- Email: [email protected]
The first Rosetta asteroid flyby
Acta Astronautica 66 (2010) 382 -- 390
Contents lists available at ScienceDirect
Acta Astronautica journal homepage: w w w . e l s e v i e r . c o m / l o c a t e / a c t a a s t r o
The first Rosetta asteroid flyby Andrea Accomazzoa, ∗ , Paolo Ferria , Sylvain Lodiota , Armelle Hubaultb , Roberto Portab , Josè-Luis Pellon-Bailona a b
European Space Agency/European Space Operations Center, Darmstadt, Germany Vega Gmbh, Darmstadt, Germany
A R T I C L E
I N F O
Article history: Received 23 January 2009 Received in revised form 5 April 2009 Accepted 12 June 2009 Available online 28 July 2009 Keywords: Rosetta Asteroid Flyby Interplanetary Comet Šteins Lutetia Churyumov-Gerasimenko European Space Agency (ESA) ESOC Optical navigation
A B S T R A C T
The International Rosetta Mission, a cornerstone mission of the European Space Agency Scientific Programme, was launched on 2 March 2004 on its 10 years journey towards a rendezvous with comet Churyumov-Gerasimenko. Once reached the comet nucleus in summer 2014, Rosetta will orbit it for about 1.5 years down to distances of a few kilometres and deliver a lander onto its surface. In the long cruise to its target, Rosetta performs four gravity assist manoeuvres, three times with Earth and once with Mars. During this flight Rosetta will cross twice the main asteroid belt, and in both occasions its trajectory is designed and controlled to flyby an asteroid at close distance. On 5 September 2008 the first of the two asteroid flybys has been conducted, with the spacecraft approaching asteroid 2867-Šteins at a minimum distance of about 800 km, and a relative velocity of 8.6 km/s. Šteins is an E-type asteroid with a diameter of about 5 km. This paper continues the period reporting of the operational experience from the Rosetta mission operations. Preparation and execution of the flyby activities are described including the first experience in optical navigation that will prove of utmost importance for the next asteroid flyby and for the comet approach phase. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Rosetta is a cornerstone mission of the European Space Agency [1], launched on 2 March 2004 on an Ariane 5G rocket. Its main scientific objective is to rendezvous with the nucleus of comet Churyumov-Gerasimenko in 2014, to orbit it for about 18 months and to deliver onto the nucleus' surface a small lander, called Philae. During the long cruise to its target Rosetta has to undergo several planet swing-bys (three times with the Earth in 2005, 2007 and 2009, and once with Mars in 2007). In addition, the mission takes advantage
∗ Corresponding author. Tel.: +49 6151 902707. E-mail addresses: [email protected] (A. Accomazzo), [email protected] (P. Ferri), [email protected] (S. Lodiot), [email protected] (A. Hubault), [email protected] (R. Porta), [email protected] (J.-L. Pellon-Bailon). 0094-5765/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2009.06.013
of the trajectory crossing twice the main asteroid belt to perform two asteroid flybys (Šteins in 2008 and Lutetia in 2010) [2,3]. After the first intense year of post-launch operations, involving long periods of in-flight commissioning of the spacecraft and the scientific payload [4], the first Earth swing-by was carried out successfully in March 2005. Previously unplanned scientific operations were also carried out in this period, the most important of which was the observation campaign in support of the encounter of NASA's Deep Impact probe with comet Tempel-1, in July 2005 [5]. As from August 2005 the so-called quiet cruise started. In this period the spacecraft was kept as much as possible in a low activity mode, the ground contact minimised, with a typical frequency of once per week, and any special operation need minimised or postponed. The purpose of this is to preserve lifetime of several on-board units and to reduce the overall operations cost for the long cruise. The cruise was
A. Accomazzo et al. / Acta Astronautica 66 (2010) 382 -- 390
not as quiet as it was originally anticipated, due to various planned activities like payload checkouts and trajectory control manoeuvres, but also due to some unexpected events like solar flares and spacecraft anomalies that kept the flight control team very busy [6,7]. In 2007 two swing-bys [8], one highly critical at Mars in February at a target altitude over the planet's surface of only 250 km and a second Earth swing-by in November, put the spacecraft onto its fourth revolution round the Sun. For the first time Rosetta is flying at distances above 2 AU from the Sun and in September 2008 crossed the asteroid belt with a close encounter with asteroid 2867-Šteins. The original Rosetta mission, due to be launched in 2003, had been planned with close encounters at asteroids Otawara and Siwa. The redesign of the mission, with launch in 2004, due to an Ariane5 ECA failure one month before the foreseen launch date, did not allow for asteroid flybys due to propellant budget limitations. Shortly after launch, when it was clear that the propellant allocated for the launcher injection dispersion was not necessary, the interplanetary trajectory was slightly modified to allow close encounters with asteroid Šteins in 2008 and asteroid Lutetia in 2010. These scenarios have been carefully analysed and defined in collaboration with the spacecraft manufacturers. The scientific community of the Rosetta mission has given a lot of importance to the flyby of asteroids and set demanding requirements for the operation itself, well beyond the scenarios originally analysed. Some of the requirements are conflicting with spacecraft constraints, pushing to the limit the spacecraft capabilities and the navigation accuracy well beyond the original baseline; nevertheless the mission operations teams have defined a new flyby scenario that is satisfying all the requirements and at the same time not violating the spacecraft constraints. The spacecraft has been designed to perform asteroid flybys; it is thus equipped with navigation cameras (needed in any case for precise navigation in the proximity of the comet) able to track the asteroid and its attitude control system is able to use this information to autonomously steer the spacecraft attitude to track the asteroid. This complex operational mode required a significant validation effort onground and in-flight with dedicated tests. After a tracking test of the Moon performed in March 2005 at the time of the first Earth swing-by, Šteins flyby operations, in view of the newly defined scenario, have been rehearsed in March 2008 in preparation of the event in September. The following sections describe how the definition, preparation, and execution of the complex asteroid flyby activities have been addressed from an operational point of view, highlighting how they will prove of fundamental importance also for the rest of the mission to the comet.
