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NASA NTRS2014

Handling Late Changes to Titan Science

The Cassini mission has been in orbit for eight years, returning a wealth of scientific data from Titan and the Saturnian system. The mission, a cooperative undertaking between NASA, ESA and ASI, is currently in its second extension of the prime mission. The Cassini Solstice Mission (CSM) extends the mission's lifetime until Saturn's northern summer solstice in 2017. The Titan Orbital Science Team (TOST) has the task of integrating the science observations for all 56 targeted Titan flybys in the CSM. In order to balance Titan science across the entire set of flybys during the CSM, to optimize and influence the Titan flyby altitudes, and to decrease the future workload, TOST went through a "jumpstart" process before the start of the CSM. The "jumpstart" produced Master Timelines for each flyby, identifying prime science observations and allocating control of the spacecraft attitude to specific instrument teams. Three years after completing this long-range plan, TOST now faces a new challenge: incorporating changes into the Titan Science Plan without undoing the balance achieved during the jumpstart.

Pitesky, Jo Eliza↗

NASA NTRS2015

Ensuring Cassini's End-of-Mission Propellant Margins

The Cassini spacecraft is in its final years. On September 15, 2017, Cassini will plunge deep into Saturn's atmosphere never to reemerge; thus concluding its second extended mission and 13 years in orbit around the ringed planet. As of October 2014, the spacecraft is four years in to its seven-year, second extended mission, the Cassini Solstice Mission (CSM). With three years left and only 2.5% of its loaded bipropellant and 37% of its loaded monopropellant remaining, the Cassini project actively manages the predicted end-of-mission propellant margins to maintain a high confidence in the spacecraft's ability to complete the CSM as designed.

Sturm, Erick J., II↗

NASA NTRS2018

Titan's Cold Case Files - Outstanding Questions After Cassini-Huygens

The entry of the Cassini-Huygens spacecraft into orbit around Saturn in July 2004 marked the start of a golden era in the exploration of Titan, Saturn's giant moon. During the prime mission (2004-2008), ground-breaking discoveries were made by the Cassini orbiter including the equatorial dune fields (flyby T3, 2005), northern lakes and seas (T16, 2006), and the large positive and negative ions (T16 & T18, 2006), to name a few. In 2005 the Huygens probe descended through Titan's atmosphere, taking the first close-up pictures of the surface, including large networks of dendritic channels leading to a dried-up seabed, and also obtaining detailed profiles of temperature and gas composition during the atmospheric descent. The discoveries continued through the Equinox mission (2008-2010) and Solstice mission (2010-2017) totaling 127 targeted flybys of Titan in all. Now at the end of the mission, we are able to look back on the high-level scientific questions from the start of the mission, and assess the progress that has been made towards answering these. At the same time, new scientific questions regarding Titan have emerged from the new discoveries that have been made. In this paper we review a cross-section of important scientific questions that remain partially or completely unanswered, ranging from Titan's deep interior to the exosphere. Our intention is to help formulate the science goals for the next generation of planetary missions to Saturn and Titan, and to stimulate new experimental, observation and theoretical investigations in the interim, before such missions arrive again at Titan.

Nixon, C. A.↗

NASA NTRS2014

Adding "Missed" Science to Cassini's Ops Plan

The phenomenal success of the Cassini Mission at Saturn is largely due to flagship instruments, in a target rich environment, for a long period of time, executing almost error free complex mission operations. A smooth transition from cruise operations through the prime science mission and extended science (Equinox) mission culminating in the currently executing Solstice mission has folded in necessary procedural alterations due to improved understanding of the spacecraft, instruments, uplink and planning systems as well as additional science objectives. These have come with the maturation of the mission along with management of workforce reductions. One important set of operational changes has been initiated due to scientific findings highlighting "missed" science opportunities. This is the case for the Titan Meteorology Campaigns and Saturn Storm Watch Campaigns. These observations involve long term monitoring of the atmospheres of Titan and Saturn while the spacecraft and science teams are focused on other high priority targets of opportunity (like Enceladus). Our objective in this paper is to emphasize how a non-invasive strategy to get additional remarkable science was conceived and implemented in a mission with an already well defined operational plan. To illustrate this we will detail Titan Meteorology Campaign and Saturn Storm Watch Campaign integration and implementation strategies as well as the scientific goals and achievements of both.

