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NASA NTRS2012

Autonomous Navigation Performance During The Hartley 2 Comet Flyby

On November 4, 2010, the EPOXI spacecraft performed a 700-km flyby of the comet Hartley 2 as follow-on to the successful 2005 Deep Impact prime mission. EPOXI, an extended mission for the Deep Impact Flyby spacecraft, returned a wealth of visual and infrared data from Hartley 2, marking the fifth time that high-resolution images of a cometary nucleus have been captured by a spacecraft. The highest resolution science return, captured at closest approach to the comet nucleus, was enabled by use of an onboard autonomous navigation system called AutoNav. AutoNav estimates the comet-relative spacecraft trajectory using optical measurements from the Medium Resolution Imager (MRI) and provides this relative position information to the Attitude Determination and Control System (ADCS) for maintaining instrument pointing on the comet. For the EPOXI mission, AutoNav was tasked to enable continuous tracking of a smaller, more active Hartley 2, as compared to Tempel 1, through the full encounter while traveling at a higher velocity. To meet the mission goal of capturing the comet in all MRI science images, position knowledge accuracies of +/- 3.5 km (3-?) cross track and +/- 0.3 seconds (3-?) time of flight were required. A flight-code-in-the-loop Monte Carlo simulation assessed AutoNav's statistical performance under the Hartley 2 flyby dynamics and determined optimal configuration. The AutoNav performance at Hartley 2 was successful, capturing the comet in all of the MRI images. The maximum residual between observed and predicted comet locations was 20 MRI pixels, primarily influenced by the center of brightness offset from the center of mass in the observations and attitude knowledge errors. This paper discusses the Monte Carlo-based analysis that led to the final AutoNav configuration and a comparison of the predicted performance with the flyby performance.

flyby↗
NASA NTRS2011

Navigation of the EPOXI Spacecraft to Comet Hartley 2

On November 4, 2010, the EPOXI spacecraft flew by the comet Hartley 2, marking the fourth time that a NASA spacecraft successfully captured high resolution images of a cometary nucleus. EPOXI is the extended mission of the Deep Impact mission, which delivered an impactor on comet Tempel-1 on July 4, 2005. EPOXI officially started in September 2007 and eventually took over 3 years of flight time and had 3 Earth gravity assists to achieve the proper encounter conditions. In the process, the mission was redesigned to accommodate a new comet as the target and changes in the trajectory to achieve better imaging conditions at encounter. Challenges in navigation of the spacecraft included precision targeting of several Earth flybys and the comet encounter, uncertainties in determining the ephemeris of the comet relative to the spacecraft, and the high accuracy trajectory knowledge needed to image the comet during the encounter. This paper presents an overview of the navigation process used for the mission.

Deep Impact (DI)↗
NASA NTRS2011

EPOXI at Comet Hartley 2

Understanding how comets work, i,e., what drives their activity, is crucial to using comets to study the early solar system. EPOXI flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus. taking both images and spectra. Unlike large, relatively inactive nuclei, this nncleus is outgassing primarily due to CO2, which drags chnnks of ice out of the nnclens. It also shows significant differences in the relative abundance of volatiles from various parts of the nucleus.

