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M A R C H / A P RI L 2 0 1 9 $ 5
Probing for Planets
Space agencies prepare
next generation
of exoplanet hunters
T H E U N I V E R S I T Y O F T E X A S AT A U S T I N M c D O N A L D O B S E R VAT O R Y
STARDATE STAFF MARCH/APRIL • Vol. 47, No. 2
EXECUTIVE EDITOR
Damond Benningfield
EDITOR
Rebecca Johnson
ART DIRECTOR
F eatures Departments
C.J. Duncan
TECHNICAL EDITOR
Dr. Tom Barnes
4 Poets, Philosophers, Queens, Astronomers
CONTRIBUTING EDITOR
Alan MacRobert Merlin 3
MARKETING MANAGER
Casey Walker Early women astronomers drafted
MARKETING ASSISTANT calendars, plotted eclipses, built Sky Calendar MARCH/APRIL 10
Joanne Duffy observatories, and helped shape
humanity’s early understanding of the The Stars in MARCH/APRIL 12
universe
For information about StarDate or other
programs of the McDonald Observatory By Jasmin Fox-Skelly
Education and Outreach Office, contact AstroMiscellany 14
us at 512-471-5285. For subscription
orders only, call 800-STARDATE.
16 Kepler Passes the Torch AstroNews 20
StarDate (ISSN 0889-3098) is published
bimonthly by the McDonald Observatory As a successful planet-hunting
Resetting the Clock on Saturn’s Rings
Education and Outreach Office, The Uni- spacecraft came to the end of its mission,
versity of Texas at Austin, 2515 Speedway,
Stop C1402, Austin, TX 78712. © 2019 a successor took flight. Several others are Chasing Away Planet Nine
The University of Texas at Austin. Annual expected to follow in the next decade Chillin’ Under the Sun
subscription rate is $26 in the United
States. Subscriptions may be paid for using By Rebecca Johnson Birth of a Black Hole, or Death by Black Hole?
credit card or money orders. The University
of Texas cannot accept checks drawn on Gaia Spies Galaxy-Hopping Stars
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On The Cover
University of Texas McDonald Observatory. Artist’s concept of Kepler-62f, the
smallest planet the Kepler spacecraft
has found orbiting in a star’s
NASA AMES/JPL-CALTECH/T. PYLE
habitable zone. Kepler’s mission
Visit StarDate Online at
stardate.org or on: recently ended, but its successors
will continue to probe exoplanets for
their secrets. For more, see Page 16.
This Page Coming Up
Hydrogen gas glows red in the Ghost Nebula, Our May/June issue brings you excerpts from
NASA/ESA/STScI/H. ARAB (UNIV. OF STRASBOURG)
seen by Hubble Space Telescope 550 light- new books in astronomy and space science
years away in the constellation Cassiopeia. to spice up your summer reading. We’ll also
The gas is heated by blasts of ultraviolet bring you summer skywatching notes and
S t a rD a t e radiation from the nearby blue giant star charts, Merlin’s answers, and more.
S t a rD a t e M a g a z i n e
Frank N. Bash Visitors Center Gamma Cassiopeiae (not shown), which is
34,000 times more powerful than the Sun.
2 MARCH/APRIL 2019
Merlin
Dear Merlin, tional force. Presumably they
Celestial bodies (planets, as- continue to draw in matter (and
teroids, comets, etc.) obey the grow larger and gravitation-
laws of gravitational physics. ally stronger) until all the
Is it possible to have a celestial nearby matter is absorbed.
object that travels askew to the Is it possible, then, that
gravitational forces of the solar the galaxies we observe
system — something that plays — billions of light-years
by rules that transcend our local distant, and therefore bil-
forces? lions of years in the past
Bill Dockery — have been consumed
Knoxville, Tennessee by a giant black hole and
no longer even exist in
ÖM
Following the laws of phys- our time frame?
STR
ND
ics isn’t just recommended, it’s Owen Daniel
LU
YN
E
required. There is no way for Fort Worth LA
any object to break those laws.
That’s a comforting thought, Possible, but highly unlikely.
because otherwise scientists Black holes in the centers
would have no way of knowing of galaxies can grow to mon-
where any object was going, so strous proportions. The larg- give birth to new stars.) Kuiper Belt, which is a dough-
they might never know if an est yet measured top 10 bil- Also, keep in mind that while nut-shaped region beyond the
object endangered Earth. lion times the mass of the Sun, 10 billion solar masses is a lot, realm of the major planets.
Having said that, Merlin will which certainly produces a it’s insignificant compared to The calculations aren’t precise
add that the motions of objects powerful gravitational pull. the mass of a major galaxy. enough to pinpoint the possible
in the solar system can be af- Yet several things conspire The Milky Way, for example, planet’s location, though, so
fected by objects outside the to prevent a black hole from has a mass of more than a tril- astronomers must scan large
solar system. A passing star, for devouring its entire host gal- lion Suns, and its disk spans regions of the sky — far from
example, can stir up the Oort axy. For one thing, as a black 100,000 light-years or more. the ecliptic — to try to find it.
Cloud, which is a vast shell of hole “feeds,” it surrounds itself All of that material exerts its Even though the planet is
comet-like bodies that extends a with a spinning disk of gas. As own gravitational pull, keep- estimated to be several times
light-year or more from the Sun. the gas gets closer to the black ing stars and gas clouds from Earth’s diameter, it is hun-
These objects can get enough of hole it moves faster, so it pro- falling toward the central dreds of times farther from the
a push to fall toward the Sun, duces friction. That heats the black hole. Sun than Earth is, so sunlight
where they could possibly run gas to millions of degrees, so it So while astronomers can’t would be only a tiny fraction
into Earth or another body. produces a lot of radiation. The be sure that black holes haven’t as strong as at Earth. It the
Such encounters strictly fol- radiation produces pressure sucked up their entire galactic planet is at its farthest pro-
low the laws of physics, how- that pushes away the material homes, they know that it’s not jected distance, in fact, sun-
ever — no cheating allowed! that’s trying to fall toward the likely. light would be less than one-
black hole. That acts as a tidy millionth as strong as at Earth,
Dear Merlin, regulator, preventing the black Dear Merlin, which means there would be
As I understand black holes, hole from overindulging. (The Why is Planet 9 so difficult little to illuminate it. Objects
they tend to exist in the centers of outward-moving pressure also to track down, especially if it is in that part of the solar sys-
galaxies, and that nothing (even can cause clouds of gas around as large as presumed? Is it too tem typically are darkened by
light) can escape their gravita- the black hole to collapse and far away from the Sun? Would it long exposure to cosmic radia-
be found on the ecliptic with the tion, too, so a possible Planet 9
other planets? would be quite dark.
