Uranus' Menagerie of Major Moons

Home , Cordelia (moon), Ophelia (moon), Solar conjunction, Voyager Golden Record

September 2019 • Astronomy.com WWW.ASTRONOMY.COM 1 Visible to the naked eye Visible with binoculars Visible with a telescope RISING MOON I A terrific trio of terraced craters SKY THIS MONTH THE WAXING CRESCENT MOON offers a THE SOLAR SYSTEM’S CHANGING LANDSCAPE AS IT APPEARS IN EARTH’S SKY. Theophilus, Cyrillus, and Catharina BY MARTIN RATCLIFFE AND ALISTER LING smorgasbord of fantastic views. From the slen- derest of arcs to First Quarter phase, Luna shows off rolling seas bordered by craters with towering OBSERVING peaks and aprons of debris blasted out during the impacts that created them. The best detail September 2019 HIGHLIGHT Theophilus appears along the terminator, where the Sun is NEPTUNE reaches its 2019 peak September 9/10, when the rising and encroaches on the dark lunar night. planet glows at magnitude 7.8 Let’s focus on the scene the evening of and spans 2.4" when viewed September 4. If you look just south of the lunar through a telescope. equator, three magnificent craters will grab your Neptune peaks attention. Northernmost Theophilus has the Mädler most classic shape of the trio: It showcases a Cyrillus sharp, almost perfectly circular rim with a dra- matic multiple peak at its center. Because it is Catharina N in Aquarius that follow close on their heels, much bigger and deeper than the crater Mädler Neptune’s can be a riveting experience. immediately to its east, Theophilus’ walls have distinct color slumped into terraces that appear most notice- E The giants of the You won’t have to strain at shows up in With Jupiter on display for solar system take all to see Jupiter. The giant amateur scopes, only a few hours each evening, able on its western flank. These three large craters form a stunning group when center stage this month. While planet shines brilliantly in though the however, only a limited number You can make some educated guesses as to the Sun first lights them up the evening of September 4. cloud features the relative ages of lunar features even without CONSOLIDATED LUNAR ATLAS/UA/LPL; INSET: NASA/GSFC/ASU Jupiter and Saturn stand out southern Ophiuchus. It stands that Voyager 2 of events are visible. Your first every September evening and 25° high in the south-southwest saw in 1989 are good look comes September 4, the tools of a geologist. Theophilus must be deserve the lion’s share of your an hour after sunset in early out of reach. when Io transits Jupiter. The younger than neighboring Cyrillus to the south- Full Moon, this topographic detail vanishes. NASA/JPL attention, Neptune ranks a September and 20° high in the innermost moon first touches west because it overlays the latter crater’s ragged Mädler seems to shelter the terrain to its east close third as it reaches peak southwest at the same time as the planet’s limb at 8:04 p.m. rim. Although Cyrillus also has a complex peak from this debris, hinting at its intermediate age. and slumped walls, the impacts of smaller objects visibility. The distant world the month closes. During the EDT, and its shadow follows The southernmost member of the crater trio over the ages have degraded them. Also note is Catharina. Once upon a time, it must have skims past a relatively bright same period, Jupiter fades from at 9:21 p.m. Both moon and that the debris apron surrounding Theophilus been sharp-featured like Theophilus, but billions star, making it easier to find magnitude –2.2 to –2.0. This shadow take about 130 minutes partially fills Cyrillus. The apron’s rough texture of years of bombardment have erased the cen- than usual. Uranus lies farther makes the planet some 20 times twilight, when the planet is Jupiter reveals a wealth of to transit the jovian disk. is obvious at this phase. Under a high Sun at tral peak and left its walls lower and softer. east and makes a worthy morn- brighter than Antares, the red high enough to avoid the poor detail through any scope. Even Giant Ganymede’s shadow ing object. Finally, Mercury and supergiant star that lies 8° to seeing conditions that typically an untrained eye will notice crosses Jupiter’s north polar Venus appear in evening twi- Jupiter’s lower right. The First interfere with objects near the two dark equatorial belts, one region starting at 11:22 p.m. light at the end of the month. Quarter Moon stands 4° to horizon. This gives you a cou- on either side of a brighter zone EDT on September 5. Although Catch the false dawn Let’s start our tour in the Jupiter’s right and 7° above ple of hours of prime observing coinciding with the equator. the gas giant is setting along the METEOR WATCH I evening sky with the toughest Antares on September 5. in early September and at least The gas giant’s disk measures East Coast, observers farther targets. Both Mercury and Jupiter appears best through an hour of good views as the 37.4" across the equator and west will get nice views of the The zodiacal light shines on autumn mornings Solar system dust particles cast an eerie glow into the predawn sky in Venus make fleeting appear- a telescope starting in late month winds down. 35.0" through the poles at mid- shadow’s 153-minute transit. late September and early October. ances in bright twilight as month, an obvious difference Jupiter’s four bright moons BARRY BURGESS September draws to a close. On once you know to look for it. all orbit in the planet’s equato- A distant world’s close encounter the 30th, Venus stands 1° above Lower-contrast details pop rial plane, so they usually line the western horizon 30 minutes into view along the edges of up. But this symmetry breaks or zodiacal light, on after sunset. Gleaming at mag- N the equatorial belts during down once in a while. Watch September mornings. nitude –3.9, it shows up only PISCES moments of steady seeing. the evening of September 19, This faint, cone-shaped glow has a broad base in because it shines so brightly. Jupiter’s fast spin drives these and you’ll see Ganymede pass Leo and tapers as it climbs You might see magnitude –0.2 features. The giant planet’s 30" due north of Callisto. through Cancer and Gemini Mercury one binocular field to equatorial regions rotate once Saturn lies 30° east of into Taurus. The glow arises Venus’ upper left. Jupiter and trails some two every 9 hours 50 minutes, tak- from sunlight reflecting off The inner planets fare poorly E φ ing five minutes less than at hours behind its sister world. fine dust particles in the because the ecliptic — the path Neptune higher latitudes. This lag is great news for Saturn plane of the solar system, of the Sun and planets across Any telescope also reveals watchers because it places the which is why it aligns with the sky — makes a shallow Jupiter’s four bright moons. All planet at its highest in the south the ecliptic. To see the light, angle to the western horizon AQUARIUS χ λ four typically show up against as darkness falls and keeps it on you need to observe from a after sunset from mid-northern ψ2 the blackness of space near the view past midnight. Saturn lies dark site shortly before twi- latitudes. Thus, the angular ψ1 planet, but occasionally one or in Sagittarius, just south of the THE ECLIPTIC’S LOW ANGLE — the ecliptic stands nearly light begins. Catch the false separation of a planet from the ψ3 1° more hides in plain sight as it constellation’s Teaspoon aster- to the western horizon after sun- straight up from the eastern hori- dawn’s ethereal glow on Sun translates mostly into dis- crosses in front of the planet. ism. The magnitude 0.4 ringed set, which keeps Mercury and zon before sunrise. This solar sys- moonless mornings, which tance along the horizon and not Neptune lies in eastern Aquarius at opposition the night of September 9/10, Watching these transits, and world shines nearly four times Venus immersed in bright twilight tem geometry affords observers run from September 27 to into altitude. just 6' from 4th-magnitudeASY-SM0919_10 Phi (ϕ) Aquarii. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY the far easier shadow transits — Continued on page 42 all month, has a favorable flip side great views of the false dawn, October 11 this year.

