Category: Comets

[/caption]Things appeared to get a little strange in the field of X-ray astronomy when the NASA/ESA ROSAT observatory started seeing emissions from a series of comets. This discovery in 1996 was a conundrum; how could X-rays, more commonly associated with hot plasmas, be produced by some of the coldest bodies in the Solar System? In 2005, NASA’s Swift observatory was launched to look out for some of the most energetic events in the observable Universe: gamma-ray bursts (GRBs) and supernovae. But in the last three years, Swift has also proven itself to be an expert comet hunter.

If X-rays are usually emitted by multi-million Kelvin plasmas, how can X-rays possibly be generated by comets composed of ice and dust? It turns out there is an interesting quirk as comets interact with the solar wind within 3AU from the solar surface, allowing instrumentation designed to observe the most violent explosions in the Universe to also study the most elegant objects closer to home…

“It was a big surprise in 1996 when the NASA-European ROSAT mission showed that comet Hyakutake was emitting X-rays,” said Dennis Bodewits, NASA Postdoctural Fellow at the Goddard Space Flight Centre. “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.” And it must have been a very big surprise for researchers who assumed ROSAT could only be used to glimpse the transient flash of a GRB or supernova, possibly spawning the birth of black holes. Comets simply did not feature in the design of this mission.

However, since the launch of another GRB hunter in 2005, NASA’s Swift Gamma-ray Explorer has spotted 380 GRBs, 80 supernovae and… 6 comets. So how can a comet possibly be studied by equipment intended for something so radically different?

As comets begin their death-defying sunward orbit, they heat up. Their frozen surfaces begin to blast gas and dust into space. Solar wind pressure causes the coma (the comet’s temporary atmosphere) to eject gas and dust behind the comet, away from the Sun. Neutral particles will be carried away by solar wind pressure, whereas charged particles will follow the interplanetary magnetic field (IMF) as an “ion tail”. Comets therefore can often be seen with two tails, a neutral tail and an ion tail.

This interaction between the solar wind and comet has another effect: charge exchange.

Charge exchange can occur in any system where a hot stream of ions interact with a cooler neutral gas. Using missions such as Swift to study the interaction of comets with the solar wind can provide a valuable laboratory for scientists to understand otherwise confusing X-ray emissions from other systems.

Source: Physorg.com

[/caption]
All comets are about the same, right? Not necessarily. Astronomer David Schleicher has been studying 150 comets, measuring the abundances of five different molecules in each comet. One of these comets, Comet 96P/Machholz 1 was different from all the rest, showing an extremely unusual chemistry. Schleicher believes the anomalous composition may reveal the existence of a new class of comets. What makes Machholz 1 different is that the molecule cyanogen, CN, is extremely depleted. In Machholz 1, CN is missing by about a factor of 72 from the average of other comets, i.e., only a little above one percent of normal. “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. The cause of the chemical anomaly is unknown.

However Schleicher, a planetary astronomer at the Lowell Observatory has come up with three intriguing scenarios to explain origins of Machholz 1, and each one will yield important but differing new constraints on the formation or evolution of comets.

One possible explanation is that Machholz 1 did not originate in our Solar System, but instead 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.”

Another possible explanation for Machholz 1’s anomalous composition is that it formed even further from the sun in a colder or more extreme environment than any other comet we have studied thus far. If this was the case, then the scarcity of such objects is likely associated with the significant difficulty of explaining how such comets moved into the inner solar system where they can then be discovered and observed.

A third possibility is that Machholz 1 originated as a carbon-chain depleted comet but that its chemistry was subsequently altered by extreme heat. 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 five 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 and orbits the sun with a period of slightly over five years, compositional measurements only took place during the comet’s recent 2007 apparition. Lowell Observatory’s program of compositional studies, currently headed by Schleicher, includes measurements of over 150 comets obtained during the past 33 years. This research is unique because it compares and contrasts Machholz 1 against this large database of 150 comets.

Currently there are two types of comets, these being identified by a program at the Lowell Obervatory in the early 1990s. 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 very fringes of our Solar System but are believed to have originally formed amidst the giant planets, particularly between 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. Since both depleted molecules, C2 and C3, are wholly composed 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 definitively 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 strongly 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.

The study is published in the November issue of the Astronomical Journal.

Source: Lowell Observatory

I wanted to see it myself before I said anything – but now it’s confirmed. Comet W1 Boattini is now visible in the northern hemisphere! So what if you have to get up before dawn? While its overall brightness is good enough to be seen with the unaided eye, I needed a lot of help, and maybe you’d like some, too?

Make no mistake. Fresh from its trip around the Sun and still holding a respectable 5.5 magnitude puts Comet W1 Boattini right in the ballpark of being visible without optical aid, but its size makes it invisible against dawn’s glow. But don’t be discouraged. If you have a decent southeastern skyline, you can catch Boattini with even small binoculars!

Let the one thing you can’t miss in the sky by your guide – the Pleiades. The view you see here is roughly what your horizon will look like before dawn. Although your own local time will vary a bit, that’s about 4:30 – 5:00 a.m. here. Take your binoculars out with you and begin scanning along the horizon for the Pleiades. Once you find them, locate Alpha Ceti. How can you be sure? It’s easy. Menkar is an optical double. Now begin looking with Menkar to the right of your field of view and scan slowly towards the Pleiades. Comet W1 Boattini will pop out and look like a small, unresolved globular cluster! It’s not big, and it doesn’t have a tail – but it sure is sweet.

