Engineering topics
Bellerose, Julie
Publications and source records attributed to Bellerose, Julie.
Double Asteroid Redirection Test (DART) Phase D Mission Design & Navigation Analysis
NASA’s Double Asteroid Redirection Test (DART) mission is the first demonstration of kinetic deflection of an asteroid. DART uses terminal guidance to impact Dimorphos, which orbits Didymos, during its 2022 close-approach to Earth. The close range to Earth allows Earth-based observations to reconstruct the impact’s effect. Light-curve data will be used to measure the resulting change in orbit period of Dimorphos due to the momentum change associated with the impact experiment. This paper describes the current DART trajectory and recent Mission Design and Navigation analysis preparing for launch.
Bennu Shape Model Validation Methods and Results
The OSIRIS-REx Independent Shape Modeling Team, staffed by the Jet Propulsion Laboratory (JPL), has used stereophotoclinometry (SPC) to produce a three-dimensional shape model of Bennu. The SPC process is informed by (but is separate from) orbit determination, so it is important to ensure that the resulting shape model is consistent with all available spacecraft tracking data. Specifically, checking the shape model’s consistency with LIDAR measurements can illuminate any discrepancies, because LIDAR measurements are highly correlated with the shape model. This study focuses on the JPL experience with these LIDAR measurements and the greater context of the shape model validation process.
Osiris-Rex Shape Model Performance During the Navigation Campaign
The Navigation Campaign of the OSIRIS-REx mission began when the first image of Bennu was recorded by the PolyCam high-resolution imager on Au-gust 17, 2018. In the ensuing months, two teams began building shape models based on imagery taken during the Approach and Preliminary survey phases to be used for the transition to landmark navigation in the Orbital A phase. The orbit determination team began analyzing and characterizing the performance and errors associated with each shape model delivery working closely to iterate on the next shape model delivery. By the end of Orbital A, shape models produced by the Altimetry Working Group and JPL exceeded pre-launch performance re-quirements. This paper provides a summary of the analysis performed during operations.
Double Asteroid Redirection Test (DART) Mission Design and Navigation for Low Energy Escape
This paper describes the evolution of the NASA Double Asteroid Redirection Test (DART) mission design and navigation. Specifically, the mission has been conceived as (1) a hydrazine bus on a ballistic trajectory, (2) a low-thrust bus launching from a geostationary transfer orbit and spiraling to escape, and (3) a lowthrust bus that launches with a small positive escape energy. This paper discusses the rationale in favor of the third concept, low energy escape, and describes the key mission design and navigation studies. In an effort to be compatible with an unknown co-manifest partner, the trajectory design must account for a large range of launch energies, orientations, and dates. The navigation approach must account for sensitive regions in the trajectory and plan for both low-thrust and chemical phases of flight. These findings are relevant to other missions pursuing low-cost interplanetary rideshare concepts.
The Cassini Mission: Reconstructing Thirteen Years of the Most Complex Gravity-Assist Trajectory Flown to Date
Cassini launched in 1997 and completed its prime mission, its Equinox first extended mission, and its Solstice second extended mission. Since its arrival at Saturn in 2004, Cassini completed almost 300 orbits around the planet. Over the span of the mission, significant improvements were made to all the major satellites ephemeris, and to Saturn gravitational and pole models. These improvements have enabled better trajectory reconstructions throughout the timeframe of the mission, although using about one hundred different models of the Saturn system. Now that the mission is over, the paper reports on the uniform reconstruction of the entire Cassini orbital mission, which uses one consistent Saturn system model and satellite ephemerides throughout. We discuss the challenges of undertaking this task, and comparison strategies for choosing the best and greatest Cassini trajectory for its very final delivery.
Preliminary Saturn Atmospheric Density Results From Cassini's Final Plunge
The Cassini spacecraft made its final descent into the planet Saturn on September 15, 2017, capping a twenty year mission full of scientific discoveries. The high gain antenna was held on Earth-point until torques from atmospheric drag caused the spacecraft to lose line-of-sight lock with Earth. The Doppler data collected during the final plunge contains information about the spacecraft’s acceleration due to atmospheric drag, and therefore, the density of Saturn’s atmosphere. In this work, we present preliminary analysis of the end of mission Doppler data and its implications regarding the density of Saturn’s upper atmosphere. Are construction of the spacecraft’s final trajectory is discussed and used to fit a model of Saturn’s atmosphere to the Doppler data taken during the final plunge. The Cassini navigation team’s experience flying the spacecraft through the final five low altitude Saturn periapses is also discussed in the context of atmospheric drag and density models.
Cassini Orbit Determination Operations Through The Final Titan Flybys and The Mission Grand Finale (February 2016 - September 2017)
This paper reports on the orbit determination performance for the final 1.5 years of the Cassini Solstice mission, including the mission’s Grand Finale. During this period, Cassini encountered its final eleven targeted flybys of Titan (T116-T126) and executed its last 62 orbits of Saturn. In these final months, the spacecraft’s inclination was gradually raised from near equatorial to near 63 degrees, critical inclination, to prevent the line of apsides from rotating out of Titan’s orbital plane. Critical inclination enables continued Titan flybys, the last of which places Cassini on an impact trajectory with Saturn, thereby satisfying planetary protection requirements. In this reporting period, the orbit period moved from 16 days to nearly 32 and, for the final 6 months, it was brought down to less than 7 days. By design, the spacecraft entered the Saturn atmosphere on its final orbit and vaporized on September 15, 2017. We also report on the particular challenges associated with a stellar occultation, a flyby of Saturn’s rocks, and the last revolutions of the mission’s Grand Finale.