Roy, Mou↗

NASA NTRS2014

Adding "Missed" Science to Cassini's Ops Plan

Roy, Mou↗

NASA NTRS2014

Skirting Saturn's Rings and Skimming Its Cloud Tops: Planning Cassini's End of Mission

In October 2010, the Cassini spacecraft embarked on the seven-year Solstice Mission. The mission will culminate with a spectacular series of orbits that bring Cassini between Saturn's innermost ring, the D ring, and the cloud tops of the planet. The spacecraft will make its closest passages ever to the planet allowing for unprecedented science to be collected on Saturn and its rings. These final orbits will expose the spacecraft to new environments, which presents a number of challenges to planning the final mission phase. While these challenges will require adaptations to planning processes and operations, they are not insurmountable. This paper describes the challenges identified and the steps taken to mitigate them to enable collection of unique Saturn system science.

Manor-Chapman, Emily↗

NASA NTRS2018

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.

Criddle, Kevin↗

NASA NTRS2011

Cassini Attitude Control Operations - Guidelines Levied on Science to Extend Reaction Wheel Life

The Cassini spacecraft was launched on October 15, 1997 and arrived at Saturn on June 30, 2004. It has performed detailed observations and remote sensing of Saturn, its rings, and its satellites since that time. Cassini deployed the European-built Huygens probe, which descended through the Titan atmosphere (Saturn's largest moon) and landed on its surface on January 14, 2005. The Cassini mission has recently been approved by NASA to continue through September of 2017. This 7-year extension is called the Solstice mission and it presents challenges to the spacecraft operations team and its ability to maintain the health of the spacecraft. To keep the spacecraft healthy for 7 more years, the spacecraft team must carefully manage hydrazine use (about 48% of the 132 kg launch load remains as of January 2011). A vital part of conserving hydrazine is to use the reaction wheel assembly (RWA) control system for precise pointing and slews wherever possible. In any given week, the Cassini spacecraft is commanded to use RWA control about 99% of the time, with about 1% of the time requiring reaction control system (RCS) thruster control (to perform Delta V course corrections or to bias the RWA momentum). Such extensive use of the RWA hardware throughout the mission requires that the RWAs be operated in a way that minimizes degradation in the RWA electronics, DC motor, and spin bearing for each reaction wheel. Three consumables in particular have been identified for the RWAs: (1) Total number of revolutions for each RWA. (2) Time spent at very low wheel speeds. At these low speeds, good elasto-hydrodynamic (EHD) film lubrication may be compromised. (3) Total number of on/off power cycles. The second of these consumables, minimizing the time spent at very low wheel speeds, is especially important to keep the spin bearing healthy and well-lubricated. These consumables are actively managed by the attitude control operations team throughout the mission. One vital management technique is to predict individual RWA momentum (given the pointing and slews that are needed to collect the best science) and to bias the RWA momentum in a way that reduces both the total number of revolutions as well as the time spent below EHD wheel speed. Another strategy to protect RWA health is to alter the planned pointing of the spacecraft (which can affect science collection) so that the RWA consumables are conserved. This paper focuses on why this second technique is needed, and discusses how guidelines have been developed by the attitude control team which affects the planned science pointing, so that science data can be most optimally collected while still minimizing RWA consumable usage.

interplanetary trajectory↗

NASA NTRS2017

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.