2 ASTRONOMY INSIGHTS • SEPTEMBER 2019

Visible to the naked eye Visible with binoculars Visible with a telescope RISING MOON I A terrific trio of terraced craters SKY THIS MONTH THE WAXING CRESCENT MOON offers a THE SOLAR SYSTEM’S CHANGING LANDSCAPE AS IT APPEARS IN EARTH’S SKY. Theophilus, Cyrillus, and Catharina BY MARTIN RATCLIFFE AND ALISTER LING smorgasbord of fantastic views. From the slen- derest of arcs to First Quarter phase, Luna shows off rolling seas bordered by craters with towering OBSERVING peaks and aprons of debris blasted out during the impacts that created them. The best detail September 2019 HIGHLIGHT Theophilus appears along the terminator, where the Sun is NEPTUNE reaches its 2019 peak September 9/10, when the rising and encroaches on the dark lunar night. planet glows at magnitude 7.8 Let’s focus on the scene the evening of and spans 2.4" when viewed September 4. If you look just south of the lunar through a telescope. equator, three magnificent craters will grab your Neptune peaks attention. Northernmost Theophilus has the Mädler most classic shape of the trio: It showcases a Cyrillus sharp, almost perfectly circular rim with a dra- matic multiple peak at its center. Because it is Catharina N in Aquarius that follow close on their heels, much bigger and deeper than the crater Mädler Neptune’s can be a riveting experience. immediately to its east, Theophilus’ walls have distinct color slumped into terraces that appear most notice- E The giants of the You won’t have to strain at shows up in With Jupiter on display for solar system take all to see Jupiter. The giant amateur scopes, only a few hours each evening, able on its western flank. These three large craters form a stunning group when center stage this month. While planet shines brilliantly in though the however, only a limited number You can make some educated guesses as to the Sun first lights them up the evening of September 4. cloud features the relative ages of lunar features even without CONSOLIDATED LUNAR ATLAS/UA/LPL; INSET: NASA/GSFC/ASU Jupiter and Saturn stand out southern Ophiuchus. It stands that Voyager 2 of events are visible. Your first every September evening and 25° high in the south-southwest saw in 1989 are good look comes September 4, the tools of a geologist. Theophilus must be deserve the lion’s share of your an hour after sunset in early out of reach. when Io transits Jupiter. The younger than neighboring Cyrillus to the south- Full Moon, this topographic detail vanishes. NASA/JPL attention, Neptune ranks a September and 20° high in the innermost moon first touches west because it overlays the latter crater’s ragged Mädler seems to shelter the terrain to its east close third as it reaches peak southwest at the same time as the planet’s limb at 8:04 p.m. rim. Although Cyrillus also has a complex peak from this debris, hinting at its intermediate age. and slumped walls, the impacts of smaller objects visibility. The distant world the month closes. During the EDT, and its shadow follows The southernmost member of the crater trio over the ages have degraded them. Also note is Catharina. Once upon a time, it must have skims past a relatively bright same period, Jupiter fades from at 9:21 p.m. Both moon and that the debris apron surrounding Theophilus been sharp-featured like Theophilus, but billions star, making it easier to find magnitude –2.2 to –2.0. This shadow take about 130 minutes partially fills Cyrillus. The apron’s rough texture of years of bombardment have erased the cen- than usual. Uranus lies farther makes the planet some 20 times twilight, when the planet is Jupiter reveals a wealth of to transit the jovian disk. is obvious at this phase. Under a high Sun at tral peak and left its walls lower and softer. east and makes a worthy morn- brighter than Antares, the red high enough to avoid the poor detail through any scope. Even Giant Ganymede’s shadow ing object. Finally, Mercury and supergiant star that lies 8° to seeing conditions that typically an untrained eye will notice crosses Jupiter’s north polar Venus appear in evening twi- Jupiter’s lower right. The First interfere with objects near the two dark equatorial belts, one region starting at 11:22 p.m. light at the end of the month. Quarter Moon stands 4° to horizon. This gives you a cou- on either side of a brighter zone EDT on September 5. Although Catch the false dawn Let’s start our tour in the Jupiter’s right and 7° above ple of hours of prime observing coinciding with the equator. the gas giant is setting along the METEOR WATCH I evening sky with the toughest Antares on September 5. in early September and at least The gas giant’s disk measures East Coast, observers farther targets. Both Mercury and Jupiter appears best through an hour of good views as the 37.4" across the equator and west will get nice views of the The zodiacal light shines on autumn mornings Solar system dust particles cast an eerie glow into the predawn sky in Venus make fleeting appear- a telescope starting in late month winds down. 35.0" through the poles at mid- shadow’s 153-minute transit. late September and early October. ances in bright twilight as month, an obvious difference Jupiter’s four bright moons BARRY BURGESS September draws to a close. On once you know to look for it. all orbit in the planet’s equato- A distant world’s close encounter the 30th, Venus stands 1° above Lower-contrast details pop rial plane, so they usually line the western horizon 30 minutes into view along the edges of up. But this symmetry breaks or zodiacal light, on after sunset. Gleaming at mag- N the equatorial belts during down once in a while. Watch September mornings. nitude –3.9, it shows up only PISCES moments of steady seeing. the evening of September 19, This faint, cone-shaped glow has a broad base in because it shines so brightly. Jupiter’s fast spin drives these and you’ll see Ganymede pass Leo and tapers as it climbs You might see magnitude –0.2 features. The giant planet’s 30" due north of Callisto. through Cancer and Gemini Mercury one binocular field to equatorial regions rotate once Saturn lies 30° east of into Taurus. The glow arises Venus’ upper left. Jupiter and trails some two every 9 hours 50 minutes, tak- from sunlight reflecting off The inner planets fare poorly E φ ing five minutes less than at hours behind its sister world. fine dust particles in the because the ecliptic — the path Neptune higher latitudes. This lag is great news for Saturn plane of the solar system, of the Sun and planets across Any telescope also reveals watchers because it places the which is why it aligns with the sky — makes a shallow Jupiter’s four bright moons. All planet at its highest in the south the ecliptic. To see the light, angle to the western horizon AQUARIUS χ λ four typically show up against as darkness falls and keeps it on you need to observe from a after sunset from mid-northern ψ2 the blackness of space near the view past midnight. Saturn lies dark site shortly before twi- latitudes. Thus, the angular ψ1 planet, but occasionally one or in Sagittarius, just south of the THE ECLIPTIC’S LOW ANGLE — the ecliptic stands nearly light begins. Catch the false separation of a planet from the ψ3 1° more hides in plain sight as it constellation’s Teaspoon aster- to the western horizon after sun- straight up from the eastern hori- dawn’s ethereal glow on Sun translates mostly into dis- crosses in front of the planet. ism. The magnitude 0.4 ringed set, which keeps Mercury and zon before sunrise. This solar sys- moonless mornings, which tance along the horizon and not Neptune lies in eastern Aquarius at opposition the night of September 9/10, Watching these transits, and world shines nearly four times Venus immersed in bright twilight tem geometry affords observers run from September 27 to into altitude. just 6' from 4th-magnitudeASY-SM0919_10 Phi (ϕ) Aquarii. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY the far easier shadow transits — Continued on page 42 all month, has a favorable flip side great views of the false dawn, October 11 this year.