Optical Navigation During Cassini's Solstice Mission
After nearly twenty years in flight, Cassini’s mission at Saturn will conclude as it purposely dives into Saturn’s atmosphere on September 15, 2017. Primarily to avoid moons potentially harboring conditions for life and with propellant very low, the intentional plunge into the atmosphere was set in motion years ago. We take this opportunity to give an overview of the optical navigation and its roles throughout the mission. The paper describes the navigation process and the evolution of optical navigation over the past thirteen years. The last equatorial phase of the Cassini mission was particularly challenging for the OD team as the Saturn system was not being estimated anymore, and it had been a few years since the last icy moon flybys. Science pictures of Enceladus one month prior to the Enceladus encounters confirmed the moon’s position to be in good agreement with the Saturn system dynamical modeling used. This reduced Enceladus’s absolute uncertainty by a factor of three, less than 1 km, and gave confidence the navigation team could achieve acceptable flybys and meet science objectives.
Update on determining the mass of Didymoon and impact parameters through radio science
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AIDA: Measuring Asteroid Binary System Parameters and DART-Imparted Deflection Using the AIM Spacecraft
The Asteroid Impact and Deflection Assessment (AIDA) mission concept would demonstrate an asteroid deflection through a high velocity spacecraft impact on the moon of the binary asteroid system Didymos. The NASA DART spacecraft would be launched on an impacting trajectory, while the ESA AIM spacecraft would be orbiting and observing the system before and after the impact. Radio science measurements with AIM provides information on the complex dynamics of the binary system. Combined with the DART experiment, the ability to measure the imparted delta-v has significant implications for how well the proposed AIDA mission would serve as a deflection demonstration. In addition, the impact-induced deflection, cratering, and mass transfer can be interpreted as indicators of surface properties. We provided preliminary analyses of the measurability of the DART impact as function of generic AIM spacecraft proximity operations and knowledge of the Didymos system from radio science techniques.
Resolution of Orbit Determination Prediction Instabilities at Titan During Cassini's Solstice Mission
The Cassini spacecraft has been in orbit about Saturn since 2004. Exploration of the Saturn system is driven by gravitational flybys of the moon Titan which alter the spacecraft trajectory. The Cassini Navigation Team receives regular updates to the Saturn satellites ephemeris from JPL's Solar System Dynamics group. The difference between subsequent ephemeris deliveries can be hundreds of meters in the position of Titan at the time of a flyby. Errors in Titan's position propagate downstream to the next flyby through the estimated spacecraft trajectory. Prior to 2013, the Cassini Orbit Determination Team estimated the Saturn satellite ephemeris parameters and used the a posteriori states and covariance of an operations arc as a priori inputs to subsequent estimation arcs. Since 2013, the OD Team has only been considering errors in the ephemeris and not estimating a correction to the satellite positions. The T119 Titan flyby exhibited a 3D miss distance of 2.44 km and the following T120 flyby yielded a smaller miss of 1.06 km at the 2.9s error level. These discrepancies between pre-flyby prediction and post-flyby trajectory reconstruction were due to errors in the Titan ephemeris. In order to improve the targeting of Titan in future flybys, the team restarted the satellite ephemeris estimation process for orbit determination solutions. Subsequent flybys had target misses of less than 1 km at the sub-3 error level. This paper describes the method of scaling the a priori satellite ephemeris covariance in the orbit determination process to allow larger corrections to the satellite system and improve the prediction of the spacecraft’s Titan-relative position at the time of encounters.
Orbit Determination Adaptations for the Cassini Grand Finale
satellite encounters. Over this period, there have been several papers describing the orbit determination process and performance up through 2016 [1-5]. In April of 2017, Cassini will enter its Grand Finale mission phase when it will traverse the gap between the D-ring and the Saturn atmosphere twenty-two times before plunging deep into the atmosphere to end the mission. The lack of targeted satellite encounters during this period necessitates updates to the nominal Cassini Orbit Determination (OD) process. This paper describes these planned adaptations for the operation of the Grand Finale. During the Equinox and Solstice Mission Phase (2008-2016), navigation analysis has been divided into segments focused on two particular targeted satellite encounters, called an “arc”. Maneuvers in an arc were usually targeted to encounter B-plane position and time, so the OD state and covariance were mapped forward to the B-plane of the encounter within the arc. Trajectory dispersions during the Grand Finale need instead to be mapped to equator crossings and targeted Cartesian positions. In addition, trajectory arcs have typically covered a few orbital revolutions (~2-8 weeks), in order to span the time between two encounters. However, the Grand Finale will encompass five months of time without an encounter which necessitates an adjusted arc strategy. A modified arc strategy was developed based on OD behavior during long multi-rev periods between encounters in the year leading up to the Grand Finale. The OD covariance study conducted for the Grand Finale mission phase will also be examined.