Engineering topics
Publications and source records attributed to Yeomans, D. K..
The criteria used to select a short period comet for possible future rendezvous space missions are stated and the selection process is outlined. For the time period 1900-2000, several candidate comets offer opportunities for spacecraft rendezvous. Two of the best candidates are periodic comets Kopff and Wild 2.
Modern techniques for making cometary astrometric observations, reducing these observations, using accurate reference star catalogs, and computing precise orbits and ephemerides are discussed in detail and recommendations and suggestions are given in each area.
In an effort to test the techniques and communication lines of the entire International Halley watch (IHW) organization, comet Crommelin was selected as a comet that would act as a trial run for the future observations of comets Halley and Giacobini-Zinner. Astrometry network members were asked to provide a few observations of comet Crommelin during a period within a few months on either side of the comet's perihelion passage. Special comet Crommelin star catalogs were distributed and each observer was asked to observe, reduce and transmit their data as quickly as possible. Some 372 astrometric observations from 42 different observatories were received at the orbit determination center. Orbit information and up-to-date ephemerides were computed and distributed by the same communication channels. The comet Crommelin trial run was successful in pointing out some areas that need to be improved to make the data more accurate and the communication channels more efficient.
Ongoing activities to improve the orbits and ephemerides of comets Halley and Giacobini-Zinner are outlined. The nongravitational acceleration model of Marsden et al (1973) is used to represent the motion of these two comets over their observed intervals, and recent orbital updates are presented.
Efforts to assure communications continuity and navigational accuracy for the ISEE-3 (now called the International Cometary Explorer - ICE) spacecraft as it heads toward a passage through the Giacobini-Zimmer (G-Z) comet tail are reviewed. Sufficient energy was gained with a series of lunar swingbys to take the rendezvous attempt possible. Communications will be by an S-band link though the upgraded Deep Space Network. The G-Z encounter is scheduled for September 1983. A 512 bps data rate will be supported by 64-3 m dishes at various global sites. The tracking will continue for 6 mos as the spacecraft moves into an orbit that parallels that of Halley's Comet. Ground-based recovery of the G-Z location in the spring of 1984 provided data for calculating the ephemerides required for configuring a major course correction for ICE 100 days before rendezvous.
A computer search for earth-approaching comets among those listed in Marsden's (1983) updated orbit catalog has identified 36 cases at which minimum separation distance was less than 2500 earth radii. A strong representation of short period comets in the sample is noted, and the constant rate of the close approaching comets in the last 300 years is interpreted to suggest the lack of long-period comets intrinsically fainter than an absolute magnitude of about 11. A comet-earth collision rate derived from the statistics of these close encounters implies an average period of 33-64 million years between any two events. This rate is comparable with the frequency of geologically recent global catastrophes which appear to be associated with extraterrestrial object impacts, such as the Cretaceous-Tertiary extinction 65 million years ago and the late Eocene event 34 million years ago.
For the planned flyby missions to Comet Halley in March 1986, the comet's ephemeris uncertainties completely dominate the spacecraft-comet miss distance. In an effort to determine realistic Comet Halley ephemeris uncertainties, a statistical covariance analysis was conducted using the actual data in 1909-1910-1911 and simulated data in 1984-1985-1986. In 1985-1986, Comet Halley's ephemeris uncertainties are very sensitive to the comet's orbital position, the optical data noise, data schedule, and whether or not the old data is included in the orbital solutions. The comet's ephemeris uncertainties in March 1986 are relatively insensitive to reasonable center of light/center of mass offsets and also to possible radar data taken in late November 1985. Accurate Space Telescope observations made in early March 1986 might significantly improve upon the comet's position uncertainties for the various intercepting spacecraft.
A ballistic sample return mission to Comet Halley was recently considered by the United States. This paper describes the navigation system and the navigation strategy which would have been employed in such a mission, assuming a launch in August of 1985, an arrival at the comet in March of 1986, and a return to earth in August of 1989. Estimates of spacecraft comet- and earth-relative orbit determination accuracies are presented as functions of time. Target-relative delivery accuracies for the spacecraft and associated trajectory correction maneuver requirements are presented also.
Since 240 B.C., Chinese observers have documented a nearly unbroken record of scientifically useful observations of Periodic Comet Halley (P/Halley). Investigations of the comet's motion by Western astronomers are discussed, taking into account the first successful prediction of a cometary return by Halley (1705), computations conducted by Rosenberger (1830), and studies performed by Cowell and Crommelin (1910). Comet Halley's motion and nongravitational forces are considered along with meteor showers associated with P/Halley. The physical properties of P/Halley are examined, giving attention to the visual observations, the light curve of P/Halley, the coma, the tails, direct photographs, spectrograms, and the emission spectrum of P/Halley. Other subjects explored are related to the cometary nucleus, the mass of P/Halley, the rotation period and axial inclination, the composition, a nominal model of P/Halley's coma, and plans for investigations in connection with the coming apparition of Comet Halley.