4 ASTRONOMY INSIGHTS • SEPTEMBER 2019

STAR DOME SEPTEMBER 2019

SUN. MON. TUES. WED. THURS. FRI. SAT.

CAMEPARAIS

M81

URSA MAJR URSA M82 HOW TO USE THIS MAP 1 2 3 4 5 6 7 This map portrays the sky as seen near 35° north latitude. Located

inside the border are the cardinal NE PERSEUS NGC 884 NGC

directions and their intermediate N 8 9 10 11 12 13 14

Polaris

NGC 869 NGC

points. To find stars, hold the map NCP

CASSIPEIA

overhead and orient it so one of MIR

VEAICI

the labels matches the direction URSA 15 16 17 18 19 20 21 Mizar

CAES

CEPEUS ARMEA

M51 you’re facing. The stars above RIAUUM the map’s horizon now match KELLY ROEN : what’s in the sky.

22 23 24 25 26 27 28

RA AC R

The all-sky map shows M33 EREICES

MA C how the sky looks at: M31 29 30

10 p.m. September 1 ASTRONOMY BY ILLUSTRATIONS 9 p.m. September 15 Note: Moon phases in the calendar vary in size due to the distance

8 p.m. September 30 ACERA from Earth and are shown at 0h Universal Time.

PISCES

Arcturus Planets are shown Deneb

at midmonth

CUS M13

PEASUS CALENDAR OF EVENTS

Vega 2 Mars is in conjunction with the Sun, 7 a.m. EDT REAIS MAP SYMBOLS C RA

ERCUES 3 Mercury is in superior conjunction, 10 p.m. EDT

E R A Open cluster P a t 5 First Quarter Moon occurs at 11:10 p.m. EDT h M57 Globular cluster o f th M27 a m e 6 Asteroid Hertha is at opposition, 2 . . EDT SERPES Diffuse nebula S CAPU u VUPECU A n M15 VIR Enif a m (e The Moon passes 2° north of Jupiter, 3 . . EDT Planetary nebula c EPIUS SAI A l M5 ip ti a m Galaxy c) 8 The Moon passes 0.04° south of Saturn, 10 . . EDT EQUUEUS Altair CEUS The Moon passes 0.08° north of Pluto, 11 p.m. EDT STAR 10 Neptune is at opposition, 3 a.m. EDT PIUCUS MAGNITUDES AQUARIUS AQUI A 13 The Moon is at apogee (252,511 miles from Earth), 9:32 a.m. EDT Sirius The Moon passes 4° south of Neptune, 2 p.m. EDT M11 IR A 0.0 3.0 M16 14 Full Moon occurs at 12:33 a.m. EDT SCU UM 1.0 4.0 M17 17 The Moon passes 4° south of Uranus, 4 p.m. EDT 2.0 5.0 SERPES CAUA Fomalhaut CAPRIC RUS M20 Jupiter 18 Saturn is stationary, 2 a.m. EDT PISCIS M22 AUSRIUS Saturn 21 Last Quarter Moon occurs at 10:41 p.m. EDT SCUPR M8 STAR COLORS Antares M4 23 Autumnal equinox occurs at 3:50 a.m. EDT A star’s color depends E M6 on its surface temperature. a m M7 25 Asteroid Vesta is stationary, 1 . . EDT SAIARIUS The hottest stars shine blue MICRSC PIUM UPUS 27 The Moon is at perigee (222,328 miles from Earth), 10:24 p.m. EDT • Slightly cooler stars appear white • C RA Asteroid Lutetia is at opposition, midnight EDT Intermediate stars (like the Sun) glow yellow AUSR AIS SC RPIUSNGC • RUS 6231 • Lower-temperature stars appear orange 28 New Moon occurs at 2:26 p.m. EDT The coolest stars glow red • IUS EESCPIUM Mercury passes 1.4° north of Spica, 7 p.m. EDT • Fainter stars can’t excite our eyes’ color receptors, so they appear white unless you 29 The Moon passes 6° north of Mercury, 6 p.m. EDT use optical aid to gather more light

BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart.

STAR DOME SEPTEMBER 2019

SUN. MON. TUES. WED. THURS. FRI. SAT.

CAMEPARAIS

M81

URSA MAJR URSA M82 HOW TO USE THIS MAP 1 2 3 4 5 6 7 This map portrays the sky as seen near 35° north latitude. Located

inside the border are the cardinal NE PERSEUS NGC 884 NGC

directions and their intermediate N 8 9 10 11 12 13 14

Polaris

NGC 869 NGC

points. To find stars, hold the map NCP

CASSIPEIA overhead and orient it so one of MIR

VEAICI

the labels matches the direction URSA 15 16 17 18 19 20 21 Mizar

CAES

CEPEUS ARMEA

M51 you’re facing. The stars above RIAUUM the map’s horizon now match KELLY ROEN : what’s in the sky.

22 23 24 25 26 27 28

RA AC R

The all-sky map shows M33 EREICES

MA C how the sky looks at: M31 29 30

10 p.m. September 1 ASTRONOMY BY ILLUSTRATIONS 9 p.m. September 15 Note: Moon phases in the calendar vary in size due to the distance

8 p.m. September 30 ACERA from Earth and are shown at 0h Universal Time.

PISCES

Arcturus Planets are shown Deneb

at midmonth

CUS M13

PEASUS CALENDAR OF EVENTS

Vega 2 Mars is in conjunction with the Sun, 7 a.m. EDT REAIS MAP SYMBOLS C RA

ERCUES 3 Mercury is in superior conjunction, 10 p.m. EDT

BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart.

PLEASE PROOF: Title Astronomy Illustrator Roen Kelly Individual illustrators, designers, art directors, Issue September 2019 Designer and editors must proof Job # MAG-ASY-SEP19 Art Dir. and sign this form. Code PC0919_D Story Ed. Proof 1 Copy Ed. Date 5-20-19 Man. Ed. Return Editor This map unfolds the entire night sky from sunset (at right) until sunrise (at left). Arrows PATHS OF THE PLANETS and colored dots show motions and locationsJULY of solar system objects 2019 during the month. SEPTEMBER 2019

UMa DRA PER LAC Callisto LYN C CYG UM o HER 1 m LM CV LM AUR e CV t GEM TRI A LYR BOÖ CNC fr COM ic COM 2 Ganymede a Europa ARI n o VUL CB Moon of the PEG SGE Jupiter Path DEL 3 LEO Uranus LEO Io ORI Amphitrite SER CM Vesta Neptune reaches its 4 Mars Metis peak in September AQL SER SEX Celestial equator OPH SEX 5 Io Eunomia VIR Venus Ganymede Sun Sun MON LIB c) ipti Laetitia SCT (ecl Mercury 6 HYA CET Pluto un CRV CRT Asteroid Lutetia reaches the S CRV CRT HYA th of LEP opposition September 27 Pa JUPITER’S CM 7 PsA CAP MOONS FOR Asteroid Hertha reaches Jupiter Dots display opposition September 5/6 Ceres 8 Europa ANT PYX COL SCL Saturn positions of SGR ANT LUP Galilean satellites PUP CAE CA SCO 9 Callisto PHE CEN p m MIC TEL at 11 . . EDT on VEL HOR ARA the date shown. South is at the 10 Dawn Midnight Early evening top to match the To locate the Moon in the sky, draw a line from the phase shown for the day view through a 11 straight up to the curved blue line. telescope. 1 30 29 28 27 12

13 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

THE PLANETS THE PLANETS 15 Uranus IN THE SKY SS IN THEIR ORBITS JupiterJupiter 16 Arrows show the inner These illustrations show the size, Saturn W E planets’ monthly motions phase, and orientation of each Saturn W E 17 and dots depict the outer planet and the two brightest dwarf panets at 0h UT for the NN planets’ positions at mid- Jupiter Neptune 18 month from high above Opposition is dates in the data table at bottom. their orbits. Saturn September 9/10 South is at the top to match the view through a telescope. 10"10" 19

20 VenusVenus Mercury MercuryMercury Mars Superior conjunction Mars Uranus Neptune Pluto 21 Pluto CeresCeres Uranus Neptune Pluto is September 3 22