A'Hearn, Michael F.↗
NASA NTRS2013

The Complex Spin State of 103P-Hartley 2: Kinematics and Orientation in Space

We derive the spin state of the nucleus of Comet 103P/Hartley 2, its orientation in space, and its short-term temporal evolution from a mixture of observations taken from the DIXI (Deep Impact Extended Investigation) spacecraft and radar observations. The nucleus is found to spin in an excited long-axis mode (LAM) with its rotational angular momentum per unit mass, M, and rotational energy per unit mass, E, slowly decreasing while the degree of excitation in the spin increases through perihelion passage. M is directed toward (RA, Dec; J2000) = 8+/-+/- 4 deg., 54 +/- 1 deg. (obliquity = 48 +/- 1 deg.). This direction is likely changing, but the change is probably <6 deg. on the sky over the approx. 81.6 days of the DIXI encounter. The magnitudes of M and E at closest approach (JD 2455505.0831866 2011-11-04 13:59:47.310) are 30.0 +/- 0.2 sq. m/s and (1.56 +/- 0.02) X 10(exp -3) sq. m /sq. s respectively. The period of rotation about the instantaneous spin vector, which points in the direction (RA, Dec; J2000) = 300 +/- 3.2deg., 67 +/- 1.3 deg. at the time of closest approach, was 14.1 +/- 0.3 h. The instantaneous spin vector circulates around M, inclined at an average angle of 33.2 +/- 1.3 deg. with an average period of 18.40 +/- 0.13 h at the time of closest approach. The period of roll around the principal axis of minimum inertia (''long'' axis) at that time is 26.72 +/- 0.06 h. The long axis is inclined to M by approx. 81.2 +/- 0.6 deg. on average, slowly decreasing through encounter. We infer that there is a periodic nodding motion of the long axis with half the roll period, i.e., 13.36+/- 0.03 h, with amplitude of 1 again decreasing through encounter. The periodic variability in the circulation and roll rates during a cycle was at the 2% and 10-14% level respectively. During the encounter there was a secular lengthening of the circulation period of the long axis by 1.3 +/- 0.2 min/d, in agreement with ground-based estimates, while the period of roll around the long axis changed by approx. -4.4 min/d at perihelion. M decreased at a rate of 0.038 (sq m/s) per day in a roughly linear fashion. Assuming a bulk density between 230-300 kg/m3 and a total volume for the nucleus of 8.09 X 10(exp 8) cubic m, the net torque acting on the nucleus was in the range 0.8-1.1 X 10(exp 5) kg m(exp 2) /s(exp 2). In order to bring the spacecraft photometric and imaging data into alignment on the direction of M, the directions of the intermediate and short principal axes of inertia had to be adjusted by 33 deg (on the sky) from the values indicated by the shape model with an assumed homogeneous interior. The adjusted direction of the intermediate axis is RA, Dec = 302 deg., -16.5 deg.. The morning and evening terminators in the images are identified, and the variation of the insolation at three regions on the nucleus associated with active areas calculated. The plume of water vapor observed in the inner coma is found to be directed close to the direction of local gravity over the sub-solar region for a range of reasonable bulk densities. The plume does not follow the projected normal to the surface at the sub-solar point.

Comets↗
NASA NTRS2011

Attitude Determination and Control Subsystem (ADCS) Preparations for the EPOXI Flyby of Comet Hartley 2

On November 4, 2010 the former "Deep Impact" spacecraft, renamed "EPOXI" for its extended mission, flew within 700km of comet 103P/Hartley 2. In July 2005, the spacecraft had previously imaged a probe impact of comet Tempel 1. The EPOXI flyby was the fifth close encounter of a spacecraft with a comet nucleus and marked the first time in history that two comet nuclei were imaged at close range with the same suite of onboard science instruments. This challenging objective made the function of the attitude determination and control subsystem (ADCS) critical to the successful execution of the EPOXI flyby.As part of the spacecraft flyby preparations, the ADCS operations team had to perform meticulous sequence reviews, implement complex spacecraft engineering and science activities and perform numerous onboard calibrations. ADCS contributions included design and execution of 10 trajectory correction maneuvers, the science calibration of the two telescopic instruments, an in-flight demonstration of high-rate turns between Earth and comet point, and an ongoing assessment of reaction wheel health. The ADCS team was also responsible for command sequences that included updates to the onboard ephemeris and sun sensor coefficients and implementation of reaction wheel assembly (RWA) de-saturations.

attitude determination and control↗
NASA NTRS2011

EPOXI Trajectory and Maneuver Analyses

The EPOXI mission is a NASA Discovery Mission of Opportunity combining two separate investigations: Extrasolar Planet Observation and Characterization (EPOCh) and Deep Impact eXtended Investigation (DIXI). Both investigations reused the DI instruments and spacecraft that successfully flew by the comet Tempel-1 (4 July 2005). For EPOCh, the goal was to find exoplanets with the high resolution imager, while for DIXI it was to fly by the comet Hartley 2 (4 Nov 2010). This paper documents the navigation experience of the earlier ma-neuver analyses critical for the EPOXI mission including statistical ?V analyses and other useful analyses in designing maneuvers. It also recounts the trajectory design leading up to the final reference trajectory to Hartley 2.