Send questions to Corrin Gani The planet might shine
Merlin
Austin brighter in the infrared, but
StarDate
Merlin is unable to send University of Texas at Austin
still would be a faint target. So,
personal replies. Answers to 2515 Speedway, Stop C1402 Astronomers have inferred as with the discovery of Pluto
many astronomy questions are Austin, TX 78712 the presence of Planet 9 from almost a century ago, if Planet
available through our web site: stardatemerlin@gmail.com the alignment and motions of 9 exists it will take a lot of pa-
stardate.org/astro-guide stardate.org/magazine a group of iceballs outside the tient searching to find it.
S ta r D at e 3
POETS, PHILOSOPHERS, QUEENS, ASTRONOMERS Early women astronomers drafted calendars, plotted eclipses, built observatories, and helped shape humanity’s early understanding of the universe By Jasmin Fox-Skelly 4 MARCH/APRIL 2019
Visitors check out Cheomseongdae,
the observatory created by Sonduk.
Opposite page, from left: Sonduk; a
modern depiction of Hypatia
TOP6BIN/WIKIPEDIA; LEFT PAGE, FROM LEFT: WIKIMEDIA, NASA/CXRO/CFA
S ta r D at e 5
H
enrietta Swan Leavitt helped make the expanding universe possible. She discovered that
a class of bright pulsating stars made perfect “standard candles” for determining the scale
of the universe: Measure such a star’s maximum brightness and the length of its pulses
and you could determine its distance. Edwin Hubble used those stars to discover that
swirling, spiral-shaped motes of matter are far outside the Milky Way galaxy, greatly increasing the size
of the known universe. He also used them as a first step in determining that the universe is expanding.
Leavitt’s discovery helped revolutionize humanity’s concept of the universe. She wasn’t the first
woman to do so, however. History is full of examples of female astronomers whose work changed the
way we think about the cosmos. In fact, some of the very first astronomers were female, and women
have been studying the stars since antiquity. From the ancient Greeks, Romans, Egyptians, and Sume-
rians, here are the stories of some of the earliest women astronomers.
EN HEDU’ANNA throughout the year, allowing her to ing the Moon, which means building
En Hedu’anna was a princess who create a lunar calendar. She also likely a table that plotted the phase and
was born about 4,300 years ago in recorded the seasons and the position position of the Moon at various times
Sumer, part of the historical region of of the Sun. of day throughout the month. It could
southern Mesopotamia, which is part “The term ‘scientist’ did not come then be used to forecast crop planting
of modern-day Iraq. Her father, King into general use until the mid-19th and harvesting dates.”
Sargon of Akkad, united the cities of century,” says Sethanne Howard, an According to Howard, it is from the
northern and southern Mesopotamia astronomer and author of the book The work of these early astronomers that
and established a mighty empire. To Hidden Giants, which tells the story of modern religious calendars developed.
help consolidate his power he ap- more than 4,000 years of women in For example, we date Easter, Passover,
pointed his daughter as head priestess science, technology, engineering, and and Ramadan using work derived by
of the temple of the Moon goddess in mathematics. “Instead, scientists were the ancient Sumerians.
the city of Ur. known as astronomers/mathemati- En Hedu’anna is perhaps most fa-
En Hedu’anna’s job involved de- cians, philosophers, or priestesses. mous as the first known poet in histo-
ciding when crops should be planted “En Hedu’anna controlled the ex- ry, earning her the title of the “Shake-
and harvested and when important tensive agricultural enterprise sur- speare of Sumeria.” Like other Sumeri-
religious festivals should take place. rounding the temple as well as reli- an writings, her poems were imprinted
To do this, she observed the Moon gious activities scheduled around the on damp clay tablets, which were left
and tracked its position across the sky year. Her calendar was based on track- to harden in the sunlight. Many of
An ancient
Sumerian disk
depicts En
Hedu’anna (third
figure from right).
IA
IPED IK
KIR/W
ZUN
6 MARCH/APRIL 2019
these still survive, including three long Pythagoras teaches a class of women, perhaps including
poems to the goddess Inanna, three Theano (foreground, facing away from Pythagoras
poems to the god Nanna, and 42 tem-
ple hymns that can be found in trans-
lation today. The most famous tablet,
honoring Inanna, is at the University
Museum in Philadelphia.
Many of her writings allude to her
work studying the Moon and stars. For
example, in one hymn she writes:
In the gipar the priestesses’ rooms
that princely shrine of cosmic order
they track the passage of the moon.
Another reads:
The true woman who possesses
exceeding wisdom,
WIKIPEDIA
She consults a tablet of lapis lazuli
She gives advice to all lands…
She measures off the heavens, She wrote books entitled Cosmology, Bicknell, a classical scholar at Monash
She places the measuring-cords on The Theorem of the Golden Mean, The University in Victoria, Australia, the-
the earth. Theory of Numbers, and The Construc- orized that Aglaonice’s disappearing
“This poem describes the work of a tion of the Universe. She also wrote a Moons could have occurred during
scientist and leader,” says Howard. “To biography about her husband, Life of a solar cycle that is different from its
measure off the heavens is to engage Pythagoras. well-known 11-year cycle of increas-
in astronomy, whilst to measure the Theano theorized that the universe ing and decreasing magnetic activity.
Earth requires surveying skills as well consists of 10 concentric spheres, Whatever the case, Aglaonice is
as astronomy. These are all technical which held the Sun, Moon, Saturn, written about in a number of ancient
subjects requiring great skill to ac- Jupiter, Mars, Venus, Mercury, Earth, Greek texts. In the book Instructions
complish.” an unseen counter-Earth, and the stars. for Married Couples, written around
En Hedu’anna was the first in a long the end of the first century AD, Plu-
line of Sumerian astronomer-priest- AGLAONICE tarch, a renowned biographer and es-
esses, of whom little is known. Her Around the time that Greek astrono- sayist, writes:
achievements are especially remark- mers first used geometry to predict “Aglaonice the daughter of Hegetor
able considering that she lived and that Earth was round, Aglaonice, the
worked 2,000 years before the Golden daughter of Hegetor (or Hegemon) of
Age of Greece and just 900 years after Thessaly, was using her observations
writing had been invented. of the phases of the Moon to master
the art of predicting lunar eclipses.
