Living systems have evolved remarkable molecular functions that can be redesigned for in vivo chemical synthesis as we gain a deeper understanding of the underlying biochemical principles for de novo ...construction of synthetic pathways. We have focused on developing pathways for next-generation biofuels as they require carbon to be channeled to product at quantitative yields. However, these fatty acid-inspired pathways must manage the highly reversible nature of the enzyme components. For targets in the biodiesel range, the equilibrium can be driven to completion by physical sequestration of an insoluble product, which is a mechanism unavailable to soluble gasoline-sized products. In this work, we report the construction of a chimeric pathway assembled from three different organisms for the high-level production of n-butanol (4,650 ± 720 mg l⁻¹) that uses an enzymatic chemical reaction mechanism in place of a physical step as a kinetic control element to achieve high yields from glucose (28%).
Dynamical simulations of the coupled rotational and orbital dynamics of binary near-Earth asteroid 66391 (1999 KW4) suggest that it is excited as a result of perturbations from the Sun during ...perihelion passages. Excitation of the mutual orbit will stimulate complex fluctuations in the orbit and rotation of both components, inducing the attitude of the smaller component to have large variation within some orbits and to hardly vary within others. The primary's proximity to its rotational stability limit suggests an origin from spin-up and disruption of a loosely bound precursor within the past million years.
The stability of equilibrium points is investigated for a particle in orbit about a model that mimics a binary asteroid. First, the equations of motion of a particle in the gravitational field of a ...sphere–ellipsoid system are derived. As in the restricted three-body problem (R3BP), five equilibrium solutions are found. In the R3BP, the stability condition for the equilateral points is given by the Routh criterion. However, in this problem, since the non-spherical mass distribution of one of the primaries is taken into account, stability is a function of the mass ratio, the distance between the bodies and the size parameters of the ellipsoid. Analytic stability criteria for the equilibria are derived and presented for a range of these parameters. In general, although some exceptions exist, it is found that the presence of the ellipsoid body reduces the stability region from the R3BP.
The problem of a binary system and a spacecraft in its gravity field is studied. As the mass distribution of the bodies is considered, the two problems are referred as the Full Two Body Problem ...(F2BP) and the Restricted Full Three Body Problem (RF3BP), respectively. The conditions for relative equilibria and their stability in the F2BP were derived for an ellipsoid–sphere system. As the non-equilibrium problem is more common in nature, we look at periodic orbits in the F2BP close to the relative equilibrium conditions. It is found that families of periodic orbits can be computed where the minimum energy state of one family is the relative equilibrium state. An approximation method was derived in order to facilitate the computation of periodic orbits near relative equilibria while keeping the interesting dynamical features. The next step is to make the connection between the dynamics of the RF3BP and the F2BP. In the current paper, we solve for the dynamics of the F2BP and substitute this model in the RF3BP. We provide a basic investigation of the dynamics of a particle in the gravitational field of this binary system. We show results in the F2BP and the RF3BP.
NASA’S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at the near-Earth asteroid (101955) Bennu, a primitive body ...that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth1. Bennu is a low-albedo B-type asteroid2 that has been linked to organic-rich hydrated carbonaceous chondrites3. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Therefore, the primary mission objective is to return a sample of Bennu to Earth that is pristine—that is, not affected by these processes4. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu’s global properties, support the selection of a sampling site and document that site at a sub-centimetre scale5,6,7,8,9,10,11. Here we consider early OSIRIS-REx observations of Bennu to understand how the asteroid’s properties compare to pre-encounter expectations and to assess the prospects for sample return. The bulk composition of Bennu appears to be hydrated and volatile-rich, as expected. However, in contrast to pre-encounter modelling of Bennu’s thermal inertia12 and radar polarization ratios13—which indicated a generally smooth surface covered by centimetre-scale particles—resolved imaging reveals an unexpected surficial diversity. The albedo, texture, particle size and roughness are beyond the spacecraft design specifications. On the basis of our pre-encounter knowledge, we developed a sampling strategy to target 50-metre-diameter patches of loose regolith with grain sizes smaller than two centimetres4. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 metres in extent, the sampling of which poses a substantial challenge to mission success.
Our knowledge of the internal structure of asteroids is, so far, indirect – relying entirely on inferences from remote sensing observations of the surface, and theoretical modeling of formation and ...evolution. What are the bulk properties of the regolith and deep interior? And what are the physical processes that shape asteroid internal structures? Is the composition and size distribution observed on the surface representative of the bulk? These questions are crucial to understand small bodies’ history from accretion in the early Solar System to the present, and direct measurements are needed to answer these questions for the benefit of science as well as for planetary defense or exploration.
