Dying comets probe the Sun

22/01/2012

Updating Magic Universe

Debris traces the solar magnetic field

What started as a bonanza for comet spotters becomes a new tool for exploring levels in the Sun’s atmosphere that have been hard to see up to now. The SOHO spacecraft (Solar and Heliospheric Observatory) has identified more than 1400 small “sungrazing” comets that fly close to the Sun and evaporate. In July last year, the comet observers using SOHO’s Large Angle and Spectrometric Coronagraph (LASCO) team alerted colleagues operating the newer SDO (Solar Dynamics Observatory) to a larger-than-usual sungrazer heading for its doom.

As he reports in the current issue of Science magazine, Karel Schrijver from the Lockheed Martin Advanced Technology Center in California tracked Comet 2011 N3 SOHO by extreme ultraviolet light with his Atmospheric Imaging Assembly on SDO, which observes highly ionized atoms. What he learned about the comet and about the Sun I’ll tell below as a concise update for Magic Universe. Meanwhile the word is that SDO also observed Comet Lovejoy last month, when it survived a close encounter with the Sun, passing behind it and reappearing on the other side.

Here are a few relevant paragraphs from my story about Comets and Asteroids in Magic Universe.

The big comet count came from another instrument on SOHO, called LASCO, developed under US leadership. Masking the direct rays of the Sun, it kept a constant watch on a huge volume of space around it, looking out primarily for solar eruptions. But it also saw comets when they crossed the Earth-Sun line, or flew very close to the Sun.

A charming feature of the SOHO comet watch was that amateur astronomers all around the world could discover new comets, not by shivering all night in their gardens but by checking the latest images from LASCO. These were freely available on the Internet. And there were hundreds to be found, most of them small ‘sungrazing’ comets, all coming from the same direction. They perished in encounters with the solar atmosphere, but they were related to larger objects on similar orbits that did survive, including the Great September Comet (1882) and Comet Ikeya-Seki (1965).

SOHO is seeing fragments from the gradual break-up of a great comet, perhaps the one that the Greek astronomer Ephorus saw in 372 BC,’ explained Brian Marsden of the Center for Astrophysics in Cambridge, Massachusetts. ‘Ephorus reported that the comet split in two. This fits with my calculation that two comets on similar orbits revisited the Sun around AD 1100. They split again and again, producing the sungrazer family, all still coming from the same direction.’

The progenitor of the sungrazers must have been enormous, perhaps 100 kilometres in diameter or a thousand times more massive than Halley’s Comet. Not an object you’d want the Earth to tangle with. Yet its most numerous offspring, the SOHO-LASCO comets, are estimated to be typically only about 10 metres in diameter.

Update January 2012

In July 2011 a larger than usual sungrazer spotted by SOHO was tracked across the face of the Sun by a newer spacecraft, the Solar Dynamics Observatory, SDO. Named as Comet 2011 N3 SOHO, it evaporated to the point of invisibility after 20 minutes, but not before the event had transformed the game from comet-spotting fun to highly productive cometary and solar physics.

Led by Karel Schrijver from the Lockheed Martin Advanced Technology Center in California, the SDO team was able to gauge the size of the comet. Initially it was up to 50 metres wide. This opened the way to investigating the sungrazers in much more detail. It should become possible to learn more about the composition of these comets, according to how they boil and rupture in the intense heat.

As for solar physics, the miniature tail of the dying comet lit up magnetic field lines at altitudes high in the solar atmosphere that otherwise are almost impossible to detect. Seeing the lines traced by sungrazers at different heights above the Sun will make it possible to trace more accurately the links between the magnetism near the visible surface and the vast field that reaches out into space and influences the Earth.

References

Karel Schrijver et al., Science 20 January 2012, vol. 335, pp. 324-328 DOI: 10.1126/science.1211688

NB: Movies are available at http://www.sciencemag.org/content/335/6066/324/suppl/


Relativity on the human scale

24/09/2010

Updating Einstein’s Universe and Magic Universe

Relativity on the human scale

The most gratifying physics I’ve seen for a while comes in today’s Science magazine, from James Chin-Wen Chou and his colleagues in the Time and Frequency Division at the National Institute of Standards and Technology in Boulder, Colorado. They detect well-known effects of relativity on the rate of time passing, but now on the scale of ordinary human activities.

