By Don Pearce, 30-Jan-05
Two comets, almost mirror images of each other, have been approaching the inner solar system for all of recorded history, and in the case of one of them, C/2003 K4 (LINEAR), probably from a distance of many thousands of astronomical units on a trip lasting several million years.
The other comet, C/2004 Q2 (Machholz), has an orbital period over 100,000 years, and, of course, has been the bright, naked eye comet visible this winter from northern skies. Each of these comets has its own characteristics, and, as will become apparent, are the ying and the yang of this season.
The first arrival, K4, many will recall, was the relatively bright comet this past summer, shining brightly in Hercules and Bootes as it approached the inner solar system. On its inbound approach it passed within 1.42 A.U. of the Earth on July 6th, not especially close for appearing fairly bright. For this is, intrinsically, a relatively bright comet, rivaling C/2001 Q4 (NEAT) in absolute brightness, and it did became barely naked eye during late July and August before it was lost in the solar glare. K4 was discovered in May of 2003 by one of the Lincoln Near Earth Asteroid Research robotic telescopes when it was 17th magnitude in Cygnus, and more than 6 A.U. from the Sun. At the time it was well above the ecliptic, but was gradually heading south, and by September 29th, 2004, reached its descending node and continued heading south (below) the ecliptic. Shortly thereafter, it reached perihelion on October 13th at 1.02 A.U. with an orbital speed of 41.6 kps (kilometers per second). It continued its downward plunge until December 24th when it reached its greatest heliocentric latitude south (-45º), and began its turn to the north. Because it has a retrograde orbit, it also made another close approach to the Earth (1.15 A.U.), this time outbound, on December 23rd, and in fact, if had not been so far below the ecliptic it would have, indeed, been a very close passage. By late January it was swinging back in view for northern hemisphere observers. K4, while now forever northbound, will, nevertheless, remain below the ecliptic for another 19 plus years, not reaching its ascending node until 2024. At that time its heliocentric distance will have reached 40 A.U., making it more distant than Pluto while its orbital speed will have slowed to 6.7 kps. Although that seems like a long time, in the big picture this comet only has a tiny fraction of its orbit below the ecliptic. K4 is on a hyperbolic orbit and should escape the solar system, altogether.
Q2 (Machholz) was discovered by Don Machholz, with a six-inch telescope, on August 27th of last year, when it was 11th magnitude and in Eridanus, well south of the ecliptic. Although it was, initially, moving towards the celestial south, that motion was deceptive and caused by perspective due to the Earth’s motion in space; in reality it was trudging northwards. A favorable set of circumstances saw the comet enter the inner solar system on a path close to the position of the Earth, with closest approach to Earth occurring within 20 days of perihelion. In fact, the Earth passed between Q2 and the Sun in late November (opposition), and this also caused a perspective problem, a foreshortened appearance of the dust tail widely observed during much of December. By coincidence, the comet reached its ascending node on the same day of closest approach to the Earth (.347 A.U.) or about 32 million miles, on January 5th, and only two days before a remarkable passage by the Pleiades on January 7th. The comet, now north of the ecliptic, reached perihelion on January 24th (1.21 A.U.) with an orbital speed of 38.4 kps. Because of its very close passage to the Earth, however, the comet appeared slightly dimmer at perihelion than at perigee, a fairly rare occurrence. Of course Q2 appeared much brighter than K4, but here, again, we have an illusion, for K4 is a full two magnitudes brighter, intrinsically. Q2 will continue its northward trek until April 24th, 2005, when it reaches its greatest heliocentric latitude north (+ 38º), and then begins its slow turn towards the south. Although it will continue in this direction for the next 50,000 plus years it will only still stay above the ecliptic for about the next 20 years, finally reaching its descending node in 2026, when it is 41 A.U. from the Sun and traveling at an orbital speed of a paltry 6.5 kps. Again, this 20-year period, its only time above the ecliptic is but a tiny fraction of the entire time spent below the ecliptic.
By now it should be apparent why I have labeled these two comets the “ying and the yang”. For they both have similar, nearly parabolic shaped orbits (Q2’s is slightly elliptical while that of K4 is slightly hyperbolic), both have similar inclinations, although one is prograde and the other retrograde, which means, of course, that they are moving in virtually opposite directions. And each spends an equivalent, relatively brief period of time either above (Q2) or below (K4) the ecliptic, and perhaps, most important, both have given us a wonderful display of cometary beauty this past year and into this year.
Note: Any directional reference is from a heliocentric perspective, and does not necessarily reflect its celestial path across the sky.