Asset Publisher

No. 102 - Start of Optical Navigation Campaign

No. 102 - Start of Optical Navigation Campaign

Report for Period 3 August to 15 August 2008With the end of the payload check-out 8 Rosetta entered the asteroid Steins fly-by navigation campaign. During the two-week reporting period, operations have been characterised by asteroid observation sessions with the navigation cameras (NAVCAMs) and OSIRIS.

During the reporting period there were four slots for asteroid observations with the spacecraft's cameras on 4, 7, 11 and 14 August. Images acquired during these sessions have been used for the first time for navigation purposes. The accuracy and the consistency of the measurements exceeded early expectations and allowed them to be used already for the execution of the first trajectory correction manoeuvre (TCM) on 14 August.

A total of four New Norcia (NNO) and eight DSN station (DSS) passes were taken during this period:

DOY Date Pass Main Activity
217 04/08/08 NNO 1620 Navigation slot 1
220 07/08/08 NNO 1623 Navigation slot 2
221 08/08/08 DSS-54 1625 Monitoring
222 09/08/08 DSS-54 1626 Monitoring
223 10/08/08 DSS-54 1627 Monitoring
224 11/08/08 NNO 1627
DSS-65 1628
Navigation slot 3
225 12/08/08 DSS-65 1629

Monitoring

226 13/08/08 DSS-54 1630 Monitoring
227 14/08/08 NNO 1630
DSS-55 1631
Navigation slot 4 and closest approach -3 weeks TCM
228 15/08/08 DSS-55 1632 Monitoring

At the end of the reporting period (DoY 228) Rosetta was at 314.5 million km from Earth (2.1 AU) and the one-way signal travel time was 1049 seconds. The distance to the Sun was 310.3 million km (2.07 AU). Asteroid Steins was at about 15.7 million km from Rosetta.

First Trajectory Control Manoeuvre (CA-3 weeks)

A series of trajectory control manoeuvres are performed leading up to the Steins fly-by in September. The first of these was successfully executed on DOY 227, three weeks before the closest approach (CA) to asteroid Steins:

Closest Approach-3 weeks TCM
Date

14 August 2008

Start of burn

11:42 UT

Length of burn

113 s

Fuel consumption

142.28 g

delta-V

12.8 (cms-1)

CA distance before TCM

554.2 km

CA distance after TCM

792.4 km

The delta-V of the TCM has adjusted Rosetta's trajectory such that the distance of the spacecraft relative to Steins at closest approach shifted from 554.2 km to 792.4 km (Figure 5). This is now very close to the target distance of 800 km, which is set by the fly-by conditions to be compatible with spacecraft performance.

Several more slots are set between now and 4 September for possible future manoeuvres, should they be required. For more details on the manoeuvres see also: "Rosetta fine-tunes its approach to asteroid Steins," linked from the right-hand side navigation.

Optical Navigation Campaign

To help determine the required delta-V of the TCM, images of the asteroid gathered by both the two navigation cameras (NAVCAM A & B) and the OSIRIS instrument, during the navigation slots in this reporting period, were used.

Figure 1. NAVCAM A image of asteroid Steins, 4 August 2008

Figure 2. OSIRIS images of asteroid Steins, 7 and 11 August

The optical data from the 4, 7, 11 and 14 August navigation slots have resulted in a progressively better determination of the asteroid's location and the relative spacecraft position in the target plane of the flyby.

The post fit statistics of the data from all the acquired images is listed in the table below, separately for the right ascension and declination determination. Stated for the three cameras are: the total number of images obtained on these four days, the mean residual of the data to the last solution, and both the root mean square and standard deviation of the residuals.

Optical Navigation post fit statistics
(residuals in millidegrees relative to solution)

Right Ascension

camera

No.

mean root mean sq. standard dev.
NAVCAM A

18

0.016

0.241

0.241

NAVCAM B

24

-0.187

0.346

0.291

OSIRIS

10

0.005

0.048

0.048

ALL

52

-0.080

0.275

0.263

Declination

NAVCAM A

18

0.122

0.261

0.231

NAVCAM B

24

-0.176

0.335

0.284

OSIRIS

10

0.023

0.054

0.049

ALL

52

-0.035

0.275

0.273

Graphical plot of the post fit residuals (right ascension and declination) for all images from the three instruments on the four navigation slot dates:

Figure 3. Post fit residuals of the optical data on 4, 7, 11 & 14 August for the three cameras (red/green: NAVCAM A/B, blue: OSIRIS)

The data from the optical navigation campaign are, in an ESA first, fed into the orbit determination of the Rosetta spacecraft and used to fine-tune its trajectory towards the asteroid. The plots below show the situation in the target plane of the 5 September fly-by, with the relative positions of Rosetta and asteroid Steins. 

Figure 4 & 5. Evolution of the estimated position of Rosetta (centre of error ellipses) relative to asteroid Steins (red, at origin in upper left corners) in the target plane of the fly-by, based on all available data up to and including 11 August (left) and 14 August (right) 2008.
The shift in the right image of Rosetta's estimated position toward the target position (green) is due to the TCM performed on 14 August.

Spacecraft

Payload
All instruments are currently OFF with the exception of OSIRIS which is permanently ON and is used to acquire images of asteroid Steins for the navigation campaign.

ALICE
The instrument is OFF.

CONSERT
The instrument is OFF.

COSIMA
The instrument is OFF.

GIADA
The instrument is OFF.

MIDAS
The instrument is OFF.

MIRO
The instrument is OFF.

OSIRIS
The instrument is permanently ON and is used to acquire images of asteroid Steins and the background stars. Operations of the instrument proceed flawlessly.

ROSINA
The instrument is OFF.

RPC
The instrument is OFF.

RSI
The instrument is muted.

VIRTIS
The instrument is OFF.

Lander Philae
The instrument is OFF.

SREM
Since DoY 184/2008 the accumulation settings are configured for active cruise mode.

Future Milestones

The cruise phase continues towards asteroid Steins which will be encountered early September 2008. Spacecraft activities continue to be focused on the operations required for the optical navigation campaign (until 4 September), with the spacecraft now at ~15.7 million km from the asteroid. Both the navigation cameras (NAVCAMs) and the OSIRIS instrument will continue taking images to feed the orbit determination process.

In week 34 (18-24 August) the first scientific observation will take place with the Steins light curve characterisation by OSIRIS.

Trajectory correction manoeuvres will be executed as required by the fly-by operations.

---
Legal disclaimer
This report is based on the ESOC mission operations report, WOR #102. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.

Last Update: 1 September 2019
13-Aug-2020 12:27 UT

ShortUrl Portlet

Shortcut URL

https://sci.esa.int/s/Wvq4pXw

Related Publications

Related Links

Documentation

Further Readings