23 PLANETS MERCURY VENUS MARS CERES JUPITER SATURN URANUS NEPTUNE PLUTO Mars 24 Solar conjunction Date Sept. 30 Sept. 30 Sept. 15 Sept. 15 Sept. 15 Sept. 15 Sept. 15 Sept. 15 Sept. 15 Venus Earth is September 2 Autumnal equinox Magnitude –0.2 –3.9 1.8 9.0 –2.1 0.4 5.7 7.8 14.3 25 is September 23 Angular size 5.2" 10.0" 3.5" 0.5" 37.4" 17.3" 3.7" 2.4" 0.1" 26 Illumination 86% 98% 100% 97% 99% 100% 100% 100% 100% Distance (AU) from Earth 1.287 1.666 2.665 2.889 5.268 9.623 19.099 28.932 33.383 27 Ceres Distance (AU) from Sun 0.465 0.723 1.664 2.847 5.266 10.044 19.832 29.935 33.880 28 PLEASEPLEASE PROOF: PROOF: Right ascension (2000.0) 13h30.9m 13h11.2m 11h15.4m 16h28.7m 16h59.3mTitleTitle Astronomy18h59.2mAstronomy 2h15.0mIllustratorIllustratorRoenRoen23h13.5m Kelly Kelly 19h28.8m IndividualIndividual illustrators, illustrators, Issue September 2019 Designer 29 Declination (2000.0) –10°27' –6°32' 5°55' designers,–23°27'designers, art art directors, directors,–22°24' Issue September–22°32' 2019 13°00'Designer –6°09' –22°23' Jupiter MAG-ASY-SEP19 andand editors editors must must proof proof JobJob # # MAG-ASY-SEP19 ArtArt Dir. Dir. andand sign sign this this form. form. CodeCode PCPC StoryStory Ed. Ed. 30 ProofProof 11 CopyCopy Ed. Ed. DateDate 5-20-195-20-19 Man.Man. Ed. Ed. ReturnReturn EditorEditor This map unfolds the entire night sky from sunset (at right) until sunrise (at left). Arrows PATHS OF THE PLANETS and colored dots show motions and locationsJULY of solar system objects 2019 during the month. SEPTEMBER 2019

13 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

20 VenusVenus Mercury MercuryMercury Mars Superior conjunction Mars Uranus Neptune Pluto 21 Pluto CeresCeres Uranus Neptune Pluto is September 3 22

Miranda

The unsolved mysteries of the

Voyager 2 captured the first good photos of Uranus’ ring system. These dark, faint rings glow in false color in this computer- enhanced image, which reveals the nine rings known before the spacecraft’s arrival. (The fainter pastel lines are processing artifacts.) Scientists have since discovered ICE GIANTS four more rings. NASA/JPL Geysers on Triton, furious storms on After the rich and complex atmospheres Neptune, and a seemingly serene Uranus of Jupiter and Saturn, Uranus seemed pretty bland, Dodd recalls. “You didn’t get that turned out to be a cosmic oddity. all the great storms you got at the other planets,” she says. Instead, Uranus “looked by Korey Haynes like a fuzzy, blue tennis ball.” Voyager did reveal a previously unde- tected magnetic field around Uranus, com- parable in strength to Earth’s. Due to its nearly 90° axial tilt, Uranus rolls around its orbit like a ball. And while Earth’s orbital and magnetic fields are offset by roughly Ariel Umbriel Oberon Titania When Voyager 2 explored Uranus in 1986, it found a bland, bluish 12°, Uranus’ are 60° apart. This results in a The planet’s Frankenstein moon, Miranda, looks like a mad scientist stitched it together from world with little contrast. But corkscrewing magnetic field trailing mil- unrelated parts. Some regions feature heavily cratered ancient terrains while others are younger the flyby occurred in the dead lions of miles behind the planet. and display canyons, cliffs, and steep ridges. Craters dominate Ariel’s surface, though this moon of summer when direct sunlight What’s more, scientists still aren’t sure also shows some major canyons and ridges. Heavily cratered Umbriel is the planet’s darkest large suppresses cloud formation. A moon; the white ring at top resides on a crater’s floor. Bright rays surround several of the big decade later, Hubble began to why the magnetic field exists at all, since craters that dot the surface of Oberon. Abundant craters also cover Uranus’ largest moon, Titania, reveal bright clouds. NASA/JPL-CALTECH Uranus lacks the standard liquid metallic though several canyons and steep cliffs break up the monotony. MIRANDA: NASA/JPL/USGS; ALL OTHERS: NASA/JPL inner layer that powers such fields on other planets. Voyager also revealed intense radiation belts around the planet, similar to fter visiting Saturn, the twin planet. The secrets that Voyager teased out the sequencing teams for Uranus and those seen at Saturn. but the star’s repeated appearance and dis- the rings are probably made mostly of ice, Voyagers parted ways substantially remain the most up-close views that scien- Neptune. The group determined exactly But the highlights, Dodd says, were appearance before it slid out of view behind like Saturn’s, but covered in organic mate- for the first time. As Voyager 1 tists have to work with. when Voyager’s instruments should take centered on Uranus’ rings and moons. the planet made astronomers realize that a rial such as methane, and then baked dark raced out of the solar system, data in order to return the information the Compared with Saturn’s rings, visible since series of rings surrounded our far-off by the planet’s radiation belts. Voyager 2 struck out on its Big blue nothing science team wanted. This meant under- the earliest days of the telescope, Uranus’ neighbor. The flyby was a chance to inves- Uranus’ moons, too, camouflage Aown toward the last two unvisited giant Voyager 2 flew past Uranus on January 24, standing in minute detail how the planets were still recent discoveries, and scientists tigate them up close. well against the dark of space. When planets: Uranus and Neptune. Smaller 1986, more than four years after the probe and their moons moved. The sequencing were eager to learn more. Voyager imaged the ring system for the Voyager left Earth, astronomers knew of and more distant than Jupiter and Saturn, visited Saturn. Following the excitement team orchestrated the various instruments Astronomers at Cornell University dis- first time, informing astronomers of its only five satellites around the planet. From these ice giant worlds were better hidden at that ringed world (and Jupiter before it), to use every second of the precious flyby covered Uranus’ ring system in early 1977, detailed structure. The spacecraft also dis- observations during its brief visit, the from Earth’s prying telescopic eyes, and scientists were eager to see what Voyager windows to image the most valuable tar- just before Voyager’s launch. The sighting covered two entirely new rings. The close- spacecraft tripled that number, yielding therefore more mysterious. And, to some would reveal at the more distant and enig- gets: the limb or edge of the planets, the was a happy accident, when a chance align- up views confirmed that Uranus’ subtle 10 new moons. extent, they remain so. No spacecraft since matic uranian system. terminators where day and night meet, the ment carried Uranus in front of a distant bands are not like Saturn’s bright icy rings; “I really think the satellites were the Voyager has visited them, and there are Suzy Dodd, the project manager for moons in their orbits, and the planets’ own star. Scientists had planned to use the they are dark and reflect little light, mak- highlight of Uranus,” says Dodd. Voyager no missions currently scheduled to either Voyager’s interstellar mission, worked on broad faces. occultation to study Uranus’ atmosphere, ing them difficult to see. Scientists think images lent the five larger known moons

