New comets are constantly being discovered, but very few are bright enough to be visible to the naked eye. Those able to capture the attention of the general public are rarer still, on average one per decade. But in January 2007 the elusive C/2006 P1 (McNaught) was, very briefly, the most spectacular comet of the past 40 years.
Comet C/2006 P1 (McNaught) was atypical in many ways: for its extreme brightness, for his long and beautiful tail, but also because it was notoriously difficult to observe. All these was because it passed too close to the Sun, which is what commands its activity, but this also meant that it could only be seen too early in the evening, near the horizon, and for a few minutes each day due to the rotation of the Earth. The prestigious magazine "Sky and Telescope", for astronomy enthusiasts, made a survey on its Internet website in which 68 % of their readers responded that unfortunately they did not get to see it.
The history of this comet begins with the project "Catalina Sky Survey", a network of three telescopes that scan different parts of the sky for objects that could threaten Earth. Two of these telescopes are in the Northern Hemisphere and the third is in Siding Springs, Australia, the only professional observatory that performs this important task of monitoring the skies of the Southern Hemisphere. This network of three telescopes discovered nearly 400 new asteroids in 2006, representing over 60 % of the world's discoveries that year.
The telescope of 50 cm diameter in Siding Springs also discovered 29 comets in nearly three years of operation. But one would be different.
On 7 August 2006 the telescope's CCD electronic camera took 4 routine photos of an area of the sky known as Ophiuchus. A computer program automatically compared this sequence of photographs, and detected something moving among the background stars. Astronomer Rob McNaught confirmed that it was a comet, and described it as a small cloud, so far from Earth that their apparent width was 180 times smaller than 1 degree. Its brightness was estimated in seventeenth magnitude, the brightest stars being of first magnitude.
The confirmation came quickly: hours later, Cristóvão Jacques and Eduardo Pimentel photographed the same comet through an amateur telescope 30 cm in diameter, from Belo Horizonte, Brazil.
The position and movement data were passed on to Brian Marsden of the Harvard-Smithsonian Center for Astrophysics, near Boston, USA, where it was officially cataloged as "C/2006 P1 (McNaught)". On 8 August, by using data from 19 consecutive observations over a little more than 1 day, the Center estimated that this comet was about 525 million kilometers from the Sun and was approaching.
Comets are leftovers from the formation of the Solar System, commonly dust and gases, which remain frozen in the depths of space, in a region known as the Oort Cloud, which envelopes the Sun and its nearby planets. In this sense, comets are fossils of the early ages of the Solar System. Due to some disturbance in their movements, the trajectories of these bodies are altered and they fall inwards, where the Sun is and where we are.
By using astrometric positions derived from 39 observations between 7 and 11 August, Marsden calculated that the new comet would get as close as just 25 million kilometer from the Sun, much closer than the Earth, Venus and even Mercury. Due to the heat of the Sun, comet's ices melt and in the vacuum of space become gases. What was a tiny block of ices a few kilometers wide will be wrapped in a giant cloud of many thousands kilometers across. The closer to the Sun, the larger and larger the cloud from that sublimated melt, apart from more illuminated by the glow of our central star. Its trajectory indicated the possibility of having a very bright comet visible from Earth in January 2007. Interest in C/2006 P1 rose, albeit cautiously, because it is known that the activity of comets is often unpredictable.
Indeed, when approaching the inner Solar System the brightness of the comet were increasing: by 26 August it was already at magnitude 16, by 12 September at magnitude 15 and ten days later at 14, about the same brightness as Pluto. The comet was looking bigger and bigger, with an apparent diameter of 30 times less than 1 degree by then.
By October the comet's brightness reached magnitude 12, but paradoxically it also started to become more difficult to observe, because at approaching the Sun, astronomers had to aim their telescopes downwards nearer to the horizon, to where the sun just set, with the resulting interference of clouds, dirt in the air and the glow of city lights.
By November the comet was very low, so close to the Sun that it had to be watched shortly after our nearby star became hidden by Earth's horizon, with the sky still lit. Despite its brightness magnitude continuing to increase until 10 and then 9, it was disappearing from our skies in the glare of the Sun.
The comet disappeared completely from the telescopes during December.
Finally one day after Christmas amateur Piotr Guzik, watching from Krosno, Poland, with a telescope of 20 cm, detected it again during the evening. He estimated its brightness at magnitude 4,5, which means that it had already passed the threshold at which humans can theoretically see an astronomical object with the naked eye, which is at sixth magnitude.
But this object was atypical because it was surrounded by the strong glare of the Sun. We would have to wait for it to get brighter still and outshine the surrounding orange twilight sky.
