Sydney amateurs plot Pluto’s shadow – a great achievement

Pluto lightcurve on the morning of Monday 23 June 2008 obtained by Ted Dobosz and Dave Gault

At 5:06am Monday morning (23 June 2008) the shadow of Pluto swept over Sydney as the tiny dwarf planet moved in front of a faint star. Two members of the Western Sydney Amateur Astronomy Group WSAAG, Ted Dobosz and Dave Gault managed to observe the event. Their observations, shown above, indicate the dip in the brightness of the star as Pluto moved in front of it. The quality of these observations are amazing. Below Dave gives details of the event and explains its significance:

At just after 5am on Monday morning, Pluto and its moon Charon would move in front of a magnitude 12.4 star in Sagittarius. The shadows created would sweep over the Earth at about 75,000km per hour.

Observers from Paris Observatory and Lowell Observatory planned to observe from Canberra, Tasmania, South Africa and La Reunion Island as well as local observers in Brisbane, Sydney, Canberra, Melbourne, Adelaide and Perth planned to lend support.

The goal was to accurately measure the rate of dimming of the starlight as Pluto and its atmosphere absorbed light. Yes, Pluto does have a feeble atmosphere, consisting of mainly nitrogen and lesser amounts of Methane and carbon monoxide and pressures are measured in micro bars. Just what kind of atmosphere exists at -235° C is hard to imagine.

Sadly, Monday morning weather was not kind to observers in Tasmania, Melbourne, Canberra and Adelaide as well as Brisbane. Luckily though local observers in Sydney and Perth had good weather and managed to have their telescopes and cameras happily taking photos at 1 exposure per second during the nearly two minutes of the event’s duration. Also fortunate were the observers on La Reunion Island where they managed to observe the occultation of Charon.

Shown above is the light curve produced by measuring the intensity of the light from the star and Pluto. The light from the star is absorbed the deeper it goes into the atmosphere and, of course, when the star is completely hidden by Pluto the amount of light recorded is from Pluto only. From this estimates of pressure and temperature are calculated and even inversion layers can be detected. All the images (called a dataset) will be sent to Paris Observatory and will aid in ongoing studies of Pluto and its atmosphere.

Dave Gault’s video of the Pluto dimming the star

Why is this important? Well apart from the pure knowledge obtained, we humans are sending a space craft to Pluto, called New Horizons and we’d like to know what to expect when it gets there.

Dave Gault