SOHO spacecraft's 2000th comet

SOHO's 2000th comet, spotted by a Polish amateur astronomer on December 26, 2010.

Credit: SOHO/Karl Battams

By Goddard Space Flight Center, Greenbelt, Maryland Published: December 29, 2010

As people on Earth celebrate the holidays and prepare to ring in the New Year, an ESA/NASA spacecraft has quietly reached its own milestone: on December 26, the Solar and Heliospheric Observatory (SOHO) discovered its 2000th comet.

Drawing on help from citizen scientists around the world, SOHO has become the single greatest comet finder of all time. This is all the more impressive since SOHO was not specifically designed to find comets, but to monitor the sun.

"Since it launched on December 2, 1995 to observe the sun, SOHO has more than doubled the number of comets for which orbits have been determined over the last three hundred years," says Joe Gurman, the U.S. project scientist for SOHO at NASA's Goddard Space Flight Center in Greenbelt, Md.

Of course, it is not SOHO itself that discovers the comets -- that is the province of the dozens of amateur astronomer volunteers who daily pore over the fuzzy lights dancing across the pictures produced by SOHO's LASCO (or Large Angle and Spectrometric Coronagraph) cameras. Over 70 people representing 18 different countries have helped spot comets over the last 15 years by searching through the publicly available SOHO images online.The 1999th and 2000th comet were both discovered on December 26 by Michal Kusiak, an astronomy student at Jagiellonian University in Krakow, Poland. Kusiak found his first SOHO comet in November 2007 and has since found more than 100.

"There are a lot of people who do it," says Karl Battams who has been in charge of running the SOHO comet-sighting website since 2003 for the Naval Research Lab in Washington, where he also does computer processing for LASCO. "They do it for free, they're extremely thorough, and if it wasn't for these people, most of this stuff would never see the light of day."

Battams receives reports from people who think that one of the spots in SOHO's LASCO images looks to be the correct size and brightness and headed for the sun – characteristics typical of the comets SOHO finds. He confirms the finding, gives each comet an unofficial number, and then sends the information off to the Minor Planet Center in Cambridge, Mass, which categorizes small astronomical bodies and their orbits.

It took SOHO ten years to spot its first thousand comets, but only five more to find the next thousand. That's due partly to increased participation from comet hunters and work done to optimize the images for comet-sighting, but also due to an unexplained systematic increase in the number of comets around the sun. Indeed, December alone has seen an unprecedented 37 new comets spotted so far, a number high enough to qualify as a "comet storm."

LASCO was not designed primarily to spot comets. The LASCO camera blocks out the brightest part of the sun in order to better watch emissions in the sun's much fainter outer atmosphere, or corona. LASCO’s comet finding skills are a natural side effect -- with the sun blocked, it's also much easier to see dimmer objects such as comets.

"But there is definitely a lot of science that comes with these comets," says Battams. "First, now we know there are far more comets in the inner solar system than we were previously aware of, and that can tell us a lot about where such things come from and how they're formed originally and break up. We can tell that a lot of these comets all have a common origin." Indeed, says Battams, a full 85% of the comets discovered with LASCO are thought to come from a single group known as the Kreutz family, believed to be the remnants of a single large comet that broke up several hundred years ago.

The Kreutz family comets are “sungrazers” – bodies whose orbits approach so near the Sun that most are vaporized within hours of discovery – but many of the other LASCO comets boomerang around the sun and return periodically. One frequent visitor is comet 96P Machholz. Orbiting the sun approximately every six years, this comet has now been seen by SOHO three times.

SOHO is a cooperative project between the European Space Agency (ESA) and NASA. The spacecraft was built in Europe for ESA and equipped with instruments by teams of scientists in Europe and the USA.

NASA's Swift Spies Comet Lulin

Comet Lulin was passing through the constellation Libra when Swift imaged it. This view merges the Swift data with a Digital Sky Survey image of the star field.
Credit: NASA/Swift/Univ. of Leicester/DSS (STScI, AURUA)/Bodewits et al.


This image of Comet Lulin taken Jan. 28 merges data acquired by Swift's Ultraviolet/Optical Telescope (blue and green) and X-Ray Telescope (red). At the time of the observation, the comet was 99.5 million miles from Earth and 115.3 million miles from the sun.
Credit: NASA/Swift/Univ. of Leicester/Bodewits et al.