If you’re good with sky charts, use this to help aid you. This is the rough track that Boattini will be following for the next few weeks – but don’t wait around to find it. In just a few days the Moon will also begin to interfere with the morning darkness and your chances of easily spotting the comet are going to become less. Once you locate it in binoculars, it’s easy to pick it up again in an optical finder on a telescope and take a closer look.

Good luck!

Many thanks to Joe Brimacombe and Guilherme Venere for the W1 Boattini images!

On June 25th, the ESA/NASA SOHO spacecraft discovered its 1,500th comet, making it more successful than all other comet discoverers throughout history, combined. But wait a minute, SOHO is the Solar and Heliospheric Observatory, designed to study solar physics. What’s it doing looking for comets? SOHO just happens to have a great vantage point to see comets as they approach the sun. Since its orbit is situated between the Sun and Earth, it has a unique view of the regions close to the sun that we can rarely see from Earth. But SOHO’s comet-finding success is just an added benefit to the extraordinary revelations this spacecraft has provided in its 13 years in space, observing the Sun and the near-Sun environment. “Catching the enormous total of comets has been an unplanned bonus,” said Bernhard Fleck, ESA SOHO Project Scientist.

About 85% of SOHO’s comet discoveries are fragments from a once-great comet that split apart in a death plunge around the Sun, probably many centuries ago. The fragments are known as the Kreutz group, which now pass within 1.5 million km of the Sun’s surface when they return from deep space.

That’s pretty close in celestial terms, and from Earth, we can only see those regions close to the Sun during an eclipse.

But that also puts them within sight of SOHO’s electronic eyes. Images of the comets are captured by the Large Angle and Spectrometric Coronograph (LASCO), one of 12 instruments on board.

Of course, LASCO itself does not make the detections; that task falls to an open group of highly-skilled volunteers who scan the data as soon as it is downloaded to Earth. Once SOHO transmits to Earth, the data can be on the Internet and ready for analysis within 15 minutes.

Enthusiasts from all over the world look at each individual image for a tiny moving speck that could be a comet. When someone believes they have found one, they submit their results to Karl Battams at the Naval Research Laboratory, Washington DC, who checks all of SOHO’s findings before submitting them to the Minor Planet Center, where the comet is cataloged and its orbit calculated.

From this mission, and with the public’s help, scientists have learned a great deal about comets.

“This is allowing us to see how comets die,” says Battams. When a comet constantly circles the Sun, it loses a little more ice each time, until it eventually falls to pieces, leaving a long trail of fragments. Thanks to SOHO, astronomers now have a plethora of images showing this process. “It’s a unique data set and could not have been achieved in any other way,” says Battams.

Most of the comet fragments are eventually destroyed when they get close enough to the Sun, evaporated by the Sun’s radiation.

Interested in helping search for SOHO’s comets? Visit the Sungrazing comets page.

Original News Source: ESA

Astoundingly, about 40,000 tons of dust particles fall to Earth each year which originates from space “leftovers,” mostly from disintegrating comets and asteroid collisions. Scientists are very interested in this dust because of its pristine nature –it is made of the original building blocks of the solar system. Some of that dust also resides in Earth’s atmosphere, and for years, NASA has routinely collected cosmic and interplanetary dust from Earth’s stratosphere with high-altitude research aircraft. NASA announced today that a new mineral has been found from this atmospheric research, in material that likely came from a comet.

Usually, any unique dust particles found in the atmosphere are difficult to trace as far as their origin, and whether it came from a comet or other space debris. But this new mineral, a manganese silicide which has been named “Brownleeite,” was discovered within an interplanetary dust particle, or IDP, that appears to have originated from comet 26P/Grigg-Skjellerup. The comet was discovered in 1902 and reappears every 5 years. A new method of collecting IDPs was suggested by space scientist Scott Messenger, from Johnson Space Center. He predicted comet 26P/Grigg-Skjellerup was a source of dust grains that could be captured in Earth’s stratosphere at a specific time of the year.

In response to his prediction, NASA performed stratospheric dust collections, using an ER-2 high-altitude aircraft flown from NASA’s Dryden Flight Research Center at Edwards Air Force Base, Calif. The aircraft collected IDPs from this particular comet stream in April 2003. The new mineral was found in one of the particles. To determine the mineral’s origin and examine other dust materials, a powerful new transmission electron microscope was installed in 2005 at Johnson.

“When I saw this mineral for the first time, I immediately knew this was something no one had seen before,” said Keiko Nakamura-Messenger, also from Johnson Space Center. “But it took several more months to obtain conclusive data because these mineral grains were only 1/10,000 of an inch in size.”

“Because of their exceedingly tiny size, we had to use state-of-the-art nano-analysis techniques in the microscope to measure the chemical composition and crystal structure of Keiko’s new mineral,” said Lindsay Keller, Johnson space scientist and a co-discoverer of the new mineral. “This is a highly unusual material that has not been predicted either to be a cometary component or to have formed by condensation in the solar nebula.”

The mineral was surrounded by multiple layers of other minerals that also have been reported only in extraterrestrial rocks. There have been 4,324 minerals identified by the International Mineralogical Association, or IMA. This find adds one more mineral to that list.

Brownleeite, is named after Donald E. Brownlee, professor of astronomy at the University of Washington, Seattle. Brownlee founded the field of IDP research. The understanding of the early solar system established from IDP studies would not exist without his efforts. Brownlee also is the principal investigator of NASA’s Stardust mission.

Brownlee says he’s always been intrigued by minerals and now “it’s great to be one.”

Original News Source: PhysOrg, AP