10 ASTRONOMY INSIGHTS • SEPTEMBER 2019 Uranus’ menagerie of major moons

Miranda

The unsolved mysteries of the

WWW.ASTRONOMY.COM 11 by a large fraction: about one-third the To mark the final flyby, NASA’s Jet planet’s radius for Uranus, and nearly half Propulsion Laboratory hosted a special a radius for Neptune. Both planets could event celebrating Voyager’s journey and have oceans of conductive icy slush that accomplishments. Scientists shared images perform the work of the liquid metallic with the public, and rock-’n’-roll legend cores at Earth and Jupiter, but inconclusive Chuck Berry, whose music lives on as part models and observations have left scientists of Voyager’s Golden Record, played in a with little more than guesswork as to what special concert. exactly drives the magnetic fields that At the edge of our planetary system, Voyager observed. 2.75 billion miles (4.43 billion km) from Voyager also detected aurorae on Earth, Voyager turned its cameras back for Neptune. Due to the strange and complex a last look, imaging farewell shots of a cres- nature of the planet’s magnetic field, these cent Neptune. Dodd recalls her reaction to aurorae don’t occur only at the poles; the images: “Wow. The planetary mission instead they are scattered across Neptune’s is done. We’re going off into the deep dark Above: Voyager captured Neptune’s three brightest rings after it flew upper atmosphere. and cold realms of space. Who knows how past the planet. (The dark bar blocks the ice giant’s crescent.) The outer long the mission will last?” ring’s clumpy nature doesn’t show up well from this angle. NASA/JPL End of an era Voyager also closed a contentious chapter Epilogue Voyager 2 captured a crescent Neptune Left: Neptune’s dynamic atmosphere surprised scientists, who expected as it sped away from its final planetary the planet to resemble its dull cousin, Uranus. The Earth-sized Great Dark in astronomy history by revising Neptune’s When she left her position with the encounter. Now, 28 years later, the Spot lies at center; several smaller storms also appear. NASA/JPL mass downward by around half a percent Neptune team, Dodd says, no one then spacecraft continues to explore the — or roughly the mass of Mars. This mis- imagined Voyager would continue as long outer realm of the solar system. NASA/JPL calculation had sent astronomers on a wild as it has. She returned to Voyager’s inter- detail and character, telling varied stories reasonable proxies for Uranus’ system, More surprises waited on Triton, goose chase through the years as they tried stellar mission in 2010, 21 years after she of violent pasts. Chancia explains. So he and Hedman Neptune’s biggest moon. Triton was to make sense of Uranus’ and Neptune’s left the project. In many ways, she admits Voyager continues to measure magnetic Two of the new moons, Cordelia and adopted techniques that planetary scientist already a hotbed of intrigue; it’s by far orbits, usually by invoking the existence of that the spacecraft is an artifact — memory fields, charged particles, plasma density, Ophelia, were identified as shepherd Mark Showalter used to find the moonlet the solar system’s largest retrograde satel- a mysterious Planet X tugging on both of and power limited, with many of its spe- and more as it cruises the solar system’s moons. They orbit on either side of Uranus’ Pan in Voyager 2’s observations of Saturn’s lite, meaning it orbits in the direction them. (Pluto was found as a direct result cialists long since retired or passed on. hinterlands, teaching scientists about the outer Epsilon ring, and their gravitational ring system. opposite to its planet’s rotation. This is of this hunt, but its small size was never Since Voyager’s departure from Neptune, subtle edges of the solar system’s boundar- pull herds the small particles in that ring They found distinct patterns in Uranus’ usually a sign of a captured object, but enough to resolve the initial problem.) many of its instruments have gone quiet. ies. Voyager 1 has passed beyond the reach along their orbital path and keeps them rings consistent with “wakes” carved by most other retrograde moons are small, Voyager settled the issue, as Neptune’s There is no need for imaging cameras in of the solar wind, and thus is sampling from dissipating into space. Uranus’ rings moonlets circling a planet within a ring misshapen asteroids. Triton is three- smaller mass means it and Uranus orbit the dark void of space. But that does not aspects of interstellar space, though it still are uncommonly narrow; without shep- system. The predicted moonlets are tiny, quarters the size of our Moon, and sur- just as they should. mean the project is defunct. lies