Pluto's atmosphere was marginally detected for the first time on August 19, 1985, during a stellar occultation observed from Israel (Brosh, MNRAS 276 , 571, 1995).

Another occultation with better signal to noise ratio, observed on July 9, 1988, revealed without ambiguity this atmosphere, during observations conducted from Australia and from the Kuiper Airborne Observatory (KAO), see Hubbard et al. Nature 336 , 452, 1988, Elliot et al. Icarus 77 , 148, 1989, and references therein.

Due to the very small angular diameter of Pluto on the sky (2400 km= 0.11 arcsec), occultations by this planet are very rare and difficult to predict well in advance. Two such events will occur on July 1st, and July 20th, 2002, and will be possibly visible from Europe/Africa on July 1st, and from Southern America on July 20th.

The magnitude of the stars are comparable for both events, V ~ 11.5-12, while Pluto has V ~ 14. Thus, the contrast of the occultation will be very good. Approximately 5 images/second would be nice to probe Pluto's atmosphere with sufficient resolution and S/N.

After the 1988 event, Pluto's atmosphere is still poorly understood. The atmosphere is composed of a dominant gas of mass 28 amu, probably N2 (as preferred over CO). Near-IR solar reflections spectra also show the presence of methane CH4 (Young, PhD thesis, 1994), which could have an abundance of up to 1% in the atmosphere.

The light curves obtained then show that the atmosphere has either (1) a strong temperature gradient near the surface (5-10 K/km), possibly due to radiative heating by methane, or (2) possesses a thick aerosol layer, with a photochemical origin. See discussions on Pluto's atmosphere in Hubbard, Yelle and Lunine, Icarus 84 , 1, 1990, Elliot and Young, Astron. J. 103 , 991, 1992, and Strobel et al. Icarus 129 , 266, 1996.

As a consequence, Pluto's radius R and the atmospheric surface pressure p_s are poorly determined till now: R ranges from about 1100 to about 1200 km, and p_s ranges from 3 to 50 microbar.

The range of possible radii quoted above implies a range of about 1.7-2.3 g/cm3 for the bulk density of Pluto. This is a large uncertainty, and improving this quantity would be most useful to better constrain the planet composition.

To discriminate between the two atmospheric models, it is necessary to observe the occultation in various wavelengths, as far apart as possible. A thermal gradient will not have chromatic effects, while a aerosol layer with absorb DIFFERENTIALLY the stellar flux in the two wavelengths during the event.

Current plans are to observe in the visible and in the near IR, for instance with CCD's and IR Adaptive Optics cameras.

The observations can be conducted from different telescopes at different wavelengths, or better, at the same telescope with a multi-canal instrument.

Thus, it would be important to look for a possible time variation of Pluto's atmosphere in 14 years.

The current predictions indicate that the central part of the shadow could go over the northern part of Chile, where dry and clear skies dominate. **The astrometry can change**, though, so that the shadow track can still shift north or south by several hundreds of km.

Along the central part of the shadow, a flash could be observed for several seconds in the middle of the event. This flash would be caused by the focalisation of the stellar rays by the lower layers of Pluto's atmosphere, if dense and transparent enough.

This flash would be most precious to detect minute distortions (some km) of Pluto's atmosphere with respect to a spherical shape.