trajectory design↗
NASA NTRS2012

Target Search and Selection for the DI/EPOXI Spacecraft

Upon completion of the Hartley 2 flyby in November 2010, the Deep Impact (DI) spacecraft resided in a solar orbit without possibility for gravity assist with any large body. Conservative estimates of remaining fuel were enough to provide only an 18 m/s impulse on the spacecraft. We present our method and results of our systematic scan of potential small body encounters for DI, and our criteria to narrow the selection to the asteroid 2002 GT as the target flyby body. The mission profile has two deterministic maneuvers to achieve the encounter, the first of which executed on November 25, 2011.

Deep Impact (DI)↗
NASA NTRS2011

Comet Ephemerides for the EPOXI and Stardust-NExT Flybys

The recently successful flybys of Comet 103P/Hartley 2 by the Deep Impact spacecraft and Comet 9P/Tempel 1 by the Stardust spacecraft each presented different challenges from the perspective of comet ephemeris prediction and spacecraft targeting. Hartley 2 is a small, highly active comet, with nongravitational accelerations that proved very difficult to model, requiring some amount of "cometchasing" by the spacecraft navigators. In contrast, Tempel 1 is a far larger and less active comet. It showed very stable ephemeris behavior and at flyby was within 1-sigma of predictions issued more than a year prior to encounter. This happenstance was fortuitous because the Stardust spacecraft had very little fuel margin available for comet ephemeris errors.

non-gravitational accelerations↗
NASA NTRS2011

EPOXI: Comet 103p/Hartley 2 Observations from a Worldwide Campaign

Earth- and space-based observations provide synergistic information for space mission encounters by providing data over longer timescales. at different wavelengths and using techniques that are impossible with an in situ flyby. We report here such observations in support of the EPOXI spacecraft flyby of comet 103P (Hartley 2. The nucleus is small and dark, and exhibited a very rapidly changing rotation period. Prior to the onset of activity, the period was approximately 16.4 hr. Starting in 2010 August the period changed from 16.6 hr to near 19 hr in December. With respect to dust composition, most volatiles and carbon and nitrogen isotope ratios, the comet is similar to other Jupiter-family comets. What is unusual is the dominance of CO2-driven activity near perihelion, which likely persists out to aphelion. Near perihelion the comet nucleus was surrounded by a large halo of water-ice grains that contributed significantly to the total water production.

Meech, K. J.↗
NASA NTRS2013

Shape, Density, and Geology of the Nucleus of Comet 103P/Hartley 2

Data from the Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) mission show Comet 103P/Hartley 2 is a bi-lobed, elongated, nearly axially symmetric comet 2.33 km in length. Surface features are primarily small mounds <40 m across, irregularly-shaped smooth areas on the two lobes, and a smooth but variegated region forming a “waist” between the two lobes. Assuming parts of the comet body approach the shape of an equipotential surface, the mean density of Hartley 2 is modeled to be 200–400 kg /cubic m.. Such a mean density suggests mass loss per orbit of >1%. The shape may be the evolutionary product of insolation, sublimation, and temporary deposition of materials controlled by the object’s complex rotation.

Extrasolar↗
NASA NTRS1984

The discovery of 1982 DB, the most accessible asteroid known

A small Apollo object was found while photographing the split comet DuToit 2-Hartley on February 27/28, 1982. Designated 1982 DB, this earth-crossing asteroid passed the earth at a distance of 4.08 million km about 1 month prior to its discovery. Asteroid 1982 DB has been determined to be the most accessible near-earth minor planet known. It provides many excellent opportunities for rendezvous and sample return missions, and a rare dual rendezvous mission with 1980 AA as the second target. For a mission to be realized, opportunities to observe 1982 DB during future apparitions must be taken.

