta Ceti (also known as Deneb Kaitos or Diphda), less far down.

What lurks below them? If you have a very good view down to a dark south horizon — and if you're not much

farther north than roughly New York, Denver, or Madrid — picture an equilateral triangle with Fomalhaut and Beta

Ceti as its top two corners. Near where the third corner would be is Alpha Phoenicis, or Ankaa, in the constellation

Phoenix. It's magnitude 2.4, not very bright but the brightest thing in its area. It has a yellow-orange tint

(binoculars help check). Have you ever it's in Gemini, with Pollux to its left and Castor above Pollux.

Source: Sky and Telescope Return to Contents

Tuesday, Nov. 19

• Last-quarter Moon (exact at 5:11 p.m. EST). The Moon, in

Leo, rises around midnight or 1 a.m. tonight with Regulus

accompanying it a few degrees to its upper right. By early

dawn Wednesday they're high in the south, with Regulus now

directly to the Moon's right (by about 7° for North America).

Wednesday, Nov. 20

• Orion clears the eastern horizon by about 8 p.m. now,

depending on how far east or west you live in your time

zone. High above Orion shines orange Aldebaran. Above

Aldebaran is the little Pleiades cluster, the size of your

fingertip at arm's length.

Far left of Aldebaran and the Pleiades shines bright Capella.

Thursday, Nov. 21

• Meteor outburst tonight? Veteran meteor-stream analysts

Peter Jenniskens and Esko Lyytinen are predicting that the

Alpha Monocerotid meteor shower, usually very minor, may

briefly erupt tonight to a rate of 100 to 400 meteors visible

per hour as seen under ideal conditions. Their model predicts

that Earth will pass through a very rich slice of the meteoroid

stream for a mere 15 to 40 minutes centered on 4:50

November 22nd UT, which is 11:50 p.m. November 21st EST.

Jupiter and Venus, the two brightest planets, will shine one over the other low in twilight by the 22nd.

Saturn looks on from afar.

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ISS Sighting Opportunities (from Denver)

Date Visible Max Height Appears Disappears

Tue Nov 19, 6:59 PM < 1 min 13° 10° above SW 13° above SW

Wed Nov 20, 6:11 PM 2 min 28° 10° above SSW 28° above SSE

Thu Nov 21, 5:23 PM 4 min 17° 10° above S 12° above E

Thu Nov 21, 7:00 PM < 1 min 25° 18° above WSW 25° above W

Fri Nov 22, 6:12 PM 2 min 84° 23° above SW 65° above NE

Sighting information for other cities can be found at NASA’s Satellite Sighting Information

NASA-TV Highlights (all times Eastern Time Zone)

November 20, Wednesday

3 p.m. – NASA Science Live: Black Hole 101

November 21, Thursday

11 a.m. – E.Z. Science Episode 3: Parker Solar Probe

November 22, Friday

5:30 a.m. - Coverage of International Space Station Expedition 61 U.S. Spacewalk # 60 to Continue

Repairs on the Alpha Magnetic Spectrometer (2nd of 4 spacewalks; spacewalk begins appx. 7:05 a.m. EST

and is expected to last at least 6 ½ hours) (All Channels)

Watch NASA TV online by going to the NASA website. Return to Contents

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Space Calendar

Nov 19 - Apollo Asteroid 2019 WH Near-Earth Flyby (0.0006 AU)

Nov 19 - Atira Asteroid 2019 AQ3 Closest Approach To Earth (0.255 AU)

Nov 19 - Colloquia: Planet Nine From Outer Space, Ithaca, New York

Nov 19 - Colloquium: Temporal Variability in Young Stellar Objects and Implications for the Early Solar System, Tucson, Arizona

Nov 19-21 - Canadian Space Summit 2019, Kanata, Canada

Nov 19-21 - Astro2020 Meeting: Panel on Electromagnetic Observations from Space, Irvine, California

Nov 19-22 - ESA/ESO SCIOPS 2019 Workshop: Cross Facilities Collaboration in the Multi Messenger Era, Madrid, Spain

Nov 20 - Apollo Asteroid 2019 WF Near-Earth Flyby (0.004 AU)

Nov 20 - Apollo Asteroid 2019 WE Near-Earth Flyby (0.008 AU)

Nov 20 - Amor Asteroid 2019 UK6 Near-Earth Flyby (0.040 AU)

Nov 20 - Lecture: Apollo 12 - A Pinpoint Landing on the Moon, London, United Kingdom

Nov 20 - Lecture: The Century of Biology on Earth and Beyond, Los Gatos, California

