As more commercial companies are getting into the satellite launching game, space is becoming a crowded place and all of these objects are creating space debris. Right now, there are approximately 2,000 satellites floating in low-Earth orbit. Space agencies have estimated that are over 100 million small particles floating in low-Earth orbit, but there are no large scale projects to clean up these pieces of space trash.  Aerospace engineer Moriba Jah and space archeologist Alice Gorman talk about framing the idea of space as another ecosystem of Earth and what environmental, cultural and political issues come along with cleaning up our space junkyard.  Saturday’s Winter Solstice, which marks not just the arbitrary beginning of a season, but also the slow return of daylight to the Northern hemisphere. Or the coming decade, as many reflect back on everything that’s happened since 2010, and prepare to mark the beginning of 2020—a completely human invention. But there’s also an invisible timekeeper inside our cells, telling us when to sleep and when to wake. These are the clock genes, such as the period gene, which generates a protein known as PER that accumulates at night, and slowly disappears over the day, approximating a 24-hour cycle that drives other cellular machinery. This insight won its discoverers the 2017 Nobel Prize in Medicine and Physiology.  These clock genes don’t just say when you snooze: from the variability of our heart rates to the ebbs and flows of the immune system, we are ruled by circadian rhythms. Erik Herzog, who studies the growing field of chronobiology at Washington University in St. Louis, explains how circadian rhythms are increasingly linked to more than our holiday jet lag or winter blues, but also asthma, prenatal health, and beyond. And he explains why the growing movement to end Daylight Savings Time isn’t just about convenience, but also saving lives. This time of year, it’s not uncommon to see a little sprig of greenery hanging in someone’s doorway. It’s probably mistletoe, the holiday decoration that inspires paramours standing beneath it to kiss. But as it turns out, we may have miscast mistletoe as the most romantic plant of the Christmas season. In reality, the plant that prompts your lover’s kiss is actually a parasite. Ira talks with evolutionary biologist Josh Der about the myth and tradition behind the parasitic plant, and what it may be up to the other 11 months of the year.