On New Year's Eve, Kenlchi Kadota, using a 25 cm telescope and a CCD camera, reported from Ageo, Japan, that he was detecting a tail that stretched for a length of 4 minutes of arc in the sky. The tail is nothing but a cloud of fine particles, leaving the cometary core as this melts, that the Sun's radiation stretches impressively, usually up to be millions of kilometers. Due to be repelled by the Sun, the tail always points in the opposite direction to our central star, which means that when the comet goes away from it the tail is in front.
In the early days of 2007 fans were already able to see it with binoculars, first with the high-power ones, but by day 5 it was within reach of common binoculars of 8x magnification x 30 mm diameter, as reported by Burkhard Leitner, whom from Austria estimated that the comet had finally caught a brightness of the first magnitude.
On the evening of Epiphany, amateur organizations began to receive messages from all kinds of ordinary people who commented that there was some strange thing in the sky, visible to the naked eye, though very low on the horizon. The Great Comet of 2007 had arrived.
Moving at thousands of kilometers per hour, on 12 January Comet C/2006 P1 (McNaught) reached perihelion, or the minimum distance to the Sun. It appeared in the cameras of the probe-satellites "Solar-Terrestrial Relations Observatories" (STEREO), launched the previous year, and of the "Solar and Heliospheric Observatory" (SOHO), which was photographing the Sun 24 hours a day since 1996.
The SOHO has photographed many comets that approached the Sun in the last 10 years, but when C/2006 P1 (McNaught) entered its field of view it was obvious that it was by far the brightest of all. Various instruments showed that comets are composed of water ice and frozen CO2 (dry ice), but also contain a variety of molecules based on carbon, hydrogen, oxygen and nitrogen, like those which are the components of living organisms. It is believed that the impact of comets with the early Earth played a very important role in the origin of life on this planet.
The comet reached its maximum apparent brightness as seen from Earth between 13 and 14 January. It was so bright that it went to the negative side of the scale of magnitudes, reaching between magnitude -5 and magnitude -6. This means that in those days it was the third brightest object in the sky after the Sun and the Moon. On Sunday 14 January 2007 comet C/2006 P1 (McNaught) made history when amateurs with some experience, even from well to the south of the equator, reported that this comet has become visible to the naked eye in broad daylight!
When turning around the Sun and beginning to recede, the comet's path led it to the South and the show began for those who live in this hemisphere of the Earth.
Day after day, traveling away from the glare of the Sun, the C/2006 P1 (McNaught) was gradually entering into increasingly darker parts of the sky seen from the ground, and the tail began to grow in view of the observers. By 17 January its length was estimated at 5 degrees; by 18 January at 15 degrees; by 22 January it reached 25 degrees. (The width of a fist seen at arm's length is about 10 degrees). On January 23, watching from places away from the light pollution of cities, the spectacular tail reached a maximum of 35 degrees at naked eye.
As the tail is composed of both gases and dust, sometimes these separate and two tails are seen: the one that is more bluish and straight are the gases pushed by the Sun's radiation, and the other, more yellowish, is the powder. Tails are often distorted by the Sun's magnetic field and gravity, so this was adopting a horse-tail shape.
The show offered in these latitudes was superior to the passage of C/1995 O1 (Hale-Bopp) in 1997, of C/1996 B2 (Hyakutake) in 1996 and the very 1P/Halley in 1986, although those were much easier to watch. For the people who succeeded in seeing it, struggling against the rain clouds of El Niño, this was probably the most spectacular comet since C/1965 S1 (Ikeya-Seki) in 1965.
In the last week of January, our Moon made its appearance on the western horizon and began to blur the view of the comet. The C/2006 P1 (McNaught), simultaneously, continued receding from the Sun and its activity was beginning to decrease. By the night of Monday 5 February, without moon again, the tail had already shrunk to a length of 12 degrees as seen from here.
The brightness was decreasing rapidly, returning to positive numbers on the scale, and during the second week of February the comet's brightness passed below the threshold of magnitude +6 again, remaining invisible to the human eye without binoculars or telescopes, even if observers traveled to places away from the light pollution of our cities.
The C/2006 P1 (McNaught) is slowly disappearing in the area of the sky known as Tucana, back to the frozen depths of space. Its elliptical orbit around the Sun is so elongated that is not expected to come back to Earth's vicinity for at least thousands of years.
But in the meantime, we can still admire the beautiful photographs.
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Originally published in ABC Color, on 12 February 2007. Photograph: Comet C/2006 P1 (McNaught) seen from North Head in Sydney Harbour, Sydney, Australia, in the early evening of 16 January 2007. Photo credit: Günther Egger (original license, of the photo only, obtained at: http://creativecommons.org/licenses/by-sa/3.0/deed.en). With permission from Günther Egger.