Pluto is now moving farther from the Sun. So you'd expect its atmosphere would be getting thinner, freezing out onto the surface. There has been worry that New Horizons would not get there before the atmosphere collapsed. Astronomers have observed stellar occultations by Pluto to keep tabs on its atmosphere repeatedly since 1988. Throughout the 2000s, the atmosphere seemed to be at a plateau -- there was some variation in its pressure, but the error bars all overlapped, so it was hard to say which direction it was going.

My previous article explained how there were three possible scenarios for Pluto's atmospheric behavior that were consistent with the data. In two of them, the atmosphere would be collapsing right now; in fact, one of the models predicted that the atmosphere would pretty much all have collapsed by the time of the New Horizons flyby.

Well, the last two stellar occultations whose data are included in this paper, including one on May 4 of this year, have shown that the atmosphere is not collapsing right now. In fact, it's still getting thicker, even as Pluto is moving farther from the Sun. Plugging these data points back into their climate models, Olkin and Young and their coauthors conclude that the only model that can explain the observed behavior of the atmosphere is one in which there is a permanent north polar cap. And in those models, the atmosphere never completely collapses. It waxes and it wanes, but it never collapses.

New Horizons will get a good look at the distribution of recent ices on Pluto. But really the best way to test Olkin and Young's models is to wait and watch more stellar occultations. "The current epoch is a time of significant change on Pluto. Most of the [permanent northern cap] models show a maximum surface pressure between 2020 and 2040. Regular observations over this time period will constrain the properties of Pluto’s substrate and the evolution of its atmosphere."