THEANO In fact, not only did she predict the
To find the next female astronomer times of lunar eclipses, she also calcu-
of whom anything is known, fast for- lated their degree of brightness. She
ward to circa 546 BC and the ancient was said to be able to predict when
Greeks. One of the scientific leaders of an eclipsed Moon would completely
the era was Pythagoras, a philosopher disappear — a trick she used to her
who founded a movement of political advantage when convincing people
and religious teachings and calculat- that she could command the Moon.
ed the numerical constant pi, which, Total “blackouts” of the Moon, in
among other things, can be used to which the eclipsed lunar disk turns
calculate the area of a circle. However, completely black instead of orange or
few know that his wife, Theano, also red, are almost unheard of, and have
was an accomplished astronomer and sometimes been attributed to volca-
mathematician. After her husband’s nic dust spewing from an eruption.
death she supposedly led his followers, However, historical records show that
the Pythagoreans. there were no such eruptions during
Theano reportedly wrote many texts Aglaonice’s time, making it a mystery
ALZINOUS/WIKIMEDIA
on medicine, physics, mathematics, as to how the sky became so dark that
A modern-day artist’s
and psychology, and was especially people believed the Moon had disap-
impression of Aglaonice
interested in issues related to children. peared. In a paper in the 1980s, Peter
S ta r D at e 7
being thoroughly conversant with the cal treatise considered by some to be actions prudent and public-spirited,
periods of the Full Moon when it is the most influential textbook ever and the rest of the city gave her suit-
subject to eclipse, and knowing be- written, and Book III of Ptolemy’s able welcome and accorded her special
forehand when the moon was due to Almagest, which described the move- respect.”
be overtaken by the Earth’s shadow, ments of the stars and planets and sug- One man with whom she developed
imposed upon audiences of women gested that the Sun revolves around a particularly close friendship was the
and made them all believe that she Earth — a model of the universe that governor of Alexandria, Orestes. Un-
drew down the Moon.” wouldn’t be overturned until the time fortunately for Hypatia, though, her
In fact, Aglaonice was so famous of Copernicus and Galileo. association with him would lead to her
in her time that she was the subject Hypatia also taught students, includ- gruesome death.
of an ancient Greek proverb, “as the ing a man named Synesius. Much of Hypatia lived in an era of extreme
Moon obeys Aglaonice.” It’s likely what we know about her comes from religious and sectarian conflict. The
that she was seen as a sorceress, and letters between the two. These letters archbishop of Alexandria, Cyril, was
later authors referred to her and other include instructions on how to con- particularly intolerant of other faiths.
female astronomers of the period as struct an astrolabe — a sort of portable A power struggle broke out between
the “witches” and “enchantresses” of astronomical calculator that can keep Cyril and Orestes, with Hypatia caught
Thessaly. track of the Sun, Moon, and stars. in the middle. The conflict between
Hypatia never married, although the them peaked after Cyril led a mob
HYPATIA Suda Lexicon, a Byzantine 10th-centu- that expelled the Jews from the city
Perhaps the most famous female as- ry encyclopedia of the Mediterranean and ransacked their homes and tem-
tronomer of ancient times was Hypa- world, describes her as being “exceed- ples, which followed the massacre of a
tia, who was born in Alexandria, ingly beautiful and fair of form ... in group of Christians by Jewish extrem-
Egypt, which at the time was con- speech articulate and logical, in her ists. Orestes complained to the Roman
trolled by Rome. Although still a cen-
ter of culture and learning, by the time
Hypatia was born, sometime between
the years 350 and 370, the city was
in decline. Its famous library, home to
half a million scrolls, had been burned
down (although it was rebuilt). By
364, the great Roman Empire had split
in half, and fighting had broken out
between Alexandria’s Christians, Jews,
and pagans.
Hypatia was the daughter of Theon,
the last known member of Alexan-
dria’s university (which was associated
with the library) and a mathematician
and astronomer. Hypatia continued
her father’s work and taught philoso-
phy, math, and astronomy. Her lec-
tures were well attended, and people
traveled from great distances to hear
her talk.
The philosopher Damascius wrote
after her death: “Donning [the robe
of a scholar], the lady made appear-
ances around the center of the city,
expounding in public to those willing
to listen on Plato or Aristotle.”
Hypatia was one of the last great
thinkers of ancient Alexandria, and
wrote commentaries on such famous
classical works as Diophantus’ Ar-
ithmetica, an ancient Greek text on An illustration of the 1893 play ‘Hypatia,’ based on an 1853
mathematics and algebra. Other texts novel. Hypatia’s life has been depicted in many books,
WIKIPEDIA
that she wrote commentaries about in- plays, and movies, including the 2009 film Agora.
clude Euclid’s Elements, a mathemati-
8 MARCH/APRIL 2019
government in Constantinople, which
led to a rift between the two men.
When Orestes refused Cyril’s attempts
at reconciliation, Cyril’s monks tried
unsuccessfully to kill him.
Unfortunately for Hypatia, a rumor
spread that she was preventing Or-
estes and Cyril from reconciling. As a
pagan who publicly spoke about non-
Christian ideas, she was an easy target,
and in the year 415 or 416, a mob of
Christian fanatics led by Peter the
Lector dragged her from her carriage
into a church, where they stripped her,
beat her to death, tore her body apart, Resources
and burned it.
Queen Books
Alexandria’s university and library
were burned not long afterward, end- Sonduk The Unforgotten Sisters: Female Astronomers and Scientists
ing the city’s long reign as one of the before Caroline Herschel, by Gabriella Bernardi, 2016
world’s leading centers of learning and The Hidden Giants: 2nd Edition, by Sethanne Howard,
science. 2013
WIKIMEDIA
Hypatia of Alexandria: Mathematician and Martyr, by
QUEEN SONDUK Michael A.B. Deakin, 2007
The final female astronomer on our wrote: “Will we ever know the truth
list is Queen Sonduk, the first female about the stars? I am too young to ven- Hypatia: Her Life and Times, by Faith L. Justice, 2013
ruler of the Korean Buddhist kingdom ture a theory about our universe. I only Hypatia: The Life and Legend of an Ancient Philosopher,
known as Shilla, who ascended to the know that I want to understand more by Edward J. Watts, 2017
throne in 632. She had developed a deeply. I want to know all I can know.
keen interest in the stars as a child, Why should it be forbidden?”