2. Evolution of the flyby strategy 2.1. Baseline scenario As soon as the two target asteroids were selected by the scientific community among 15 different combinations, the spacecraft manufactures studied and analysed the flyby scenarios.
383
In the case of asteroid Šteins the following parameters characterise the scenario: • • • • •
relative velocity 8.6 km/s Sun/spacecraft/asteroid angle at approach 141.5◦ Earth/spacecraft/asteroid angle at approach 164.2◦ Sun distance 2.14 AU Earth distance 2.41 AU.
The spacecraft design is such that the remote sensing instruments have their line of sight aligned with the spacecraft Z axis (refer to Fig. 1 for spacecraft reference frame definition). The side walls (+Y and −Y faces) host the radiators and so does the −X face; these faces have therefore significant restrictions in terms of Sun exposure. The launch vehicle adapter (LVA) is installed on the −Z face, thus making also this face very sensitive to Sun exposure. The only safe side of the spacecraft for what concerns Sun exposure is therefore the +X face where the high gain antenna (HGA) is installed. The case studied by the spacecraft manufacturer included several constraints, such as: • limitation of the excursion of the solar arrays in the regions defined as “cold” from a spacecraft thermal point of view (i.e. Sun in the −Z/+X and −Z/−X quadrants). This was done in order to mitigate the effects of having a solar array stuck in such a position • selection of the flyby distance to replicate the angular rate and acceleration profile of the future Lutetia asteroid flyby, considered scientifically more interesting. This was done to make the Šteins flyby an in-flight test of the Lutetia case. The flyby scenario with these constraints was then limited to tracking the object from few hours to circa 3 min before the closest approach (CA) (see Fig. 2) when the Sun would have moved in the “undesired” region. The proposed strategy provided good illumination conditions and observations at phase angle zero, however, with a large minimum observation distance and no observation possible at closest approach and beyond. 2.2. Scientific requirements This scenario was judged of limited scientific value by the scientists of the Rosetta community which in turn reiterated their requirements for the flyby as follows: • • • • •
good illumination conditions observation at phase angle zero closest possible distance observation at closest approach (i.e. range rate zero) continuous observation from before to after closest approach • “good” pointing performance • “good” synchronisation of payload operations with flight events. This request raised several problems to the spacecraft operators since it would mean • pushing the spacecraft to its performance limits in terms of attitude dynamics (e.g. reaction wheels torques, rotational speed of appendages, etc.)
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Fig. 1. Spacecraft reference frame.
Fig. 2. Original Šteins flyby geometry.
• violating thermal constraints (i.e. exposure of “cold” faces to the Sun) • increasing the accuracy required for the optical navigation (i.e. impact of positional errors on attitude accuracy increased) • introducing demanding constraints for what concerns the planning of the operations (i.e. need for late orbit determination and correction manoeuvres, last minute commands updates, etc.).
3. Improved flyby scenario In reply to the request of the scientists the operations team at the European Space Operations Center (ESOC) designed a new scenario for the flyby itself that could satisfy most of, if not all, the requirements. The fundamental change of the new scenario proposed by ESOC was the introduction of a so called “attitude flip manoeuvre” shortly before the closest approach, in order to
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Fig. 3. “Inverted” Šteins flyby geometry.
minimise the time in which the `cold” faces of the spacecraft are exposed to Sun illumination. This strategy, called “inverted” because the spacecraft attitude is inverted during the flyby with respect to the Sun, gave the possibility for the spacecraft to track the asteroid from well before the closest approach till a few hours after. Fig. 3 shows how with a 180◦ rotation around the +Z axis (pointed to the asteroid) before the closest approach the spacecraft inverts the side exposed to the Sun. In this way the spacecraft keeps tracking the asteroid with the Sun that illuminates the “cold” face between the flip manoeuvre and the “phase angle zero” point. In order to respect the thermal constraints and at the same time guarantee a robust transition to autonomous tracking phase the flip manoeuvre has been placed between 40 and 20 min before closest approach so that the “cold” −X face is exposed to the Sun for a very limited period and with shallow angle. When the spacecraft is in asteroid flyby mode (AFM) the guidance for the attitude control system (AOCS) is provided by the measurements provided by the on-board camera that is tracking the asteroid. This was planned to start several hours before the flyby; however, the spacecraft design does not allow performing the “flip manoeuvre” in AFM, meaning that this mode can only be activated after the manoeuvre itself and with a pre-defined steering guidance before that. Due to the small pointing error allowed in the transition to AFM, the accuracy of orbit determination had to be higher than planned to allow a smooth transition between preprogrammed and autonomous guidance; for the same reason the operations timeline had to include a late opportunity to update attitude parameters. Following the scientists request to fly as close as possible to the asteroid, the minimum flyby distance was also analysed by the operations teams against the spacecraft capabilities (mainly driven by the maximum angular acceleration of the spacecraft that could be sustained by the reaction wheels). The result was a target flyby distance of 800 km. This scenario was immediately accepted by the scientific community since it satisfied most of the requirements and became the baseline for the development of the operational timeline. 4. Asteroid flyby implementation 4.1. Overall definition and validation Since the adopted flyby scenario significantly deviates from the one originally designed and validated by the spacecraft manufactures before launch, the operations teams had
to prepare an implementation plan that could achieve a similar level of confidence. The flight dynamics team at ESOC performed a set of analyses, tests, and partial simulations in order to consolidate the overall scenario and its features. The flight control team further processed these data to define the spacecraft activities to execute the full operation. Once the overall timeline for activities was consolidated a validation plan was defined including: • simulations with the ESOC software simulator emulating the AOCS software • simulations with the software simulator at avionics manufacturer premises • in-flight test replicating dynamics and Sun exposure conditions. The validation plan required a non-negligible effort by the various parties but it provided the necessary confidence on the robustness and safety of the scenario. The in-flight test was performed on 24 March 2008 when the position of Rosetta relative to the Earth and the Sun was the same as the one of the actual flyby. The test started shortly before the flip manoeuvre, replicating the closest approach dynamics, and ending 1 h later. The test was also used to check and confirm the tracking performance of the camera and the star tracker with the Sun approaching or violating the limit defined for nominal performance. In both cases the on-board units performed nominally. Simulations performed at the manufacturer premises with the test bed used to validate the AOCS subsystem during the design phase also confirmed that the proposed scenario could be flown as planned and gave the final confirmation to go ahead for the detailed implementation of the operation.
