Before 1979, there were less than a dozen known
sungrazing comets – comets that swing by incredibly close to the sun.
But that was before the ESA/NASA Solar and Heliospheric Observatory
launched in 2000. Since then, SOHO has become the greatest sungrazing
comet hunter of all time with comet finds numbering in the thousands. Credits: NASA/Duberstein. Download video
A sun grazing comet as witnessed by the ESA/NASA
Solar Heliospheric Observatory, or SOHO, as it dived toward the sun on
July 5 and July 6, 2011. SOHO is the overwhelming leader in spotting
sungrazers, with almost 3000 spotted to date. SOHO can see the faint
light of a comet, because the much brighter light of the sun is blocked
by what's known as a coronograph. Credits: ESA&NASA/SOHO
One of the more well-known comets observed by SOHO
is Comet ISON, seen in the this time lapse photo from Nov. 28, 2013.
Comet ISON comes in from the bottom right and moves out toward the upper
right, getting fainter and fainter. The image of the sun at the center
is from NASA's Solar Dynamics Observatory. Credits: ESA/NASA/SOHO/SDO/GSFC. hi-res image
In 1995, a new solar observatory was launched. A joint project of ESA
and NASA, the Solar and Heliospheric Observatory – SOHO – has been
sending home images of our dynamic sun ever since. SOHO was planned to
open up a new era of solar observations, dramatically extending our
understanding of the star we live with. . . and it delivered.
But no one could have predicted SOHO's other observational triumph:
In the last two decades, SOHO has become the greatest comet finder of
all time. In August 2015, SOHO is expected to discover its 3000th
comet. Prior to the SOHO launch, only a dozen or so comets had ever
even been discovered from space, and some 900 had been discovered from
the ground since 1761.
"SOHO has a view of about 12 and a half million miles beyond the
sun," said Joe Gurman, the mission scientist for SOHO at NASA's Goddard
Space Flight Center in Greenbelt, Maryland. "So we expected it might
from time to time see a bright comet near the sun. But nobody dreamed
we'd approach 200 a year."
More than just a celebrated bright vision in the night sky, comets
can tell scientists a great deal about the place and time where they
originated. Comets are essentially a clump of frozen gases mixed with
dust. They are often pristine relics that can hold clues about the very
formation of our solar system. On the other hand, if they have made
previous trips around the sun, they can hold information about the
distant reaches of the solar system through which they've traveled. We
have a variety of tools to determine what comets are made of from afar.
One is to watch how material evaporates off its surface when it comes
close to the sun, and here's where SOHO can provide remarkable
information.
SOHO is unique in that it is able to spot comets that skim extremely
close to the sun, known as sungrazers. One of SOHO's instruments, called
a coronagraph, specifically blocks out the bright light of the sun to
examine its atmosphere – which is a billion times fainter than the star
itself. To this day, SOHO is one of our best sources for views of the
giant explosions regularly produced by the sun called coronal mass
ejections, or CMEs, which can hurl a million tons of solar particles off
into space. This field of view is large enough to see a sungrazing
comet as it sling shots around the sun.
The overwhelming bulk – some 85% -- of SOHO's comet discoveries are
what's called Kreutz comets. Scientists think a single extremely large
sungrazing comet broke up thousands of years ago, leading to thousands
of leftover fragments, which continue to follow the same Kreutz path. On
average, a new member of the Kreutz family is discovered every three
days. Unfortunately, the long journey for these fragments invariably
ends as they pass the sun. If they're close enough to the sun to be seen
by SOHO, they're too close to survive.
"They just disintegrate every time we observe one," said Karl
Battams, a solar scientist at the Naval Research Labs in Washington,
D.C., who has been in charge of running the SOHO comet-sighting website
since 2003. "There's only one Kreutz comet that made it around the sun –
Comet Lovejoy. And we are pretty confident it fell apart a couple of
weeks afterwards."
Other, non-Kreutz comets have survived, however. One frequent visitor
is comet 96P Machholz. Orbiting the sun approximately every 6 years,
SOHO has now seen this comet four times. Such comets survive by virtue
of the fact that they don't travel as close to the sun – so they
experience less intense solar radiation and are not subject to
gravitational stretching and pulling from the sun. Information about the
composition of comets is something SOHO can help with. Depending on how
a comet reacts to the sun gives clues about the very substance out of
which these visitors from the outer solar system are made.
Watching these sungrazing comets also help us learn about the sun.
Their tails of ionized gas illuminate magnetic fields around the sun, so
they can act as a tracer that helps scientists observe these invisible
fields. Such fields have even ripped off comet tails allowing
astronomers to watch the lost tails blowing in the steady outpouring of
solar particles streaming off the sun. The tails act as a giant windsock
in this solar wind, showing researchers the details of the wind's
movement.
SOHO's great success as a comet finder is, of course, dependent on
the people who sift through SOHO's data – a task open to the world as
the data is publicly available online in real time. A cadre of volunteer
amateur astronomers dedicate themselves to searching the data via the
NASA-funded Sungrazer Project. While scientists often search the imagery
for very specific events, various members of the astronomy community
choose to comb through all the imagery in fine detail. The result: 75%
of SOHO comets have been found by these citizen scientists.
Whenever someone spots a comet, they report it to Battams. He goes
over the imagery to confirm the sighting and then submits it to the
Central Bureau for Astronomical Telegrams, which gives it an official
name. While comets spotted from the ground are named after the person
who first discovered them, comets first observed by a space-based
telescope are named after the spacecraft.
"As I joined the team when we already had found 500 comets, I've been
in charge of confirming 2,500 so far," said Battams. "I think it's safe
to say I've looked at more images of comets than any other person in
history. Each comet is visible in at least 15 images, so that's more
than 40,000 images of comets."
SOHO has also helped provide images for comets discovered by others.
In 2012, a sungrazer was found the old-fashioned way – from the ground.
Known as Comet ISON, scientists quickly realized it would make a swing
by the sun close enough to be spotted by a variety of solar telescopes
including SOHO. A large campaign of observations was launched, as
telescopes from around the world and across the solar system watched the
comet -- a fossil from the original days of the solar system formation –
sweep in. The final observatory to see Comet ISON was SOHO, which
watched the comet curve in toward the sun. . . and disintegrate.
Observations from SOHO were key to helping describe ISON's last hours – something that no other observatory captured.
"When SOHO launched, its sensors were some 100 times more sensitive
than previous imagers," said Gurman. "That was crucial to seeing the
faint light from the solar particles in a CME. SOHO allowed us to see a
range of brightness and details never before seen. It was great luck
that the same exposures allowed us to see comets – not just extremely
bright ones, but a whole range of fainter ones, too."
At almost 20 years old, the SOHO mission is a respected elder in
NASA's Heliophysics System
Observatory – the fleet of spacecraft that
both watch the sun and measure its effects near Earth and throughout the
solar system. SOHO is a cooperative effort between ESA and NASA.
Mission control is based at NASA Goddard. The Large Angle and
Spectrometric Coronagraph Experiment, or LASCO, which is the instrument
that provides comet imagery, was built at the Naval Research Lab in
Washington, D.C.
For more about SOHO: http://www.nasa.gov/soho
For more on the SOHO Sungrazer Project: http://sungrazer.nrl.navy.mil
Source: NASA/SOHO