but was prevented from studying as- After she became queen, Sonduk Internet
tronomy because women were seen as created Cheomseongdae (“tower of Women Astronomers Through History
subordinate to men, and education for the stars”), an observatory that was adsabs.harvard.edu/full/2008POBeo..85..207D
girls was considered improper. When built according to her design. The Theano
she was 15, it is rumored that Sonduk milk bottle-shaped tower, which is www.agnesscott.edu/lriddle/women/theano.htm
asked the Chinese ambassador, Lin about 30 feet tall, was constructed
Hypatia of Alexandria
Fang, who was also an astronomer, to from 365 stones, one for each day of
http://www-history.mcs.st-andrews.ac.uk/Biographies/
teach her more about the universe. He the year. Observations of the Sun and
Hypatia.html
reportedly replied, “Surely you cannot Moon probably helped determine the
imagine that I would converse on such planting and harvesting seasons, while Hypatia, Ancient Alexandria’s Great Female Scholar
a serious subject with a young lady? It observations of the stars, which were www.smithsonianmag.com/history/hypatia-ancient-alexan-
would be unnatural and wholly against important to astrology, would have drias-great-female-scholar-10942888
the laws of propriety.” had important political significance. Hypatia of Alexandria: The Passing of Philosophy to
The relationship between Sonduk The observatory is still standing — the Religion
and Lin Fang deteriorated further oldest surviving astronomical observa- www.ancient.eu/article/76/hypatia-of-alexandria-the-passing-
when Lin Fang convinced the king to tory in East Asia. of-philosophy-to
introduce a new official calendar based The Unforgotten Sisters: Sonduk, the astronomer queen
on the Chinese Sui calendar. After These five examples show that cosmosmagazine.com/space/the-unforgotten-sisters-sonduk-
Sonduk correctly predicted when a despite facing incredible obstacles, the-astronomer-queen
solar eclipse was to occur, an event women have shared the exciting world
Queen Seondeok
missed by the official calendar, the am- of science with men from the be-
www.ancient.eu/Queen_Seondeok
bassador became angry and reportedly ginnings of written history, making
said, “Astronomy is not for women. important contributions to human- UNESCO: Cheomseongdae Observatory
Go do something female like look ity’s ever-increasing knowledge of the www3.astronomicalheritage.net/index.php/show-entity?identi
after silkworms.” universe. ty=19&idsubentity=1
The king consequently forbade Son-
duk to study astronomy. At the age Jasmin Fox-Skelly is a science writer in
of 15, in a message she placed in her Cardiff, Wales, and a previous contribu-
grandmother’s ancestral jar, Sonduk tor to StarDate.
S ta r D at e 9
by Alan MacRobert
S pring has sprung, and its stars are making their
move into prominence. Regulus, Leo’s bright
heart, is moving high across the south. Spica, shining
the height of the planet’s os-
tentatious display last sum-
mer, it was three times bright-
MARCH 16 - 31
Sirius, brightest spark of
the evening sky, now shines
er than Sirius! due south as the stars begin
in Virgo, rises high by March’s end. In April, look for Look to the upper right to come out. How early can
the Moon and Mars to stage appearances alongside of Sirius for Orion. Procyon you spot Sirius in the fading
Taurus, the bull, and its bright eye Aldebaran. shines to Sirius’ upper left. Be-light of day? Face south. To
telgeuse, Orion’s Mars-colored tell how high to look, subtract
shoulder, teams up with Sirius your latitude from 73 degrees.
MARCH 1 - 15 the seasonal westward turn- and Procyon to form the Win- For example, Austin is at lati-
Is a planet ever unwelcome? ing of the constellations as ter Triangle, showy and equi- tude 30 degrees. So at Austin
When Mars is in the evening Earth goes around the Sun. lateral. you’ll find Sirius 43 degrees
sky, it’s always that guest who But by now we’ve far out- Leo stands high in the east above horizontal when it’s
just stays and stays. And, like run Mars and left it in the to southeast, with Regulus as due south.
the inebriated life of the party distance, small and faint. So his modestly bright forefoot. And how high is that? For
who never gets the message, in early March, as we see it Leo’s Sickle asterism extends most people, your fist at arm’s
Mars just keeps turning dim- crossing Aries, it’s a paltry to the upper left from Regulus. length is about 10 degrees tall.
mer and less interesting. magnitude +1.2 (paltry for a And since it’s now almost That’s a good enough mea-
Last summer, the Red Plan- naked-eye planet). And in a spring, the Big Dipper stands suring tool to get you close.
et was the night-life dazzler, telescope it looks like a tiny, on its handle high in the Sirius is the brightest star
attracting everyone’s atten- disappointing little blob. northeast as soon as the stars of Canis Major, the big dog.
tion with its closest, More of the con-
brightest opposition stellation comes
in 15 years. Those March 26 into view as night
days are gone, but March 27 arrives. Look for
Mars isn’t. It re- Murzim 5 degrees
mains high in the Saturn Jupiter to Sirius’ right.
west in early eve- Antares Its name means
ning, a mediocre March 28 “The Announcer;”
remnant of its for- March 29 Murzim always pre-
mer self. cedes Sirius across
Why does Mars al- the sky, like a herald
ways do this? The sit- clearing the way for
uation is not caused the king.
by Mars itself, but Look about 10
by our viewing plat- degrees below Siri-
form from the mov- us, and a bit to the
ing Earth. We’re the left, for the triangle
next planet closer to of the dog’s hind-
CJ DUNCAN
the Sun, and not by 30 minutes before sunrise S quarters and tail-
very much as solar- tip. These four stars
system distances go. So, after Spot it in the west after come out. plus Sirius form a big-bladed
opposition, Mars moves along dark. It’s the brightest point For early risers, three plan- meat cleaver with a short
its orbit only a little slower below the Pleiades. As always, ets await in early dawn. Ju- handle at the bottom; Sirius is
than Earth does on its. This its fire-orange color helps to piter dominates the south as a sparkle on the cleaver’s top
means that compared to the give it away. The waxing cres- morning twilight begins, with back corner.
other outer planets, Mars cent Moon hangs under it on Antares and the head of Scor- With the arrival of deep
takes a long time to fall behind the evening of March 10 and pius to its right. Look two or night in an un-light-pollut-
us in its orbit. to the left of it on the 11th. three fists to Jupiter’s lower ed sky, the full stick figure of
Here’s another way to view Sirius shines much brighter left for Saturn, dimmer and Canis Major comes into view.
the situation. Mars’ fast mo- in early evening in the south. yellowish. Follow this line far- He’s seen in profile, prancing
tion eastward against the eve- The star sparkles white with ther to the lower left and you on his hind feet. Sirius is a
ning constellations, from our 12 times as much light (at hit bright Venus, lower every shiny dog tag on his chest,
viewpoint, partly counteracts magnitude –1.5) as Mars. At morning. Murzim is his forefoot, and
10 MARCH/APRIL 2019the last several weeks. By April is perhaps the most famous down when you face north.
Meteor Watch 1, the Pleiades is passing a cou- double star for amateur tele- Look below the Big Dipper
ple of finger widths to Mars’ scopes, though its compo- for the much dimmer Little
The Shower
right. The cluster moves down nents are a less interesting Dipper. It’s highlighted only
Lyrids
to the lower right thereafter. icy white. by Polaris, its handle-end on
Peak Look to the left of Mars for the left, and the two stars
Night of April 22 Aldebaran, slightly brighter. APRIL 16 - 30 forming the end of its bowl
On the evening of April 8, the The Arch of Spring is form- on the right: Kochab and
Notes waxing crescent Moon hangs ing up nicely high in the west. Pherkad, the “Guardians of
The Lyrids are modest, with around under the arc of Mars, Al- Pollux and Castor, nearly hor- the Pole,” forever circling Po-
one or two dozen meteors per hour debaran, and the Pleiades. On izontal, form its top. The south laris.
