How has the vast expanse of space influenced the story of our planet? This week on Planetary Radio, Sarah Al-Ahmed of The Planetary Society delves into this intriguing question with historian Dagomar Degroot. Degroot’s new book, Ripples on the Cosmic Ocean, explores how cosmic events, from Martian dust storms to solar flares, have significantly impacted life and history on Earth.
Degroot, a professor of environmental history at Georgetown University, introduces the concept of “cosmic environmental history,” suggesting that understanding our place in the solar system can help us better protect our world. His book, published by Harvard University Press on October 28, 2025, invites readers to view the solar system as an interconnected ecosystem that has continuously shaped life on Earth.
Understanding Cosmic Environmental History
Degroot argues that our environment extends far beyond Earth, encompassing the entire solar system. He traces how solar cycles, Venus’ greenhouse effect, Martian dust storms, and even Cold War fears of nuclear winter have influenced human civilization. From the asteroid that ended the dinosaurs to the science that saved the ozone layer, Degroot demonstrates how cosmic events have shaped human history.
In an interview with Sarah Al-Ahmed, Degroot defines environmental history as the study of the reciprocal relationships between humanity and everything else in the universe. This perspective challenges the traditional Earth-focused view of environmental history, encouraging us to consider the broader cosmic influences on our planet’s past, present, and future.
The Role of the Sun
The sun, as a major driver of Earth’s climate, plays a crucial role in Degroot’s narrative. He highlights how solar variability and celestial cycles, such as the Milankovitch cycle, have shaped larger systems like ice ages, human migration, and civilization. Degroot emphasizes the importance of understanding these solar influences to grasp the broader patterns of human history.
“The sun is one of the greatest dictators of our local climate,” Degroot explains, “and its influence extends far beyond what we typically consider.”
Venus and Climate Change
Degroot also explores how Venusian exploration has shaped our understanding of climate change. The discovery of Venus as a hellish world with a runaway greenhouse effect served as a stark warning for Earth. This realization, largely driven by Carl Sagan’s work, highlighted the potential risks of unchecked greenhouse gas emissions on our planet.
“Venus became a kind of worst-case scenario in the 1980s,” Degroot notes, “a warning of what could befall Earth if we continue to burn fossil fuels.”
Asteroids, Comets, and Existential Threats
The book also delves into the existential threats posed by asteroids and comets. Degroot recounts how events like the Shoemaker-Levy 9 impact on Jupiter in 1994 heightened awareness of these cosmic dangers. This realization led to increased efforts in planetary defense, including the tracking of near-Earth objects and the development of asteroid redirection missions.
“For a very modest amount of money, scientists were able to detect and track thousands of near-Earth asteroids, dramatically reducing the risk,” Degroot explains.
The Future of Space Exploration
As humanity continues to explore space, Degroot emphasizes the need for careful consideration of space conservation and planetary protection. He advocates for a balanced approach that supports both robotic and human exploration while safeguarding the environments of other worlds.
Degroot’s work challenges us to reconsider our relationship with the cosmos and the interconnectedness of our solar system. By understanding the cosmic influences on Earth’s history, we can better navigate the challenges of the future and protect our planet for generations to come.
For those interested in exploring these themes further, Ripples on the Cosmic Ocean offers a comprehensive and thought-provoking look at the cosmic forces that have shaped human history. As Degroot concludes, “Earth is a node in a much bigger set of environments, and that’s why outer space research matters so much.”
