Q: We already send rovers to Mars. Why did NASA test a helicopter there?
—Marjorie Mathews | Silver Spring, Maryland
As the former director of the National Air and Space Museum, Ellen Stofan hates to say anything critical of rovers—but rovers can’t move as fast as people do. “Just think how powerful it will be for future astronauts on Mars to have little drone helicopters to help answer questions like, ‘What’s over the next ridge? Should I go there?’” she says. It will be a while before that happens. Ingenuity, the four-pound helicopter that hitched a ride on NASA’s Perseverance probe, was only there for flight tests. But its success at staying aloft in the thin Martian atmosphere was a huge breakthrough. The helicopter is being left behind on Mars, but Stofan, who is now the Smithsonian’s under secretary for science and research, hopes an astronaut on a future human mission will pick up Ingenuity and bring it back to Earth so it can be displayed “next to the Wright Flyer, where it belongs.”
Q: When and why did humans evolve to have a gag reflex?
—Justin Munleeuw | Fort Collins, Colorado
Skeleton fossils can’t tell us when our early ancestors started having these throat contractions, says Rick Potts, paleoanthropologist at the National Museum of Natural History, but looking at changes in their diet provides helpful clues. Scientists believe the gag reflex evolved when our early ancestors started trying new foods between two million and four million years ago. Those who had a gag reflex were less likely to consume rotting meat, and they passed this advantage along to their descendants. Now the trait is so reliable that even smells can trigger it.
Q: How do we know that lost nuclear weapons are really lost? Could some of them be in the hands of foreign governments?
—George Pantagis | Englewood, New Jersey
Six nuclear weapons were lost by the United States in the 1950s and ’60s and never recovered. One rolled off the deck of a ship. Others vanished in plane crashes. The U.S. government searched for the components of these bombs but was unable to find them. It’s not absolutely impossible that someone else did, says Frank Blazich, curator of political and military history at the National Museum of American History. If another country got hold of our lost nukes, it might be able to study their design and “leapfrog ahead,” Blazich says. But it’s far more likely that the bombs really are lost forever, whether deep in a swamp or at the bottom of the ocean.
Q: Why do some species migrate while others don’t?
—Steve Heffelfinger | Derry, New Hampshire
Animals evolved different ways of dealing with changing seasons, says Alfonso Alonso, a conservation biologist at the Smithsonian Conservation Biology Institute. Snowshoe rabbits grow thick fur that keeps them warm and helps them blend in with the snow. Bears eat extra in the fall so they can hibernate. Cold-blooded turtles conserve energy by burrowing into the ground and remaining inactive. But some animals, like birds, need to head to warmer climates. Certain types of fish migrate to reproduce. Many insects die in the winter, while their eggs lie waiting to hatch in warmer weather. But adult monarch butterflies migrate to Mexico to escape the cold. They fly north again in March to lay eggs near the only food source their caterpillars can eat: milkweed.
*Editor's Note, July 16, 2021: An ealier version of this piece stated that Homo sapiens emerged between 2 and 4 million years ago. This was an editing error. Homo sapiencs only emerged hundreds of thousands of years ago. Dr. Potts was referring to early human ancestors that preceded Homo sapiens.