at best. The Moon climbs into view the 9th, the Moon poses above end of the Arch is Procyon, to If you have an open hori-
before midnight and is about 90 Aldebaran. their lower left. The other end, zon to the north, look low on
percent full, so it will wash out most With the coming of April, farther flung to their right, the other side of Polaris from
of the meteors. Arcturus, “the spring star,” consists of Menkalinan (Beta the Big Dipper for the dipper’s
twinkles in the east. Spica, Aurigae) and Capella. eternal opposite: W-shaped
his faint, pointy-nosed head not quite as bright, is well off The Arch of Spring is the Cassiopeia, lying low for the
is a long triangle to Sirius’ to its right or lower right. If last, trailing half of the even warm season.
left and upper left. The stick you live as far south as Miami, bigger Winter Hexagon. Turn around. Look high
figure becomes obvious once you’ll find Arcturus and Spica But not all the other Hexa- overhead in the south for
you know it. equally high. Farther north of gon stars are yet gone. Sirius Leo. We see the lion in profile,
Now that winter is turning there your eastern horizon is sparkles brightly, low in the stalking westward, with its
to spring, Orion tips south- tilted differently with respect southwest, for a while dur- tail-star Denebola on the left
westward, to the right of Sir- to the stars, so Arcturus ap- ing and after twilight. From and the Sickle pattern form-
ius as soon as night is fully pears higher. there, look to the right for ing the lion’s head, mane, and
dark — and Orion’s Belt is in High to the upper left of Orion with his horizontal belt. forequarters on the right.
its horizontal springtime ori- Arcturus, in the northeast, Can you still spot Rigel down Leo always announces
entation. The belt points to the Big Dipper is starting under the belt? Continuing to spring. Now that spring is
the left, back to Sirius, and to turn over to dump April
a little farther to the right, showers. The dipper’s curv-
toward Aldebaran in Taurus. ing handle arcs back toward Springing a New Season
Look to the lower right of Arcturus.
Spring begins in the northern hemisphere at 4:58 p.m. CDT March
Aldebaran for Aldebaran- The dipper handle can also
colored Mars. How closely, guide you to a lesser-known
20 as the Sun crosses the equator heading north. For many of us in
to your eyes, do their colors sight. If you picture the han- modern times, that moment, the spring equinox, is little more than a
match? To me, the tint of dle and the side of the dipper notation on the calendar. But to many cultures in bygone centuries, it
Mars looks just a trace deeper bowl that it’s attached to as was a time for celebration, as the Sun warmed the earth and banished
orange. a segment of a (very rough) the long, cold nights of winter.
circle, then near the circle’s
APRIL 1 - 15 center is modestly bright Cor
Mars remains in the west Caroli, a grand yellow-and-vi- the right from the belt, you here, he’s already walking
during and just after twi- olet double star in a telescope. come to Aldebaran, now with westward on his way to de-
light, as it will continue to do It’s in the dim constellation Mars above or to its upper parting into the sunset come
through May. But now it has Canes Venatici, the hunting right. summer.
an interesting partner. You dogs. Meanwhile, the Big Dipper
may have noticed the Pleiades And the star at the bend is assuming its highest posi- Alan MacRobert is a senior edi-
cluster descending toward it for of the dipper’s handle, Mizar, tion after dark, floating upside tor of Sky & Telescope.
6 14 20 27
MARCH
10:04 am 5:27 am 8:43 pm 11:10 pm Moon phase times
are for the
5 12 19 26
Central Time Zone.
APRIL
3:50 am 2:06 pm 6:12 am 5:18 pm
S t a r D a t e 11How to use these charts: February 20 11 p.m.
1. Determine the direction you are facing. March 5 10 p.m.
2. Turn the chart until that direction is at the bottom.
March 20 8 p.m.*
NORTH
* Daylight Saving Time begins March 10.
CE PH EU S A
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12 MARCH/APRIL 2019How to use these charts: March 20 11 p.m.
1. Determine the direction you are facing. April 5 10 p.m.
2. Turn the chart until that direction is at the bottom.
April 20 9 p.m.
NORTH
A ld e
ram CE PH EU S
in
IA
Et PE
am DR IO S
in AC AS C
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te
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SOUTH
S t a r D a t e 13Astro Miscellany
The Arecibo radio dish; Right:
NSF/NAIC ARECIBO OBSERVATORY
A colorized version of the 1974
message; Bottom: Teams must
turn this sequence of numbers
into an 18-character password
WIKIPEDIA
to enter the contest.
A New Message for the Galaxy
L
ike a person updating The Arecibo Observatory, numbers 1-10, information the competing teams must
her voicemail greet- which operates a 1,000-foot about DNA, a figure of a per- be accompanied by a nar-
ing, the radio observa- radio telescope in Puerto son, and details on the Are- rative that explains why the
tory that beamed the first Rico, beamed out the origi- cibo telescope. message is important, ad-
greeting from Earth into nal message on November The new message will dresses possible risks of send-
space is getting ready to 16, 1974. It was designed by be composed by one of 45 ing it to another civilization,
update the message. It has Frank Drake, a pioneer in the teams that make the initial and provides such practical
started a contest that calls on search for extraterrestrial in- cut. To reach that point, the details as where to aim the
groups of students — from telligence, with help from as- teams must decode a pass- telescope (the first message
kindergarten through grad- tronomer and science popu- word that allows them to was beamed toward M13, a
uate school — to prepare larizer Carl Sagan. It consist- enter the contest and solve a globular star cluster 25,000
their own greetings to other ed of 1,679 ones and zeroes puzzle about a mystery in the light-years from Earth).
civilizations. The winning arranged in a grid of 73 rows telescope control room, then The winner will be an-
message will be transmitted and 23 columns. Among register by late March. nounced on the September
in November. other details, it included the Completed messages by equinox.
www.areciboobservatory.org/challenge
00000000000000000000000000000000000000000000000000001010
10001110010101000111001010100011100101010000100010101001
11110101010101110110101010111011010101010101101000101110
11010001011101101000101110110100010101011010001010101100
00000101001000000010100100000001010010000000101001000000
01010010000000101001000000010100100000011011000000000000
00000000000000000000000000000000000000000
14 MARCH/APRIL 2019Picturing a Destiny in Space
U ntil Surveyors and Mariners and Voyagers
began beaming back close-range pictures of the
worlds of the solar system, most Americans saw the
teamed with rocket pioneers Willy Ley and Wernher
von Braun to publish a book and a series of maga-
zine articles that showed how humans could expand
Moon, Mars, and other worlds through the eyes of into the solar system.