4.2. Payload check-out On request of the scientists regular payload check-out phases are scheduled during the mission, the major ones linked to operational events like planetary swing-bys and asteroid flybys. In the case of the Šteins flyby a check-out campaign of four weeks has been conducted in July 2008. This phase included a significant amount of check-out, calibration, on-board SW maintenance, and interference testing activities that kept the team under a constant workload for several weeks. The planning, with the involvement of the Rosetta Science Operations Center (RSOC) located in Spain,
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Table 1 Overall timeline. Week 32
Week 33
Week 34
Week 35
Week 36
Start of optical navigation campaign
Bi-weekly optical navigation imaging TCM at-3 week
Bi-weekly optical navigation imaging Characterisation of Šteins light curve by OSIRIS instrument
Daily navigation imaging
Daily navigation imaging
TCM at -1 week
TCM at 3 days, -36 h, -12 h
Flyby with scientific operations
Fig. 4. Post-fit angular residuals of optical data.
proved to be extremely robust and efficient allowing all instrument teams to complete their planned activities in time. At the end of the campaign all relevant instruments confirmed their readiness to participate in the Šteins flyby operations. 4.3. Navigation campaign As soon as the payload check-out phase was over, the operations plan focused on the navigation campaign that started on 4 August. Table 1 gives an overview of the timeline of the five weeks preceding the flyby. For the first time in the mission the traditional radiometric navigation was complemented with optical data. Images were taken twice a week for the first three weeks of the campaign; the frequency of image taking was increased to daily in the last two weeks before the flyby. The orbit determination process utilised images coming from the on-board navigation camera and from the more powerful Narrow Angle Camera (NAC) of the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS). This required a detailed definition of the interface between the ESOC operations team and the OSIRIS scientific team. The interface proved to work extremely well and became an operational validation of the same type of activities planned for the early comet detection and approach phase,
planned for 2014. The OSIRIS camera allowed a clear early detection of the asteroid, which was however also, unexpectedly, visible by the navigation cameras at a distance of ca. 26 million km. Fig. 4 reports the deviations (in terms of declination and right ascension) between the predicted and the measured asteroid position when derived from the images of the two navigation cameras and the OSIRIS camera. The blue points, typically very close to zero, clearly show the improvement in performance the OSIRIS camera can provide to the navigation process when compared to the spacecraft navigation camera and the increasing knowledge of the orbit solution over time. Fig. 5 shows the evolution of the orbit determination (projected in the B-plane) and the associated uncertainty ellipse as computed by the ESOC navigation team after the first week of navigation: • the black dashed line is the solution using radiometric data only (this is affected by the error in the knowledge of the asteroid position) • the orange dashed line is the solution obtained including the images of the navigation camera of the 4th of August (here the asteroid actual position starts being taken into account)
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Fig. 5. Rosetta targeting 11 August 2008.
• the magenta dashed line is the solution obtained with all the data available on the 7th of August, i.e. including the first images from OSIRIS (the significant performance improvement can be easily seen) • the blue dashed line is the solution obtained with the data up to 11 August.
Further analyses also showed that, in the absence of the optical data, the prediction would have been 137.0 km from the target corresponding to a miss-distance of 916.5 km from Šteins and a minimum phase angle of 3.00◦ .
4.4. Operations timeline and results The consistency of the measurements gave the operations team the necessary confidence to use them already for the first trajectory correction manoeuvre executed on 14 August. Fig. 6 reports the sequence of the orbit solutions in the days preceding the flyby and the correction manoeuvre executed 36 h before the closest approach. The last orbit solution, estimated to be less than 2 km away from the B-plane aim point, is the small violet ellipse obtained with optical data of 4 September. Orbit reconstruction was conducted during the post flyby phase with the help of the on-board attitude estimation data and the scientific images acquired by the OSIRIS instrument that, given the reduced distance to the asteroid, could give valuable attitude information for the minutes around the closest approach. Fig. 7 shows a discrepancy between the pre- and post-flyby estimated position. The final navigation result was 6.6 km from the target and corresponded to a miss-distance of 802.6 ± 0.2 km (3-sigma) from Šteins. The distinct separation between the orange and black error ellipses means that the predicted and final results are inconsistent with each other. This is most likely due to systematic errors in the direction measurements derived from the optical data during approach, particularly those at the end of the approach phase since they have the highest information content.
In addition to the activities required for the navigation campaign, the flyby preparation activities were characterised by an extensive planning process by the flight control team. For the first time in the mission the detailed scheduling of spacecraft commands of a critical phase had to be performed in an incremental way, whereas previous operations could always be scheduled in detail well before the operation. Following the incremental approach of commands availability, the scheduling process of the last 3 days included several sessions of commands generation and uplink windows to the spacecraft. The operations timeline also included a dense set of activities planned for the last days: • three slots for trajectory correction manoeuvres (TCM at CA-3days, CA-36 h, CA-12 h plus two at -3 and -1 weeks) with late availability of commands (typically few hours before each manoeuvre); • availability of preliminary asteroid flyby commands at CA3days • availability of the final asteroid flyby commands at CA12 h, i.e. following the last orbit determination done with images taken between CA-30 and CA-24 h • final confirmation to go for closed loop tracking of the asteroid between CA-7 and CA-2 h (the fall-back solution being a flyby based on a fully pre-defined attitude profile).
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Fig. 6. Last Rosetta targeting before flyby.
Fig. 7. Reconstructed Rosetta targeting.
