Australian Sky & Telescope - June 2018

Bennu

Ryugu

Earth

s

Mercury

Sun

http://www.skyandtelescope.com.au 29

Earth will reveal precious new information that even our vast

collection of fallen meteorites cannot provide.

Petite and primitive

The diminutive destinations of the Hayabusa 2 and Osiris-

REX missions are, respectively, the near-Earth asteroids

162173 Ryugu (formerly designated 1999 JU 3 ) and 101955

Bennu (provisionally tagged as 1999 RQ 36 when it was

discovered). Scientists chose these targets not only for their

compositions, but also because both are comparatively easy

to reach from an orbital mechanics perspective. Both follow

moderately eccentric orbits that carry them away from the

Sun to the distance of Mars and bring them just interior to

Earth’s orbit, crossing our planet’s path in the process.

Ryugu is the larger of the two at just shy of a kilometre

across. We don’t know much about it, but ground-based

observations reveal a 7.6-hour rotation period and a spectral

marker in reflected sunlight that’s characteristic of iron-

bearing clay minerals. Since clays form in the presence of

S ASTEROIDS AMONG US Ryugu and Bennu’s orbits

nestle among those of the inner planets, shown here at their

locations for July 1st. They are only two of the nearly 4,000

known asteroids whose orbits lie within that of Mars.

rendezvous

This winter, two spacecraft will bear down

on their target asteroids, preparing to snatch

rubble and bring it home to scientists.

OSIRIS-REX: NASA GODDARD SPACE FLIGHT CENTER CONCEPTUAL IMAGE LAB; O

RBITS: GREGG DINDERMAN /

S&T