ESA Science & Technology - No. 137 - Start of navigation campaign

During the reporting period, mission operations have been conducted with support of the ESA New Norcia (NNO) ground station. Additional tracking passes were performed with the NASA DSN ground stations in Canberra (DSS-34, DSS-45) and Madrid (DSS-65) as part of the navigation campaign for asteroid Lutetia:

At the end of the reporting period (DoY 155) Rosetta was at 346.24 million km from Earth (2.31 AU) and the one-way signal travel time was 1155 seconds. The distance to the Sun was 364.8 million km (2.43 AU).

Optical Navigation Campaign

Rosetta will fly by asteroid (21) Lutetia on 10 July 2010 at a distance of less than 3200 km from the asteroid. In addition to the spacecraft tracking and usage of radiometric data for navigation purposes, in the final weeks leading up to this flyby Rosetta's on-board cameras visually track the asteroid in an optical navigation campaign. The optical observations will be fed into the orbit determination process to refine Lutetia's predicted location and to optimise Rosetta's trajectory for the upcoming flyby.

The three cameras that are used for these observations are Rosetta's two navigation cameras (NAVCAM A and B) and the narrow angle camera (NAC) of the OSIRIS instrument.

Figure 1. NAVCAM A image of asteroid (21) Lutetia

A series of time slots have been allocated for the optical navigation campaign and the first observation slot was completed in this reporting period, on 31 May, DOY 151. 
 
One of the first images of Lutetia taken by NAVCAM A on 31 May is seen here on the right. Lutetia was at a distance of about 53 million kilometres from Rosetta when the observation was made.

The OSIRIS camera was not yet switched ON during the first observation slot on 31 May. This was due to too low temperatures on the instrument's electronics box. The spacecraft attitude was adjusted to allow the components to warm up by increasing the Sun's illumination on that side of the spacecraft. The camera was subsequently switched ON on 2 June (DOY 153) and is now ready for use during the coming slots of the optical navigation campaign.

The optical navigation campaign continues with all three cameras, initially with two observation slots per week, followed by daily observation slots starting late June up to the end of the campaign.

Navigation status

Information on Rosetta's current trajectory comes from telemetry transmitted by Rosetta and from data received from the ground stations that are tracking the spacecraft regularly. From this data the spacecraft projected trajectory is calculated up to the time of the flyby. The orbit of asteroid Lutetia and its predicted position on 10 July are derived from a long record of ground-based astrometric observations of the asteroid, plus the new input from Rosetta's optical navigation campaign.

Five slots for possible trajectory correction manoeuvres (TCM), should they be required, are reserved at 3 weeks, 1 week, 3 days, 40 hours, and 12 hours before the closest approach to Lutetia.

Below are two plots showing the predicted situation for 10 July in the target plane of the Rosetta - Lutetia flyby, based on all data up to 27 May (before the start of the optical navigation). The closest approach to the asteroid is currently expected on 10 July 2010 at 15:45:59.02 UTC.

Figure 2 & 3. Plot of the projected position of Rosetta (centre of error ellipses) relative to asteroid Lutetia (brown, at the origin) in the target plane of the flyby, based on all available radiometric data for the spacecraft up to and including 27 May 2010 (prior to the start of the optical navigation campaign). The plot on the right is a zoom on the target region from the plot on the left.

The target point for Rosetta in these plots is indicated in green. The red curve corresponds to a miss distance of 3160 km from Lutetia. This is the minimum allowed flyby distance for Rosetta. If the spacecraft would pass closer to Lutetia the asteroid would fill up the entire field of view of Rosetta's cameras during closest approach. This would impact the spacecraft's attitude control (the images guide the attitude control system) and also the scientific observations which are prepared to be performed from within a pre-set distance from the asteroid.

Spacecraft

Payload

ALICE
The instrument is currently OFF.

CONSERT
The instrument is currently OFF.

COSIMA
The instrument is currently OFF.

GIADA
The instrument is currently OFF.

MIDAS
The instrument is currently OFF.

MIRO
The instrument is currently OFF.

OSIRIS
Since DOY 153 the instrument is ON. The instrument will now be regularly used to take images of asteroid Lutetia as part of the optical navigation campaign.

ROSINA
The instrument is currently OFF.

RPC
The instrument is currently OFF.

RSI
The ultra-stable oscillator (USO) is ON/muted since DOY 032/2010.

VIRTIS
The instrument is currently OFF.

LANDER Philae
The instrument is currently OFF.

SREM
The instrument is ON with standard settings since DOY 032/2010.

Future Milestones

Rosetta is now fully configured for the navigation campaign towards asteroid (21) Lutetia. The closest approach to the asteroid is currently expected on 10 July 2010 at 15:45:59.02 UTC.

The navigation cameras and OSIRIS narrow angle camera on-board Rosetta will take images of the asteroid twice a week, in preparation for an eventual trajectory correction manoeuvre on 18 June 2010, three weeks before closest approach (CA - 3 weeks). Later in the navigation campaign images will be taken daily, with four additional slots available for trajectory correction manoeuvres should they be required.

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Legal disclaimer
This report is based on the ESOC mission operations report, WOR #137. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.