Context. Near-Earth asteroid (25143) Itokawa was visited by the Hayabusa spacecraft in 2005, resulting in a highly detailed shape and surface topography model. This model has led to several ...predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should be decelerating. Aims. To detect changes in rotation rate that may be due to YORP-induced radiative torques, which in turn may be used to investigate the interior structure of the asteroid. Methods. Through an observational survey spanning 2001 to 2013 we obtained rotational lightcurve data at various times over the last five close Earth-approaches of the asteroid. We applied a polyhedron-shape-modelling technique to assess the spin-state of the asteroid and its long term evolution. We also applied a detailed thermophysical analysis to the shape model determined from the Hayabusa spacecraft. Results. We have successfully measured an acceleration in Itokawa’s spin rate of dω/dt = (3.54 ± 0.38) × 10-8 rad day-2, equivalent to a decrease of its rotation period of ~45 ms year-1. From the thermophysical analysis we find that the centre-of-mass for Itokawa must be shifted by ~21 m along the long-axis of the asteroid to reconcile the observed YORP strength with theory. Conclusions. This can be explained if Itokawa is composed of two separate bodies with very different bulk densities of 1750 ± 110 kg m-3 and 2850 ± 500 kg m-3, and was formed from the merger of two separate bodies, either in the aftermath of a catastrophic disruption of a larger differentiated body, or from the collapse of a binary system. We therefore demonstrate that an observational measurement of radiative torques, when combined with a detailed shape model, can provide insight into the interior structure of an asteroid. Futhermore, this is the first measurement of density inhomogeneity within an asteroidal body, that reveals significant internal structure variation. A specialised spacecraft is normally required for this.
•New scheme to correct for position and proper motion errors in 19 star catalogs.•Reference catalog subset of PPMXL corresponding to 2MASS based astrometry.•Improvement in residual statistics for ...Apophis, Bennu, Golevka, and Pan-STARRS.•Improvement in ephemeris prediction errors.•New weighting scheme to mitigate correlations between observation errors.
We provide a scheme to correct asteroid astrometric observations for star catalog systematic errors due to inaccurate star positions and proper motions. As reference we select the most accurate stars in the PPMXL catalog, i.e., those based on 2MASS astrometry. We compute position and proper motion corrections for 19 of the most used star catalogs. The use of these corrections provides better ephemeris predictions and improves the error statistics of astrometric observations, e.g., by removing most of the regional systematic errors previously seen in Pan-STARRS PS1 asteroid astrometry. The correction table is publicly available at ftp://ssd.jpl.nasa.gov/pub/ssd/debias/debias_2014.tgz and can be freely used in orbit determination algorithms to obtain more reliable asteroid trajectories.
We review the results of an extensive campaign to determine the physical, geological, and dynamical properties of asteroid (101955) Bennu. This investigation provides information on the orbit, shape, ...mass, rotation state, radar response, photometric, spectroscopic, thermal, regolith, and environmental properties of Bennu. We combine these data with cosmochemical and dynamical models to develop a hypothetical timeline for Bennu's formation and evolution. We infer that Bennu is an ancient object that has witnessed over 4.5 Gyr of solar system history. Its chemistry and mineralogy were established within the first 10 Myr of the solar system. It likely originated as a discrete asteroid in the inner Main Belt approximately 0.7–2 Gyr ago as a fragment from the catastrophic disruption of a large (approximately 100‐km), carbonaceous asteroid. It was delivered to near‐Earth space via a combination of Yarkovsky‐induced drift and interaction with giant‐planet resonances. During its journey, YORP processes and planetary close encounters modified Bennu's spin state, potentially reshaping and resurfacing the asteroid. We also review work on Bennu's future dynamical evolution and constrain its ultimate fate. It is one of the most Potentially Hazardous Asteroids with an approximately 1‐in‐2700 chance of impacting the Earth in the late 22nd century. It will most likely end its dynamical life by falling into the Sun. The highest probability for a planetary impact is with Venus, followed by the Earth. There is a chance that Bennu will be ejected from the inner solar system after a close encounter with Jupiter. OSIRIS‐REx will return samples from the surface of this intriguing asteroid in September 2023.
•We deal with the short-term hazard from newly discovered asteroids.•We present an orbit determination technique suitable for short observation arcs.•Systematic ranging allows the early detection of ...short-term impact threats.•We developed a system to routinely analyze objects on the MPC NEO Confirmation Page.
We describe systematic ranging, an orbit determination technique suitable to assess the near-term Earth impact hazard posed by newly discovered asteroids. For these late warning cases, the time interval covered by the observations is generally short, perhaps a few hours or even less, which leads to severe degeneracies in the orbit estimation process. The systematic ranging approach gets around these degeneracies by performing a raster scan in the poorly-constrained space of topocentric range and range rate, while the plane of sky position and motion are directly tied to the recorded observations. This scan allows us to identify regions corresponding to collision solutions, as well as potential impact times and locations. From the probability distribution of the observation errors, we obtain a probability distribution in the orbital space and then estimate the probability of an Earth impact. We show how this technique is effective for a number of examples, including 2008 TC3 and 2014 AA, the only two asteroids to date discovered prior to impact.