Friday, February 20, 2009

While waiting for high-energy outbursts and cosmic explosions, NASA's Swift Gamma-ray Explorer satellite is monitoring Comet Lulin as it closes on Earth. For the first time, astronomers are seeing simultaneous ultraviolet and X-ray images of a comet.

"We won't be able to send a space probe to Comet Lulin, but Swift is giving us some of the information we would get from just such a mission," said Jenny Carter, at the University of Leicester, U.K., who is leading the study.

"The comet is releasing a great amount of gas, which makes it an ideal target for X-ray observations," said Andrew Read, also at Leicester.

A comet is a clump of frozen gases mixed with dust. These "dirty snowballs" cast off gas and dust whenever they venture near the sun. Comet Lulin, which is formally known as C/2007 N3, was discovered last year by astronomers at Taiwan's Lulin Observatory. The comet is now faintly visible from a dark site. Lulin will pass closest to Earth -- 38 million miles, or about 160 times farther than the moon -- late on the evening of Feb. 23 for North America.

On Jan. 28, Swift trained its Ultraviolet/Optical Telescope (UVOT) and X-Ray Telescope (XRT) on Comet Lulin. "The comet is quite active," said team member Dennis Bodewits, a NASA Postdoctoral Fellow at the Goddard Space Flight Center in Greenbelt, Md. "The UVOT data show that Lulin was shedding nearly 800 gallons of water each second." That's enough to fill an Olympic-size swimming pool in less than 15 minutes.

Swift can't see water directly. But ultraviolet light from the sun quickly breaks apart water molecules into hydrogen atoms and hydroxyl (OH) molecules. Swift's UVOT detects the hydroxyl molecules, and its images of Lulin reveal a hydroxyl cloud spanning nearly 250,000 miles, or slightly greater than the distance between Earth and the moon.

The UVOT includes a prism-like device called a grism, which separates incoming light by wavelength. The grism's range includes wavelengths in which the hydroxyl molecule is most active. "This gives us a unique view into the types and quantities of gas a comet produces, which gives us clues about the origin of comets and the solar system," Bodewits explains. Swift is currently the only space observatory covering this wavelength range.

In the Swift images, the comet's tail extends off to the right. Solar radiation pushes icy grains away from the comet. As the grains gradually evaporate, they create a thin hydroxyl tail.

Farther from the comet, even the hydroxyl molecule succumbs to solar ultraviolet radiation. It breaks into its constituent oxygen and hydrogen atoms. "The solar wind -- a fast-moving stream of particles from the sun -- interacts with the comet's broader cloud of atoms. This causes the solar wind to light up with X rays, and that's what Swift's XRT sees," said Stefan Immler, also at Goddard.

This interaction, called charge exchange, results in X-rays from most comets when they pass within about three times Earth's distance from the sun. Because Lulin is so active, its atomic cloud is especially dense. As a result, the X-ray-emitting region extends far sunward of the comet.

"We are looking forward to future observations of Comet Lulin, when we hope to get better X-ray data to help us determine its makeup," noted Carter. "They will allow us to build up a more complete 3-D picture of the comet during its flight through the solar system."

Other members of the team include Michael Mumma and Geronimo Villanueva at Goddard.

NASA's Goddard Space Flight Center in Greenbelt, Md., manages the Swift satellite. It is being operated in collaboration with partners in the U.S., the United Kingdom, Italy, Germany and Japan. NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics observatory developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the U.S.

Comet Threat More Constant Than Thought

Diagram showing the position of the Oort Cloud. Credit: Southwest Research Institute

11 December 2008

It certainly captures the imagination: a star passing silently by our solar system knocks a deadly barrage of comets towards Earth. However, recent simulations by one group of researchers has shown that these star-induced comet showers may not be as dramatic as once thought.The idea of nearby stars influencing comets goes back to 1950, when the astronomer Jan Hendrik Oort hypothesized an invisible repository of comets — the so-called Oort cloud — swarming around the solar system out to a distance of 100,000 AU (one AU is the distance between the sun and the Earth).
Oort assumed that stars passing through the cloud would cause a fresh batch of comets to fall in towards the sun, where they become visible to astronomers. Such a disturbance could have long-term effects."The comets we see now could be from a stellar passage hundreds of millions of years ago," said Hans Rickman of the Uppsala Astronomical Observatory in Sweden.However, Rickman and his colleagues have confirmed that star encounters alone cannot explain comet behavior. Using a computer model of the Oort cloud, they show that gravity effects from the galaxy are equally important. The results are reported in a recent article in the journal Celestial Mechanics and Dynamical Astronomy.