The determination of cometary ephemerides is considered with reference to proposed missions to Comet Halley. It is noted that the ability of spacecraft without onboard navigation capability to fly close to target comets is limited chiefly by the comet's ephemeris uncertainty. The situation is complicated by nongravitational forces acting on a comet's nucleus and the paucity of observers currently making astrometric observations of comets. The nongravitational forces affecting Comet Halley are consistent with the rocket effect of an outgassing water ice nucleus; the nucleus is apparently rotating in a direct sense about a stable spin axis. It is emphasized that accurate Comet Halley ephemerides for close spacecraft flybys will require continued efforts to refine the existing nongravitational force model. In addition, the flyby mission to Comet Halley will require a well-organized network of astrometric observers. For this purpose, an Astrometry Network is being set up within NASA's International Halley Watch program.
Beginning with studies related to the 1835 appearance of comet Halley, it became apparent that comets may undergo substantial nongravitational perturbations due to the rocket-like effect of an outgassing cometary nucleus. Marsden (1968, 1969) modeled these nongravitational effects with a semiempirical term in the cometary equations of motion. The most successful orbital solution was consistent with an outgassing water ice nucleus in direct rotation. For the conduction and coordination of accurate astrometric observations of comet Halley, an Astrometry Network is being established within the International Halley Watch program. Attention is given to the support of the Halley flight projects provided by the Astrometry Network, and the support of ground based observers by this network.
The orbital motion of comet Halley is numerically integrated back to 1404 BC. Starting with an orbit based on the 1759, 1682, and 1607 observations of the comet, the integration was run back in time with full planetary perturbations and nongravitational forces taken into account at each 0.5 day time-step. Small empirical corrections were made to the computed perihelion passage time in 837 and to the osculating orbital eccentricity in 800. In nine cases, the perihelion passage times calculated by Kiang (1971) from Chinese observations have been redetermined, and osculating orbital elements are given at each apparition from 1910 back to 1404 BC.
With the use of narrow field-of-view instrumentation on faint comets, the accuracy requirements upon computed ephemerides are increasing. It is not uncommon for instruments with a one arc minute field-of-view to be tracking a faint comet that is not visible without a substantial integration time. As with all ephemerides of solar syste objects, the computed motion and reduction of these observations, the computed motion of a comet is further depenent upon effects related to the comet's activity. Thus, the ephemeris of an active comet is corrupted by both observational errors and errors due to the comet's activity.
Radar observations of the asteroid Apollo, at 3.5 cm wavelength, indicate a radius of 600 m and a rotation period of 3.0 hr. The data are consistent with a rough surface of either hard ice or of regolith softened rock.
The distribution of dust surrounding periodic comet Tempel-Tuttle has been mapped by analyzing the associated Leonid meteor shower data over the 902-1969 interval. The majority of dust ejected from the parent comet evolves to a position lagging the comet and outside the comet's orbit. Hence, it appears that radiation pressure and planetary perturbations, rather than ejection processes, control the dynamic evolution of the Leonid particles. Significant Leonid meteor showers are possible roughly 2500 days before or after the parent comet reaches perihelion, but only if the comet passes closer than 0.025 AU inside or 0.010 AU outside the earth's orbit. Although the conditions in 1998-1999 are optimum for a significant Leonid meteor shower, the event is not certain because the dust particle distribution near the comet is far from uniform. The orbit of comet Tempel-Tuttle has been redetermined for the 1366-1966 observed interval.
The orbit of Comet Halley is described as well as its expected physical behavior (brightness, tail lengths, coma diameters) in 1985-1986 during which time its preperihelion positon will allow better conditions for Northern Hemisphere observers. Southern Hemisphere observers will prefer post perihelion observation. Ephemeris data for 1981-1987 are presented in tables.
The broad impact of a cometary mission is assessed with particular emphasis on scientific interest in a fly-by mission to Halley's comet and a rendezvous with Tempel 2. Scientific results, speculations, and future plans are discussed.
The dynamical behavior of comet Tempel 2 is investigated and the comet is found to be very well behaved and easily predictable. The nongravitational forces affecting the motion of this comet are the smallest of any comet that is affected by nongravitational forces. The sign and time history of these nongravitational forces imply (1) a direct rotation of the comet's nucleus and (2) the comet's ability to outgas has not changed substantially over its entire observational history. The well behaved dynamical motion of the comet, the well observed past apparitions, the small nongravitational forces and the excellent 1988 ground based observing conditions all contribute to relatively small position and velocity errors in 1988 -- the year of a proposed rendezvous space mission to this comet. To assist in planned ground based and earth orbital observations of this comet, ephemerides are given for the 1978-79, 1983-84 and 1988 apparitions.