The bulk of the interactive activities took place during ground contacts via the ESA New Norcia station, with a significant contribution from the NASA DSN Madrid station in the weeks preceding the flyby. In the last day before closest approach the available network included the ESA Cebreros station and the NASA DSN Canberra and Goldstone stations,
resulting in a continuous coverage for ca. 36 h around the flyby. Fig. 8 shows the busy timeline of activities in the last 36 h before the closest approach (5 September 2008 at 18:38). It can bee seen how the sequence for the orbit determination process had a very tight timeline for the images downlink and almost real time processing of these to
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Fig. 8. Activities timeline.
Fig. 9. Attitude errors.
feed the results into the commanding chain for the eventual TCM. Of the allocated TCM slots only the ones at -3 weeks and -36 h were actually used to set the spacecraft onto the right trajectory towards the asteroid. Both manoeuvres had a magnitude of ca 12 cm/s using a total of 270 g of fuel. Scientific observations began mid August with a light curve characterisation of the asteroid by the OSIRIS camera, with the remaining instruments activated only few days before the flyby for a set of calibration activities before entering the main science acquisition phase of several hours around the closest approach. Operations have been executed for all of the Rosetta instruments with the exception of two. In the last 12 h before the encounter a significant effort had to be put in the fine-tuning of the parameters of the navigation camera to properly track the asteroid. The noise level in the background was higher than expected due to the presence of warm pixels, i.e. pixels with considerably higher signal rate above the mean background. This required
an ad-hoc modification in the exposure time of the camera and AOCS parameters. After few hours of tests the engineers at ESOC had the camera tracking the asteroid and satisfying all the criteria required to allow the autonomous tracking during the closest approach and the commands to enable this mode were released to the spacecraft only 3 h before the event. The spacecraft kept its pre-programmed attitude till 18 min before the closest approach time when, as programmed, performed the transition to autonomous tracking. At this stage the spacecraft was at a distance of ca. 10 000 km from the asteroid with a tracking angle (angle between the asteroid direction and the flight direction) of ca. 5◦ . The autonomous tracking continued throughout the flyby as planned. Detailed performance analysis of the AOCS did show results outside the expectations with off-pointing during the approach phase as reported in Fig. 9.
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Fig. 10. Šteins as seen by the OSIRIS/Wide Angle Camera (ESA/OSIRIS).
The in-plane miss-pointing reached a maximum of a little more than 0.4◦ about 95 s before the time of closest approach. According to this analysis, the out-of-plane miss-pointing did not exceed about 25 mdeg that occurred very near the time of closest approach. This unexpected behaviour was given by the modified settings which had the side effect of causing over-exposure of the camera in the hour preceding the closest approach. The camera images were affected by blooming effects which off-pointed the on-board controller. Preliminary results delivered by the scientific instruments are very valuable and the images revealed a diamond shaped body of ca. 5 km diameter (see Fig. 10). Scientific operations continue after the flyby for ca. 4 weeks with the observation of the gravitational microlensing events by the OSIRIS instrument. 5. Conclusions The Šteins flyby constituted the first planned scientific objective of the mission and as such posed the control teams in front of a new approach to operations planning and conduct. For the first time in a critical mission phase the operations did not have as prime objective the optimisation of the navigation towards the final target, but the whole planning process and mission operations had to be designed around the intended scientific observation plan. The lessons learned from the preparation and execution of the activity will be invaluable to minimise the risks and optimise the operations for the Lutetia flyby, planned for July 2010. The scheme adopted and validated for this flyby
is generic enough that it can be easily re-used for Lutetia as well, with obvious benefits in terms of workload and confidence. The demanding planning and scheduling activities posed several problems to the operation teams and raised several interesting ideas for the definition of the planning concept for the comet operations phase. The main areas to be looked into for improvements are the capability to schedule operations with very late availability of products (e.g. manoeuvres) and the capability to re-schedule on short notice operations already loaded on-board. The results obtained with the first optical navigation campaign are well beyond the expectations and constitute a solid and comfortable baseline for the next flyby and for the definition of the more demanding and critical operations to be conducted during the comet detection, approach, and mapping phase in 2014. This operation highlighted once more how fundamental is a direct and close collaboration between the scientific teams and the operation teams at ESOC. The performance of the spacecraft and mainly of the navigation camera was not as expected during the autonomous tracking phase. This aspect is being further analysed and inflight tests have already been planned to optimise the performance for the Lutetia flyby. The Rosetta flyby of Šteins was the first space operation of this kind carried out by a European spacecraft and operations centre. This does not only mark a historical moment in European spaceflight, but also provided invaluable experience for the continuation of the Rosetta mission and for future European space enterprises. References [1] B. Gardini, C. Berner, J. van Casteren, The technical and programmatic challenge of the international Rosetta mission, in: IAF 50th Congress, Session Q.5, Amsterdam, October 1999. [2] M. Warhaut, P. Ferri, E. Montagnon, Rosetta ground segment and mission operations, Space Science Reviews 128 (2007) 189–204. [3] P. Ferri, Mission operations for the new Rosetta, Acta Astronautica 58 (2006) 105–111. [4] P. Ferri, M. Warhaut, First in-flight experience with Rosetta, in: IAF 55th Congress, IAC-04-Q.5.03, Vancouver, October 2004. [5] E. Montagnon, P. Ferri, Rosetta on its way to the outer solar system, Acta Astronautica 59 (2006) (2005) 301–309. [6] P. Ferri, E. Montagnon, J. Morales, The first Rosetta passive cruise: approach and experience, in: IAF 57th Congress, IAC-06-A3.5.03, Valencia (E), October 2006. [7] E. Montagnon, P. Ferri, Rosetta ground contact minimisation in cruise, in: Proceedings of the Sixth Symposium on Reducing the Cost of Spacecraft Ground Systems and Operations, Darmstadt (D), June 2005. [8] P. Ferri, A. Accomazzo, E. Montagnon, J. Morales, Rosetta in the year of the swing-bys, in: IAF 58th Congress, IAC-07-A3.5.02, Hyderabad, India, September 2007.
