For a brief moment in time in July 2018, the solar system aligned to show Earth the fully sunlit disk of Pluto, an arrangement that won't occur again for 161 years.
Planetary scientist Bonnie Buratti was ready: She had been waiting for a decade for the opportunity to catch the rare sight in hopes of filling a gap in knowledge that even the carefully planned New Horizons mission couldn't tackle. The result is an enigmatic plot of light from Pluto and its moon, Charon.
"We grasped this once-in-a-lifetime — well it's once in more than a lifetime, once in two centuries — opportunity to see Pluto fully illuminated," Buratti, who works at NASA's Jet Propulsion Laboratory in California and is the lead author on a new paper presenting the observations, told Space.com.
Observations of a solar system body at and around the point of its maximum illumination allow scientists to measure what they call the "opposition surge," a sudden increase in brightness of an object as it is fully illuminated, disproportionate to the extra sliver of surface area being illuminated.
And opposition surge isn't just a curious optical phenomenon: Scientists believe the pattern of the surge is influenced by the density of material on a world's surface. "By looking at how much an object brightens when it gets full, you can tell something about the surface texture and what the surface is like — is it fluffy? Is it snowy? Is it compact?" Buratti said.
For example, the opposition surge of the full moon is caused by the loose, dusty regolith at its surface, she said. "These particles cast shadows and those shadows rapidly disappear as the face becomes illuminated to the observer."
A fully illuminated Pluto is difficult to catch from Earth because the inclination of the dwarf planet's orbit compared to our own. Skywatchers use the term "opposition" to refer to the point in Earth's year when a particular solar system object appears opposite to the sun in our skies. However, a celestial body doesn't necessarily appear fully illuminated at that time, particularly something like Pluto that is often above or below the plane of Earth's orbit around the sun.
Hence the 161 year wait for the next such opportunity and Buratti's care to make a note of the 2018 opportunity so far in advance. She and her colleagues arranged for time using the 200-inch Hale telescope at the Palomar Observatory near San Diego. The Hale telescope is equipped with an optics system that could pick apart Pluto and Charon, rather than seeing one blob of the two worlds.
And the team managed to catch super-illuminated observations of Pluto on July 12, 2018, as well as other days that month and in July 2019. Buratti and her colleagues had more measurements planned for last summer, but the COVID-19 pandemic shut down the observatory.
The researchers still aren't allowed to use the telescope in person, but finally, Buratti and her colleagues again have observations on the calendar, beginning on Saturday (June 19). She hopes those measurements, plus more next month and in October, will clarify just how Pluto's opposition surge occurs, giving scientists the details they need to understand what might be happening on the ground to cause the visual effect.
Buratti said her suspicion is that the sharp opposition surge is related to how dynamic a world the New Horizons spacecraft discovered during its 2015 flyby. "Pluto is much more active than we thought," she said. "We saw stuff we never saw before there."
It's the first solar system world beyond Earth known to sport glaciers, for example. Ices vaporize and refreeze, sometimes moving between Pluto and Charon in the process. "There might be snow on it [Pluto], there's a lot of frost moving around, it might have a really fluffy, textured surface."
Although the relatively close alignment of spacecraft flyby and full illumination from Earth is a pure coincidence, Buratti said the combination of observations is another example of how planetary science thrives best when using ground-based and space-based tools in tandem.
New Horizons saw night-side and halfway illuminated views of Pluto during its maneuver, she noted, but couldn't see the fully illuminated disk because of the trajectory of its brief flyby. "You combine it with these ground-based observations and you have the full thing, you have the full package," she said. "These are totally complementary."
And for Buratti, the new observations are a poignant reminder of her own work during the New Horizons flyby, since it's rare that scientists get to use equipment powerful enough to differentiate Pluto and Charon when observing the pair from Earth.
"We saw Pluto and Charon separately for the first time since the encounter," Buratti said. "It's kind of emotional for me ... Here's a thing that was just a point of light, and then in a day or two it becomes this geologic world and you feel like you're there. It's an intimate thing. But then it goes back to being a point again."
The observations are described in a paper published June 8 in the journal Geophysical Research Letters.
Email Meghan Bartels at firstname.lastname@example.org or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.