well within the Sun’s gravitational herd moons, the small particles would dis- only 2 to 9 miles (4 to 14 kilometers) vived its capture intact. Scientists wanted influence. Voyager 2, following a slower perse over long timescales. across. And they are likely dark, like close-up views of the satellite, and since it trajectory from its two-planet detour, tags Over the years, astronomers have used the rest of the moons and ring system. was the last target, they were free to adjust behind, still sampling the solar wind. From imagery, mostly from Hubble, to add more Confirming the moonlets will be a chal- Voyager’s trajectory as needed. So the their distance, it takes more than 15 hours moons to Uranus’ count, which currently lenge. But even 30 years later, Voyager is spacecraft swooped only 3,075 miles for their signals to reach Earth. stands at 27. But Cordelia and Ophelia still helping to crack Uranus’ secrets. (4,950km) above Neptune’s north pole — Sometime in the next decade, the space- remain the only observed shepherds of the its closest approach to any object during craft will lose power and begin to shut ring system. Astronomers have long won- Final surprises the mission — and flew toward its encoun- down. Dodd’s team will turn the Voyagers’ dered whether more moons are hiding from Voyager’s last planetary encounter came ter with Triton. heaters off first, and one by one, the science view, or whether other forces are at work. August 24, 1989. Far from Uranus’ “fuzzy The last world Voyager 2 visited instruments will succumb to the cold of Last year, astronomers from the tennis ball,” Neptune was alive with storms stunned scientists. The moon boasted a space. But the spacecraft themselves and University of Idaho revisited the Voyager and bright, quick-moving clouds, delight- thin atmosphere, polar caps, and active their Golden Records will journey on, car- data. Thirty years after the flyby, they ing unsuspecting astronomers. Clouds not geysers that spewed icy material miles rying humanity’s imprint into the cosmos. found evidence for two more tiny moonlets only appeared clearly in Voyager images, high. The active cryovolcanism puts Triton It will be years before any spacecraft shaping Uranus’ rings. “Nobody — or not but they also cast shadows on deeper cloud in a select group of satellites, in the com- retreads Voyager’s path to Uranus or many people — had looked at this in a very layers, allowing scientists to measure the pany of other dynamic moons such as Neptune. With at least half a century of long time,” says Robert Chancia, who led planet’s atmosphere in great detail. The Europa and Enceladus. technological advances behind it, any the investigation. In fact, the Voyager data “Great Dark Spot,” as astronomers termed Voyager also discovered six new moons future craft will undoubtedly revolutionize were taken before he was born. Chancia the largest tempest, was as big as Earth, orbiting Neptune and delivered clear pic- our understanding of the ice giants all over and his adviser, Matthew Hedman, usually swirling in Neptune’s southern hemi- tures of its ring system for the first time, again. But it’s safe to say that nothing will study Saturn’s rings. But recent discoveries sphere and boasting wind speeds as high revealing the rings to be clumpy but com- match Voyager for sheer adventure and by the Cassini spacecraft have added as 750 mph (1,200 km/h). In the decades plete, unlike those at Uranus. scope. Decades after its primary mission, greatly to astronomers’ understanding of since, that storm has died, while new And as it did at Uranus, Voyager dis- Voyager continues to teach, to inspire, planetary rings. So Chancia and Hedman storms have risen in its place. covered that Neptune’s magnetic pole is and to explore. decided to take another look at the Voyager “That was a bit of a surprise to me misaligned from its rotational pole, causing Neptune’s largest moon, Triton, boasts some of the solar system’s most unusual landscapes. The unique “cantaloupe terrain” in the top half of this image is riddled with crevices and findings, applying new theories to old data. when you consider the Great Red Spot on extreme variations in its magnetic field as depressions but few impact craters. The south polar region at bottom shows dark streaks Korey Haynes is a contributing editor to “There are several narrow ringlets Jupiter has been going on for 400 years,” the planet rotates. Furthermore, both plan- deposited by huge geysers that were active during the Voyager 2 flyby. NASA/JPL/USGS Astronomy. You can find her on Twitter within the rings of Saturn” that provide Dodd says. ets’ magnetospheres are offset from center @weird_worlds.

