Chapter 9: Observing the Sun, Moon and Comets

CHAPTER 9: Observing the Sun, Moon and Comets

What’s New?

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Observing the SUN, MOON and COMETS — what’s New?

Comet Hartley 2 / 103P - The Best Comet of 2010

What might prove to be the best comet of the year is approaching Earth (as of early October 2010), bound for a close approach to the Sun and Earth in late October. Officially it is known as Comet 103P/Hartley 2. This normally obscure comet was discovered in 1986 by Australian astronomer Malcolm Hartley. So we haven’t known about it for very long, surprising since it orbits the Sun every 6.4 years as one of Jupiter’s family of comets, objects orbiting the Sun but clearly trapped by Jupiter and governed by its gravitational influence. But it seems up until recently Hartley 2 never came close enough to the Sun to become very bright.
In 2010, the comet reaches its closest point to the Sun on October 28, at a distance just outside Earth’s orbit. That’s still not nearly as close to the Sun as many comets get. But, the great thing about Hartley is that about 10 days earlier on its inbound path it passes just 18 million kilometres from Earth. So this time round Hartley will be bright by virtue of the fact it is so close to us.

What makes Comet Hartley’s appearance so favourable is that it passes Earth just outside our orbit, so it appears opposite the Sun, placing it in a dark sky. Its position well above the plane of Earth’s orbit (the ecliptic) puts it high in our northern sky. Being close, it should also appear quite large, perhaps like Comet Holmes did in October 2007.

How bright? That’s always the risky bit. Some predictions put it at between 4th and 5th magnitude at its brightest in the third week of October. That’s good enough to be a naked eye object. However, that’s also when the Full Moon interferes with the view. Earlier in the month, before the Moon enters the sky, is the prime season for Hartley. For example, around Thanksgiving weekend October 8 through 11, the comet appears in a dark sky and might shine at a decent 5th or 6th magnitude. However, other predictions say it might be no better than 7th or 8th magnitude. Only time will tell if we have a dud or a delight in our sky.

Prime comet season comes in early October, with Comet Hartley perhaps as bright as 5th magnitude. The Moon is out of the way until at least mid-October. Look for “photo ops” as the comet passes deep-sky objects in the Milky Way, such as the passage by the showpiece Double Cluster on October 7.

One thing we can be sure of is where the comet will be. And that’s where we have very good news. Because it passes us just outside our orbit Comet Hartley appears opposite the Sun in our sky. That places it in a dark sky, not buried in twilight. What’s more, because Hartley’s inclined orbit keeps it above the plane of Earth’s orbit in September and most of October, we see the comet high in the northern constellations of Cassiopeia and Perseus. From Canada, the comet is actually circumpolar and already high in the east as it gets dark each evening. By 3 a.m. it shines almost straight overhead as it travels down the scenic Milky Way in Perseus. We couldn’t ask for better viewing geometry. Now we just need a bright comet sporting a prominent dust tail.

As a result of Hartley’s ideal orbit, we see it high in the east during early evening hours in the first half of October, the prime nights for viewing the comet as it reaches nearly its brightest magnitude and with the Moon absent from the sky. During October the comet travels down the autumn Milky Way, affording us some wonderful views.

For updated information on Hartley’s predicted brightness, see Japanese comet observer Seiichi Yoshida’s web page at http://www.aerith.net/comet/catalog/0103P/2010.html. For actual reports of its current brightness see the International Comet Quarterly page at http://www.cfa.harvard.edu/icq/CometMags.html.

Diagrams and text from September-October issue of SkyNews magazine.

COMET CHASER

Remarkably, the brightest comet of the year is also the target for NASA’s next comet probe. On November 4 the craft formerly known as Deep Impact will fly close enough to Comet Hartley to return images of the nucleus that should show craters and the sources of the plumes of dust and gas that create the tail of the comet. The craft was the same vehicle that delivered the probe that impacted Comet Tempel 1 on July 3, 2005. With that comet mission accomplished the probe was re-directed to Comet Hartley for a second comet encounter. Images will be returned to Earth during the days following its closest approach to the icy nucleus. To learn more visit the mission’s website at http://epoxi.umd.edu/index.shtml.

This is what Comet Hartley looked like as of September 30. It’s the cyan glow to the right of the Pacman Nebula, NGC 281, in Cassiopeia. Hartley is fainter than predicted and appears as a diffuse glow. It will brighten through October 2010 but may remain a telescopic object and one for binoculars if you know just where to look and what to look for. Even in dark skies it can be easy to miss, as it lacks any distinctive comet tail. Photo by Alan Dyer