Chesley Bonestell, an architect, designer, and artist. The documentary, which premiered last year, in-
Bonestell and his work are profiled in a docu- cludes clips of Bonestell himself, as well as interviews
mentary that will appear at several venues this with rocket scientists, filmmakers, artists, and other
year. “Chesley Bonestell: A Brush With the Future,” notables, who discuss his influence on our concept
chronicles his career and his influence on space of worlds beyond our own.
exploration.
Bonestell began painting depictions of the Moon UPCOMING SHOWINGS:
and planets, rockets, and other space-related sub- March 24 The Filoli Estate, Woodside, California
jects in the 1930s. His work appeared in maga- March 27-31 Sonoma International Film Festival
zines and books and in “War of the Worlds,” “Des- April 23 Mount Diablo Astronomical Society, Walnut Creek, Calif.
tination Moon,” and other classic sci-fi movies. He July 18 Wings Over the Rockies Museum, Denver
www.chesleybonestell.com/home.html
Getting to Know the Moon Bit by Bit
M ore than 30,000 years ago,
the Aurignacians, the earliest
modern humans in present-day Eu-
Sky at Night, it is a compendium of
just about every Moon-related topic.
Aderin-Pocock talks about observ-
rope, notched a series of holes and ing the Moon, the history of moon-
crescents in a short segment of ea- watching (including the Abri Blan-
gle’s bone. The bone was discovered card calendar), the Moon in folklore
in the early 20th century at Abri and science fiction, the history of
Blancard, a rock shelter in France. lunar exploration, lunar science,
Decades later, an archaeologist con- and the possible future commer-
cluded that the notches represented cialization of lunar resources, from
the phases of the Moon, and that the water and solar power to helium-3,
bone was the earliest known lunar a possible fuel for nuclear fusion
calendar. reactors.
People have been recording the Although each section is brief, it
The Book of the Moon Moon ever since, on walls, moun- provides a thorough overview of
A Guide to Our Closest Neighbor
tainsides, canvases, video screens, each topic without getting bogged
By Maggie Aderin-Pocock
Penguin Books, $24.99 (April 1)
and especially paper. One of the lat- down in too much detail. So read-
www.abramsbooks.com/ est is The Book of the Moon: A Guide ers can learn about everything from
product/book-of-the- to Our Closest Neighbor. Written by moonquakes to H.G. Wells to lunar
moon_9781419738494 Maggie Aderin-Pocock, a scientist volcanism to werewolves in a quick,
and host of BBC television’s weekly easy package.
S t a r D a t e 15As a successful planet-hunting
spacecraft came to the end of its
mission, a successor took flight.
Several others are expected to
follow in the next decade
By Rebecca Johnson
NASA AMES/JPL-CALTECH/T. PYLE
NASA AMES
16 MARCH/APRIL 2019O
n October 30, NASA announced that its pre- work through all of them.
Borucki “converted me from a skep-
mier planet hunter, Kepler, had run out of fuel tic to a true believer” in the concept,
and could no longer conduct science. A few Cochran says. Cochran served as co-
investigator on “all four proposals that
weeks later, the agency sent its final com-
got turned down, until the fifth one was
mands to the spacecraft, severing its communication successful,” he recalls.
link with Earth. Kepler’s mission ended on November 15, By the time Kepler launched, astrono-
mers had found a few handfuls of planets
388 years to the day after the death of its namesake, the orbiting other stars using ground-based
astronomer Johannes Kepler. The spacecraft is now drift- telescopes (using a different technique
ing in a safe orbit around the Sun, 94 million miles from from Kepler). So the question of whether
other planets were out there — long be-
Earth. After nine years and thousands of planets, Kepler lieved, but never proven — had been an-
is passing the torch to newer planet hunters. swered. But there weren’t nearly enough
known planets to do any kind of statis-
tical studies — to know the variety of
K
epler began its journey with a team found a workaround, which in- planets or their parent stars.
night launch from Cape Canaver- volved allowing the spacecraft to change Kepler produced a torrent of new plan-
al on March 6, 2009. It soon began its field of view every few months. A new et discoveries. By early February, 2,662
searching for planets around other stars mission, called K2, then began as Kepler planets discovered with Kepler have been
by looking for transits. In these events, continued hunting planets, now all across confirmed, and a roughly equal number
a planet passes in front of a star, briefly the sky. All told, the spacecraft probed of candidates await confirmation. About
eclipsing a small portion of the star’s sur- more than half a million stars for orbiting 3,000 scientific papers based on Kepler
face, causing it to grow a tiny bit fainter. planets. research have been published so far.
Over its 3.5-year primary mission, Ke- What the spacecraft found has changed “Kepler has shown that virtually all
pler kept a constant eye on the light of the picture of planets in our galaxy. of the stars that we see in the sky prob-
150,000 stars in the constellations Cyg- “When we started conceiving this mis- ably host planetary systems,” Cochran
nus and Lyra. The mission’s larger goals sion 35 years ago we didn’t know of a sin- says. “Now we are discovering that a
were to use this sample to extrapolate gle planet outside our solar system,” says significant number of those systems are
the total number of Earth-like planets in Bill Borucki, who created the Kepler con- very much like our own and may have the
the Milky Way galaxy, and to better un- cept and served as the mission’s original capability of being habitable.”
derstand the characteristics of other solar principal investigator, based at NASA’s After launch, Cochran and his team at
systems to help scientists put our own in Ames Research Center in California. McDonald Observatory, along with as-
context. McDonald Observatory research pro- tronomers at other observatories around
Soon after its primary mission ended, fessor Bill Cochran worked with Borucki the world, conducted follow-up obser-
Kepler’s science halted when one of its on planning the mission, discussing the vations to confirm or disprove Kepler’s
reaction wheels, which helped keep it scientific and technical hurdles they discoveries. Their observations could dis-
NASA GSFC/FRANCIS REDDY
pointed at its target, failed. The Kepler would have to overcome and helping to tinguish between a genuine planet transit
and other behaviors in stars that can
The planet K2-288Bb, seen at bottom in this artist’s concept, orbits two red dwarf mimic it, such as starspots.
stars. It’s the latest find in the Kepler data archive. Left: Kepler and TESS hunt This ground-based work studies these
for planets using the transit method, looking for a dip in a star’s light output that stars in a completely different way than
indicates a planet is passing in front of it. Far left: This artist’s concept shows how Kepler, using a technique called the “radi-
Kepler’s field of view changed positions across the sky during the K2 mission. al velocity” method. It entails monitoring
S t a r D a t e 17the star’s motion toward and away from Kepler’s landslide of discoveries has not exist in our own solar system.
