Jupiter, resplendent low in the ESE at nightfall and climbing into the S by midnight, is the only bright planet on view tonight. Through binoculars view a dim star just a Moon's width above and to its left, while the planet Uranus stands three moonwidths away in the same direction.
Binoculars should also reveal a comet swooping within 19 million km of the Earth on the 20th on its way to perihelion eight days later. Comet Hartley 2, or formally 103P/Hartley, was discovered from Australia by the British-born astronomer Malcolm Hartley in 1986 and takes 6.46 years to orbit the Sun, roughly between the paths of the Earth and Jupiter. Perihelion lies 158 million km from the Sun, outside the Earth's orbit so we are in no danger of a collision.
It is only a small comet and lacks any appreciable tail. Instead, a small knot of greenish luminosity surrounding its icy nucleus is set within an extensive circular glow. The latter is hard to see unless the sky is free of light pollution and moonlight. It is certainly growing, though, appearing wider than the Moon last week and helping to make the comet a naked eye object for those under ideal skies. I suspect, though, that most of us will need binoculars.
Our chart shows a 70°- high window of our E sky at midnight and shows Comet Hartley's motion from Perseus, through Auriga and Gemini. Ticks along its path mark its place every two nights, beginning tonight when it lies 5° above-let of Mirfak, the brightest star in Perseus, and stands 20 million km away. Note that it sweeps less than 3° to the right of Capella on the 18th though by then moonlight is becoming an issue.
The comet is the target for Nasa's Epoxi mission, with a fly-by planned for 5 November. Epoxi is the name for the extended mission of the Deep Impact spacecraft that visited Comet Tempel 1 in 2005 and watched the explosive results as it fired a projectile into the nucleus. Next month, though, there will be no such impact and only the briefest of inspections as the probe speeds to within 700 km of Hartley's nucleus at 12.3km per second.
The stars on our chart climb higher during the night so that Capella stands overhead by 05:00 as Orion straddles the meridian. It may be interesting to check the brightness of the variable star Algol in Perseus. This usually shines at magnitude 2.1, a little fainter than Mirfak, but every 2 days and 21 hours it fades threefold in brightness to magnitude 3.4 as the fainter of its two component stars eclipses its companion. Each dip in light lasts ten hours and takes Algol below the mag 2.9 of Epsilon (see chart).