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No. 104 - Asteroid Steins Flyby

No. 104 - Asteroid Steins Flyby

Report for Period 30 August to 5 September 2008The reporting period was characterised by the final navigation of Rosetta towards asteroid Steins, culminating in the successful flyby on the evening of 5 September 2008 at 18:38 UTC (the exact time of closest approach to be confirmed after detailed analysis of telemetry data). The navigation was completed with a trajectory control manoeuvre (TCM) executed at about 36 hours before the closest approach (CA).

The navigation images from the last navigation slot on 4 September 2008, after the TCM, confirmed that the spacecraft was well on track for the asteroid encounter. The fly-by was conducted successfully and as expected, with the spacecraft autonomously tracking asteroid Steins. No single error event was reported by the spacecraft in the reporting period. A detailed analysis of the spacecraft performance is being conducted with the data recorded on-board.

The bulk of the scientific operations was conducted in the hours around the encounter (see also the links in the right-hand menu, for the first images and results).

Mission operations were conducted with the support of the ESA New Norcia (NNO) and Cebreros (CEB) ground stations and with NASA DSN ground stations (DSS).

DOY Date Pass Main Activity
243 30/08/08 NNO 1646
DSS-54 1647
Navigation slot 12
244 31/08/08 NNO 1647
DSS-54 1648

Navigation slot 13

245 01/09/08 NNO 1648
DSS-25 1649
Navigation slot 14
246 02/09/08 NNO 1649
DSS-54 1650
DSS-24 1650
Navigation slot 15
Monitoring (pass deleted)
247 03/09/08 NNO 1650
DSS-24 1651
Navigation slot 16
248 04/09/08 NNO 1651
CEB 1316
TCM at CA-36 hours
Navigation slot 17
249 05/09/08 DSS-24 1652
NNO 1652
DSS-45 1652
CEB 1317

DSS-15 1653

Verification of NAVCAM tracking
Back-up uplink station
Uplink of AFM telecommands
Fly-by of asteroid Steins (CA)
Science data downlink

At the end of the reporting period (DoY 249) Rosetta was at 358.9 million km from Earth (2.39 AU) and the one-way signal travel time was 1197 seconds. The distance to the Sun was 319.2 million km (2.13 AU).

Asteroid Steins Fly-by

Figure 1. Spacecraft attitude angle rates during flip manoeuvre and asteroid Steins fly-by.

Rosetta's fly-by of asteroid Steins marks an historical moment for European space flight. For the first time ever a European spacecraft flew next to an asteroid, performed an optical navigation campaign, and autonomously tracked the asteroid by means of its on-board camera. 

Just before closest approach, Rosetta entered in asteroid fly-by mode (AFM) in which the attitude of the spacecraft is automatically driven by an on-board navigation camera. This mode was rehearsed in March this year and has continuously kept asteroid Steins in the field of view of Rosetta's imaging instruments, while the spacecraft flew past the asteroid at a relative speed of 8.6 kms-1. Figure 1 shows the angle rates for the spacecraft's three main axes between about 17:44 and 19:14 UT on 5 September 2008.

Figure 2. Schematic diagram illustrating the manoeuvres during the asteroid fly-by, with time running from left to right and the spacecraft's attitude indicated by its +Z axis (toward position of asteroid Steins - red marker) and +X axis (blue). The actual measured rotation rates about the Z axis (green) and the Y axis  (red) during the 5 September fly-by are given in Figure 1.

The following table is a simplified version of the operations timeline that characterised the fly-by, listing the main events leading up to closest approach and main events afterward until end of pointing.

4 September 2008 (DoY 248)
Time* Event/Activity
05:00  s Start TCM/reaction wheel off-loading slot (CA-36 hours)
09:00  s End TCM/reaction wheel off-loading slot (CA-36 hours)
11:00  s Start last image acquisition for navigation (slot #17)
16:00  s Last image acquisition for navigation (slot #17)
5 September 2008 (DoY 249)
06:20  g Start navigation camera (NAVCAM) asteroid tracking attempts
09:00  g Uplink of updated fly-by commands
14:20  g Final settings for NAVCAM defined
15:00  g GO decision for asteroid fly-by mode (AFM) taken
16:00  g NAVCAM A selected for AFM, final telecommands uplinked
17:58  s Start spacecraft attitude flip manoeuvre
18:18  s End spacecraft attitude flip manoeuvre
18:19  s Start asteroid closed loop tracking (autonomous tracking)
18:28  s High-gain antenna stops tracking the Earth - loss of telemetry signal
18:30  s Fault detection, isolation and recovery (FDIR) disabled on-board
18:36  s Phase angle zero crossing
18:38  s

Closest approach (CA)

19:38  s End of asteroid closed loop tracking
19:48  s Fault detection, isolation and recovery (FDIR) re-enabled on-board
19:54  s High-gain antenna resumes tracking the Earth (acquisition of telemetry signal on the ground)
23:20  g Start science data downlink
7 September 2008 (DoY 251)
14:00  s Planned end of pointing

*All times are in UT. An 's' or 'g' after the time indicates an event at the spacecraft or on ground respectively. The one-way light time was about 20 minutes for these dates.

