Engineering topics
Johnston, M. D.
Publications and source records attributed to Johnston, M. D..
The Mars Reconnaissance Orbiter Mission: 10 years of exploration from Mars orbit
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The Mars Reconnaissance Orbiter Mission
This paper provides the current status of MRO, including a summary of the launch activities, a detailed description of the cruise phase, and a brief summary of the remaining mission phases
Overview of the Mars Reconnaissance Orbiter Mission
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The Mars Reconnaissance Orbiter Mission
The Mars Reconnaissance Orbiter (MRO) will be launched in August 2005 by an Atlas HIB expendable launch vehicle from Cape Canaveral Air Force Station. It will deliver to Mars orbit a payload to conduct remote sensing science observations, identify and characterize sites for future landers, and provide critical telecom/navigation relay capability for follow-on missions.
Trajectory design for the Mars Reconnaissance Orbiter mission
This paper describes the analysis and design evolution of the Mars Reconnaissance Orbiter trajectory from launch to end-of-mission.
Primary science orbit design for the Mars Reconnaissance Orbiter mission
This paper addresses the constraints, analysis and design evolution of the Mars Reconnaissance Orbiter Primary Science Orbit.
Trajectory design for the Mars Reconnaissance Orbiter Mission
This paper describes the analysis and design evolution of the Mars Reconnaissance Orbiter trajectory from launch to end-of-mission. The mission uses a combination of propulsive maneuvers and aerobraking techniques to deliver the orbiter to a low-altitude frozen orbit at Mars. The launch/arrival space for the 2005 Earth-Mars opportunity will be described as well as the particular launch strategy chosen for this mission. Details of the aerobraking profile will be provided. Finally, the Primary Science Orbit will be examined, and the trade-offs between science objectives and orbiter capability will be presented.
The Strategy for the Second Phase of Aerobraking Mars Global Surveyor
On February 19, 1999, the Mars Global Surveyor (MGS) spacecraft was able to propulsively establish its mapping orbit. This event followed the completion of the second phase of aerobraking for the MGS spacecraft on February 4, 1999. For the first time, a spacecraft at Mars had successfully employed aerobraking methods in order to reach its desired pre-launch mapping orbit. This was accomplished despite a damaged spacecraft solar array. The MGS spacecraft was launched on November 7, 1996, and after a ten month interplanetary transit was inserted into a highly elliptical capture orbit at Mars on September 12, 1997. Unlike other interplanetary missions, the MGS spacecraft was launched with a planned mission delta-V ((Delta)V) deficit of nearly 1250 m/s. To overcome this AV deficit, aerobraking techniques were employed. However, damage discovered to one of the spacecraft's two solar arrays after launch forced major revisions to the original aerobraking planning of the MGS mission. In order to avoid a complete structural failure of the array, peak dynamic pressure levels for the spacecraft were established at a major spacecraft health review in November 1997. These peak dynamic pressure levels were roughly one-third of the original mission design values. Incorporating the new dynamic pressure limitations into mission replanning efforts resulted in an 'extended' orbit insertion phase for the mission. This 'extended' orbit insertion phase was characterized by two distinct periods of aerobraking separated by an aerobraking hiatus that would last for several months in an intermediate orbit called the "Science Phasing Orbit" (SPO). This paper describes and focuses on the strategy for the second phase of aerobraking for the MGS mission called "Aerobraking Phase 2." This description will include the baseline aerobraking flight profile, the trajectory control methodology, as well as the key trajectory metrics that were monitored in order to successfully "guide' the spacecraft to its desired mapping orbit. Additionally, the actual aerobraking progress is contrasted to the planned aerobraking flight profile. (A separate paper will describe the navigation aspects of MGS aerobraking in detail.) Key to the success of the MGS mission is the delivery of the spacecraft to its final mapping orbit and the synergy the instrument complement provides to its scientific investigators when science data is returned from that orbit. The MGS mapping orbit is characterized as a low altitude, near-circular, near-polar orbit that is Sun-synchronous with the descending equatorial crossing at 2:00 AM local mean solar time (LMST).