Earth to detect any wobbles caused by the helped astronomers learn a lot about Other solar systems often have many
gravitational pull of orbiting planets. planets in the Milky Way. planets orbiting very close to their parent
The McDonald observations helped There are more planets than stars in stars, making our solar system look sparse
confirm the first Earth-sized planet orbit- the Milky Way, for example, and small in comparison. While our system has four
ing in the habitable zone of a star (Ke- planets are common. In fact, as many as small, rocky planets close to the Sun,
pler-22); the most Earth-like planet yet half of the stars in the Milky Way may some have as many as eight planets with-
seen orbiting another star (Kepler-452); have small, rocky planets orbiting in in the same distance. Astronomers want
the first multi-planet system orbiting a the habitable zone, the distance from a to know if these packed-in planets formed
binary star (Kepler-47); a system of six star where temperatures are just right for close to their parent stars, or formed far-
planets, some near-Earth-sized, orbiting liquid water to exist on a planet’s surface ther out and later migrated inward.
in the habitable zone of a star (Ke- — a likely prerequisite for life, at least life Kepler’s findings provide a tantalizing
pler-11); and an ancient system of five as currently understood. beginning for future exoplanet searches.
Earth-sized planets that dates to the dawn Planets come in diverse forms. And the “Now that we know planets are every-
of the Milky Way galaxy (Kepler-444). most common form, which appears to be where,” Borucki says, “Kepler has set us
between Earth and Neptune in size, does on a new course that’s full of promise for
future generations to explore our galaxy.”
Indeed, one of Kepler’s successors is
already hard at work.
Artist concepts of HD 21749b (left)
K
and a lava world like LHS 3844b
epler’s immediate follow-up mis-
sion was launched from Cape Ca-
naveral April 18. As its name in-
dicates, the Transiting Exoplanet Survey
Satellite (TESS) is searching for planets
NASA/2
using the same transit method as Kepler.
TESS Logs Busy Start
What’s different is the types of target
stars. While Kepler looked at faint stars
between 300 and 3,000 light-years away,
T
ESS began logging exo- 23 times the mass of Earth but exploding stars. TESS is studying closer stars that are 30
planet discoveries just only three times Earth’s diameter, Some of its planet hunting and to 100 times brighter.
a few months after its which places it in the same cat- astrophysics work overlap. Over its initial two-year mission, TESS
launch. By late January, it had egory as Neptune, the outermost For example, it discovered will survey about 200,000 nearby red-
dwarf stars, covering an area of sky 400
discovered eight confirmed plan- of the Sun’s major planets. It’s a planet orbiting the star HIP
times larger than Kepler did. Red dwarfs
ets and almost 350 additional separated from its star by just 20 116158. It also used a technique are smaller and cooler than the Sun, but
candidates. It also recorded more percent of the Earth-Sun distance, called asteroseismology to dis- they are the most common type of star in
than two dozen exploding stars, though, so its surface is too hot to cover dozens of sets of vibrations the galaxy.
flares on the surfaces of red- be inhabited by Earth-like life. on the star’s surface, which are Astronomers at McDonald Observa-
dwarf stars, and scores of other A second, unconfirmed, planet caused by the motions of gas and tory are planning to follow up TESS
flickering objects. in the system appears to be about other processes deep within the planet discoveries, too. A new instrument
“The torrent of data is starting the same size as Earth but about star. The vibrations revealed that for the Hobby-Eberly Telescope, one of
to flow already,” said George 2.5 times as massive, making it a the star is 1.2 times the mass of the world’s largest optical telescopes, has
Ricker, TESS principal investigator, “super-Earth.” the Sun and 2.9 times its diam- been designed to study red-dwarf stars
during a press conference at the LHS 3844b is an Earth-sized eter, about 4.9 billion years old, and their potential planets. Called the
American Astronomical Society world orbiting a stellar pipsqueak and that it’s beginning the transi- Habitable Zone Planet Finder (HPF), it
known as a red dwarf. Although is being commissioned now.
meeting January 7 in Seattle. tion to its red-giant phase, when it
Michael Endl is a senior research sci-
“TESS is going to be a robust the star is a faint ember, the will get much bigger and brighter.
entist at McDonald Observatory who
planet finder and a supernova planet, whose mass has not yet The combination of observa- has worked on Kepler follow-up with
machine,” added Thomas Bar- been determined, is so close to it tions, along with follow-up work Cochran, as well as projects using ground-
clay, a mission scientist at NASA’s that “it’s probably a lava world,” from the ground, then produced based telescopes to look for planets
Goddard Space Flight Center. said Chelsea Huang, a mission sci- a dossier on the planet, which is around red dwarfs. He was involved in
One of the most recent dis- entist at the Kavli Institute at MIT. classified as a “hot Saturn”: It’s the discovery of Proxima b, the planet
coveries is HD 21749b, orbiting TESS also has discovered about 9.2 times Earth’s diameter orbiting the nearest star to the Sun, a red
a small, cool star about 53 light- many other types of astronomi- but 60 times Earth’s mass, so it’s dwarf called Proxima Centauri.
years away. The planet is about cal objects, including dozens of big and fluffy. DB “TESS and HPF are working together,”
Endl says. “TESS announces a candidate,
18 MARCH/APRIL 2019NASA ARC/J. DOTSON/W. STENZEL
NASA (2)
W
and then everybody jumps to follow up.” A graphic from December 2017 (left) shows ith Kepler, the study of exo-
TESS has begun to return some results how Kepler provided a landslide of planet planets got its first motherlode
(see sidebar). Its planets should be close finds (yellow) compared to missions before of data, providing key new in-
enough to Earth for astronomers to begin and since (blue, purple). Technicians prepare sights. And now it’s time for the new kids
looking for signs of life using spectroscopy TESS for its April 18 launch (right). on the block to make their contributions.