To monitor the spacecraft status after the fly-by, the Cebreros tracking of DoY 249 was extended beyond the planned time with the antenna following the spacecraft below 10° elevation. This action allowed the operations team to confirm successful fly-by as soon as the telemetry was received at 20:14 UT (ground time).

Trajectory Control Manoeuvre (CA-36 hours)

A final trajectory control manoeuvre was performed on DoY 248, around 36 hours before closest approach to asteroid Steins:

Closest Approach-36 hours TCM

4 September 2008

Start of burn

07:26 UT

Length of burn

103.5 s

Fuel consumption

127.10 g


11.8 (cms-1)

CA distance before TCM

791.4 km

CA distance after TCM

800.7 km

The delta-V of the TCM adjusted Rosetta's trajectory such that the estimated distance of the spacecraft relative to Steins at closest approach shifted from 791.4 km to 800.7 km (see Figure 2 below). The target distance was 800 km, which is set by the fly-by conditions to be compatible with spacecraft performance.

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, as part of the optical navigation campaign, were used.

Optical Navigation Campaign

The optical navigation campaign started 4 August. Images were taken twice a week up to 25 August, from which date onward images were taken daily up to and including 4 September 2008. This reporting period covers the last six navigation slots (no. 12 / no. 17).

The post fit statistics of the data from all the images acquired by the NAVCAMs and OSIRIS is listed in the table below, separately for the right ascension and declination determination. It includes the observations from all seventeen navigation slot dates between 4 August and 4 September inclusive. (Note, only the NAVCAM images from 4 September are not yet included).

Stated for the three cameras are: the total number of images obtained on these seventeen 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



mean root mean sq. standard dev.









































Figure 3 shows a plot of the post fit residuals (right ascension and declination) for all images obtained by the three cameras on all seventeen navigation slot dates (excluding the NAVCAM images from 4 September).

Figure 4 shows the situation in the asteroid Steins fly-by target plane as it evolved between 25 August and 4 September, including the effect of the TCM on Rosetta's fly-by distance. 

Figure 3. Plot of the post fit residuals (for both right ascension and declination) for all images obtained by the two NAVCAMs (red and green) and OSIRIS (blue) on the seventeen navigation slot dates from 4 August 2008 until 4 September 2008.

Figure 4. Evolution of the estimated position of Rosetta relative to asteroid Steins in the target plane of the fly-by, based on all available data up to and including 4 September 2008.
The shift of Rosetta's estimated position toward the target position (green) is due to the TCM performed on 4 September.


The spacecraft behaved very well during the asteroid fly-by and no single parameter indicated a problem. Detailed analysis of the dynamic behaviour of the spacecraft will now be performed once all the data are available.

Most of the instruments have been activated as part of the fly-by campaign:

The instrument was active during the fly-by and operations were conducted according to the plan.

The instrument is OFF.

The instrument is OFF.

The instrument was active during the fly-by and operations were conducted according to the plan.

The instrument is OFF.

The instrument was active during the fly-by and operations were conducted according to the plan.

The instrument was extensively used as part of the navigation campaign required for the approach to Steins. The last images for this activity were taken on DoY 248 between 11:00 and 13:00 UT. These images allowed the mission control team at ESOC to assess the final targeting before the fly-by the next day.

The instrument was later operated as part of the fly-by operations. Both cameras (narrow-angle and wide-angle) have been operating during the fly-by, with the NAC autonomously switching to safe mode at about 9 minutes before closest approach. The PI team could deliver shortly after the fly-by astonishing images taken with the WAC. After the successful fly-by, the instrument remained active for the gravitational microlensing event observation.

The instrument was active during the fly-by and operations were conducted according to the plan.

The instrument was active during the fly-by and operations were conducted according to the plan.

The instrument is muted.

The instrument was active during the fly-by and operations were conducted according to the plan.

The instrument was active during the fly-by and operations were conducted according to the plan.

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

Future Milestones

The fly-by operations will continue now for a few days with gravitational microlensing observations using the OSIRIS instrument. The data downlink phase will last until the beginning of October, interleaved with OSIRIS science observations. The spacecraft will then continue its 4th orbit around the Sun reaching aphelion on 17 December 2008 at a distance of 2.26 AU to head back to the Earth for the mission's third and last Earth swing-by on 13 November 2009.

Legal disclaimer
This report is based on the ESOC mission operations report, WOR #104. 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 11:03 UT

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