Years of uncertainty about Pluto's exact size were being resolved as humanity's familiar emissary approached this dwarf planet, described in the article as the most distant object yet explored at close range.
Before New Horizons, researchers using instruments such as ground-based telescopes and the Hubble Space Telescope could not determine Pluto's diameter precisely and instead published approximate estimates. One reason measurements from Earth were difficult was Pluto's thin atmosphere. The article describes that atmosphere as having roughly one hundred-thousandth to one millionth the density of Earth's atmosphere and as containing nitrogen, methane, and carbon monoxide.
The source cites an early 1993 measurement by Millis and colleagues estimating Pluto's diameter at 2,390 kilometres. In subsequent years other researchers produced different estimates, some of which placed Pluto below the approximate 2,326-kilometre diameter then given for Eris, another trans-Neptunian object discovered in 2005.
The article then discusses the International Astronomical Union's 2006 classification decision. It states that the IAU adopted a resolution defining eight known Solar System bodies as planets according to three conditions and placed bodies such as Pluto, which did not satisfy all three, in categories including dwarf planets and other small Solar System bodies. It also mentions Neil deGrasse Tyson in the historical discussion and notes the controversy surrounding Pluto's reclassification. [source]
As New Horizons approached Pluto, scientists finally had the opportunity to measure its diameter more directly using the spacecraft's Long Range Reconnaissance Imager (LORRI). Based on New Horizons observations made around July 13, the article gives Pluto's diameter as 2,370 kilometres. [source] [source] [source]
The accompanying comparison shows Pluto relative to Earth. The author notes that Pluto's diameter is roughly comparable with the width of Iran, while a sphere of that diameter has a surface area comparable in scale to the area of Russia.
With that measurement, the article concludes that Pluto was confirmed as the largest trans-Neptunian object then known, while astronomers continued searching for still larger objects beyond Neptune.