Contents lists available at ScienceDirect
Acta Astronautica journal homepage: w w w . e l s e v i e r . c o m / l o c a t e / a c t a a s t r o
The first Rosetta asteroid flyby Andrea Accomazzoa, ∗ , Paolo Ferria , Sylvain Lodiota , Armelle Hubaultb , Roberto Portab , Josè-Luis Pellon-Bailona a b
European Space Agency/European Space Operations Center, Darmstadt, Germany Vega Gmbh, Darmstadt, Germany
A R T I C L E
I N F O
Article history: Received 23 January 2009 Received in revised form 5 April 2009 Accepted 12 June 2009 Available online 28 July 2009 Keywords: Rosetta Asteroid Flyby Interplanetary Comet Šteins Lutetia Churyumov-Gerasimenko European Space Agency (ESA) ESOC Optical navigation
A B S T R A C T
The International Rosetta Mission, a cornerstone mission of the European Space Agency Scientific Programme, was launched on 2 March 2004 on its 10 years journey towards a rendezvous with comet Churyumov-Gerasimenko. Once reached the comet nucleus in summer 2014, Rosetta will orbit it for about 1.5 years down to distances of a few kilometres and deliver a lander onto its surface. In the long cruise to its target, Rosetta performs four gravity assist manoeuvres, three times with Earth and once with Mars. During this flight Rosetta will cross twice the main asteroid belt, and in both occasions its trajectory is designed and controlled to flyby an asteroid at close distance. On 5 September 2008 the first of the two asteroid flybys has been conducted, with the spacecraft approaching asteroid 2867-Šteins at a minimum distance of about 800 km, and a relative velocity of 8.6 km/s. Šteins is an E-type asteroid with a diameter of about 5 km. This paper continues the period reporting of the operational experience from the Rosetta mission operations. Preparation and execution of the flyby activities are described including the first experience in optical navigation that will prove of utmost importance for the next asteroid flyby and for the comet approach phase. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Rosetta is a cornerstone mission of the European Space Agency [1], launched on 2 March 2004 on an Ariane 5G rocket. Its main scientific objective is to rendezvous with the nucleus of comet Churyumov-Gerasimenko in 2014, to orbit it for about 18 months and to deliver onto the nucleus' surface a small lander, called Philae. During the long cruise to its target Rosetta has to undergo several planet swing-bys (three times with the Earth in 2005, 2007 and 2009, and once with Mars in 2007). In addition, the mission takes advantage
∗ Corresponding author. Tel.: +49 6151 902707. E-mail addresses: [email protected] (A. Accomazzo), [email protected] (P. Ferri), [email protected] (S. Lodiot), [email protected] (A. Hubault), [email protected] (R. Porta), [email protected] (J.-L. Pellon-Bailon). 0094-5765/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2009.06.013
of the trajectory crossing twice the main asteroid belt to perform two asteroid flybys (Šteins in 2008 and Lutetia in 2010) [2,3]. After the first intense year of post-launch operations, involving long periods of in-flight commissioning of the spacecraft and the scientific payload [4], the first Earth swing-by was carried out successfully in March 2005. Previously unplanned scientific operations were also carried out in this period, the most important of which was the observation campaign in support of the encounter of NASA's Deep Impact probe with comet Tempel-1, in July 2005 [5]. As from August 2005 the so-called quiet cruise started. In this period the spacecraft was kept as much as possible in a low activity mode, the ground contact minimised, with a typical frequency of once per week, and any special operation need minimised or postponed. The purpose of this is to preserve lifetime of several on-board units and to reduce the overall operations cost for the long cruise. The cruise was
A. Accomazzo et al. / Acta Astronautica 66 (2010) 382 -- 390
not as quiet as it was originally anticipated, due to various planned activities like payload checkouts and trajectory control manoeuvres, but also due to some unexpected events like solar flares and spacecraft anomalies that kept the flight control team very busy [6,7]. In 2007 two swing-bys [8], one highly critical at Mars in February at a target altitude over the planet's surface of only 250 km and a second Earth swing-by in November, put the spacecraft onto its fourth revolution round the Sun. For the first time Rosetta is flying at distances above 2 AU from the Sun and in September 2008 crossed the asteroid belt with a close encounter with asteroid 2867-Šteins. The original Rosetta mission, due to be launched in 2003, had been planned with close encounters at asteroids Otawara and Siwa. The redesign of the mission, with launch in 2004, due to an Ariane5 ECA failure one month before the foreseen launch date, did not allow for asteroid flybys due to propellant budget limitations. Shortly after launch, when it was clear that the propellant allocated for the launcher injection dispersion was not necessary, the interplanetary trajectory was slightly modified to allow close encounters with asteroid Šteins in 2008 and asteroid Lutetia in 2010. These scenarios have been carefully analysed and defined in collaboration with the spacecraft manufacturers. The scientific community of the Rosetta mission has given a lot of importance to the flyby of asteroids and set demanding requirements for the operation itself, well beyond the scenarios originally analysed. Some of the requirements are conflicting with spacecraft constraints, pushing to the limit the spacecraft capabilities and the navigation accuracy well beyond the original baseline; nevertheless the mission operations teams have defined a new flyby scenario that is satisfying all the requirements and at the same time not violating the spacecraft constraints. The spacecraft has been designed to perform asteroid flybys; it is thus equipped with navigation cameras (needed in any case for precise navigation in the proximity of the comet) able to track the asteroid and its attitude control system is able to use this information to autonomously steer the spacecraft attitude to track the asteroid. This complex operational mode required a significant validation effort onground and in-flight with dedicated tests. After a tracking test of the Moon performed in March 2005 at the time of the first Earth swing-by, Šteins flyby operations, in view of the newly defined scenario, have been rehearsed in March 2008 in preparation of the event in September. The following sections describe how the definition, preparation, and execution of the complex asteroid flyby activities have been addressed from an operational point of view, highlighting how they will prove of fundamental importance also for the rest of the mission to the comet.
