SMART-1 Complets Its First Orbit Around The Moon

http://sci.esa.int/science-e/www/obj...objectid=36101

SMART-1 completes its first orbit around the Moon
European Space Agency
November 19, 2004

Today, 19 November 2004, at 10:58 UTC, SMART-1 passed its second
perilune and successfully completed its first orbit around the Moon. The
first orbit started on 15 November at 17:47 UTC when SMART-1 passed its
first perilune.

The next perilunae will occur with increasingly shorter intervals as the
orbital period of the spacecraft continuously decreases. This is due to
the thrust provided by the electric propulsion system, which is used to
reduce the semi-major axis of the orbit.

The first firing of the electric propulsion engine started on 15
November at 05:23 UTC and the engine has performed flawlessly since. It
will remain on until 17:48 UTC today, providing a total thrust time of
108 hours.


Upcoming events

Perilune passages

Event Day and Time (UTC) Period (h)
Perilune Passage 1 15-11-2004 @ 17:47:38.825Z 89
Perilune Passage 2 19-11-2004 @ 10:58:46.976Z 75
Perilune Passage 3 22-11-2004 @ 14:09:38.299Z 69
Perilune Passage 4 25-11-2004 @ 11:11:06.898Z 59

Electric propulsion burns

Event Day and Time (UTC) Duration (s)
EPP Start 15-11-2004 @ 05:23:53 390 299
EPP End 19-11-2004 @ 17:48:52 0
EPP Start 20-11-2004 @ 19:09:52 39 095
EPP End 21-11-2004 @ 06:01:27 0
EPP Start 21-11-2004 @ 22:54:03 27 651
EPP End 22-11-2004 @ 06:34:54 0
EPP Start 22-11-2004 @ 13:15:54 26 268
EPP End 22-11-2004 @ 20:33:42 0
EPP Start 23-11-2004 @ 19:11:49 33 388
EPP End 24-11-2004 @ 04:28:17 0
EPP Start 24-11-2004 @ 18:30:12 87 455


Eclipse period

Another important event in the near future is a fairly long eclipse
caused by the satellite crossing the Moon's umbra for 1 hour and 50
minutes on 22 November starting at 08:52 UTC. The spacecraft has proved,
during the transfer orbit, to be able to withstand even longer eclipses.
However, since the batteries have not been discharged for that
long since many months, the ESOC flight control team and the ESTEC and
industry specialists will closely follow the event.


Orbital/Trajectory information

The following osculating orbital elements of the first orbit have been
determined by a tracking campaign:

EPOCH (UTC) 2004/11/15 17:47:38.7

Elements WRT Moon and its equator of date

Pericentre Distance (km) 6 704.286533
Apocentre Distance (km) 53 207.630341
Semi Major Axis (km) 29 955.958437
Eccentricity 0.776195
Inclination (deg) 81.077151
Asc. Node (deg) 246.524941
Arg. of Pericentre (deg) 308.011816
True Anomaly (deg) 0.000000
Osc. Orbital Period (h) 129.234981


[Graphic]
SMART-1 first lunar orbit, both predicted (green line) and measured
(blue line)

These elements are extremely close to the calculated elements of the
planned orbit (see Status Report No. 31). This accurate prediction
is evidence of both the good performance of the electric propulsion
engine (about 1% above nominal) and the accuracy of the trajectory
calculation by the flight dynamics team. In the diagram the predicted
and measured osculating first lunar orbits are plotted. The accuracy is
so good that the two lines coincide (the difference is smaller than the
thickness of the lines).

One interesting point to note is that although the osculating orbital
period is 129 hours, the actual orbit was only 89 hours. This
illustrates the significant impact that the engine burns have on the
orbit and marks the meaning of the osculating orbit, which is the orbit
that would be travelled by the spacecraft if at that instant all
perturbations, including EP thrust, would cease.