Subject: Hayabusa-2's Journey to Ryugu
From: Daniel Adamo <[email-masked]>
Date: Sun, 29 Nov 2015 15:13:51 -0700

ATIG Members-

At 4:22 UT on 3 Dec 2014, Japan launched Hayabusa-2, its second
ion-propelled spacecraft designed to visit a near-Earth asteroid (NEA).  The
destination for this mission is a volatile-rich NEA recently named Ryugu
(dubbed 1999 JU3 when discovered) and estimated to be 920 m in diameter.

For the last year, Hayabusa-2 has been in a slightly more elliptical orbit
than Earth's about the Sun.  This orbit also has a 1 : 1 resonance with
Earth's, facilitating an Earth flyby and gravity assist into Ryugu's orbit
plane on 3 Dec 2015.  Thereafter, Hayabusa-2 will slowly close with Ryugu
from behind and sunward of it, arriving in mid-2018.  This week's Earth
flyby trajectory, together with a series of yearly heliocentric trajectory
plots, are attached to illustrate Hayabusa-2's outbound journey.  Trajectory
data are based on a 28 Nov 2015 update to JPL's Horizons ephemeris server at
http://ssd.jpl.nasa.gov/?horizons.

After about 18 months surveying and sampling Ryugu, Hayabusa-2 is to begin a
one-year journey back to Earth in late 2019.  The return date, when samples
are to be delivered by an atmospheric entry capsule, will be very close to 3
Dec 2020.  The attached heliocentric plots illustrate why major Hayabusa-2
mission events involving Earth tend to fall on December 3.  This point in
Earth's orbit is only about 240,000 km (roughly 60% of the Moon's geocentric
distance) from Ryugu's ecliptic plane descending node.  On 29 Dec 2020,
Ryugu will be very close to this node and only 9 million km from Earth.
That geometry makes Hayabusa-2's return propulsion requirements easy to
satisfy, even if Ryugu departure date is postponed.

As this may be 2015's last ATIG upload, I'll take the opportunity to wish
you happy holidays and a joyous 2016!

-Dan