All Eyes on Pluto

By: Bruno Sicardy July 23, 2003 0

Even if your telescope won’t show faint Pluto, it might show a star’s light being cut off as the planet or its moon Charon passes in front. July’s events involve 12th-magnitude stars lying about one low-power field west of the 6.9-magnitude orange star SAO 160261 just off the left edge of the expanded chart. That star, in turn, is 1.4° west and 3.0° north of Eta (h) Ophiuchi. Tick marks on Pluto’s path show its location at 0 hours Universal Time on each date. The chart shows stars to magnitude 15, fainter than Pluto (magnitude 13.8) itself. Click on the image to see the complete chart.

Sources: U.S. Naval Observatory, Hubble Guide Star Catalog.

Twice in July, tiny Pluto — the least understood of the nine planets — passes extremely close to a 12th-magnitude star. Whether Pluto or its moon, Charon, will occult either star for a specific spot on Earth is still not known with complete certainty, but much has been learned since David W. Dunham’s first alerted readers of Sky & Telescope ("Planetary Occultation Highlights for 2002").

In a nutshell, the occultation initially predicted for European observers on the night of July 1–2 did not occur. But a second event on July 19–20, not mentioned in Dunham’s article because it was thought to be a near miss, probably will produce an occultation for observers in a wide swath across South America.

The Big One

On the evening of July 19–20, when Pluto and Charon pass in front of a 12th-magnitude star, the track for Pluto itself sweeps directly across South America although the exact path is uncertain. The Universal Times given here refer to the middle of Pluto or Charon’s disk.

Data source courtesy Bruno Sicardy.

As if to make up for the letdown on July 1st, prospects have dramatically reversed for the month’s second encounter. It involves a slightly fainter and very red star at 17h 00.3m, -12° 42'. The contrast of the occultation is still very good. The best prediction at this writing, based on the star’s position from Stone and that of Pluto from the Jet Propulsion Laboratory’s HORIZONS On-Line Ephemeris System, gives a highly favorable path across South America around 1:47 Universal Time on July 20th, which is Friday evening, July 19th (local date). Pluto could make the star disappear for up to about 180 seconds. However the exact path remains uncertain and I have posted three alternate possibilities on my Web site.

This event can be timed visually with a 20-centimeter (8-inch) telescope that clearly and steadily shows the star. Better yet, Dunham and Wolfgang Beisker of the International Occultation Timing Association (IOTA) suggest videotaping the event with a sensitive camera like the PC-164C (available for about $130 from Supercircuits) on at least a 25-cm telescope. Pluto is known to be 2,300 to 2,400 km in diameter, and careful timings could help to refine this value.

This occultation also presents a rare opportunity to probe Pluto’s atmosphere, believed to be mostly nitrogen and perhaps 1 percent methane. But light curves obtained by the Kuiper Airborne Observatory during the 1988 occultation left planetary scientists with a puzzle. Either Pluto’s atmosphere has a strong temperature gradient near the surface, possibly due to radiative heating by the methane, or there is a thick aerosol layer. If an observer can record the event at five images per second in two or more widely separated wavelengths (including near infrared), we should find out which model is correct.

Another question is whether Pluto’s atmosphere has cooled and shrunk in the last 14 years. The planet reached the perihelion point of its orbit in 1989 and is now heading back into deep space. It already receives 6 percent less sunlight than in 1989, enough to make the ground temperature decrease by ½° Celsius and shrink the atmosphere significantly.

An observer precisely centered in the occultation path may be able to record a brightening of the light curve for several seconds in the middle of the event. This "central flash," due to starlight focused by the lower layers of Pluto’s atmosphere, is expected to go over northern Chile, where dry and clear skies prevail. It could yield precious data on Pluto’s atmosphere, including any departure from spherical shape by as little as a few kilometers.