2. Evolution of the flyby strategy 2.1. Baseline scenario As soon as the two target asteroids were selected by the scientific community among 15 different combinations, the spacecraft manufactures studied and analysed the flyby scenarios.
383
In the case of asteroid Šteins the following parameters characterise the scenario: • • • • •
relative velocity 8.6 km/s Sun/spacecraft/asteroid angle at approach 141.5◦ Earth/spacecraft/asteroid angle at approach 164.2◦ Sun distance 2.14 AU Earth distance 2.41 AU.
The spacecraft design is such that the remote sensing instruments have their line of sight aligned with the spacecraft Z axis (refer to Fig. 1 for spacecraft reference frame definition). The side walls (+Y and −Y faces) host the radiators and so does the −X face; these faces have therefore significant restrictions in terms of Sun exposure. The launch vehicle adapter (LVA) is installed on the −Z face, thus making also this face very sensitive to Sun exposure. The only safe side of the spacecraft for what concerns Sun exposure is therefore the +X face where the high gain antenna (HGA) is installed. The case studied by the spacecraft manufacturer included several constraints, such as: • limitation of the excursion of the solar arrays in the regions defined as “cold” from a spacecraft thermal point of view (i.e. Sun in the −Z/+X and −Z/−X quadrants). This was done in order to mitigate the effects of having a solar array stuck in such a position • selection of the flyby distance to replicate the angular rate and acceleration profile of the future Lutetia asteroid flyby, considered scientifically more interesting. This was done to make the Šteins flyby an in-flight test of the Lutetia case. The flyby scenario with these constraints was then limited to tracking the object from few hours to circa 3 min before the closest approach (CA) (see Fig. 2) when the Sun would have moved in the “undesired” region. The proposed strategy provided good illumination conditions and observations at phase angle zero, however, with a large minimum observation distance and no observation possible at closest approach and beyond. 2.2. Scientific requirements This scenario was judged of limited scientific value by the scientists of the Rosetta community which in turn reiterated their requirements for the flyby as follows: • • • • •
good illumination conditions observation at phase angle zero closest possible distance observation at closest approach (i.e. range rate zero) continuous observation from before to after closest approach • “good” pointing performance • “good” synchronisation of payload operations with flight events. This request raised several problems to the spacecraft operators since it would mean • pushing the spacecraft to its performance limits in terms of attitude dynamics (e.g. reaction wheels torques, rotational speed of appendages, etc.)
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Fig. 1. Spacecraft reference frame.
Fig. 2. Original Šteins flyby geometry.
• violating thermal constraints (i.e. exposure of “cold” faces to the Sun) • increasing the accuracy required for the optical navigation (i.e. impact of positional errors on attitude accuracy increased) • introducing demanding constraints for what concerns the planning of the operations (i.e. need for late orbit determination and correction manoeuvres, last minute commands updates, etc.).
3. Improved flyby scenario In reply to the request of the scientists the operations team at the European Space Operations Center (ESOC) designed a new scenario for the flyby itself that could satisfy most of, if not all, the requirements. The fundamental change of the new scenario proposed by ESOC was the introduction of a so called “attitude flip manoeuvre” shortly before the closest approach, in order to
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Fig. 3. “Inverted” Šteins flyby geometry.
minimise the time in which the `cold” faces of the spacecraft are exposed to Sun illumination. This strategy, called “inverted” because the spacecraft attitude is inverted during the flyby with respect to the Sun, gave the possibility for the spacecraft to track the asteroid from well before the closest approach till a few hours after. Fig. 3 shows how with a 180◦ rotation around the +Z axis (pointed to the asteroid) before the closest approach the spacecraft inverts the side exposed to the Sun. In this way the spacecraft keeps tracking the asteroid with the Sun that illuminates the “cold” face between the flip manoeuvre and the “phase angle zero” point. In order to respect the thermal constraints and at the same time guarantee a robust transition to autonomous tracking phase the flip manoeuvre has been placed between 40 and 20 min before closest approach so that the “cold” −X face is exposed to the Sun for a very limited period and with shallow angle. When the spacecraft is in asteroid flyby mode (AFM) the guidance for the attitude control system (AOCS) is provided by the measurements provided by the on-board camera that is tracking the asteroid. This was planned to start several hours before the flyby; however, the spacecraft design does not allow performing the “flip manoeuvre” in AFM, meaning that this mode can only be activated after the manoeuvre itself and with a pre-defined steering guidance before that. Due to the small pointing error allowed in the transition to AFM, the accuracy of orbit determination had to be higher than planned to allow a smooth transition between preprogrammed and autonomous guidance; for the same reason the operations timeline had to include a late opportunity to update attitude parameters. Following the scientists request to fly as close as possible to the asteroid, the minimum flyby distance was also analysed by the operations teams against the spacecraft capabilities (mainly driven by the maximum angular acceleration of the spacecraft that could be sustained by the reaction wheels). The result was a target flyby distance of 800 km. This scenario was immediately accepted by the scientific community since it satisfied most of the requirements and became the baseline for the development of the operational timeline. 4. Asteroid flyby implementation 4.1. Overall definition and validation Since the adopted flyby scenario significantly deviates from the one originally designed and validated by the spacecraft manufactures before launch, the operations teams had
to prepare an implementation plan that could achieve a similar level of confidence. The flight dynamics team at ESOC performed a set of analyses, tests, and partial simulations in order to consolidate the overall scenario and its features. The flight control team further processed these data to define the spacecraft activities to execute the full operation. Once the overall timeline for activities was consolidated a validation plan was defined including: • simulations with the ESOC software simulator emulating the AOCS software • simulations with the software simulator at avionics manufacturer premises • in-flight test replicating dynamics and Sun exposure conditions. The validation plan required a non-negligible effort by the various parties but it provided the necessary confidence on the robustness and safety of the scenario. The in-flight test was performed on 24 March 2008 when the position of Rosetta relative to the Earth and the Sun was the same as the one of the actual flyby. The test started shortly before the flip manoeuvre, replicating the closest approach dynamics, and ending 1 h later. The test was also used to check and confirm the tracking performance of the camera and the star tracker with the Sun approaching or violating the limit defined for nominal performance. In both cases the on-board units performed nominally. Simulations performed at the manufacturer premises with the test bed used to validate the AOCS subsystem during the design phase also confirmed that the proposed scenario could be flown as planned and gave the final confirmation to go ahead for the detailed implementation of the operation.