Nov 20 - ICTP Colloquium on 30 TeV: The Quest to Reach the Next Milestone in Elementary Particle Physics, Trieste, Italy

Nov 20 - Lecture: Technosignatures vs. Biosignatures - Which Will Succeed First?, Menlo Park, California

Nov 20 - Edwin Hubble's 130th Birthday (1889)

Nov 20-21 - SpaceCom Conference, Houston, Texas

Nov 20-22 - Astro2020 Meeting: Panel on Particle Astrophysics and Gravitation, Washington DC

Nov 20-22 - Astro2020 Meeting: Panel on State of the Profession and Societal Impacts, Washington DC

Nov 20-23 - American Society for Gravitational and Space Research (ASGSR) Annual Meeting, Denver, Colorado

Nov 21 - Comet 260P/McNaught At Opposition (0.708 AU)

Nov 21 - Apollo Asteroid 2019 VK Near-Earth Flyby (0.015 AU)

Nov 21 - Aten Asteroid 481394 (2006 SF6) Near-Earth Flyby (0.029 AU)

Nov 21 - Apollo Asteroid 2011 UB Near-Earth Flyby (0.088 AU)

Nov 22 - Inmarsat 5 F5/ TIBA 1 Ariane 5 Launch

Nov 22 - Aten Asteroid 1994 WR12 Near-Earth Flyby (0.074 AU)

Nov 22 - Teleconference: Astro2020: Panel on Cosmology Briefing Development

Nov 22 - Event: Stories in the Stars, Atlanta State Park, Atlanta, Texas

Nov 23 - Beidou 3 M19/M20 CZ-3B/YZ-1 Launch

Nov 23 - Apollo Asteroid 2008 EA9 Near-Earth Flyby (0.027 AU)

Source: JPL Space Calendar Return to Contents

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Food for Thought

With Mars Methane Mystery Unsolved, Curiosity Serves

Scientists a New One: Oxygen

Credits: Melissa Trainer/Dan Gallagher/NASA Goddard

For the first time in the history of space exploration, scientists have measured the seasonal changes in the

gases that fill the air directly above the surface of Gale Crater on Mars. As a result, they noticed something

baffling: oxygen, the gas many Earth creatures use to breathe, behaves in a way that so far scientists cannot

explain through any known chemical processes.

Over the course of three Mars years (or nearly six Earth years) an instrument in the Sample Analysis at Mars

(SAM) portable chemistry lab inside the belly of NASA’s Curiosity rover inhaled the air of Gale Crater and

analyzed its composition. The results SAM spit out confirmed the makeup of the Martian atmosphere at the

surface: 95% by volume of carbon dioxide (CO2), 2.6% molecular nitrogen (N2), 1.9% argon (Ar), 0.16%

molecular oxygen (O2), and 0.06% carbon monoxide (CO). They also revealed how the molecules in the

Martian air mix and circulate with the changes in air pressure throughout the year. These changes are caused

when CO2 gas freezes over the poles in the winter, thereby lowering the air pressure across the planet

following redistribution of air to maintain pressure equilibrium. When CO2 evaporates in the spring and

summer and mixes across Mars, it raises the air pressure.

Within this environment, scientists found that nitrogen and argon follow a predictable seasonal pattern, waxing

and waning in concentration in Gale Crater throughout the year relative to how much CO2 is in the air. They

expected oxygen to do the same. But it didn’t. Instead, the amount of the gas in the air rose throughout

spring and summer by as much as 30%, and then dropped back to levels predicted by known chemistry in fall.

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This pattern repeated each spring, though the amount of oxygen added to the atmosphere varied, implying

that something was producing it and then taking it away.

“The first time we saw that, it was just mind boggling,” said Sushil Atreya, professor of climate and space

sciences at the University of Michigan in Ann Arbor. Atreya is a co-author of a paper on this topic published on

November 12 in the Journal of Geophysical Research: Planets.

As soon as scientists discovered the oxygen enigma, Mars experts set to work trying to explain it. They first

double- and triple-checked the accuracy of the SAM instrument they used to measure the gases: the

Quadrupole Mass Spectrometer. The instrument was fine. They considered the possibility that CO2 or water

(H2O) molecules could have released oxygen when they broke apart in the atmosphere, leading to the short-

lived rise. But it would take five times more water above Mars to produce the extra oxygen, and CO2 breaks up

too slowly to generate it over such a short time. What about the oxygen decrease? Could solar radiation have

broken up oxygen molecules into two atoms that blew away into space? No, scientists concluded, since it

would take at least 10 years for the oxygen to disappear through this process.

