2018-06-29 16:39:56 UTC
WHAT WOULD IT BE LIKE TO STAND ON THE SURFACE OF RYUGU?
Contributed by Phil Plait
Jun 29, 2018
The Japanese Space Agency mission Hayabusa 2 is now orbiting the small
This grand moment was achieved just after midnight (UTC) on June 27,
2018, when the spacecraft's thrusters shut down after a course change to
place it in a circular trajectory around the asteroid. It is currently
about 20 kilometers (12 miles) from Ryugu, which itself is about 900
meters (roughly half a mile) in diameter.
When it was still 40 km away, it took this extremely interesting image
of the small asteroid:
The small (<1 km wide) asteroid Ryugu, seen by the Hayabusa
2spacecraft when they were separated by 40 km. Credit: JAXA Zoom In
The small (<1 km wide) asteroid Ryugu, seen by the Hayabusa 2spacecraft
when they were separated by 40 km. Credit: JAXA
We've known for some time Ryugu was roughly spherical, but more
accurately described as diamond shaped. I'd actually say it's like two
squat truncated cones sitting base-to-base (in fact, each hemisphere is
about the same shape as a space capsule like Apollo or Dragon).
The surface is weird. You can see one whopping great crater there on the
left, about 300 meters wide. A crater this big relative to the parent
body is relatively common (see Phobos, Mimas, and Tethys), created in
this case when something biggish smacked into Ryugu. In many cases, you
don't get craters bigger than this because if the impact were any harder
(due to a bigger impactor or a higher velocity at impact) it would
literally shatter the asteroid. But I wonder…
The surface of Ryugu isn't smooth. It looks like the surface of a
construction site, where smaller pebbles and such make the base, with
bigger lumps of rock scattered here and there. This is very persuasive
evidence that Ryugu is a "rubble pile" asteroid! Instead of a solid
piece of rock, it's more like a big collection of individual rocks bound
together by their own gravity. This can happen when a small asteroid
suffers lots of little impacts, which riddles it with cracks. Some
impacts could even just barely shatter the asteroid, enough to break it
up into pieces but not enough to make them fly away. What you're left
with is something that looks like a bag of a bunch of different sized
rocks without the actual bag.
That's why I wonder if you couldn't get a bigger crater on Ryugu; if
it's crunchy instead of solid, it can absorb a bigger impact. The force
of impact would go into jostling the loose material instead of
Note that the big crater's rim isn't sharp either; it's softer and
rounder. I'd expect that from a rubble pile too. We do have close-ups of
another rubble pile object, Itokawa, an asteroid visited by another
Japanese spacecraft… Hayabusa! The first Hayabusa, that is. Here are the
two asteroids compared (put together and scaled by my friend Emily
Ryugu (right) compared to the asteroid Itokawa (left) at the same
scale. Both look like rubble piles, but Itokawa has large smooth
regions, possibly because of dust flowing along the surface. Zoom In
Ryugu (right) compared to the asteroid Itokawa (left) at the same scale.
Both look like rubble piles, but Itokawa has large smooth regions,
possibly because of dust flowing along the surface. Credit: JAXA,
University of Tokyo, Koichi University, Rikkyo University, Nagoya
University, Chiba Institute of Technology, Meiji University, University
of Aizu and AIST....
A key observation Hayabusa 2 can make to confirm or contradict this is
to measure the gravity of Ryugu. A rubble pile has a lower density than
a solid rock (due to there being voids, empty pockets, inside it), so
for a given size a rubble pile has less mass than a monolithic asteroid,
and therefore lower gravity. That actually shouldn't take too long to
determine; the orbit of Hayabusa 2 depends on the gravity, so that's a
number we should be getting soon.
The shape is interesting too. There's more at work here than gravity!
Imagine you're standing on its surface. Gravity there is incredibly
weak; assuming it has a density of about twice that of water (like a low
density rock) then the gravity you feel would be less than 0.0002 times
that of Earth's! I have a mass of about 80 kilograms, which is a weight
of 175 pounds or so on Earth. On Ryugu I'd only weigh about 1/30th of an
ounce! If I jumped I could easily escape the asteroid altogether (the
escape velocity is about 30 cm/sec, less than 1 mph).
But, as I pointed out in my previous post, Ryugu is a fast rotator,
spinning once every 7.5 hours or so. That's important! If you're on a
spinning object there's an acceleration (called centripetal
acceleration) that you feel as a force pushing you away from the center
of the spin. It's the same thing as when you're in a car making a turn
and you feel a force in the direction opposite the turn (if you turn
left you're thrown right, which is the same as saying you want to go
straight but the car turning left makes it feel as if you're being
pushed to the right).
This counteracts the force of gravity down, toward the center of Ryugu.
I played with the math a bit, and found that on the equator the force
outward is about 1/5th or so of gravity! So you'd weigh noticeably less
on the equator than you would at the poles just because of this.
If you're not on the equator, that force outward points at an angle away
from the center of Ryugu. The net effect is it would feel like you're
standing on a slope, downhill toward the equator, even if the ground
were perfectly flat! So if you're a rock sitting on Ryugu halfway
between the equator and pole, say, and there's a small impact by another
asteroid, the ground would shake, dislodging you, and you'd roll toward
I strongly suspect that's why Ryugu has the shape it does. Stuff has
rolled "downhill" toward the equator, piling up there, forming that
ridge all the way around, giving it a more diamond-like shape than
spherical. If it's a rubble pile, there's a lot of loose material
available to do this, too. So these ideas hang together.
We'll get a better idea as Hayabusa studies Ryugu further, especially
when it releases the landers/rovers! Then we'll get amazing close-up
images of the surface, and learn a vast amount more about this weird
little rock orbiting the Sun. A lot of the smaller asteroids are likely
to be similar to Ryugu, so it'll be a proxy for a huge collection of
tiny worldlets out there. It's time to explore them.