Mission design for an Orbiting Volcano Observatory
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Explosive nucleosynthesis in massive stars - Comparison with the Cassiopeia A fast-moving knots
If the ejecta of a Type II supernova do not undergo extensive mixing, then, based on the explosion of current presupernova models, only a small fraction approximately equal to or less than 0.1 solar mass of the mantle of a massive star can yield abundances similar to those observed in the fast-moving knots of Cas A. This is shown to be independent of the detailed structure of the mantle and the supernova energy. Lack of mixing in Cas A is indicated by strong upper limits on the abundance ratios Ne/O, and Fe/O. If this is confirmed by further observations, then either Cas A is not the result of a standard progenitor of approximately equal to or less than 25 solar masses disrupted by a Type II supernova, or the picture of the last stages of stellar evolution in massive stars needs to be modified substantially.
Identification of the fast, high galactic latitude X-ray transient A 0000 + 28 with the RS CVn system HD 224085
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The Ophiuchus cluster - A bright X-ray cluster of galaxies at low galactic latitude
The discovery of an extended X-ray source identified with a cluster of galaxies at low galactic latitude is reported. The source, designated the Ophiuchus cluster, was detected near 4U 1708-23 with the HEAO 1 Scanning Modulation Collimator, and identified with the cluster on the basis of extended X-ray size and positional coincidence on the ESO/SRC (J) plate of the region. An X-ray flux density in the region 2-10 keV of approximately 25 microJ was measured, along with an X-ray luminosity of 1.6 x 10 to the 45th ergs/sec and an X-ray core radius of approximately 4 arcmin (0.2 Mpc) for an assumed isothermal sphere surface brightness distribution. The X-ray spectrum in the range 2-10 keV obtained with the HEAO 1 A-2 instrument is well fit by a thermal bremsstrahlung model with kT = 8 keV and a 6.7-keV iron line of equivalent width 450 eV. The steep-spectrum radio source MSH 17-203 also appears to be associated with the cluster, which is the closest and brightest representative of the class of X-ray clusters with a dominant central galaxy.
The chemical abundances of the Cassiopeia A fast-moving knots - Explosive nucleosynthesis on a minicomputer
A simplified nuclear reaction network for explosive nucleosynthesis calculations is described in which only the most abundant nuclear species and the most important reactions linking these species are considered. This scheme permits the exploration of many cases without excessive computational effort. Good agreement with previous calculations employing more complex reaction networks is obtained. This scheme is applied to the observed chemical abundances of the fast-moving knots in the supernova remnant Cassiopeia A and it is found that a wide range of initial conditions could yield the observed abundances. The abundances of four of the knots with significant and different amounts of elements heavier than oxygen are consistent with an origin in material of the same initial composition but processed at different peak temperatures and densities. Despite the observed high oxygen abundances and low abundances of light elements in the knots, they did not necessarily undergo incomplete oxygen burning; in fact, it is not even necessary that oxygen have been present in the initial composition. The agreement between the calculated and observed chemical abundances in Cas A and similar supernova remnants depends primarily upon the relevant nuclear physics and does not provide strong evidence in favor of any particular model of the supernova event.
Optical identification of H0123 + 075 and 4U 1137-65 - Hard X-ray emission from RS Canum Venaticorum systems
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Unidentified galactic X-ray sources positioned with the HEAO-1 scanning modulation collimator
Observations made with the HEAO-1 scanning modulation collimator are used to determine the positions of 16 unidentified X-ray sources at low galactic latitudes. Positions were obtained to within 10-30 arcsec in one dimension from two four-grid collimator observations made at six-month intervals. Positions determined in addition for eight sources identified with optical objects verify the method employed. Of the unidentified sources, the positional uncertainties of ten are reduced by factors of two to eight, the precise X-ray positions (within 10 arcsec) obtained by lunar occultation experiments are confirmed for three and multiple precise positions (within 20 arcsec) which reduce the previous location uncertainties by factors of greater than 100 are obtained for the remaining three.
HEAO 1 identification of the X-ray emitting poor cluster of galaxies, AWM 7
The X-ray source 2A 0251 + 413 is identified with the AWM 7 group of galaxies. The source is extended, with a FWHM of roughly 9 arcmin if an isothermal sphere profile is assumed. The spectrum is best fitted by thermal bremsstrahlung at 4.0 + or - 0.5 keV, with a significant Fe line emission of equivalent width 0.63 keV centered at 6.6 keV. This temperature is consistent with a measured dispersion velocity of 600 km/s of the central galaxies.