July 24, 2013
On July 19, 2013, NASA spacecraft got
not one but two rare and unique views of Earth from opposite ends of the solar
system. While exploring Saturn, NASA’s Cassini spacecraft took the top image of
Earth from a distance of about 1.45 billion kilometers (898 million miles)
away. NASA’s MESSENGER spacecraft was 98 million kilometers (61 million miles)
from Earth, in orbit around Mercury, when it acquired the lower image.
The Cassini view is the third-ever
image of Earth from the outer solar system. Views of Earth from distant planets
are rare because our planet is so close to the Sun. Sunlight would damage the
spacecraft's sensitive imagers, so they are rarely pointed homeward. On July
19, however, Cassini was positioned so that Saturn blocked the Sun’s light
while Earth was within the spacecraft’s field of view. Sunlight glimmers around
the giant planet’s limb and lights its icy, dusty rings. The sunlit Earth is
light blue. The Moon is a faint white dot to the side, but is more clearly
visible in the narrow-angle camera view.
Cassini was launched in 1997 to study
Saturn and its moons and rings. The July 19 image that includes Earth was part
of a wider mosaic of the Saturn system as it was backlit by the Sun. This view
will allow scientists to see particles and patterns in Saturn’s rings that are
not often visible. The mosaic will take weeks to compile.
On the other side of the solar system,
MESSENGER (Mercury Surface, Space, Environment, Geochemistry, and Ranging) was
looking for potentially dim, small moons around Mercury when it acquired the
lower, black and white image. The sensor required a long exposure time to
capture these dim objects, which means Earth and Moon are overexposed. They
appear exceptionally bright and large when, in reality, both are less than a
pixel in size in this image.
“Cassini’s
picture reminds us how tiny our home planet is in the vastness of space, and
also testifies to the ingenuity of the citizens of this tiny planet to send a
robotic spacecraft so far away from home to study Saturn and take a look-back
photo of Earth,” said Cassini project scientist Linda Spilker. That observation
holds true for the view from Mercury, as well.
References
EarthSky (2013, July 23) Earth seen
from Saturn July 19, 2013. Accessed July 23, 2013.
NASA (2013, July 18) NASA
interplanetary probes to take pictures of Earth from space. Accessed July 23,
2013.
NASA Jet Propulsion Laboratory (2013,
July 22) NASA releases images of Earth taken by distant spacecraft. Accessed
July 23, 2013.
Johns Hopkins Applied Physics
Laboratory (2013, July 22) Looking back at us. Accessed July 23, 2013.
Cassini
image courtesy NASA/JPL-Caltech/Space Science Institute. MESSENGER image
courtesy NASA
One
Special Day in the Life of Planet Earth
Earth
is an ocean planet. Our home world's abundance of water -- and life -- makes it
unique in our solar system. Other planets, plus a few moons, have ice,
atmospheres, seasons and even weather, but only on Earth does the whole
complicated mix come together in a way that encourages life -- and lots of it
...
An
artist's impression of our solar system with separate representations of scale
and size
...
A
true-color image of Jupiter taken by the Cassini spacecraft. The Galilean moon
Europa casts a shadow on the planet's cloud tops
Jupiter,
the most massive planet in our solar system -- with dozens of moons and an
enormous magnetic field -- forms a kind of miniature solar system. Jupiter does
resemble a star in composition, but it did not grow big enough to ignite. The
planet's swirling cloud stripes are punctuated by massive storms such as the
Great Red Spot, which has raged for hundreds of years.
Jupiter's
appearance is a tapestry of beautiful colors and atmospheric features. Most
visible clouds are composed of ammonia. Water vapor exists deep below and can
sometimes be seen through clear spots in the clouds. The planet's
"stripes" are dark belts and light zones created by strong east-west
winds in Jupiter's upper atmosphere.
...
A Solar System can be defined as a
star and all the objects orbiting it as well as all the material in that
system. Our solar system includes the Sun together with the eight planets and
their moons as well as all other celestial bodies that orbit the Sun.
From our small world we have gazed
upon the cosmic ocean for thousands of years. Ancient astronomers observed
points of light that appeared to move among the stars.
This
image of Comet C/2001 Q4 (NEAT) was taken at Kitt Peak National Observatory
near Tucson, Ariz. in 2004
.
Comets are cosmic snowballs of frozen gases, rock and dust roughly the
size of a small town. When a comet's orbit brings it close to the sun, it heats
up and spews dust and gases into a giant glowing head larger than most planets.
The dust and gases form a tail that stretches away from the sun for millions of
kilometers.
...
NASA's Opportunity rover found this
meteorite on Mars. It is about the size of a basketball
. Little
chunks of rock and debris in space are called meteoroids. They become meteors
-- or shooting stars -- when they fall through a planet's atmosphere; leaving a
bright trail as they are heated to incandescence by the friction of the
atmosphere. Pieces that survive the journey and hit the ground are called
meteorites
...
