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Small bodies such as the near-Earth asteroid Bennu drift in their orbit due to thermal radiation forces (the Yarkovsky effect). Ground-based observations have indicated a nonzero probability of Bennu impacting Earth, depending on how its orbit evolves. Thus, among the goals of the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission to Bennu were to precisely measure the Yarkovsky effect and refine the impact hazard assessment for this body. Here we address these objectives. Using OSIRIS-REx spacecraft tracking data, we derive meter-level constraints on the distance between Earth and Bennu from January 2019 to October 2020. While these data greatly improve the knowledge of the trajectory of Bennu, they also require an unprecedented fidelity for the modeling of an asteroid’s trajectory. In particular, special care is needed to take into account the contribution of 343 small-body perturbers and the uncertainty in their masses. Radiation effects such as the Poynting–Robertson drag, so far only considered for interplanetary dust dynamics, now become a consideration for modeling the trajectory of a 500-m asteroid such as Bennu. By employing a thermophysical model based on OSIRIS-REx’s characterization of Bennu, we estimate a semimajor axis drift of − 284 . 6 ± 0 . 2 m/yr (signal-to-noise ratio ∼ 1400) at epoch 2011 January 1 caused by the Yarkovsky effect. The largest source of modeling error is solar wind drag, which may lower the magnitude of the semimajor axis drift from the Yarkovsky effect by up to 0.16 m/yr. The Yarkovsky-related semimajor axis drift varies by roughly ± 1 m/yr as the orbit of Bennu evolves due to planetary perturbations from 1900 to 2135. The Yarkovsky thermophysical model proves to be extremely accurate by predicting a bulk density estimate within 0.1% of that estimated through gravity science analysis. Compared to the information available before the OSIRIS-REx mission, the knowledge of the circumstances of the scattering Earth encounter that will occur in 2135 improves by a factor of 20, thus allowing us to rule out many previously possible impact trajectories. However, there remain some impact trajectories compatible with the data. Prior to the spacecraft encounter, the overall impact probability through 2200 was 3 . 7 × 1 0 − 4 (1 in 2700). As a result of our analysis, the cumulative impact probability through 2300 becomes 5 . 7 × 1 0 − 4 (1 in 1750) and the most significant individual impact solution is for September 2182, with an impact probability of 3 . 7 × 1 0 − 4 (1 in 2700). Both Bennu and (29075) 1950 DA have a Palermo scale value of − 1 . 42 and share the distinction as the currently most hazardous object in the asteroid catalog.

Due to its eccentric orbit about Jupiter, Europa experiences periodic tidal deformation, which causes changes in its gravitational field and induces both radial and transverse displacements of the surface. The amplitude and phase of these tidal changes are diagnostic of internal structure, and can be measured with sufficient radiometric and optical tracking of a spacecraft during a series of flyby encounters with Europa. This paper presents results of the simulated accuracy for recovery of the tides of Europa through measuring the second-degree tidal Love numbers k 2 , h 2 , and l 2 . A reference trajectory, which consists of a total of 45 close flybys, was considered and a detailed covariance analysis was performed. The study was based on Earth-based Doppler tracking during ± 2 h of each periapsis passage and surface imaging data taken below 500 km altitude. The result shows that the formal uncertainty of the second-degree tidal Love numbers can be estimated to be σ k 2 = 0.01 , σ h 2 = 0.02 , and σ l 2 = 0.01 , which is sufficient to constrain the global ice thickness to about 10 km under reasonable assumptions. Moreover, the forced librations of Europa can be measured to 0.3″ accuracy, which can further constrain Europa's interior structure.

Deep Impact successfully met its primary mission objective on 4 July 2005 when the smart impactor guided itself into the path of Comet 9P/Tempel 1. The mother flyby spacecraft then observed and recorded the breathtaking collision and subsequent plume development. Ground-based navigators targeted the prerelease trajectory using optical navigation image planning and data analysis, trajectory correction maneuver design and evaluation, and orbit determination using both radiometric and optical data. In-flight improvements to theTempel 1 ephemeriswere also a critical part of the overall navigation design and operations success. The achieved navigation accuracy established a new standard for comet encounters, and this difficult task taught several important lessons. This definitive work provides a mission overview, summarizes the navigation requirements, compares the achieved navigation performance with a baseline design that reflects in-flight updates, and identifies operational procedures that may benefit future comet-bound navigators.

On July 4, 2005 at 05:44:34 UTC the Impactor spacecraft (s/c) impacted comet 9P/Tempel 1 with a relative speed of more than 10 km/s. The Flyby s/c captured the impact event, using both the medium resolution imager and the high resolution imager, and tracked the impact site for the entire observing period following impact. The objective of the Impactor s/c was to impact in an illuminated area viewable from the Flyby s/c and telemeter high-resolution context images of the impact site prior to impact. The Flyby s/c had two primary objectives: (1) capture the impact event in order to observe the ejecta plume expansion dynamics and (2) track the impact site for at least 800 s to observe the crater formation and capture high-resolution images of the fully developed crater. All of these objectives were met by estimating the trajectory of each spacecraft relative to 9P/Tempel 1 using the autonomous navigation system, precise attitude information from the attitude determination and control subsystem, and allowing each spacecraft to independently select the same impact site. This paper describes the challenges of targeting and tracking comet 9P/Tempel 1. © 2007 Wiley Periodicals, Inc.