Criddle, Kevin↗

NASA NTRS2017

The Cassini Grand Finale Mission: Planning for a New Mission Environment

The Cassini F-Ring & Proximal Orbits (FRPO) is a new and unique mission; to ensure the highest priority science gets implemented, the POST (Proximal Orbit Science Team) was created to pre-allocate the time around periapse for all 22 proximal orbits. The F-ring orbits, and proximal time outside of POST, were handled similar to Cassini’s Solstice Mission using the Pre-Integrated Event (PIE) process. The new and unique properties of the spacecraft’s trajectory required much forethought to be flown safely while still planning for the most and best science return possible. Some ring-plane crossings (RPX) will be protected against dust impacts by turning the high gain antenna (HGA) to the dust RAM direction (HGA2RAM). If on the first proximal RPX higher than expected dust readings are seen then the Project Office may choose to require more (all) subsequent RPX to be HGA2RAM, implemented via a real-time command overlay for uplinked sequences. The pointing uncertainties will be larger than usual after the final targeted flyby; some of the process changes to address this include adding extra orbit trim maneuvers (OTMs) (fuel permitting) to resync to the reference trajectory and reduce pointing uncertainties; and movable blocks of commands to be used for some periapses where atmospheric drag may cause large timing shifts Changes made for FRPO to address perceptions that these sequences will be hard to implement include requiring early pointing designs (during integration) for certain types of observations, requiring teams to check early on that they can turn to and from their observation attitude, and that their attitude is safe, and adjusting the Implementation process to give more time for science observation designers. This paper will discuss these process changes and lessons learned so far.

Ray, Trina↗

NASA NTRS2016

Cassini Navigation: The Road to Consistent Subkilometer Accuracy Satellite Encounters

This paper reviews the orbit determination performance for the last five years of the Cassini Mission Solstice Tour. During this period of time, Cassini had more than 30 satellite encounters, including Titan, Rhea, and Dione. We report on the navigational flyby accuracy, comparing post-flyby reconstructions and encounter predictions, and discuss the performance improvement and challenges over the years. Finally, we give an overview of the "Grand Finale" end of mission planned for 2017.The Cassini mission has been in orbit in the Saturnian system for more than 11 years, and hasreturned a wealth of discoveries and operational knowledge in the outer Solar System. In this paper, we reported on the last three years of navigation operations focusing on orbit determinationand encounter performance. Modeling and strategy changes over the past years now allow us tonavigate bodies at 100s of meters in accuracy, and consider a miss larger than 1 km an outlier.The Cassini Grand Finale will be spectacular - do not miss it!

Bellerose, Julie↗

NASA NTRS2013

Cassini Returns to Saturn's Poles: Seasonal Change in the Polar Vortices

High inclination orbits during Cassini's solstice mission (2012) are providing us with our first observations of Saturn's high latitudes since the prime mission (2007). Since that time, the northern spring pole has emerged into sunlight and the southern autumn pole has disappeared into winter darkness, allowing us to study the seasonal changes occurring within the polar vortices in response to these dramatic insolation changes. Observations from the Cassini Composite Infrared Spectrometer] have revealed (i) the continued presence of small, cyclonic polar hotspots at both spring and autumn poles; and (ii) the emergence of an infrared-bright polar vortex at the north pole, consistent with the historical record of Saturn observations from the 1980s (previous northern spring).

Cassini↗

NASA NTRS2018

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.

Wagner, Sean↗

NASA NTRS2011

Cassini Mission Sequence Subsystem (MSS)

This paper describes my work with the Cassini Mission Sequence Subsystem (MSS) team during the summer of 2011. It gives some background on the motivation for this project and describes the expected benefit to the Cassini program. It then introduces the two tasks that I worked on - an automatic system auditing tool and a series of corrections to the Cassini Sequence Generator (SEQ_GEN) - and the specific objectives these tasks were to accomplish. Next, it details the approach I took to meet these objectives and the results of this approach, followed by a discussion of how the outcome of the project compares with my initial expectations. The paper concludes with a summary of my experience working on this project, lists what the next steps are, and acknowledges the help of my Cassini colleagues.

