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SMART-1 completes its first orbit around the Moon

Status Report From: European Space Agency
Posted: Sunday, November 21, 2004

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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


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.

// end //

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