“We’re struggling to explain this,” said Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight

Center in Greenbelt, Maryland who led this research. “The fact that the oxygen behavior isn’t perfectly

repeatable every season makes us think that it’s not an issue that has to do with atmospheric dynamics. It has

to be some chemical source and sink that we can’t yet account for.”

To scientists who study Mars, the oxygen story is curiously similar to that of methane. Methane is constantly in

the air inside Gale Crater in such small quantities (0.00000004% on average) that it’s barely discernable even

by the most sensitive instruments on Mars. Still, it’s been measured by SAM’s Tunable Laser Spectrometer.

The instrument revealed that while methane rises and falls seasonally, it increases in abundance by about

60% in summer months for inexplicable reasons. (In fact, methane also spikes randomly and dramatically.

Scientists are trying to figure out why.)

With the new oxygen findings in hand, Trainer’s team is wondering if chemistry similar to what’s driving

methane’s natural seasonal variations may also drive oxygen’s. At least occasionally, the two gases appear to

fluctuate in tandem.

“We’re beginning to see this tantalizing correlation between methane and oxygen for a good part of the Mars

year,” Atreya said. “I think there’s something to it. I just don’t have the answers yet. Nobody does.”

Oxygen and methane can be produced both biologically (from microbes, for instance) and abiotically (from

chemistry related to water and rocks). Scientists are considering all options, although they don’t have any

convincing evidence of biological activity on Mars. Curiosity doesn't have instruments that can definitively say

whether the source of the methane or oxygen on Mars is biological or geological. Scientists expect that non-

biological explanations are more likely and are working diligently to fully understand them.

Trainer’s team considered Martian soil as a source of the extra springtime oxygen. After all, it’s known to be

rich in the element, in the form of compounds such as hydrogen peroxide and perchlorates. One experiment

on the Viking landers showed decades ago that heat and humidity could release oxygen from Martian soil. But

that experiment took place in conditions quite different from the Martian spring environment, and it doesn’t

explain the oxygen drop, among other problems. Other possible explanations also don’t quite add up for now.

For example, high-energy radiation of the soil could produce extra O2 in the air, but it would take a million

years to accumulate enough oxygen in the soil to account for the boost measured in only one spring, the

researchers report in their paper.

“We have not been able to come up with one process yet that produces the amount of oxygen we need, but

we think it has to be something in the surface soil that changes seasonally because there aren’t enough

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available oxygen atoms in the atmosphere to create the behavior we see,” said Timothy McConnochie,

assistant research scientist at the University of Maryland in College Park and another co-author of the paper.

Credits: Melissa Trainer/Dan Gallagher/NASA Goddard

The only previous spacecraft with instruments capable of measuring the composition of the Martian air near

the ground were NASA’s twin Viking landers, which arrived on the planet in 1976. The Viking experiments

covered only a few Martian days, though, so they couldn’t reveal seasonal patterns of the different gases. The

new SAM measurements are the first to do so. The SAM team will continue to measure atmospheric gases so

scientists can gather more detailed data throughout each season. In the meantime, Trainer and her team hope

that other Mars experts will work to solve the oxygen mystery.

“This is the first time where we’re seeing this interesting behavior over multiple years. We don’t totally

understand it,” Trainer said. “For me, this is an open call to all the smart people out there who are interested

in this: See what you can come up with.”

Source: NASA Return to Contents

Sunset at the Viking Lander 1 site, 1976.

Credits: NASA/JPL

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Space Image of the Week

The Star Streams of NGC 5907 Image Credit & Copyright: R Jay Gabany (Blackbird Observatory) - collaboration; D.Martinez-

Delgado (IAC, MPIA), J.Penarrubia (U.Victoria) I. Trujillo (IAC) S.Majewski (U.Virginia), M.Pohlen (Cardiff)

Explanation: Grand tidal streams of stars seem to surround galaxy NGC 5907. The arcing structures form tenuous loops extending more than 150,000 light-years from the narrow, edge-on spiral, also known as the Splinter or Knife Edge Galaxy. Recorded only in very deep exposures, the streams likely represent the ghostly trail of a dwarf galaxy - debris left along the orbit of a smaller satellite galaxy that was gradually torn apart and merged with NGC 5907 over four billion years ago. Ultimately this remarkable discovery image, from a small robotic observatory in New Mexico, supports the cosmological scenario in which large spiral galaxies, including our own Milky Way, were formed by the accretion of smaller ones. NGC 5907 lies about 40 million light-years distant in the northern constellation Draco.

Source: NASA APOD Return to Contents