| Discovery | |
|---|---|
| Discovery date: | August 7, 2006 |
| Alternate designations: | C/2006 P1, Comet McNaught, Great Comet of 2007 |
| Orbital characteristics A | |
| Epoch: | 2454113.2961 (January 20, 2007) |
| Aphelion: | ~4100 AU[1][a] |
| Perihelion: | 0.17075400 AU 25,544,000 km |
| Semi-major axis: | ~2050 AU[1][a] |
| Eccentricity: | 1.000019[2] (Hyperbolic trajectory) |
| Orbital period: | ~92,600 yr[1][3][a] |
| Inclination: | 77.82768004° |
| Last perihelion: | January 12, 2007[2] |
| Next perihelion: | Unknown[2] |
Comet McNaught, also known as the Great Comet of 2007 and given the designation C/2006 P1, is a non-periodic comet discovered on August 7, 2006 by British-Australian astronomer Robert H. McNaught.[4] It was the brightest comet for over 40 years, and was easily visible to the naked eye for observers in the Southern Hemisphere in January and February 2007.
With an estimated peak magnitude of -5.5, the comet was the second brightest since 1935.[5] Around perihelion on January 12, it was visible worldwide in broad daylight. Its tail measured an estimated 35 degrees in length at its peak.[6]
The brightness of C/2006 P1 near perihelion was enhanced by forward scattering.[7]
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McNaught discovered the comet in a CCD image on August 7, 2006 during the course of routine observations for the Siding Spring Survey, which searches for Near-Earth Objects that might represent a collision threat to Earth. The comet was discovered in Ophiuchus, shining very dimly at a magnitude of about +17. From August through November 2006, the comet was imaged and tracked as it moved through Ophiuchus and Scorpius, brightening as high as magnitude +9, still too dim to be seen with the unaided eye.[6] Then, for most of December, the comet was lost in the glare of the sun.
Upon recovery, it became apparent that the comet was brightening very rapidly, reaching naked-eye visibility in early January 2007. It was visible to northern hemisphere observers, in Sagittarius and surrounding constellations, until about January 13. Perihelion was January 12 at a distance of 0.17 AU. This was close enough to the Sun to be observed by the space-based Solar and Heliospheric Observatory (SOHO).[8] The comet entered SOHO's LASCO C3 camera's field of view on January 12,[8] and was viewable on the web in near real-time. The comet exited SOHO's field of view on January 16.[8] Due to its proximity to the sun, the Northern Hemisphere ground-based viewers had a short window for viewing, and the comet could be spotted only during bright twilight.
As it reached perihelion on January 12, it became the brightest comet since Comet Ikeya-Seki in 1965.[5] The comet was dubbed the Great Comet of 2007 by Space.com.[9] On January 13 and 14, 2007, the comet attained an estimated maximum apparent magnitude of -5.5.[10]
The comet was visible in daylight about 5°- 10° southeast of the Sun from January 12 to 14, with a peak brightness of magnitude -5.5.[11] Perigee (closest approach to the Earth) was January 15, 2007, at a distance of 0.82 AU.[12]
After passing the sun, McNaught became visible in the Southern hemisphere. In Australia, according to Siding Spring Observatory at Coonabarabran, where the comet was discovered, it was to have reached its theoretical peak in brightness on Sunday January 14 just after sunset,[13] when it would have been visible for 23 minutes. On January 15 the comet was observed at Perth Observatory with an estimated apparent magnitude of -4.0.
The Ulysses spacecraft made an unexpected pass through the tail of the comet on February 3, 2007.[14] Evidence of the encounter was published on the October 1, 2007 issue of the Astrophysical Journal.[15] Ulysses flew through McNaught's ion tail 160 million miles from the comet's core and instrument readings showed that there was "complex chemistry" in the region.
The Solar Wind Ion Composition Spectrometer (SWICS) aboard Ulysses measured the composition speed of the comet tail and solar wind, and detected unexpected ions within the comet's tail and found that it had a major impact on the surrounding solar wind. It's the first time that researchers have detected O3+ oxygen ions (atoms of oxygen with a positive charge because they have five electrons instead of eight) near a comet. This suggested that the solar wind ions, originally missing most of their electrons, picked up some of their missing electrons as they passed through McNaught's atmosphere.
Besides that, SWICS found that even at 160 million miles from the comet's nucleus, the tail had slowed the solar wind to half its normal speed. The solar wind should usually be about 435 miles (700 km) per second at that distance from the Sun, but inside the comet's ion tail, it was less than 250 miles (400 km) per second.
"This was very surprising to me. Way past the orbit of Mars, the solar wind felt the disturbance of this little comet. It will be a serious challenge for us theoreticians and computer modellers to figure out the physics,"
—space science professor, Michael Combi.[14]
Prof. George Gloeckler, the principal investigator on the Solar Wind Ion Composition Spectrometer (SWICS), said the discovery was important as the composition of comets told them about conditions approximately 4.5 billion years ago when the solar system was formed.
"Here we got a direct sample of this ancient material which gives us the best information on cometary composition. We're still in the process of figuring out what it tells us. We're contributing part of the whole puzzle".
"The benefits of such an observation are important. They constrain the interactions of such comets with the Sun, including how the comets lose mass. They also examine the question of how a sudden injection of neutral and cold material interacts with hot solar-like plasmas. That occurs in other places of the universe and we were able to study it right here,"
—space science professor, Thomas Zurbuchen.[14]
Comet C/2006 P1 is in a hyperbolic trajectory (with an osculating eccentricity larger than 1)[2] during its passage through the inner solar system. After leaving the influence of the planets, the eccentricity will drop below 1 and it will remain bound to the solar system as an Oort cloud comet.
Given the orbital eccentricity of this object, different epochs can generate quite different heliocentric unperturbed two-body best-fit solutions to the aphelion distance (maximum distance) of this object.[b] For objects at such high eccentricity, the Suns barycentric coordinates are more stable than heliocentric coordinates. Using JPL Horizons the barycentric orbital elements for epoch 2029 generate a semi-major axis of 2050 AU and a period of approximately 92,600 years.[1]
