I had to share this first incredible photo taken by one of the MINERVA robots we talked about in Friday’s blog. Both of the MINERVAs landed successfully on the surface of the asteroid Ryugu, and this image was taken the very next day (Sept. 22). It captures a view of Ryugu’s forbidding surface in midair snapped by the robot after its first successful bounce. The robots remain airborne for up to 15 minutes before settling back down to the surface because gravity is next to nothing there.
You can figure out how much you weigh on different worlds if you know the mass and density of the object, its speed of rotation and how far you are from its center. For instance , a 220-pound (100 kg) human standing on the surface of comet 67P/Churyumov-Gerasimenko, which measures 2.5 miles long with a rather fluffy density compared to that of Earth, weighs just 1/25th of an ounce or very close to 1 gram. That’s a pinch of salt or the weight of a dollar bill.
As part of the Rosetta mission to the comet, a lander named Philae was dispatched to the surface. Engineers built anchoring harpoons into Philae so it would attach itself to the comet’s surface and not float away. In the end, a thruster applying downward force as the lander landed malfunctioned, the harpoons weren’t deployed and the probe bounced off the comet several times before finally landing in a shady crevice. Gravity (or lack of it) matters.
Ryugu is only about a quarter the size of the comet and composed of what appears to be loose rocks held together by their combined gravitational attraction, what astronomers call a “rubble pile” asteroid. Still, because of its rocky nature, it’s 2.4 times as dense as the comet, so that a 220-pound person would also weigh very close to a gram standing on Ryugu.
But there’s more afoot here. Ryugu rotates quickly, making one spin every 7.5 hours. If you’re standing on a fast spinning object you feel a force called centripetal acceleration. Picture yourself as a kid seated at the edge of a spinning merry-go-round. Remember how you had to hold onto the bars to keep from sliding off? Centripetal force did that.
I searched around and found this to be one of the best explanations of centripetal force — with a little sass for fun.
On a rotating asteroid, comet or planet, centripetal force counteracts the force of gravity, which wants to keep you planted on your feet. It’s greatest at the equator, which spins faster, compared to the mid-latitudes and poles. According to astronomer Phil Plait in his Bad Astronomer blog, the centripetal force at Ryugu’s equator shaves a 1/5th off gravity’s pull, so if you’d weigh even less than a gram there compared to the poles.
It would be interesting to take a walk on Ryugu. If you started at the north pole and walked toward the equator — a distance of only 1,500 feet (914 meters) by the way — you’d feel yourself becoming noticeably lighter. The increasing centripetal force would also make it feel like you were walking downhill the entire time even though the ground around you would appear flat.
Since the circumference of Ryugu is 1.8 miles (2.9 km), most of us could walk around the whole asteroid, carefully picking our way between rocks, in about an hour. Of course, you’d have to do this gingerly so as not to launch yourself off the giant space rock. But if you wanted to leave it wouldn’t take much. Just a jump. The asteroid’s escape velocity is just a mile an hour. One push and you’d never return.