This article originally appeared on the Hayabusa2 Project website and is reposted here with permission.

We are now only a short distance from asteroid Ryugu! On February 26, 2018, an image of Ryugu was successfully captured using the Optical Navigation Camera - Telescopic (ONC-T) onboard Hayabusa2 (story and images here). Although the asteroid can currently only be seen as a bright point, it is very encouraging for the Hayabusa2 Project Members to spot the destination!

Nobody yet knows what Ryugu will look like. Even the Hayabusa2 Project Members and experts from all around the world do not know! This is why we are hosting a contest to ask you all to imagine what Ryugu will be like before Hayabusa2 arrives at the asteroid. But what do we already know about Ryugu, and why is it so hard to know what it looks like? Here I will summarise what you might find helpful when picturing Ryugu.

Note: This explanation is quite long and sometimes a bit technical. For imagining what Ryugu will look like, it may be better not to have your ideas tangled up in all this information! So if you find this a tricky read, feel free to just skip to the last summary.

Introduction

The reason we do not yet understand Ryugu is because asteroids appear only as points even when viewed with a large telescope. This may seem a little strange! How are we able to see the shape of a galaxy tens of millions or even billions of light years away, but we do not know the shape of celestial bodies within our Solar System? Actually, a quick calculation can show you why this is true.

Let's consider an asteroid with size 1 km at the same distance as the Moon, which is about 380,000 km from the Earth. The apparent size (how big it looks to us on Earth) is about 0.5 "arc seconds" or arcsec, which is the size of the angle the asteroid fills on the sky. An angle of 1 second is 1/3600 of a degree, so 0.5 arcsec covers just half that on the sky! Even to large ground-based telescopes (telescopes here on Earth rather than in space), such a small size looks like a point, even when the asteroid is as close as the Moon. Normally, asteroids are much further away than the Moon so they become even smaller points. For comparison, the diameter of the Moon is about 3,500 km, which is about 0.5 degrees (30 minutes or 1800 arcsec) when seen from Earth. The elliptical galaxy, M87, in the Virgo Cluster of galaxies, is a staggering 60 million light years away, but the galaxy is very big and has an apparent size of about 7 minutes (420 arcsec). This is big enough to see the galaxy's shape through a telescope.

So how can we know the shape of an asteroid without travelling much closer? There are two different methods for deciphering the shape at a distance. The first is to estimate the shape by examining changes to the asteroid’s brightness. Asteroids are bright because they reflect sunlight. Like the Earth, they also rotate on their own axes (they spin). If the asteroid's shape is irregular, the brightness changes as the asteroid rotates and different parts of its body come into view. This data from the change in brightness is called a "light curve" (or "light intensity curve"). By acquiring many light curves, information about the asteroid's rotation (such as the rotation rate and orientation of the rotation axis (direction of spin)) and the shape of the asteroid can be found. The second method for finding the shape uses radar.

An example: Asteroid Itokawa

As an example of how shape-finding works, let's look at the asteroid explored by Hayabusa; the predecessor of Hayabusa2. This was asteroid Itokawa. Itokawa originally had the moniker "1998 SF36" before it was named "Itokawa". We call this code name the "provisional designation" of an asteroid. Although we will not unpick the full meaning of the provisional designation here, the "1998" is easily understood as it refers to the year of discovery.

The asteroid explorer, Hayabusa, also had a code name and was called "MUSES-C" before launch. Many observations of 1998 SF36 were made once the asteroid was chosen as the destination for MUSES-C and one important one was the observation of the light curve.