SMART-1 Mission Operations Until Moon Impact

http://sci.esa.int/science-e/www/obj...objectid=39395

SMART-1 Mission Operations Until Moon Impact
European Space Agency
22 Jun 2006

Report for Period 14 June to 3 September 2006

This is the final operational report for the SMART-1 mission and covers
all the activities up to the end of the mission and a controlled impact
with the lunar surface.

SMART-1 was launched on 27 September 2003 and took 14 months to reach
the Moon using a Solar Electric Propulsion system. Lunar orbit was
achieved on 15 November 2004 and since then a number of orbit
optimisations have been carried to maximise the science return from the
mission.

If the current orbit is allowed to continue without further correction
the spacecraft will impact the lunar far side away from ground contact
and visible observations. To allow scientists to observe the impact a
series of perilune rasing manoeuvres are required to shift the impact
location to the lunar near side and a date of 3 September 2006. In
total
a delta-velocity (?v) of ~12 ms-1 is required to achieve this result.

graphic

Plot of the spacecraft-to-Moon-centre distance from capture to impact.
The current reboost phase is marked in blue.

Current Spacecraft Status

All SMART-1 subsystems are nominal and are expected to continue
functioning properly until the Moon impact. There will be operational
thermal constraints for both the spacecraft platform and payloads at
low
Moon altitudes.

Mission End Activities

The selected mission scenario for an impact with the lunar surface on 3
September 2006 will require special manoeuvres with the attitude
hydrazine subsystem to raise the orbit by around 90 km. It is no longer
possible to reboost the orbit using the Electric Propulsion System
since
all Xenon reserves were used in the previous reboost phase. The
adjustments will be made so as to maximize the scientific and
observational requirements.

The initial state prior to perilune raising manoeuvres, on 19 June
2006:

* Radius of perilune: 2016 km (Moon radius = 1738 km)
* Radius of apolune: 4820 km
* Inclination: 90.7??
* Right ascension of ascending node: 239.4??
* Argument of perilune: 226.3??
* Xenon left: 0.260 kg (0.060 kg usable with special operations =
1.6 m s-1)
* Hydrazine left: 5.8 kg (out of 8.09 kg at launch)

The current plan for the perilune raising manoeuvres comprises:

* 74 revolutions with about 3 hours of intermittent thrust centred
at apolune
* Thrust direction along the velocity with some in-plane tilt when
away from apolune
* Slew duration to re-orient X-axis = 15 minutes; rate = 0.22??
s-1
* Total ?V: 11.8 m s-1
* Total hydrazine usage: 2.5 kg
* Total perilune raising: 90 km
* Total duration: 16 days + 3 days (halt for orbit determination
and
?V calibration)
* Start date: 19 June 2006 05:23:44 UTC
* End date: 7 July 2006
* Slots for trim: 26-28 July 2006, 25 August 2006 and 1 September
2006
* Current impact time prediction: 3 September 2006 01:26 UTC

AOCS
In the next three months until impact, AOCS operations will be nominal
except for the series of ?V manoeuvres that will be executed using
the
hydrazine subsystem in order to raise the perilune altitude.

graphic

Illustration of part of the reboost strategy: series of reaction wheel
offloadings that transfer spacecraft angular momentum between the
spacecraft +Y and -Y axis, generating a small ?V in the direction of
flight (to the right).

Payload
The only constraint on payload operations in SMART-1's final orbits is
a
routine one associated with the need to download the data generated
onboard the spacecraft. This requires that the MGA be pointed at the
Earth - which prohibits any payload specific pointing at that time.
Payload operations will also be suspended during the pericentre raising
?V manoeuves scheduled for June/July.

Thermal
The operations of the thermal subsystem will continue as normal until
impact and there are no constrains or special operations planned.



Timeline of Events

Date Activity
May Manoeuvres preparation
13 June Remove solar array 35?? offset
Mid June SMART-1 perilune height 300 km; platform constraints
19 June Start of perilune raising manoeuvres; payload activities
stopped
5 July SMART-1 Ground Operations Automation System (GOAS) real
time
demonstration at ESOC
7 July End of manoeuvres
8 July Resume payload operations
~20 July SMART-1 perilune height 200 km; platform and payload
constraints
26-28 July Trim manoeuvres to adjust time of Moon impact
Mid August SMART-1 perilune height 100 km; platform and payload
constraints
1 September Last trim manoeuvre
3 September Real time Moon impact;
World observatories coordination from ESOC
Last observations
5 September SMART-1 press conference at ESOC

Orbital Information

The interactive diagrams below show SMART-1's orbit evolution from 19
June to 3 September, plus details on the projected final orbit. In
addition details for the adjacent two orbits of the nominal impact
orbit
are given.

Around impact, the perilune distance reduces by 1.2 km per
revolution. Clementine LIDAR (Laser Image Detection And Ranging) data
are used for the Moon topography. The accuracy of this data ranges from
100 m at the measurement points to 2 km in-between the Clementine
ground
tracks.