Standard atomic clocks employ microwaves to ensure their regularity, but Chou’s team used laser light in a pair of aluminium-27 optical clocks (invented in 2005), which gives about 100 times better accuracy. In one experiment, they used an electric field to jiggle the aluminium ion at the heart of a clock and showed that time passed more slowly in accordance Einstein’s Special Relativity theory, about the effect of motion on time. The effect of atomic motion as slow as 8 metres per second (about 30 km/h) was detectable.

Raising a clock makes it run a little faster. Credit: Chou et al., Science, 24 September 2010 – see reference.

Especially pleasing for me was another experiment, in which one clock was jacked up just 33 cm relative to the other. The clock gaining height ran faster because it was further from the Earth’s centre of gravity, and the gravitational field was slightly weaker, in accordance with General Relativity. As the change in clock rate was only about 40 parts in a billion billion (1018), its detection was a tour de force for the NIST team.

This effect of altitude on time was the key to the efforts by Martin Freeth of BBC-TV and me to make Einstein’s theory of gravity, General Relativity, comprehensible to the public, in our film “Einstein’s Universe” (1979).

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Dark matter’s lens

20/08/2010

Updating Magic Universe

Dark matter’s lens on the cosmic scenery

Since 1996 the efforts of the French astrophysicist Jean-Paul Kneib to exploit natural lenses in the sky, created by the dark matter that surrounds clusters of galaxies, have fascinated me. While other stargazers used the “gravitational lenses”, bending light in the Einsteinian manner, to see galaxies far beyond the range of unaided telescopes, Kneib’s aim was to chart the mysterious dark matter itself. He wanted to see how visible matter and the far weightier dark matter have interacted through cosmic time – to see “the whole history of the Universe from start to finish”, as Kneib remarked to me in 2002.

It’s been taxing work, but now Kneib is one of the team reporting in today’s Science magazine about the dark matter around one the richest known clusters of galaxies. Abell 1689 lies 2.2 billion light-years away in the Virgo constellation, and a couple of years ago its extraordinary lensing power revealed a very distant and early object in the sky, Galaxy A1689-zD1, 12.8 billion light-years away. But that’s by the way

The new report not only gauges the cluster’s dark matter but uses the galaxies beyond it to infer the overall nature of space-time itself, dominated by the even more massive dark energy that drives the accelerating expansion of the Universe.

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Comets from sister stars

09/07/2010

Upating Comets

Did our comets come from sister stars?

How did the Solar System acquire its never-ending supply of comets to keep startling us? An explanation comes in today’s Science magazine, from Harold Levison and David Kaufmann of the Southwest Research Institute in Boulder, Colorado, working with Canadian and French colleagues.

The presumed source of supply is a very distant cloud of 100 billion or more comets, loosely bound to the Sun, called the Oort Cloud. The new report suggests that, in the tight cluster of stars in which the Sun was born, comets were scattered hither and yon in close encounters between stars, and many of our comets were captured from the Sun’s sisters.

In this extract from Comets I am at pains to stress that Jan Oort wasn’t the inventor of the distant comet cloud.

Ernst Öpik is an Estonian astronomer and musician who has recently been running the Armagh Observatory in Northern Ireland. For most of his long life he has adopted the role of cosmic garbage-sorter, concerning himself with the stray material of the Solar System. In 1932 he calculated that an invisible cloud of comets and meteors, surrounding the Sun at enormous distances, could survive throughout the long lifetime of the Solar System. In 1950 the doyen of Dutch astronomers, Jan Oort of Leiden, who is better known for classic work on the nature of galaxies, reworked Öpik’s idea. He emphasised a different aspect of it, namely that passing stars would cause a few of the objects to fall out of the cloud and into the heart of the Solar System, to become observable as ‘new’ comets.

Thus was the fabulous Öpik-Oort Cloud conceived, as the source of the comets. I abridge the name to the Öoo Cloud and defend this coinage on grounds of sight and sound. It looks like an untidy collection of roughly round objects of various sizes, and it is pronounced ‘Er, oh!’ – just what a neophyte comet lover is liable to utter when he is first told that there are many billions of the things out there.

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