Two stars passing in the night

Although Earth has almost certainly been hit by comets throughout its history, it is not all that clear how often that has happened. Much of the crater history on Earth has been erased because of erosion or tectonic activity. The remaining craters could have come from asteroids instead of comets."It's quite difficult to tell a comet-induced crater from an asteroid one, since the impactor gets essentially vaporized," Rickman said.Comet impacts are, however, likely to be more energetic (and therefore more damaging), since comets are moving much faster than asteroids when they pass by Earth.Comet orbits can be altered whenever another star comes within 10,000 AU of our sun. Such a close encounter — occurring every 100 million years or so — will not typically disturb asteroids or planets, but it definitely "shakes up the whole Oort cloud," Rickman said.Most scientists have presumed that these star crossings will lead to a shower of comets raining down on the Earth and the rest of the inner solar system. Some have even claimed to find evidence of periodic mass extinctions that might be explained by a single (as-yet-unidentified) star in an elliptical orbit around the sun.To study the effect of stellar perturbations, Rickman and his colleagues model the Oort cloud with a sample of one million comets (the true number of cloud comets is unknown, but certainly much higher). The simulations are allowed to run for a time period corresponding to the 5-billion-year age of the solar system.The results show that stars can induce comet showers, but the contrast with non-shower periods is less than what people have thought before, Rickman said. This leveling out in comet activity is due to the influence of the gravitational field of the Milky Way.

Galactic tide

Astronomers have known for some time that our galaxy's gravity has an influence on the Oort cloud. Specifically, the cloud experiences a tidal effect due to the fact that the gravitational field is stronger the closer one is to the plane of the galaxy.The simulations by Rickman and colleagues show how the galactic tide constantly gives a small nudge to the cloud's comets. Some of these comets are in rather unstable orbits to begin with, so the slight push can send them on a sun-bound trajectory. Eventually, however, all these unstable comets are ejected from the solar system.And this is where stellar encounters become important. They scramble the Oort cloud, so that the galactic tide has a new crop of unstable comets to funnel into the inner solar system."The general picture spawned by our results is that injection of comets from the Oort Cloud is essentially to be seen as a teamwork involving both tides and stars," the scientists write in their paper.

This star-tide collaboration keeps a relatively steady supply of comets zooming nearby, so the threat from comet impacts probably does not change much over time.

Lowell Observatory astronomer confirms new class of comets

Comet Machholz swings past the sun every 5.24 years. Its latest perihelion, on January 8, 2002, brings it just 0.12 AU from the sun. SOHO / LASCO / NASA / ESA

December 2, 2008

Provided by Lowell Observatory, Flagstaff, Arizona

Comet 96P/Machholz 1's anomalous compositional characteristics help pinpoint its origin to one of three intriguing scenarios. David Schleicher, Lowell Observatory planetary astronomer, measured abundances of five molecular species in the comae of 150 comets and discovered that one comet, 96P/Machholz 1, has an unusual chemistry. The cause of this chemical anomaly remains unknown, but each of three possible explanations yields important but differing constraints on the evolution of comets.

One possible explanation is that Machholz 1 did not originate in our solar system, but escaped from another star. In this scenario, the other star's proto-planetary disk might have had a lower abundance of carbon, resulting in all carbon-bearing compounds having lower abundances. "A large fraction of comets in our own solar system have escaped into interstellar space, so we expect that many comets formed around other stars would also have escaped," said Schleicher. "Some of these will have crossed paths with the Sun, and Machholz 1 could be an interstellar interloper."

The discovery of comet Machholz 1's anomalous composition reveals the existence of a new class of comets. Astronomers identified two other classes in the 1990s. While Machholz 1 also has strongly depleted C2 and C3 carbon species, what makes it anomalous is that the molecule cyanogen (CN) is depleted. In Machholz 1, cyanogen is missing by about a factor of 72 from the average of other comets. "This depletion of CN is much more than ever seen for any previously studied comet, and only one other comet has even exhibited a CN depletion," said Schleicher.

Another possible explanation for Machholz 1's anomalous composition is that it formed even farther from the Sun in a colder or more extreme environment than other comets studied. The scarcity of such objects likely is associated with the difficulty of explaining how such comets moved into the inner solar system.

