Today's APOD isn't a photo—it's a time-lapse video: Mars grows from a thin sliver of light into a crater-covered sphere, then slowly shrinks back into darkness.
Image: NASA Astronomy Picture of the Day (APOD) · Video Credit: NASA/JPL-Caltech/ASU/True Story Films · Text: Keighley Rockcliffe · Page for today.
The footage was captured by NASA's Psyche spacecraft. On May 15 this year, it swept past Mars, coming within about 4,600 kilometers of the surface—roughly the distance from Beijing to Singapore. Throughout May, the onboard camera kept shooting Mars; stitch those images together in sequence, and you get the clip you're watching today.
Doing nothing, gaining speed for free
The spacecraft didn't swing by Mars for the view. It was there to borrow a boost.
Mars orbits the Sun at close to 87,000 kilometers per hour. As Psyche passed by, Mars's gravity acted like a hand, pulling the spacecraft forward along the planet's own direction of travel—speeding it up and nudging its trajectory slightly. The process burned virtually no fuel.
Astronomers call this a "gravity assist." Here's a rough analogy: you stand by the road and gently toss a tennis ball at the front of an oncoming truck; the ball bounces back much faster than you threw it. The extra speed was "loaned" by the truck. A planet is that truck—except it's moving faster than any truck ever could, and it doesn't noticeably slow down as a result.
This trick isn't new. Humanity has been using it since the Soviet Union's Luna 3 in 1959. The Voyager twins and Cassini reached the distant places they did not on propellant alone—there's a hard limit to how much fuel you can carry, but gravity is free.
Where it's headed
Psyche's destination is an asteroid in the belt between Mars and Jupiter, also named Psyche, with an expected arrival in 2029.
What makes this asteroid special: it may be composed primarily of metal. Most asteroids in the solar system are rock or ice, but Psyche is thought to be the exposed metallic core of a "planetesimal"—a building block of rocky planets—that was shattered by collisions. If that's true, humanity will have its first chance to directly examine what the heart of a planet looks like. Earth's core lies buried beneath thousands of kilometers of rock, forever out of reach—yet this one may be sitting bare in open space.
So the Mars flyby doubled as a dress rehearsal. The team treated Mars as a stand-in for the asteroid and switched on every instrument for a full test run: the neutron spectrometer detected the expected signal increase near closest approach; the magnetometer recorded a celestial body's magnetic field for the first time—the "bow shock" formed where the solar wind slams into Mars's magnetic field; and the camera picked out Mars's two tiny moons from a great distance, a practice run for searching for small satellites near Psyche. According to NASA, every instrument performed at or above expectations, and they collected some new data for Mars science along the way.
Watching that video, Mars grows larger and larger in the center of the frame, then smaller and smaller again. Nothing dramatic happens. Yet in those few hours, a spacecraft permanently altered its trajectory at nearly zero cost. It turns out the most effortless way to move through the universe is simply to pass by.
One more thing—APOD's note at the end today reminds us: this August's Perseid meteor shower won't be washed out by moonlight. It's worth finding somewhere free of light pollution and looking up.