solstice mission↗

NASA NTRS2019

Titan Surface Temperatures During the Cassini Mission

By the close of the Cassini mission in 2017 the Composite Infrared Spectrometer had recorded surface brightnesstemperatures on Titan for 13 yr (almost half a Titan year). We mapped temperatures in latitude from pole to pole inseven time segments from northern mid-winter to northern summer solstice. At the beginning of the mission thewarmest temperatures were centered at 13 S where they peaked at 93.9 K. Temperatures fell off by about 4 Ktoward the north pole and 2 K toward the south pole. As the seasons progressed the warmest temperatures shiftednorthward, tracking the subsolar point, and at northern summer solstice were centered at 24 N. While moving norththe peak temperature decreased by about 1 K, reaching 92.8 K at solstice. At solstice the fall-off toward the northand south poles were 1 K and 3 K, respectively. Thus the temperature range was the same 2 K at the two poles. Ourobserved surface temperatures agree with recent general circulation model results that take account of methanehydrology and imply that hemispherical differences in Titan's topography may play a role in the north?southasymmetry on Titan.

Jennings, D. E.↗

NASA NTRS1993

Solar-Stellar Irradiance Comparison Experiment 1. II - Instrument calibrations

The science objective for the Solar-Stellar Irradiance Comparison Experiment (SOLSTICE) is to accurately measure the full disk solar spectral irradiance in the ultraviolet (UV) spectral region over a long time period. The SOLSTICE design was driven by the requirement for long-term, precise solar photometry conducted from space. The SOLSTICE 1 is on the Upper Atmosphere Research Satellite (UARS), launched in September 1991 with the possibility for a 10-year operational mission. The in-flight calibration for SOLSTICE to meet its primary objective is the routine measurements of the UV radiation from a set of early-type stars, using the identical optical elements employed for the solar observations. The extensive preflight calibrations of the instrument have yielded a precise characterization of the three SOLSTICE channels. Details of the preflight and in-flight SOLSTICE calibrations are discussed in this paper.

Woods, Thomas N.↗

NASA NTRS1988

In-flight calibration of solar irradiance measurements by direct comparison with stellar observations

The Solstice instrument aboard the upper atmosphere research satellite (UARS) is designed to correct the deficiencies of both long-term instrument drift and of intercomparison of solar data sets taken at different times by different instruments. The Solstice will make repeated measurements, spaced throughout the mission, of a number of bright early-type stars. In addition, it will determine the ratio of the solar flux to the average flux from an ensemble of stars. This ratio will be monitored throughout the UARS mission and archived.

Rottman, Gary J.↗

NASA NTRSDate not supplied

Mapping the seasonal variability of Titan’s upper atmospheric gases using ALMA

We present a study of the impact of seasonal effects on the spatial distribution of Titan’s atmospheric gases HNC, CH3CN, and HC3N over a timescale of less than a Titan month (9 Earth months). These molecules are synthesized primarily in the upper and middle atmosphere as a result of CH4 and N2 photochemistry, and are redistributed around Titan’s globe as a result of diffusion and zonal and meridional circulation. With observations from the Atacama Large Millimeter/submillimeter Array (ALMA) obtained in August 2016 [1] and May 2017 [2], we produced high-resolution, spectrally and spatially resolved maps of the emission from these gases. These observations are contemporaneous with the end of the Cassini mission (around the time of the northern summer solstice), allowing for studies of seasonal variations in Titan’s organic chemistry through analysis of the production, destruction, and transport of complex molecules. We observed a strong reduction of 41% ± 4% in the HC3N flux coming from the north pole, while CH3CN and HNC remained relatively stable (within errors) over the 9 month period of our observations. In the south pole, CH3CN presented a 31% ± 10% increase, while HNC saw a reduction of 20% ± 8%, and HC3N remained fairly stable (within errors). The HNC emission comes primarily from the highest (thermospheric) altitudes, and is distributed more uniformly about Titan’s limb than HC3N and CH3CN; the latter two molecules show strong polar enhancements due to meridional circulation. The steep decline in the HC3N north polar concentration is consistent its short photochemical lifetime (compared to HNC and CH3CN), combined with a reduction in meridional transport of gases produced at lower latitudes, towards the northern (summer) pole. The loss of emission of HC3N in the northern hemisphere is also consistent with the breakdown of Titan’s northern polar vortex during the onset of the northern summer, leading to a loss of confinement of these species [3].

Titan↗