The third possibility is that Machholz 1 originated as a carbon-chain depleted comet but extreme heat altered its chemistry. While no other comet has exhibited changes in chemistry due to subsequent heating by the Sun, Machholz 1 has the distinction of having an orbit that now takes it to well inside Mercury's orbit every 5 years. (Other comets get even closer to the Sun, but not as often). "Since its orbit is unusual, we must be suspicious that repeated high temperature cooking might be the cause for its unusual composition," said Schleicher. "However, the only other comet to show depletion in the abundance of CN did not reach such high temperatures. This implies that CN depletion does not require the chemical reactions associated with extreme heat."

Although comet 96P/Machholz 1 was first sighted in 1986, compositional measurements only took place during the comet's 2007 apparition. Lowell Observatory's program of compositional studies, currently headed by Schleicher, includes measurements of more than 150 comets obtained during the past 33 years. This research compares and contrasts Machholz 1 against this large database of 150 comets.

In the early 1990s, Lowell Observatory's long-term program first identified the existence of two compositional classes of comets. One class, containing the majority of observed comets, has a composition called "typical." Most members of this typical class have long resided in the Oort Cloud at the fringes of our solar system, but they are believed to have formed amidst the giant planets, particularly among Saturn, Uranus, and Neptune. Other members of this compositional class arrived from the Kuiper Belt, located just beyond Neptune.

The second compositional class of comets has varying depletions in two of the five chemical species measured. Because both depleted molecules, C2 and C3, are composed wholly of carbon atoms, this class was named "carbon-chain depleted." Moreover, nearly all comets in this second class have orbits consistent with their having arrived from the Kuiper Belt. For this and other reasons, the cause of the depletion is believed to be associated with the conditions that existed when the comets formed, perhaps within an outer, colder region of the Kuiper Belt.

Comets are widely thought to be the most pristine objects available for detailed study remaining from the epoch of solar-system formation. As such, comets can be used as probes of the proto-planetary material that was incorporated into our solar system. Differences in the current chemical composition among comets can indicate either differences in primordial conditions or evolutionary effects.

Although the location of origin cannot be determined for any single comet, Machholz 1's short orbital period means that astronomers can search for additional carbon-bearing molecular species during future apparitions. "If additional carbon-bearing species are also depleted, then the case for its origin outside of our solar system would be strengthened," said Schleicher. The next opportunity for observations will be in 2012.

Swift Looks to Comets for a Cool View

Swift's Ultraviolet/Optical Telescope (UVOT) captured Comet 73P/Schwassmann-Wachmann 3's fragment C as it passed the famous Ring Nebula (oval, bottom) on May 7, 2006. Swift watched as the crumbling comet left dusty blobs behind. Credit: NASA/Swift/Stefan Immler and Dennis Bodewits

NASA's Swift Gamma-ray Explorer satellite rocketed into space in 2004 on a mission to study some of the highest-energy events in the universe. The spacecraft has detected more than 380 gamma-ray bursts, fleeting flares that likely signal the birth of a black hole in the distant universe. In that time, Swift also has observed 80 exploding stars and studied six comets.

Comets? ... Comets are "dirty snowballs" made of frozen gases mixed with dust. X-rays come from superhot plasmas. What do cold comets have in common with exploding stars or the birth of black holes?

"It was a big surprise in 1996 when the NASA-European ROSAT mission showed that comet Hyakutake was emitting X-rays," says Dennis Bodewits, a NASA Postdoctural Fellow at the Goddard Space Flight Center in Greenbelt, Md. "After that discovery, astronomers searched through ROSAT archives. It turns out that most comets emit X-rays when they come within about three times Earth's distance from the sun."

Bodewits is working with the Swift team at NASA's Goddard Space Flight Center in Greenbelt, Md., to study comets using data from the spacecraft's Ultraviolet/Optical Telescope (UVOT) and X-Ray Telescope (XRT). "Swift is an excellent platform for studying dynamic processes in comets," he says.

Ultraviolet wavelengths let astronomers identify the chemical composition of the comet's atmosphere, observe the structure of dust emission, and identify the rotation of the comet's icy nucleus. X-rays reveal the structure of the comet's gas and the state of the solar wind, a stream of charged particles that flows from the sun at speeds upwards of 900,000 mph.

This movie combines Swift UVOT observations of Comet 73P/Schwassmann-Wachmann 3's fragment C from May 1 to May 11, 2006. Look for blobs of dust moving down the comet's inner tail (orange and yellow). Credit: NASA/Swift/Stefan Immler and Dennis Bodewits
> Watch video Swift's UVOT captured a striking sequence that shows unresolved blobs of dust trailing from a crumbling comet. In early May 2006, when the largest fragment of Comet 73P/Schwassmann-Wachmann 3 (SW3) passed Earth, Swift monitored its approach.

