ESA Science & Technology - Lunar Orbit

The table below shows SMART-1's orbital elements with respect to the Moon and its equator for two dates: the first lunar orbit after capture on 15 November 2004 and the first operational orbit on 28 February 2005. The final operational orbit was an elliptical polar orbit with a period of nearly 5 hours.

Note: SMART-1's altitude at perilune and apolune for this first operational orbit were respectively 471 kilometres and 2880 kilometres. The perilune passage occured in the southern hemisphere at a latitude of 74° south.

Once in operational orbit, the Electric Propulsion (EP) system was no longer used and SMART-1 followed a natural evolving orbit around the Moon. 

The orbital elements varied during the operational period under the effect of the Earth gravity perturbations. The perilune slowly decreased to about 300 km and the apolune increased to about 3000 km, while the argument of perilune drifted around the south pole.

First Re-boost Phase

On 2 August 2005 at 10:23 UT the EP was restarted to reboost the spacecraft orbit. In order to extend the orbital lifetime by 1 year, the EP was fired repeatedly from 2 August to 17 September 2005. The firing strategy consisted of 2 pulses of 68 minutes each per orbit, one on the ascending arc and one on the descending arc. The period covered over 200 orbits (of ~5h each).

After the re-boost phase, SMART-1 was left in a natural orbit determined by lunar gravity and perturbations caused by gravitational influence of the Earth and Sun.

End of Ion Engine Operations

Virtually all the remaining xenon was used during the re-boost phase. The end of the re-boost phase therefore also marked the end of the electric propulsion system's active live. The EP was fired for the last time on 17 September 2005 at 18:45 UTC.

Second Re-boost Phase

Without any changes to its orbit, SMART-1 would impact the Moon on 17 August 2006. To optimise scientific return, however, this date was extended through a second reboost phase to 3 September 2006. To achieve this, the perilune distance was increased by a series of raising manoeuvres from 19 June to 2 July 2006.

As the ion engine was no longer availble for providing thrust, another method was adopted to increase the spacecraft's velocity and boost the orbit. The orbit perilune was raised by about 90 km using only the attitude thrusters of the AOCS system in combination with a series of reaction wheel offloadings to provide a deltaV in the direction of SMART-1's motion.

The accumulative effect of the small imparted deltaV's over the two week period - covering 64 lunar orbits - resulted in the desired total acceleration, and SMART-1's orbit was pushed outward. After the re-boost phase, the natural evolution of the orbit causes the spacecraft to spiral in again toward the lunar surface, but it is now set to impact the Moon at a favourable time and location - observable from the Earth.

Status Reports Coverage

The period from the first lunar orbit up to the end of the second re-boost phase is covered in the status reports no. 35 to no. 54