Helin, E. F.↗
NASA NTRS2011

Optical Navigation for the EPOXI Mission

The Deep Impact spacecraft flew by comet Hartley 2 on November 4, 2010 as part of its extended mission called EPOXI. Successful navigation depended critically on the quality and timing of optical navigation data processing, since pictures of the comet provided the most precise comet-relative position of the spacecraft. This paper describes the planning, including the picture timing and pointing; the methods used to determine the center of the comet image in each picture; and the optical navigation results, which provided the necessary information to allow the cameras to accurately target the comet for science imaging at encounter.

small-body mission↗
NASA NTRS2012

EPOXI and Stardust NExT: The Management Challenges of Two Comet Flybys in Three Months

The EPOXI and Stardust NExT missions were missions of opportunity utilizing the Deep Impact and Stardust spacecraft, respectively. These new missions took advantage of the cost savings of utilizing spacecraft that were already flying for new science investigations. Both were retargeted to fly by an additional comet. EPOXI visited Hartley 2, significantly smaller than the other Jupiter family comets visited previously. Stardust NExT flew by Tempel 1, providing a second look at the comet previously studied by Deep Impact in 2005. Both projects were part of NASA's Discovery Program. In order to further save costs, the projects were combined into a single project office at JPL. This provided some efficiencies due to the similarity of the missions, but having the flybys space only three months apart posed challenges for the project management team to ensure each project was ready for its critical event and ensuring each received the proper support from the management team. The project office relied on an integrated calendar for tracking and scheduling meetings, reviews, and other key events. The project management team also coordinated their availability for both projects to maintain involvement with each team to ensure effective risk identification and management.

NExT (New Exploration of Tempel 1)↗
NASA NTRS2011

Risk Balance: A Key Tool for Mission Operations Assurance

The Mission Operations Assurance (MOA) discipline actively participates as a project member to achieve their common objective of full mission success while also providing an independent risk assessment to the Project Manager and Office of Safety and Mission Success staff. The cornerstone element of MOA is the independent assessment of the risks the project faces in executing its mission. Especially as the project approaches critical mission events, it becomes imperative to clearly identify and assess the risks the project faces. Quite often there are competing options for the project to select from in deciding how to execute the event. An example includes choices between proven but aging hardware components and unused but unproven components. Timing of the event with respect to visual or telecommunications visibility can be a consideration in the case of Earth reentry or hazardous maneuver events. It is in such situations that MOA is called upon for a risk balance assessment or risk trade study to support their recommendation to the Project Manager for a specific option to select. In the following paragraphs we consider two such assessments, one for the Stardust capsule Earth return and the other for the choice of telecommunications system configuration for the EPOXI flyby of the comet Hartley 2. We discuss the development of the trade space for each project's scenario and characterize the risks of each possible option. The risk characterization we consider includes a determination of the severity or consequence of each risk if realized and the likelihood of its occurrence. We then examine the assessment process to arrive at a MOA recommendation. Finally we review each flight project's decision process and the outcome of their decisions.

operations↗
NASA NTRS2012

Target Search & Selection for the DI/EPOXI Spacecraft

Upon completion of the Hartley 2 flyby in November 2010, the Deep Impact (DI) spacecraft resided in a solar orbit without possibility for gravity assist with any large body. Conservative estimates of remaining fuel were enough to provide only an 18 m/s impulse on the spacecraft. We present our method and results of our systematic scan of potential small body encounters for DI, and our criteria to narrow the selection to the asteroid 2002 GT as the target flyby body. The mission profile has two deterministic maneuvers to achieve the encounter, the first of which executed on November 25, 2011.

Grebow, Daniel J.↗
NASA NTRS1974

NO and O3 measurements in the lower stratosphere from a U-2 aircraft

In situ measurements of nitric oxide and ozone were carried out at altitudes up to 21 km during two flights of a U-2 aircraft on Dec. 6 and 18, 1973, over the west coast of the United States. A chemiluminescent detector based on the luminescent reaction of nitric acid with excess ozone was used in nitric acid measurements. Ozone concentration measurements were based on the absorption of the Hg line at 2,537 A in the Hartley continuum. The results are compared with available data.

Loewenstein, M.↗