Vega Spacecraft Research Articles

Comets in the space age

view Abstract Citations (24) References (101) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Comets in the space age Whipple, F. L. Abstract The historical development of the study of the nature of comets and their origin is discussed, emphasizing the use of aerospace technology in cometary science. The use of satellites to study the Comet Kohoutek 1973 XII, advances between Kohoutek and P/Halley, and studies of P/Halley during its 1986 return are examined. Consideration is given to data from ground, air, and space sensors, and from the Giotto and Vega spacecraft missions. Also, the physical structure of the nucleus of Comet Halley is described. Publication: The Astrophysical Journal Pub Date: June 1989 DOI: 10.1086/167466 Bibcode: 1989ApJ...341....1W Keywords: Comet Nuclei; Cometary Atmospheres; Comets; Giotto Mission; Light Curve; Vega Project; Fractals; Interplanetary Dust; Kohoutek Comet; Magnetohydrodynamics; COMETS; HISTORY; PHYSICAL PROPERTIES; COMET NUCLEI; ORIGIN; FORMATION; DIAGRAMS; KOHOUTEK; HALLEY; ASTRONOMY; LIGHTCURVES; STRUCTURE; COMAE; COMET TAILS; SHAPE; COMPOSITION; DUST; SPACECRAFT OBSERVATIONS; REVIEW; EARTH-BASED OBSERVATIONS; GRAINS; MODELS; HYPOTHESES; COMETESIMALS; COMPARISONS; SATELLITE OBSERVATIONS; Astrophysics; Comets full text sources ADS |

Confirmation of dust clusters in the coma of Comet Halley

Analysis of the time of arrival and mass distributions of Comet Halley dust particles based on measurements by the dust counter and mass analyzer (DUCMA) on the VEGA-1 and VEGA-2 spacecraft has led to the discovery that the time distribution of dust particle detection is not random, both within and beyond the dust envelopes. The dust is detected as ‘clusters’ of particles (∼ 10 s duration). We now report a preliminary analysis of the data from the dust instrument of the Space Research Institute, Moscow (SP-1), which was also onboard the VEGA spacecrafts and whose mass range overlaps that of DUCMA (∼ 10 −13 g to ≥ 10 −10 g). We report that the sequence of pulses recorded by the SP-1 instruments in some cases show deviations from a random distribution and that some of the non-random sequences (‘clusters’) observed by SP-1 arrive in the same time intervals as clusters observed by the DUCMA instrument. These results support the earlier conclusion of Simpson et al . that breakup of conglomerates of dust particles is an important phenomenon both within and beyond the coma envelope.

Pair of plasma discontinuities inside Halley's bow shock as seen by the plasma wave experiment

Plasma wave experiment APV-N (Vega mission) has detected clear effects correlating with two internal boundaries (at ∼770 000 and ∼350 000 km from the nucleus) of the comet Halley plasma environment. Important signatures of these boundaries were found by some other experiments aboard the Giotto and Vega spacecraft. Taking into account all the effects we have proposed the following interpretation: the charge-exchange cooling of cometary plasma leads to formation of a rarefaction wave (the region between ∼900 and ∼800 000 km) and of a weak shock wave (probably at ∼350 000 km) in the supersonic region inside the bow shock flanks: at the inner edge of the rarefaction wave as well as at the shock front the nonlinear (solitary) waves, with length ∼20 000 km, are generated; reflection of ions at the nonlinear waves gives rise to the cumulation of α-particles, plasma turbulence, electron heating.

Impacts of large dust particles on the Vega spacecraft

The two Vega spacecraft carried electric field and Langmuir probes on booms extending from the outer edges of the solar panels. Impulsive signal increases were observed simultaneously with all sensors and occurred more frequently towards closest approach. These events have been interpreted as impacts of dust particles (m > 10 −10 g) on the spacecraft, which acted as a large dust collector, with a cross-section area of about 10 m 2 perpendicular to the ram direction. These results are discussed on the background of other Vega dust measurements.

Multifluid simulations of the solar wind-comet interaction

Solar wind-comet interaction is studied in an multifluid approach which treates electrons, protons and cometary ions as separate fluids coupled by macroscopic electromagnetic fields. As a striking feature, the model predicts the absence of a staedy-state cometary bow shock, contrary to results of single fluid studies. The solar wind is gradually slowed down due to mass loading up to the Alfvenic Mach point (≈ 10 5km from the nucleus for Halley conditions). The calculated plasma structures in this region show many similarities with the features observed by the VEGA and GIOTTO spacecrafts near the so-called cometopause, which is suggested to represent a multifluid shock. In addition, travelling weak shocks have been found to occur in the super-Alfvenic region as transient phenomena. Many of their properties are consistent with observations which hithertoo have been interpretet as cometary bow shock. Hybrid code simulations support the model results concerning travelling multifluid shocks.