The piece, known as fragment C, is believed to be the comet's main body, which began splintering in 1995. In 2006, astronomers counted 66 fragments. Telescopes -- including NASA's Hubble and Spitzer -- revealed dust and condensations trailing several pieces. But fragment C showed no unusual changes -- except to Swift's ultraviolet eye. "It's subtle, but Swift caught clouds of dust and perhaps small pieces that no one else was able to," Immler says.

The UVOT also includes an ultraviolet grism, which combines a grating with a prism to separate incoming light by wavelength. "Swift's grism spans the wavelengths where carbon-bearing molecules and the hydroxyl molecule are most active. This gives us a unique view into the types and quantities of gas a comet produces, and that gives us clues about the origin of comets and the solar system," Bodewits explains. In fact, with the failure of the Hubble Space Telescope's ultraviolet spectrograph in 2004, Swift is currently the only space observatory covering this wavelength range.

As a comet's surface warms near the sun, the ices turn to gas and form a tenuous atmosphere, or coma, measuring hundreds of thousands of miles across. The solar wind pushes this gas back to form a comet's glowing gas tail. X-ray emission is a side effect of this interaction.

The X-rays arise through a process called charge exchange. Fast-moving ions in the solar wind snatch electrons from uncharged atoms in the comet's atmosphere. The solar-wind ions give off X-rays as the relocated electrons settle into their new home. Because the interaction occurs over such a broad region, the total power output of these emissions can reach one billion watts.

Charge exchange may play important roles in any objects where hot, expanding gas collides with cooler gas. One example: Young stars interacting with the gas and planets that might surround them. Comets provide excellent laboratories to explore these interactions.

When Comet 17P/Holmes underwent a surprising outburst in October 2007, Bodewits tasked both Swift and NASA's Chandra X-ray Observatory to observe it. "The comet was too bright to observe with the UVOT. We were afraid we'd damage the instrument," Bodewits says. "Despite this, we're still not sure whether we detected Holmes with the XRT or Chandra."

At the time of the outburst, Holmes was about 19 degrees above the ecliptic, the plane in which the planets orbit the sun. At that elevation, the comet was probably experiencing a cooler, steadier flow from the solar wind. "The source of this cooler flow wasn't hot enough to produce the ions Holmes needed to make X-rays," Bodewits notes.

Four years ago today, Swift captured its first x-rays. The radiation came from Cygnus X-1, one of sky's strongest sources at these energies. The system, located within our galaxy, contains a hot, blue-giant star orbited by a black hole.

"Swift has operated two years longer than we had hoped," says Neil Gehrels, the mission's lead scientist at NASA's Goddard Space Flight Center. "And while gamma-ray bursts and stellar explosions are the satellite's bread and butter, it's clear that Swift has a lot to contribute to other areas of astronomy."

Comet 73P Schwassman-Wachmann

photo: Comet 73P Schwassman-Wachmann

This infrared image from NASA's Spitzer Space Telescope shows the broken Comet 73P/Schwassman-Wachmann 3 skimming along a trail of debris left during its multiple trips around the sun. The flame-like objects are the comet's fragments and their tails, while the dusty comet trail is the line bridging the fragments.

Comet 73P /Schwassman-Wachmann 3 began to splinter apart in 1995 during one of its voyages around the sweltering sun. Since then, the comet has continued to disintegrate into dozens of fragments, at least 36 of which can be seen here. Astronomers believe the icy comet cracked due the thermal stress from the sun.

The Spitzer image provides the best look yet at the trail of debris left in the comet's wake after its 1995 breakup. The observatory's infrared eyes were able to see the dusty comet bits and pieces, which are warmed by sunlight and glow at infrared wavelengths. This comet debris ranges in size from pebbles to large boulders. When Earth passes near this rocky trail every year, the comet rubble burns up in our atmosphere, lighting up the sky in meteor showers. In 2022, Earth is expected to cross close to the comet's trail, producing a noticeable meteor shower.

Astronomers are studying the Spitzer image for clues to the comet's composition and how it fell apart. Like NASA's Deep Impact experiment, in which a probe smashed into comet Tempel 1, the cracked Comet 73P/Schwassman-Wachmann 3 provides a perfect laboratory for studying the pristine interior of a comet.

This image was taken from May 4 to May 6 by Spitzer's Multiband Imaging Photometer, using its 24-micron wavelength channel.