See Comet Hartley 2!

Perhaps you were able to observe Comet McNaught this past June and July.  In case you missed that comet, however, another brighter than average comet has approached the Earth this October. Comet Hartley 2 is now visible in binoculars, and could be a naked-eye object around the time of its closest approach to Earth on October 20. As its name indicates, this is the second comet found by Malcolm Hartley. He discovered this comet in 1986 at the Siding Spring Observatory in Australia. Comet Hartley 2 is a small object only about 1.5km across. It orbits the sun once in about 6.5 years, attaining a maximum (aphelion) distance from the sun of about 5.87 AU (beyond Jupiter’s orbit) and a minimum (perihelion) distance of about 1.05 AU (just beyond Earth’s orbit). This orbit places it in the Jupiter family of comets, which orbit in the same direction as the planets with periods of less than 20 years.   In fact, there is evidence that interaction with Jupiter has shortened Hartley 2′s orbital period from over 9 years to the present 6.5.

In October 2010, Hartley 2 comes to perihelion while Earth is on the same side of the sun. This brings Hartley 2 within 11.2 million km of the Earth, close enough to make it visible in our skies—possibly even to the naked eye.  Many comets come inside the Earth’s orbit as they approach the sun, which means we look more or less in the sun’s direction when seeing them at their brightest.  Hartley 2, on the other hand, has a perihelion just outside Earth’s orbit, so Earth is passing roughly between the sun and the comet this month.  Therefore, we can observe it while looking away from the sun in our sky. 

 Halley’s Comet with tail

Comets are made of ice and dust and are often called ‘dirty snowballs.’  We believe they are left over from the formation of the solar system.  As comets approach the sun, ice changes into gas and dust embedded in the ice is released.  A cloud of particles expands out to form a coma around the comet’s solid nucleus. This coma may be a hundred thousand miles across. Radiation pressure of sunlight and the powerful solar wind sweep gases and dust away from the comet’s head, forming tails pointed away from the sun.  Comets have bluish gas tails and yellowish dust tails.   Since Earth is passing more or less between the sun and Hartley 2, however, its tails will be mostly oriented away from us and foreshortened from our point of view. 

Hartley 2′s coma is now quite large in our sky, so you should look for a fuzzy area, perhaps bigger than the full moon, not a single point of light. The total brightness of the comet is about that of the dimmest stars visible, so the farther you can get from city lights, the better.  The large coma means that Hartley 2′s brightness is diffused over a large area, and therefore the comet may look dimmer than its total brightness would suggest.  Averted vision, which involves looking slightly away from the comet’s actual position, may help you see Hartley 2 if it is at the threshold of visibility.   If you choose a viewing site far from big cities and a night with no moon, you may see the comet with the unaided eye.  The extended coma has a soft, diffuse look comparable to the Milky Way band.  

October 20, the day of closest approach to Earth, has a large waning gibbous moon approaching its full phase on October 22.  Before the full moon, it may be easier to spot Hartley 2 at dawn, after the moon has set.  Or, you can look towards the end of the month, with the moon is at Last Quarter.   Here  is a chart showing Hartley 2′s position through November 3, 2010.  (Note that the dates given are in Universal Time, which corresponds to the previous evening for us.)  Of the constellations shown, Perseus rises in the northeast at dusk in mid-October, while Auriga comes up at about 9:30, also in the northeast.  Gemini, the Twins, rises closer to midnight. Keep in mind that predictions for a comet’s brightness are just that–predictions.  many comets appear significantly brighter or dimmer than expected. 

 Tempel 1 as photographed by Deep Impact

Amateur astronomers who get out and observe this comet won’t be alone in observing Hartley 2.  NASA has retargeted its Deep Impact spacecraft to fly past Hartley 2 on November 4, 2010.  Back in 2005, NASA used Deep Impact to study comet Tempel 1.  In that mission, scientists released a probe to impact Tempel 1 and study the material released.  Deep Impact will simply fly by Hartley 2, however, taking advantage of this opportunity to study yet another comet up close.

With its short orbital period, Hartley 2 should return for its next perihelion near April 20, 2017.  Earth, however, will not be on the same side of the sun as Hartley 2 in 2017, so the comet will be much dimmer in our sky.  Hopefully, our skies will cooperate, and Hartley 2 will brighten as expected or even more, and we’ll all get to appreciate a fascinating sight in the fall 2010 sky.