Dust photometry in the near nucleus region of Comet Halley

The dust-jet radial brightness has been analysed along jet cores on images taken by the TV experiment aboard VEGA-2 spacecraft on 9 March 1986 during the close encounter with Comet Halley. Applying the RFIT code - used in reactor physics - to the data, it has been shown that there is a breakpoint in the radial brightness distribution which occurs at about 40 km above the surface. We interpret this result as being due to heat-shock included desintegration of dust particles as they are ejected into the near-nucleus environment.

Solar System VLBI Astrometry: The Venus Balloon Experiment

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere by the VEGA spacecraft.

Electron impact ionization in the vicinity of comets

The solar wind interacts very strongly with the extensive cometary coma, and the various interaction processes are initiated by the ionization of cometary neutrals. The main ionization mechanism far outside the cometary bow shock is photoionization by solar extreme ultraviolet radiation. Electron distributions measured in the vicinity of comets Halley and Giacobini‐Zinner by instruments on the VEGA and ICE spacecraft, respectively, are used to calculate electron impact ionization frequencies. Ionization by electrons is of comparable importance to photoionization in the magnetosheaths of Comets Halley and Giacobini‐Zinner. The ionization frequency in the inner part (radial distance ≈ 104 km) of the cometary plasma region of comet Halley is several times greater than the photoionization value. Tables of ionization frequencies as functions of electron temperature are presented for H2O, CO2, CO, O, N2, and H.

Thermooptical processes in mirror-lens objectives. I. Scheme for synthesis of a thermostable telescope

Special features of the designing of a thermostable telescope are discussed on the example of the objective for the narrow-angle television camera of the Vega spacecraft.

Initial estimates of Venus winds from ground-based radio tracking of the VEGA balloons

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere by the VEGA spacecraft. Initial analysis of only radial velocities indicates that each balloon was blown westward about 11,500 kilometers (8000 kilometers on the night side) by zonal winds with a mean speed of about 70 meters per second. Excursions of the data from a model of constant zonal velocity are generally less than 3 meters per second; however, a much larger variation is evident near the end of the flight of the second balloon. Consistent systematic trends in the residuals for both balloons indicate the possibility of a solar-fixed atmospheric feature.

Vega landing sites: Venera 15/16 unit analogs from Pioneer Venus reflectivity and RMS slope data

Pioneer Venus radar data on surface properties have been used to compare the Vega spacecraft landing sites with the northern 1/4 of Venus mapped by the orbiters Venera 15 and 16. The regions surrounding both landing sites possess surface reflectivity and small‐scale roughness properties most similar to those of mapped volcanoes and volcanic plains regions and different surface properties than those of mapped tectonic units. Regions analogous to the Vega 1 site are relatively rare, covering 2.8% of the mapped surface. Vega 2 analogs are much more common and cover 22.6% of the surface. Neither landing site is representative of the nearby highlands of Aphrodite, but the Vega 2 landing site is similar to much of the northern plains of Venus.

Vega project: Re-entry vehicle of “VEGA” spacecraft

The VEGA mission aims at an encounter with the comet Halley after a “fly by” over Venus, during which part of the spececraft enters the atmosphere, divided in a Lander and Cruising Balloon. The paper gives a short description of both, as well as the re-entry, landing and cruising programs. After a definition of the preliminary ground tests, the main results obtained by the two VEGA spacecraft are listed.

Comet Halley dust environment from SP-2 detector measurements

The SP-2 experiment carried by the Vega spacecraft during their encounter with comet Halley was designed to determine the spatial distribution of cometary dust particles and their mass distribution over the range ∼10−16–10−6 g. The first infrequent dust particle impacts recorded by the SP-2 sensors indicated that Vega 1 entered the cometary dust coma at a distance of ∼280,000 km from the nucleus. Although the subsequent rapid and even spectacular developments were not unexpected and the results of the in situ measurements agree well with the general predictions of cometary dust environment models1,2, a number of remarkable features were revealed, such as the high density of sub-micrometre-sized particles, considerable variations in the particle mass distribution observed in different regions of the coma and particularly in jets, and the asymmetry relative to the comet–Sun line.