Many boulders on (101955) Bennu, a near‐Earth rubble pile asteroid, show signs of in situ disaggregation and exfoliation, indicating that thermal fatigue plays an important role in its landscape ...evolution. Observations of particle ejections from its surface also show it to be an active asteroid, though the driving mechanism of these events is yet to be determined. Exfoliation has been shown to mobilize disaggregated particles in terrestrial environments, suggesting that it may be capable of ejecting material from Bennu's surface. We investigate the nature of thermal fatigue on the asteroid, and the efficacy of fatigue‐driven exfoliation as a mechanism for generating asteroid activity, by performing finite element modeling of stress fields induced in boulders from diurnal cycling. We develop a model to predict the spacing of exfoliation fractures and the number and speed of particles that may be ejected during exfoliation events. We find that crack spacing ranges from ~1 mm to 10 cm and disaggregated particles have ejection speeds up to ~2 m/s. Exfoliation events are most likely to occur in the late afternoon. These predictions are consistent with observed ejection events at Bennu and indicate that thermal fatigue is a viable mechanism for driving asteroid activity. Crack propagation rates and ejection speeds are greatest at perihelion when the diurnal temperature variation is largest, suggesting that events should be more energetic and more frequent when closer to the Sun. Annual thermal stresses that arise in large boulders may influence the spacing of exfoliation cracks or frequency of ejection events.
Plain Language Summary
Soon after its rendezvous with the asteroid Bennu, the OSIRIS‐REx spacecraft observed the asteroid to be ejecting tiny particles of material. Bennu is a rubble‐pile asteroid covered in boulders of varying size. Many of these boulders show evidence of exfoliation, a process where thin layers of material are shed from their surfaces. Exfoliation is one consequence of thermal fatigue, which is the slow and progressive lengthening of cracks caused by the daily variation in boulder temperature from exposure to the Sun. Here we explore how thermal fatigue may cause the degradation and fracturing of boulders on Bennu and how the specific process of exfoliation could lead to the ejection of particles from the asteroid surface. We develop a model to predict the timing, number, and speeds of particles that may be ejected during exfoliation events and compare our results to the spacecraft observations of the ejection events from Bennu's surface. Our results suggest that particles ejected from boulder surfaces during exfoliation can have speeds up to ~2 m/s and are most likely occur when Bennu is closest to the Sun and during the late afternoon, consistent with spacecraft observations.
Key Points
We simulated stress fields in boulders to assess the nature and efficacy of thermal breakdown on Bennu, including by exfoliation
Our model predicts that exfoliation is capable of ejecting centimeter‐scale particles from the asteroid at speeds of meters per second
This mechanism is consistent with observations of particle ejection at Bennu and is a viable explanation for Bennu's activity
In May of 2011, NASA selected the
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rigins,
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pectral
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nterpretation,
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esource
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dentification, and
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ecurity–
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egolith
Ex
plorer (OSIRIS-REx) asteroid sample return mission as the third mission ...in the New Frontiers program. The other two New Frontiers missions are
New Horizons
, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and
Juno
, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.
The geophysical environment of Bennu Scheeres, D.J.; Hesar, S.G.; Tardivel, S. ...
Icarus (New York, N.Y. 1962),
09/2016, Volume:
276
Journal Article
Peer reviewed
•The OSIRIS-REx mission to Asteroid Bennu will achieve an unparalleled investigation of a small, primitive rubble pile asteroid.•The geophysics of these bodies is largely unknown and unconstrained, ...and will shed fundamental new insight into the geophysical evolution of primitive materials in the solar system.•This paper sets the stage for the geophysical investigation of this body, laying out a range of possible hypotheses for its characteristic shape that ensures its future relevance once the actual measurements are made.
An analysis of the surface and interior state of Asteroid (101955) Bennu, the target asteroid of the OSIRIS-REx sample return mission, is given using models based on Earth-based observations of this body. These observations have enabled models of its shape, spin state, mass and surface properties to be developed. Based on these data the range of surface and interior states possible for this body are evaluated, assuming a uniform mass distribution. These products include the geopotential, surface slopes, near-surface dynamical environment, interior stress states and other quantities of interest. In addition, competing theories for its current shape are reviewed along with the relevant planned OSIRIS-REx measurements.
We observed the near‐Earth asteroid (101955) Bennu from the ground in 1999 and 2005, and with the Hubble Space Telescope (HST) in 2012, to constrain its rotation rate. The data reveal an acceleration ...of 2.64±1.05 × 10−6deg/day2, which could be due to a change in the moment of inertia of Bennu or to spin up from the Yarkovsky‐O'Keefe‐Radzievskii‐Paddack effect or other source of angular momentum. The best solution is within 1 σ of the period determined by Nolan et al. (2013, https://doi.org/10.1016/j.icarus.2013.05.028). The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS‐REx) mission will determine the rotation state independently in 2019.Those measurements should show whether the change in rotation rate is a steady increase (due, e.g., to the Yarkovsky‐O'Keefe‐Radzievskii‐Paddack effect) or some other phenomenon. The precise shape and surface properties measured by the OSIRIS‐REx science team will allow for a better understanding of variations in rotation rate of small asteroids.
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