Subject: Re: "Dawn" Achieves Ceres Orbit
From: Daniel Adamo <[email-masked]>
Date: Mon, 22 Jun 2015 13:43:00 -0700

ATIG Members-

A first update to the March 30 Dawn ephemeris (referenced in the plot
accompanying April 4's ATIG upload below) was posted to JPL's Horizons
server circa June 18.  This reflects initial as-flown trajectory
reconstruction of arrival in the RC3 orbit (13,600 km in altitude) and a
spiral down to the Survey orbit (4400 km in altitude), completed on June 3.
These milestones are plotted with respect to Ceres, as viewed from its
equatorial plane, in the attached PDF.

On June 22, Dawn is completing her sixth southbound data collection pass
over the dayside of Ceres in the Survey orbit.  After the eighth Survey
orbit is complete late in June, Dawn is to begin lowering her altitude once
again.

-Dan

From: Daniel Adamo <[email-masked]>
Date: Sat, 04 Apr 2015 12:11:40 -0700
Conversation: "Dawn" Achieves Ceres Orbit
Subject: Re: "Dawn" Achieves Ceres Orbit

ATIG Members-

Updating Dawn's approach to (1) Ceres, the attached plot of Ceres-centered
motion has SEP thrusting modeled into the mid-April time frame.  This plot
is viewed from Ceres' equatorial plane in a direction looking "up" in the
plot sent March 8.  From the updated trajectory, SEP thrusting to achieve
Dawn approach over Ceres' north rotational pole is evident.  Planned SEP
thrusting in late April (not modeled in this plot's ephemeris) will achieve
a circular "RC3" orbit at 13,500 km altitude on April 23.  You can read more
about Dawn's Ceres approach at http://dawnblog.jpl.nasa.gov/?p=2437.

-Dan

From: Daniel Adamo <[email-masked]>
Date: Sun, 08 Mar 2015 21:45:51 -0700
Conversation: "Dawn" Achieves Ceres Orbit
Subject: "Dawn" Achieves Ceres Orbit

ATIG Members-

About the time most of us in the U.S. were arising on Friday morning, March
6, the Dawn spacecraft was achieving orbit about dwarf planet asteroid (1)
Ceres.  Because Dawn uses solar electric propulsion (SEP), its thrust duty
cycles are highly flexible and equally uncertain because of the low
accelerations exerted.

The attached plot of Dawn's Ceres-centered motion depicts orbit capture, but
SEP thrusting doesn't appear to be modeled much after March in the
associated ephemeris posted March 6 to http://ssd.jpl.nasa.gov/?horizons.
According to http://dawnblog.jpl.nasa.gov/2015/03/06/dawn-journal-march-6/,
Dawn should be in its circular RC3 orbit about Ceres (13,500 km altitude,
polar inclination, ascending node at local midnight) on April 23.  I'll
attempt to pass along subsequent plots reflecting SEP thrusting leading to
RC3 as they become available.

-Dan