4.2. Payload check-out On request of the scientists regular payload check-out phases are scheduled during the mission, the major ones linked to operational events like planetary swing-bys and asteroid flybys. In the case of the Šteins flyby a check-out campaign of four weeks has been conducted in July 2008. This phase included a significant amount of check-out, calibration, on-board SW maintenance, and interference testing activities that kept the team under a constant workload for several weeks. The planning, with the involvement of the Rosetta Science Operations Center (RSOC) located in Spain,
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Table 1 Overall timeline. Week 32
Week 33
Week 34
Week 35
Week 36
Start of optical navigation campaign
Bi-weekly optical navigation imaging TCM at-3 week
Bi-weekly optical navigation imaging Characterisation of Šteins light curve by OSIRIS instrument
Daily navigation imaging
Daily navigation imaging
TCM at -1 week
TCM at 3 days, -36 h, -12 h
Flyby with scientific operations
Fig. 4. Post-fit angular residuals of optical data.
proved to be extremely robust and efficient allowing all instrument teams to complete their planned activities in time. At the end of the campaign all relevant instruments confirmed their readiness to participate in the Šteins flyby operations. 4.3. Navigation campaign As soon as the payload check-out phase was over, the operations plan focused on the navigation campaign that started on 4 August. Table 1 gives an overview of the timeline of the five weeks preceding the flyby. For the first time in the mission the traditional radiometric navigation was complemented with optical data. Images were taken twice a week for the first three weeks of the campaign; the frequency of image taking was increased to daily in the last two weeks before the flyby. The orbit determination process utilised images coming from the on-board navigation camera and from the more powerful Narrow Angle Camera (NAC) of the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS). This required a detailed definition of the interface between the ESOC operations team and the OSIRIS scientific team. The interface proved to work extremely well and became an operational validation of the same type of activities planned for the early comet detection and approach phase,
planned for 2014. The OSIRIS camera allowed a clear early detection of the asteroid, which was however also, unexpectedly, visible by the navigation cameras at a distance of ca. 26 million km. Fig. 4 reports the deviations (in terms of declination and right ascension) between the predicted and the measured asteroid position when derived from the images of the two navigation cameras and the OSIRIS camera. The blue points, typically very close to zero, clearly show the improvement in performance the OSIRIS camera can provide to the navigation process when compared to the spacecraft navigation camera and the increasing knowledge of the orbit solution over time. Fig. 5 shows the evolution of the orbit determination (projected in the B-plane) and the associated uncertainty ellipse as computed by the ESOC navigation team after the first week of navigation: • the black dashed line is the solution using radiometric data only (this is affected by the error in the knowledge of the asteroid position) • the orange dashed line is the solution obtained including the images of the navigation camera of the 4th of August (here the asteroid actual position starts being taken into account)
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Fig. 5. Rosetta targeting 11 August 2008.
• the magenta dashed line is the solution obtained with all the data available on the 7th of August, i.e. including the first images from OSIRIS (the significant performance improvement can be easily seen) • the blue dashed line is the solution obtained with the data up to 11 August.
Further analyses also showed that, in the absence of the optical data, the prediction would have been 137.0 km from the target corresponding to a miss-distance of 916.5 km from Šteins and a minimum phase angle of 3.00◦ .
4.4. Operations timeline and results The consistency of the measurements gave the operations team the necessary confidence to use them already for the first trajectory correction manoeuvre executed on 14 August. Fig. 6 reports the sequence of the orbit solutions in the days preceding the flyby and the correction manoeuvre executed 36 h before the closest approach. The last orbit solution, estimated to be less than 2 km away from the B-plane aim point, is the small violet ellipse obtained with optical data of 4 September. Orbit reconstruction was conducted during the post flyby phase with the help of the on-board attitude estimation data and the scientific images acquired by the OSIRIS instrument that, given the reduced distance to the asteroid, could give valuable attitude information for the minutes around the closest approach. Fig. 7 shows a discrepancy between the pre- and post-flyby estimated position. The final navigation result was 6.6 km from the target and corresponded to a miss-distance of 802.6 ± 0.2 km (3-sigma) from Šteins. The distinct separation between the orange and black error ellipses means that the predicted and final results are inconsistent with each other. This is most likely due to systematic errors in the direction measurements derived from the optical data during approach, particularly those at the end of the approach phase since they have the highest information content.
In addition to the activities required for the navigation campaign, the flyby preparation activities were characterised by an extensive planning process by the flight control team. For the first time in the mission the detailed scheduling of spacecraft commands of a critical phase had to be performed in an incremental way, whereas previous operations could always be scheduled in detail well before the operation. Following the incremental approach of commands availability, the scheduling process of the last 3 days included several sessions of commands generation and uplink windows to the spacecraft. The operations timeline also included a dense set of activities planned for the last days: • three slots for trajectory correction manoeuvres (TCM at CA-3days, CA-36 h, CA-12 h plus two at -3 and -1 weeks) with late availability of commands (typically few hours before each manoeuvre); • availability of preliminary asteroid flyby commands at CA3days • availability of the final asteroid flyby commands at CA12 h, i.e. following the last orbit determination done with images taken between CA-30 and CA-24 h • final confirmation to go for closed loop tracking of the asteroid between CA-7 and CA-2 h (the fall-back solution being a flyby based on a fully pre-defined attitude profile).
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Fig. 6. Last Rosetta targeting before flyby.
Fig. 7. Reconstructed Rosetta targeting.