— spreading the light from the planet “As NASA’s first planet-hunting mis-
into its component wavelengths to look large, bright, and relatively far from their sion, Kepler has wildly exceeded all our
for chemical elements and compounds parent stars. expectations and paved the way for our
in its atmosphere that could have been Because missions can take decades of exploration and search for life in the solar
produced by living organisms. planning and technology development, system and beyond,” said Thomas Zurbu-
space agencies are already planning mis- chen, associate administrator of NASA’s
A
few more planet-hunting and sions with exoplanet-studies goals that Science Mission Directorate. “Not only
-studying spacecraft are sched- won’t launch until well into the next did it show us how many planets could
uled to launch soon. decade or beyond. be out there, it sparked an entirely new
CHEOPS (CHaracterizing ExOPlanets WFIRST (Wide Field InfraRed Survey and robust field of research that has
Satellite) is a joint project of the European Telescope) will take on multiple sci- taken the science community by storm.
Space Agency (ESA) and the Swiss Space ence missions. In addition to studying Its discoveries have shed a new light on
Office, with the participation of several dark energy, it will study exoplanets in our place in the universe, and illuminated
other nations. Set to launch in October or a couple of ways. First, it will conduct a the tantalizing mysteries and possibilities
November from the Guiana Space Center survey by staring at a dense region of stars among the stars.”
on the South American coast, its mission toward the center of the galaxy to look
will last at least 3.5 years. for brightenings caused when two stars Rebecca Johnson is editor of StarDate.
Unlike Kepler or TESS, which use sur- exactly align. Such alignments, called
veys of many stars to discover exoplanets, microlensing events, also reveal the pres-
CHEOPS will study systems with already
known planets. Specifically, it will target
ence of planets around the stars, and mis-
sion planners expect WFIRST to detect
Resources
exoplanets smaller than Saturn that orbit more than 2,000 worlds. The telescope Internet
bright stars with orbital periods of less also will take direct images of perhaps 50 Exoplanet Exploration
than 50 days. By zeroing in on such a planets around other stars. http://exoplanets.nasa.gov
specific set of exoplanets, mission plan- Europe’s PLATO (PLAnetary Tran- Kepler & K2
ners hope to narrow down the structure sits and Oscillations of stars) mission is http://nasa.gov/kepler
of these planets and figure out how they scheduled to launch in 2026. It’s a follow-
formed and evolved. up to CHEOPS, and will build on that TESS
http://nasa.gov/tess
James Webb Space Telescope (JWST) mission’s findings. “With PLATO, we
is another near-term space mission that are focusing on Earthlike planets orbit- JWST
will follow up on known exoplanets. ing in the habitable zone around other http://jwst.nasa.gov
Unlike CHEOPS, though, it’s not made stars which are similar to our Sun,” said CHEOPS
exclusively for planet work. JWST, which Johann-Dietrich Wörner, director gen- http://sci.esa.int/cheops
currently is scheduled for launch in 2021, eral of ESA. “This will be a major step
WFIRST
will study a wide range of phenomena in towards finding another Earth.” http://wfirst.gsfc.nasa.gov
infrared light, including in-depth follow- A few years later, ESA will launch
up observations of exoplanets. The tele- ARIEL (Atmospheric Remote-sensing In- PLATO
scope will take spectra of these planets frared Exoplanet Large-survey). This satel- http://sci.esa.int/plato
to study their atmospheres. It may even lite is designed to study the atmospheres of ARIEL
be able to take images of planets that are a diverse group of about 1,000 exoplanets. http://sci.esa.int/ariel
S t a r D a t e 19by Damond Benningfield and Rebecca Johnson
Resetting the Clock on Saturn’s Rings
B
eauty is ephemeral. the rings and Saturn itself. ring, the D ring, “rains” into could make the rings vanish in
Consider, for example, Previous estimates of the age Saturn’s upper atmosphere. as little as 100 million years.
the rings of Saturn. of the rings have varied widely, Much of this falls toward the Researchers also used obser-
These bands of ice and dust with some suggesting the rings equator. (The D ring may be vations of Saturn’s rings to de-
are among the most beautiful are as old as Saturn itself. resupplied by material from the termine the length of the plan-
features in the solar system, The trajectory measure- next ring out, the C ring.) et’s day: 10 hours, 33 minutes,
yet they could be short lived. ments showed that the rings Cassini found, however, 38 seconds.
Recent research suggests they are about 40 percent the mass that a substantial amount of Scientists have been unable
are no more than 100 mil- of Mimas, a small moon of material rains toward higher to determine the rate of Sat-
lion years old, and could disap- Saturn, or roughly half the latitudes, roughly 45 degrees urn’s rotation because there
pear in as little as 100 million mass of the ice sheet cover- north and south of the equa- are no solid landmarks on the
years more — a mere blink ing Antarctica. According to tor. gas-giant planet to follow. At-
in the planet’s 4.5 billion-year models of ring formation, the When researchers combined tempts to determine the rota-
lifetime. low mass suggests a recent ori- the Cassini observations with tion rate using variations in
The Cassini spacecraft pro- gin — probably in the last 10 some older views of Saturn Saturn’s magnetic field yielded
vided new details on the rings million to 100 million years. from a ground-based telescope, estimates that varied by as
during the last phases of its The rings probably formed they found changes in the much as 10 minutes.
13-year reconnaissance of the when a small comet or one or planet’s ionosphere, an elec- Cassini scientists, though,
Saturn system, which ended more small moons passed too trically charged layer of the determined that the rings reso-
when the craft plunged into close to Saturn. The planet’s atmosphere. The changes may nate to motions deep within
Saturn’s atmosphere in Sep- powerful gravity pulverized the be caused by bits of ice in the Saturn itself, creating waves
tember 2017. Cassini passed unlucky progenitor. Over time, rings falling toward Saturn. that ripple through the rings.
between Saturn and the inner collisions ground the debris to The particles have their own The waves were difficult to sep-
edge of the rings during each smaller and smaller particles, electric charge, so they’re guid- arate from waves caused by the
of its 22 final orbits of the giant which spread out to form the ed by Saturn’s magnetic field. gravitational influence of Sat-
planet. The rings’ minuscule magnificent rings. As they hit the atmosphere, urn’s moons, but careful anal-
gravitational pull allowed sci- Today, the rings consist pri- they wash away particles of ysis revealed the waves caused
entists to “weigh” them more marily of tiny bits of ice, with a haze, leaving dark bands de- by the planet’s own gravita-
precisely than ever before. The smattering of dust and pebbles. tected by the ground-based tional field, yielding what the
passages also provided a close Some of the material at the observations. The researchers researchers say is the best mea-
view of interactions between inner edge of the closest major calculated that this icy rain surement of Saturn’s day. DB
A Cassini view of the
rings, with Saturn in
the background
NASA/JPL/SSI
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