Uranus' moon Ariel (white dot) and its
shadow (black dot) were caught crossing the face of Uranus in this Hubble Space
Telescope image.
Uranus is the only giant planet whose
equator is nearly at right angles to its orbit. A collision with an Earth-sized
object may explain Uranus' unique tilt. Nearly a twin in size to Neptune,
Uranus has more methane in its mainly hydrogen and helium atmosphere than
Jupiter or Saturn. Methane gives Uranus its blue tint.
...
Eyes on the Solar System:
They called these objects planets,
meaning wanderers, and named them after Roman deities -- Jupiter, king of the
gods; Mars, the god of war; Mercury, messenger of the gods; Venus, the goddess
of love and beauty; and Saturn, father of Jupiter and god of agriculture.
The stargazers also observed comets
with sparkling tails, and meteors -- or shooting stars apparently falling from
the sky.
Since the invention of the telescope,
three more planets have been discovered in our solar system: Uranus (1781),
Neptune (1846) and Pluto (1930). Pluto was reclassified as a dwarf planet in
2006. In addition, our solar system is populated by thousands of small bodies
such as asteroids and comets. Most of the asteroids orbit in a region between
the orbits of Mars and Jupiter, while the home of comets lies far beyond the
orbit of the dwarf planet Pluto, in the Oort Cloud.
The four planets closest to the sun --
Mercury, Venus, Earth, and Mars -- are called the terrestrial planets because
they have solid rocky surfaces. The four large planets beyond the orbit of Mars
-- Jupiter, Saturn, Uranus, and Neptune -- are called the gas giants.
Beyond Neptune, on the edge of the
Kuiper Belt, tiny, distant, dwarf planet Pluto has a solid but icier surface
than the terrestrial planets.
...
Water-ice
clouds, polar ice, polar regions, and geological features can be seen in this
full-disk image of Mars
Mars is a cold desert world. It is half the
diameter of Earth and has the same amount of dry land. Like Earth, Mars has
seasons, polar ice caps, volcanoes, canyons and weather, but its atmosphere is
too thin for liquid water to exist for long on the surface. There are signs of
ancient floods on Mars, but evidence for water now exists mainly in icy soil
and thin clouds.
Sun-scorched Mercury is only slightly larger
than Earth's Moon. Like the Moon, Mercury has very little atmosphere to stop
impacts, and it is covered with craters. Mercury's dayside is super-heated by
the sun, but at night temperatures drop hundreds of degrees below freezing. Ice
may even exist in craters. Mercury's egg-shaped orbit takes it around the sun
every 88 days.
Magellan spacecraft radar data enabled
scientists to penetrate Venus' thick clouds and create simulated views of the
surface
Venus is a dim world of intense heat and
volcanic activity. Similar in structure and size to Earth, Venus' thick, toxic
atmosphere traps heat in a runaway "greenhouse effect." The scorched
world has temperatures hot enough to melt lead. Glimpses below the clouds
reveal volcanoes and deformed mountains. Venus spins slowly in the opposite
direction of most planets.
...
Voyager
2 captured this image of Neptune in 1989.
Dark,
cold and whipped by supersonic winds, Neptune is the last of the hydrogen and
helium gas giants in our solar system. More than 30 times as far from the sun
as Earth, the planet takes almost 165 Earth years to orbit our sun. In 2011
Neptune completed its first orbit since its discovery in 1846.
...
A
Hubble Space Telescope image of Pluto and its moons. Charon is the largest moon
close to Pluto. The other four bright dots are smaller moons discovered in
2005, 2011 and 2012.
Discovered
in 1930, Pluto was long considered our solar system's ninth planet. But after
the discovery of similar intriguing worlds deeper in the distant Kuiper Belt,
icy Pluto was reclassified as a dwarf planet. This new class of worlds may
offer some of the best evidence about the origins of our solar system. Pluto is
also a member of a group of objects that orbit in a disc-like zone beyond the
orbit of Neptune called the Kuiper Belt. This distant realm is populated with
thousands of miniature icy worlds, which formed early in the history of our
solar system. These icy, rocky bodies are called Kuiper Belt objects or
transneptunian objects.
...
The
Galileo spacecraft sent back this image of the Moon as it headed into the outer
solar system. The distinct bright ray crater at the bottom of the image is the
Tycho impact bas
Our
Moon makes Earth a more livable planet by moderating our home planet's wobble
on its axis, leading to a relatively stable climate, and creating a rhythm that
has guided humans for thousands of years. The Moon was likely formed after a
Mars-sized body collided with Earth and the debris formed into the most
prominent feature in our night sky.
...
The
moon's sometimes larger look when it is close to the horizon is an optical
illusion.