Radar is one of the main instruments capable of sounding asteroids to characterize internal structure from sub-meter to global scale. In this paper, we review the science case for direct observation of the deep internal structure and regolith of a rocky asteroid of kilometer size or smaller. We establish the requirements and model dielectric properties of asteroids to outline a possible instrument suite, and highlight the capabilities of radar instrumentation to achieve these observations. We then review the expected science return including secondary objectives contributing to the determination of the gravitational field, the shape model, and the dynamical state. This work is largely inherited from MarcoPolo-R and AIDA/AIM studies.
The gravity field of a small body provides insight into its internal mass distribution. We used two approaches to measure the gravity field of the rubble-pile asteroid (101955) Bennu: (i) tracking ...and modeling the spacecraft in orbit about the asteroid and (ii) tracking and modeling pebble-sized particles naturally ejected from Bennu's surface into sustained orbits. These approaches yield statistically consistent results up to degree and order 3, with the particle-based field being statistically significant up to degree and order 9. Comparisons with a constant-density shape model show that Bennu has a heterogeneous mass distribution. These deviations can be modeled with lower densities at Bennu's equatorial bulge and center. The lower-density equator is consistent with recent migration and redistribution of material. The lower-density center is consistent with a past period of rapid rotation, either from a previous Yarkovsky-O'Keefe-Radzievskii-Paddack cycle or arising during Bennu's accretion following the disruption of its parent body.
Purpose: To use a flat panel electronic portal imaging device (EPID) for intensity map verification as part of an intensity modulated radiation therapy (IMRT) QA program. Method and Materials: An ...a‐Si EPID was used to image each field shape segment of a head and neck IMRT treatment. The DICOM images produced from each gantry angle were summed together and weighted according to the number of MU measured. Open field portal images were acquired at the end of the treatment, for each gantry angle, and subtracted from each composite image. A GUI was created in the MATLAB environment to read and sort each image according to gantry angle and MU measured, perform the image summations/subtractions, produce and display a final composite image. Results: Subtraction of the open field portal image from the field shape composite image removed the anatomy from the treatment area and produced a true intensity map. Outside of the treatment area the anatomy was visible, thus providing verification of correct radiation delivery location. The composite images were compared to other composite images produced using the same treatment plan delivered with and without a phantom and were found to be in excellent agreement. The images were also compared to the intensity map images created in the treatment planning system. Imaging of each segment of an IMRT treatment required no additional radiation be delivered to the patient. The open field portal images require additional radiation, however each image needs only to be acquired once during a course of treatment. Conclusion: This work indicates that flat panel EPIDs may be used to image an IMRT treatment and accurately and efficiently verify the intensity and location of the delivered radiation. Future investigations will use the 2D images to produce a 3D intensity “cloud” image.
Purpose: Megavoltage Cone‐Beam CT (MVCBCT) has recently been introduced in the clinic to improve patient alignment prior to dose delivery. The objective of this research was to evaluate the dose ...delivered to patients for MVCBCT acquisition. We also studied the possibility of making simple plan modifications to compensate for the dose delivered by daily MVCBCT imaging. Method and Materials: Because MVCBCT uses the treatment beam, conventional CT scans (pelvis and head and neck patients) were imported in a treatment planning system (Phillips, Pinnacle) to simulate an MVCBCT acquisition. To validate the dose obtained from Pinnacle, a simple water‐equivalent cylindrical phantom with spaces for MOSFETs and an ion chamber was used to measure the actual dose delivered during MVCBCT. Results: The MVCBCT dose delivered to the phantom, calculated from Pinnacle, was within 3% to all the MOSFET measurement points. The difference between Pinnacle and the ion chamber was 0.2%. For a typical MVCBCT (arc: 270° to 110°) the delivered dose forms an anterior‐posterior gradient. Head and neck patients receive dose ranging from 0.7 to 1.2 cGy per MVCBCT monitor unit (MU). The range is 0.6 to 1.2 cGy per MVCBCT MU for pelvis patients. The total dose for daily positioning using MVCBCT can be reduced and made uniform by alternating between two opposed imaging arcs. Dose‐volume histograms of a compensated plan for a pelvis patient imaged with 10 MU MVCBCTs for 40 fractions show no additional dose to the target and small increases at low doses. Conclusion: Given that clinical MVCBCTs are currently performed at doses ranging from 2–15 MU, simple plan modifications, such as reducing the total number of MU, can be used to nearly eliminate the dose used for daily positioning. Results for other body sites will also be presented. Conflict of Interest: Research sponsored by Siemens OCS.
In the past, small body exploratory science missions sent back images of a variety of asteroids and comets, expanding our understanding and awareness of the vast diversity of these objects in our ...solar system. On its way to Jupiter, the Galileo spacecraft imaged a binary asteroid system: Ida and its moon Dactyl Mathilde was later imaged by the NEAR spacecraft before reaching its main target, the asteroid Eros. Asteroids such as these are typically between 50 and 60 kilometers on their longest axis, and reside within the main-belt asteroid population between Mars and Jupiter. Ida and Dactyl's discovery showed scientists that asteroids can be "mini planetary systems," bringing questions about evolutionary processes within the small body population.
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