The bulk of the interactive activities took place during ground contacts via the ESA New Norcia station, with a significant contribution from the NASA DSN Madrid station in the weeks preceding the flyby. In the last day before closest approach the available network included the ESA Cebreros station and the NASA DSN Canberra and Goldstone stations,
resulting in a continuous coverage for ca. 36 h around the flyby. Fig. 8 shows the busy timeline of activities in the last 36 h before the closest approach (5 September 2008 at 18:38). It can bee seen how the sequence for the orbit determination process had a very tight timeline for the images downlink and almost real time processing of these to
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Fig. 8. Activities timeline.
Fig. 9. Attitude errors.
feed the results into the commanding chain for the eventual TCM. Of the allocated TCM slots only the ones at -3 weeks and -36 h were actually used to set the spacecraft onto the right trajectory towards the asteroid. Both manoeuvres had a magnitude of ca 12 cm/s using a total of 270 g of fuel. Scientific observations began mid August with a light curve characterisation of the asteroid by the OSIRIS camera, with the remaining instruments activated only few days before the flyby for a set of calibration activities before entering the main science acquisition phase of several hours around the closest approach. Operations have been executed for all of the Rosetta instruments with the exception of two. In the last 12 h before the encounter a significant effort had to be put in the fine-tuning of the parameters of the navigation camera to properly track the asteroid. The noise level in the background was higher than expected due to the presence of warm pixels, i.e. pixels with considerably higher signal rate above the mean background. This required
an ad-hoc modification in the exposure time of the camera and AOCS parameters. After few hours of tests the engineers at ESOC had the camera tracking the asteroid and satisfying all the criteria required to allow the autonomous tracking during the closest approach and the commands to enable this mode were released to the spacecraft only 3 h before the event. The spacecraft kept its pre-programmed attitude till 18 min before the closest approach time when, as programmed, performed the transition to autonomous tracking. At this stage the spacecraft was at a distance of ca. 10 000 km from the asteroid with a tracking angle (angle between the asteroid direction and the flight direction) of ca. 5◦ . The autonomous tracking continued throughout the flyby as planned. Detailed performance analysis of the AOCS did show results outside the expectations with off-pointing during the approach phase as reported in Fig. 9.
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Fig. 10. Šteins as seen by the OSIRIS/Wide Angle Camera (ESA/OSIRIS).
The in-plane miss-pointing reached a maximum of a little more than 0.4◦ about 95 s before the time of closest approach. According to this analysis, the out-of-plane miss-pointing did not exceed about 25 mdeg that occurred very near the time of closest approach. This unexpected behaviour was given by the modified settings which had the side effect of causing over-exposure of the camera in the hour preceding the closest approach. The camera images were affected by blooming effects which off-pointed the on-board controller. Preliminary results delivered by the scientific instruments are very valuable and the images revealed a diamond shaped body of ca. 5 km diameter (see Fig. 10). Scientific operations continue after the flyby for ca. 4 weeks with the observation of the gravitational microlensing events by the OSIRIS instrument. 5. Conclusions The Šteins flyby constituted the first planned scientific objective of the mission and as such posed the control teams in front of a new approach to operations planning and conduct. For the first time in a critical mission phase the operations did not have as prime objective the optimisation of the navigation towards the final target, but the whole planning process and mission operations had to be designed around the intended scientific observation plan. The lessons learned from the preparation and execution of the activity will be invaluable to minimise the risks and optimise the operations for the Lutetia flyby, planned for July 2010. The scheme adopted and validated for this flyby
is generic enough that it can be easily re-used for Lutetia as well, with obvious benefits in terms of workload and confidence. The demanding planning and scheduling activities posed several problems to the operation teams and raised several interesting ideas for the definition of the planning concept for the comet operations phase. The main areas to be looked into for improvements are the capability to schedule operations with very late availability of products (e.g. manoeuvres) and the capability to re-schedule on short notice operations already loaded on-board. The results obtained with the first optical navigation campaign are well beyond the expectations and constitute a solid and comfortable baseline for the next flyby and for the definition of the more demanding and critical operations to be conducted during the comet detection, approach, and mapping phase in 2014. This operation highlighted once more how fundamental is a direct and close collaboration between the scientific teams and the operation teams at ESOC. The performance of the spacecraft and mainly of the navigation camera was not as expected during the autonomous tracking phase. This aspect is being further analysed and inflight tests have already been planned to optimise the performance for the Lutetia flyby. The Rosetta flyby of Šteins was the first space operation of this kind carried out by a European spacecraft and operations centre. This does not only mark a historical moment in European spaceflight, but also provided invaluable experience for the continuation of the Rosetta mission and for future European space enterprises. References [1] B. Gardini, C. Berner, J. van Casteren, The technical and programmatic challenge of the international Rosetta mission, in: IAF 50th Congress, Session Q.5, Amsterdam, October 1999. [2] M. Warhaut, P. Ferri, E. Montagnon, Rosetta ground segment and mission operations, Space Science Reviews 128 (2007) 189–204. [3] P. Ferri, Mission operations for the new Rosetta, Acta Astronautica 58 (2006) 105–111. [4] P. Ferri, M. Warhaut, First in-flight experience with Rosetta, in: IAF 55th Congress, IAC-04-Q.5.03, Vancouver, October 2004. [5] E. Montagnon, P. Ferri, Rosetta on its way to the outer solar system, Acta Astronautica 59 (2006) (2005) 301–309. [6] P. Ferri, E. Montagnon, J. Morales, The first Rosetta passive cruise: approach and experience, in: IAF 57th Congress, IAC-06-A3.5.03, Valencia (E), October 2006. [7] E. Montagnon, P. Ferri, Rosetta ground contact minimisation in cruise, in: Proceedings of the Sixth Symposium on Reducing the Cost of Spacecraft Ground Systems and Operations, Darmstadt (D), June 2005. [8] P. Ferri, A. Accomazzo, E. Montagnon, J. Morales, Rosetta in the year of the swing-bys, in: IAF 58th Congress, IAC-07-A3.5.02, Hyderabad, India, September 2007.