On a bitterly cold February night in 1930 at Lowell Observatory in northern Arizona, farm boy turned astronomer Clyde Tombaugh was practicing his newly embraced science. He collected photons on his long-exposure photographic plates from a point of light that would turn out to be Pluto, orbiting just beyond the reach of Neptune. By the end of the 20th century, Pluto would prove to be just the tip of an iceberg, the brightest member of a large population of dwarf planets orbiting in the Kuiper belt. Eighty-five years after Pluto’s discovery, a small and speedy robotic probe would turn it from a point of light into a real world, revealing a surprisingly delightful diversity of surface, atmospheric, and ionospheric processes at work.
In Chasing New Horizons: Inside the Epic First Mission to Pluto, leading planetary scientists Alan Stern and David Grinspoon tell the insiders’ story of that 85-year span from the discovery of Pluto to its initial reconnaissance by the New Horizons spacecraft. Stern is the principal investigator on the New Horizons mission and is by any measure the father of that project. His friend and colleague Grinspoon has been helping with public outreach and press interactions during the mission and is an author of previous popular science books. Together, they make a great writing and storytelling team.
And what a story! All of NASA’s most famous missions of planetary exploration have, at one point or another, faced existential crises or cancellation, only to be resurrected by political champions, leading scientists, or public pressure. New Horizons was no exception. Plutophiles at NASA endured 26 years of pitching the mission, seeing it approved, and then having it cancelled. They survived that cycle multiple times before New Horizons was finally really approved in 2003. As the authors point out, the key to getting the spacecraft off the launchpad was the rare alignment of a strong scientific case; a lean and nimble spacecraft engineering team and operations plan; support from NASA, congressional leaders, and citizen groups like the Planetary Society; and persistence, persistence, persistence.
Stern, Grinspoon, and colleagues could be forgiven for being bitter about how long it took to make the mission happen, but fortunately any such bitterness was ultimately blown away by the sweet winds of success. After a successful and scientifically rewarding gravitationally assisted flyby of Jupiter in 2007 and then eight more years of cruising to Pluto, the spacecraft finally encountered its target in July 2015. The flyby through the Pluto system produced a spectacular cornucopia of images, spectroscopic data, and information about particles and fields, the most detailed of which was collected during a manic but beautifully orchestrated half-hour sprint close to the planet. It will likely take decades or more to fully understand everything planetary scientists have learned from the New Horizons mission.
Stern and Grinspoon’s book is a comprehensive, entertaining, and educational story about the birth, glorious prime of life, and continuing adventures of a unique mission to the farthest realm of our solar system. The spacecraft is on course to fly by another, much smaller Kuiper belt object (KBO) on New Year’s Day 2019. The authors do an excellent job of describing New Horizons’ results in language appropriate for a general audience. In a nod to the professional scientists who will also read it, the book includes an appendix describing the major discoveries and surprises of the mission in more technical detail. For me, the biggest surprise is Pluto’s internal heat, which drives still-active surface geologic and atmospheric processes. The source of that heat is a puzzle that might only be solvable if we return to Pluto.
Is Pluto a planet, as Tombaugh claimed and schoolkids throughout the 20th century were taught? Stern and Grinspoon make the case that it is, despite the poorly managed decision of the International Astronomical Union’s leadership to demote it to dwarf planet status back in 2006. That demotion was based partly on the discovery of so many other Pluto-sized KBOs.
But I agree with the authors that planets should be judged on what they are like, not where they happen to be. I believe that many—perhaps most—planetary scientists would classify the small but supremely interesting and dynamic world of Pluto as a full-fledged planet, and I would include large KBOs beyond Pluto, the large moons of the giant planets, and Pluto’s own moon Charon in that category as well. Plutophiles, and the fans of the 40 or so other should-be planets that we know of so far in our solar system, should not give up the fight.
Jim Bell is an astronomer and planetary scientist in Arizona State University’s School of Earth and Space Exploration and president of the Planetary Society, the world's largest citizen space organization. He is also a member of the International Astronomical Union, despite disagreeing with some of its decisions. His most recent book is The Ultimate Interplanetary Travel Guide: A Futuristic Journey Through the Cosmos (2018), which includes Pluto as a prime destination for future space tourists.
