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Saturday, 17 January 2009

More Comet Lulin

Comet Lulin was still only about 11th or 12th magnitude when Michael Jaeger took this image on Sept. 2, 2008. He used an 8-inch f/2.8 ASA Astrograph with a SXV H9 CCD camera for this stacked pair of 4-minute exposures.

Michael Jaeger Comet C/2007 N3 (Lulin), discovered in July 2007, should be the highlight comet of this season. It's predicted to reach about 5th magnitude in late February, so it should be easily seen in binoculars. It may even become detectable with the unaided eye in a dark, moonless sky.

Below is a calendar of the comet's doings in the coming weeks (and here's an ephemeris: ). But don't assume that the brightnesses are trustworthy. The comet's brightness behavior may be unpredictable, because it's on a nearly parabolic orbit that suggests this is its first visit to the inner solar system. You never know what a pristine comet might do.

January: Pre-dawn, and Brightening

The start of 2009 found Comet Lulin glowing at 7th magnitude in Libra, near Lambda (&lambda) Librae not far from the border of Scorpius. Day by day Lulin is getting higher in the southeast (as seen from mid-northern latitudes) just before start of dawn. It was hidden in the solar glare during November and most of December. Prior to that, observers had followed it from as early as May 2008, when it was 14th magnitude, through late October, when it had risen to 8th.

Moving westward across Libra, the comet passes 0.1° south of 47 Librae, and 0.9° north of Lambda Librae, on the morning of January 8th. The comet is at perihelion (closest approach to the Sun) on January 10th, at a solar distance of 1.2 a.u. (181 million km).

Rather than fading after perihelion, Lulin is expected to brighten further as its diminishing distance from us more than compensates for it moving away from the Sun.

February: Peak Brightness, Peak Speed

As Comet Lulin nears Earth its speed across the sky will increase. The beginning of February sees the 6th- or 7th-magnitude comet rising around midnight, and it passes within 1° of the wide binocular double star Alpha Librae on the night of February 5–6. Initially moving at about 1° per day, Lulin will be creeping westward at 2° per day by February 11th, when it crosses into Virgo and passes within a quarter degree of Lambda Virginis.

Five days later, on the 16th, comet Lulin — now perhaps 5th or 6th magnitude — will pass 3° north of Spica, and the comet's speed will have increased to 3° per day.

On the night of February 23rd, near its peak brightness, Comet Lulin is passing 2° south-southwest of Saturn.

Lulin's closest approach to Earth, 0.41 a.u. (61 million km), occurs on February 24th, when the comet may reach a peak of magnitude 5. By now it's visible in late evening (after rising around the end of astronomical twilight) and remains in view for the rest of the night.

And it's speeding along at just over 5° per day! That's about 1 arcsecond every 5 seconds of time, enough to show obvious motion during a short telescopic observing session. Similarly, that's 1 arcminute per 5 minutes of time if you're using binoculars.

After that Lulin moves away from both Earth and the Sun, so it fades quickly. The evening of February 27 will see it at 5th or 6th magnitude within 1° of Regulus.

Weird Orbital Geometry

Strangely, as you may have noticed on the charts, this comet is traveling almost exactly along the ecliptic — backward! Could this really be just be a coincidence? The comet's nearly parabolic orbit indicates that it has never really interacted with the planets at all. Yet its orbital inclination is 178.4°, meaning that it's orbiting in the opposite direction from the planets just 1.6° from the ecliptic plane. (Manipulable 3-D orbit diagram).

Tails and Antitails

Because the comet stays nearly on the ecliptic, its tail (which points away from the Sun) aligns with the ecliptic and with the comet's own direction of motion across the sky. This is indicated by the direction the tail is pointed on the comet symbols on the finder charts linked to above.

Moreover, because Earth remains in the comet's own orbital plane, we're likely to see the comet with a very thin tail and an antitail, a spike pointing in almost the opposite direction from the main tail, for months on end. Why? In three dimensions a comet's dust tail is often wide but it's always thin, confined to the comet's orbital plane. When we are in or near this plane, we can sometimes see parts of the wide, thin dust tail on opposite sides of the comet's head. We pass through most comets' orbital planes briefly. But this time, the situation will last and last.

And indeed, as of January 7th Lulin did have an antitail, as shown in this image taken by Karzaman Ahmad in Malaysia with a 20-inch scope (image courtesy Here's another image, from Ernesto Guido, Giovanni Sostero and Paul Camilleri, taken January 8th. Here's a photo gallery of more.

A comet's blue-green gas tail, on the other hand, always points nearly in a straight line away from the Sun in space. Cometary gas is blown directly away from the Sun at high speed by the solar wind.

March: Following Lulin Out

Comet Lulin crosses from Leo into Cancer at the beginning of March. The night of March 5 sees the 6th-magnitude comet within 2° of both Delta (δ) Cancri and the Beehive Cluster (M44). It will make for a fine sight, particularly in high-powered binoculars. Think photo opportunity.

Another nice conjunction occurs on March 16th when the comet, now around 7th or 8th magnitude (and setting in the early hours of the morning), is 1° from Delta Geminorum.

As Comet Lulin recedes, its passage across our sky will slow. Indeed, from the end of March to the end of May (when Lulin may have faded to 11th magnitude) it will stay within a narrow, 3° strip of sky bounded by Epsilon (ε), Mu (μ), and 36 Geminorum.

Comet Lulin won't return again to the inner solar system for more than a thousand years.

Cooperative Discovery

Comet C/2007 N3 (Lulin) was discovered by Quanzhi Ye, a student (age 19) at Sun Yat-sen University in mainland China, as an apparently asteroidal object on images taken by Chi Sheng Lin (National Central University, Taiwan) with a 16-inch telescope at Lulin Observatory in Taiwan on the night of July 11, 2007. A week later, confirming images revealed the telltale presence of a coma. In China and Taiwan, the comet has been hailed as the "Comet of Cooperation."

Lulin was one of 223 comet discoveries on images taken from the ground and in space in 2007, an all-time record.

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