Pathfinder: accuracy improvement of comet Halley trajectory for Giotto navigation

The ‘Pathfinder’ project represents a cooperative effort involving the space agencies Intercosmos (Soviet Union), the National Aeronautics and Space Administration (NASA; United States) and the European Space Agency (ESA; Europe) for the benefit of the Giotto mission, the context of which was the stringent requirement for Giotto to fly past comet Halley at a distance of ∼500 km on the sunward side. The uncertainties1–5 in the comet's trajectory were such that this requirement could not be fulfilled with the necessary accuracy using only ground-based astronomical observations; early studies6–8,20 revealed that a considerable improvement could be attained by incorporating space-borne observations. As the two Vega spacecraft were to reach the comet a few days before Giotto, optical observations performed by the Vega cameras could be used for this purpose. These, combined with a very accurate determination of the Vega trajectories using VLBI (very-long-baseline interferometry)9,10, permitted a final fly-by distance of 600km to be achieved with an uncertainty of only ±40 km (1σ probability level).

Dust counter and mass analyser (DUCMA) measurements of comet Halley's coma from Vega spacecraft

The Vega encounters with comet Halley allowed the first direct measurements of the spatial and temporal distributions of the masses and fluxes of dust particles originating from the comet nucleus. These data are fundamental for establishing the physical processes of dust emission from the nucleus, their propagation to form a coma, and the behaviour of dust jets. The measurements reported here were made with instruments employing a new principle of dust detection1,2, which have a high time-resolution (∼4µs) over a large range of dust fluxes and masses. The dust coma, whether quiescent (as seen by Vega 2) or containing a major jet structure (as seen by Vega 1), displays large, short-term variations throughout, which are at times quasi-periodic. The integral mass spectra increase in intensity to the lowest masses measured (contrary to some theoretical models), and the flux levels lie approximately in the ranges estimated previously from ground-based observations6. The coma is highly dynamical on all spatial and temporal scales, suggesting a complex structure of localized regions of dust emission from the nucleus.

Television observations of comet Halley from Vega spacecraft

The television system (TVS) aboard the spacecraft Vega 1 and Vega 2 observed comet Halley from 4 to 11 March 1986, and transmitted ∼1,500 images to Earth. The best images of the cometary nucleus were obtained from distances of 8,000–9,000 km. The nucleus is an irregular potato-shaped body, ∼14 × 7.5 × 7.5 km in size, which rotates with a period of 53±3 h. The surface is rather dark; the measured albedo is ∼0.04−0.01+0.02. Active emission of matter was observed on the sunlit side of the nucleus.

Magnetic field observations in comet Halley's coma

During the encounter with comet Halley, the magnetometer (MISCHA) aboard the Vega 1 spacecraft observed an increased level of magnetic field turbulence, resulting from an upstream bow wave. Both Vega spacecraft measured a peak field strength of 70–80 nT and observed draping of magnetic field lines around the cometary obstacle. An unexpected rotation of the magnetic field vector was observed, which may reflect either penetration of magnetic field lines into a diffuse layer related to the contact surface separating the solar-wind and cometary plasma, or the persistence of pre-existing interplanetary field structures.

Vega spacecraft encounters with comet Halley

The spacecraft Vega 1 and Vega 2 encountered comet Halley on 6 and 9 March 1986. Their scientific payload comprised 14 instruments, which collected data concerning the comet's optical characteristics, dust emission, and neutral gas, plasma and electromagnetic field environment. The main features of the encounters are described and highlights of the first scientific results are presented.

Determination of Venus Winds by Ground-Based Radio Tracking of the VEGA Balloons

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere near Venus midnight by the VEGA spacecraft on 11 and 15 June 1985. Initial analysis of only radial velocities indicates that each balloon was blown westward about 11,500 kilometers (8,000 kilometers on the night side) by zonal winds with a mean speed of about 70 meters per second. Excursions of the data from a model of constant zonal velocity were generally less than 3 meters per second; however, a much larger variation was evident near the end of the flight of the second balloon. Consistent systematic trends in the residuals for both balloons indicate the possibility of a solar-fixed atmospheric feature. Rapid variations in balloon velocity were often detected within a single transmission (330 seconds); however, they may represent not only atmospheric motions but also self-induced aerodynamic motions of the balloon.

Halley's comet: Vega spacecraft reveal more dust than expected

Halley's comet: Vega spacecraft reveal more dust than expected