12 ASTRONOMY INSIGHTS • SEPTEMBER 2019 FMS 2020 CLA FMS 2020 CLA FMS 2020 CLA FMS Parent Matrix Item

by a large fraction: about one-third the To mark the final flyby, NASA’s Jet planet’s radius for Uranus, and nearly half Propulsion Laboratory hosted a special a radius for Neptune. Both planets could event celebrating Voyager’s journey and have oceans of conductive icy slush that accomplishments. Scientists shared images perform the work of the liquid metallic with the public, and rock-’n’-roll legend cores at Earth and Jupiter, but inconclusive Chuck Berry, whose music lives on as part models and observations have left scientists of Voyager’s Golden Record, played in a with little more than guesswork as to what special concert. exactly drives the magnetic fields that At the edge of our planetary system, Voyager observed. 2.75 billion miles (4.43 billion km) from Voyager also detected aurorae on Earth, Voyager turned its cameras back for Neptune. Due to the strange and complex a last look, imaging farewell shots of a cres- nature of the planet’s magnetic field, these cent Neptune. Dodd recalls her reaction to aurorae don’t occur only at the poles; the images: “Wow. The planetary mission instead they are scattered across Neptune’s is done. We’re going off into the deep dark Above: Voyager captured Neptune’s three brightest rings after it flew upper atmosphere. and cold realms of space. Who knows how past the planet. (The dark bar blocks the ice giant’s crescent.) The outer long the mission will last?” ring’s clumpy nature doesn’t show up well from this angle. NASA/JPL End of an era Voyager also closed a contentious chapter Epilogue Voyager 2 captured a crescent Neptune Left: Neptune’s dynamic atmosphere surprised scientists, who expected as it sped away from its final planetary the planet to resemble its dull cousin, Uranus. The Earth-sized Great Dark in astronomy history by revising Neptune’s When she left her position with the encounter. Now, 28 years later, the Spot lies at center; several smaller storms also appear. NASA/JPL mass downward by around half a percent Neptune team, Dodd says, no one then spacecraft continues to explore the — or roughly the mass of Mars. This mis- imagined Voyager would continue as long outer realm of the solar system. NASA/JPL calculation had sent astronomers on a wild as it has. She returned to Voyager’s inter- detail and character, telling varied stories reasonable proxies for Uranus’ system, More surprises waited on Triton, goose chase through the years as they tried stellar mission in 2010, 21 years after she of violent pasts. Chancia explains. So he and Hedman Neptune’s biggest moon. Triton was to make sense of Uranus’ and Neptune’s left the project. In many ways, she admits Voyager continues to measure magnetic Two of the new moons, Cordelia and adopted techniques that planetary scientist already a hotbed of intrigue; it’s by far orbits, usually by invoking the existence of that the spacecraft is an artifact — memory fields, charged particles, plasma density, Ophelia, were identified as shepherd Mark Showalter used to find the moonlet the solar system’s largest retrograde satel- a mysterious Planet X tugging on both of and power limited, with many of its spe- and more as it cruises the solar system’s moons. They orbit on either side of Uranus’ Pan in Voyager 2’s observations of Saturn’s lite, meaning it orbits in the direction them. (Pluto was found as a direct result cialists long since retired or passed on. hinterlands, teaching scientists about the outer Epsilon ring, and their gravitational ring system. opposite to its planet’s rotation. This is of this hunt, but its small size was never Since Voyager’s departure from Neptune, subtle edges of the solar system’s boundar- pull herds the small particles in that ring They found distinct patterns in Uranus’ usually a sign of a captured object, but enough to resolve the initial problem.) many of its instruments have gone quiet. ies. Voyager 1 has passed beyond the reach along their orbital path and keeps them rings consistent with “wakes” carved by most other retrograde moons are small, Voyager settled the issue, as Neptune’s There is no need for imaging cameras in of the solar wind, and thus is sampling from dissipating into space. Uranus’ rings moonlets circling a planet within a ring misshapen asteroids. Triton is three- smaller mass means it and Uranus orbit the dark void of space. But that does not aspects of interstellar space, though it still are uncommonly narrow; without shep- system. The predicted moonlets are tiny, quarters the size of our Moon, and sur- just as they should. mean the project is defunct. lies well within the Sun’s gravitational herd moons, the small particles would dis- only 2 to 9 miles (4 to 14 kilometers) vived its capture intact. Scientists wanted influence. Voyager 2, following a slower perse over long timescales. across. And they are likely dark, like close-up views of the satellite, and since it trajectory from its two-planet detour, tags Over the years, astronomers have used the rest of the moons and ring system. was the last target, they were free to adjust behind, still sampling the solar wind. From imagery, mostly from Hubble, to add more Confirming the moonlets will be a chal- Voyager’s trajectory as needed. So the their distance, it takes more than 15 hours moons to Uranus’ count, which currently lenge. But even 30 years later, Voyager is spacecraft swooped only 3,075 miles for their signals to reach Earth. stands at 27. But Cordelia and Ophelia still helping to crack Uranus’ secrets. (4,950km) above Neptune’s north pole — Sometime in the next decade, the space- remain the only observed shepherds of the its closest approach to any object during craft will lose power and begin to shut ring system. Astronomers have long won- Final surprises the mission — and flew toward its encoun- down. Dodd’s team will turn the Voyagers’ dered whether more moons are hiding from Voyager’s last planetary encounter came ter with Triton. heaters off first, and one by one, the science view, or whether other forces are at work. August 24, 1989. Far from Uranus’ “fuzzy The last world Voyager 2 visited instruments will succumb to the cold of Last year, astronomers from the tennis ball,” Neptune was alive with storms stunned scientists. The moon boasted a space. But the spacecraft themselves and University of Idaho revisited the Voyager and bright, quick-moving clouds, delight- thin atmosphere, polar caps, and active their Golden Records will journey on, car- data. Thirty years after the flyby, they ing unsuspecting astronomers. Clouds not geysers that spewed icy material miles rying humanity’s imprint into the cosmos. found evidence for two more tiny moonlets only appeared clearly in Voyager images, high. The active cryovolcanism puts Triton It will be years before any spacecraft shaping Uranus’ rings. “Nobody — or not but they also cast shadows on deeper cloud in a select group of satellites, in the com- retreads Voyager’s path to Uranus or many people — had looked at this in a very layers, allowing scientists to measure the pany of other dynamic moons such as Neptune. With at least half a century of long time,” says Robert Chancia, who led planet’s atmosphere in great detail. The Europa and Enceladus. technological advances behind it, any the investigation. In fact, the Voyager data “Great Dark Spot,” as astronomers termed Voyager also discovered six new moons future craft will undoubtedly revolutionize were taken before he was born. Chancia the largest tempest, was as big as Earth, orbiting Neptune and delivered clear pic- our understanding of the ice giants all over and his adviser, Matthew Hedman, usually swirling in Neptune’s southern hemi- tures of its ring system for the first time, again. But it’s safe to say that nothing will study Saturn’s rings. But recent discoveries sphere and boasting wind speeds as high revealing the rings to be clumpy but com- match Voyager for sheer adventure and by the Cassini spacecraft have added as 750 mph (1,200 km/h). In the decades plete, unlike those at Uranus. scope. Decades after its primary mission, greatly to astronomers’ understanding of since, that storm has died, while new And as it did at Uranus, Voyager dis- Voyager continues to teach, to inspire, planetary rings. So Chancia and Hedman storms have risen in its place. covered that Neptune’s magnetic pole is and to explore. decided to take another look at the Voyager “That was a bit of a surprise to me misaligned from its rotational pole, causing Neptune’s largest moon, Triton, boasts some of the solar system’s most unusual landscapes. The unique “cantaloupe terrain” in the top half of this image is riddled with crevices and findings, applying new theories to old data. when you consider the Great Red Spot on extreme variations in its magnetic field as depressions but few impact craters. The south polar region at bottom shows dark streaks Korey Haynes is a contributing editor to “There are several narrow ringlets Jupiter has been going on for 400 years,” the planet rotates. Furthermore, both plan- deposited by huge geysers that were active during the Voyager 2 flyby. NASA/JPL/USGS Astronomy. You can find her on Twitter within the rings of Saturn” that provide Dodd says. ets’ magnetospheres are offset from center @weird_worlds.