Halley Multicolour Camera Research Articles

Comparison of the dust distributions in the innermost comae of comets—1P/Halley and 19P/Borrelly spacecraft observations

We present a comparative study of the inner comae of comets 1P/Halley and 19P/Borrelly using data from the Halley Multicolour Camera (HMC) onboard Giotto and the Miniature Integrated Camera and Spectrometer onboard Deep Space 1 (DS1). We show that the dust brightness dependence as a function of radial distance is different for both comets. We suggest that optical depth or fragmentation effects dominate the brightness distribution at comet 1P/Halley whereas acceleration or non-point source geometry effects dominate at comet 19P/Borrelly. The nightside profiles of comet 19P/Borrelly suggest a continuing non-radial outflow from the nucleus out to several tens of kilometres. This modifies the observed dayside to nightside brightness ratio with distance and offers a further constraint on dust emission models. By setting up a linear system of equations to fit the dust intensity distribution, better fits could be obtained by incorporating acceleration into the equation of free-radial outflow. Finally, we estimate the dust production rate of 19P/Borrelly at the time of DS1 encounter as no higher than 324 kg/s based on comparisons with HMC intensity measurements.

Comparative study of the dust emission of 19P/Borrelly (Deep space 1) and 1P/Halley

Images obtained by the Miniature Integrated Camera and Imaging Spectrometer (MICAS) experiment onboard the Deep Space 1 spacecraft which encountered comet 19P/Borrelly on September 22nd 2001 show a dust coma dominated by jets. In particular a major collimated dust jet on the sunward side of the nucleus was observed. Our approach to analyse these features is to integrate the observed intensity in concentric envelopes around the nucleus. The same procedures has been used on the Halley Multicolour Camera images of comet 1P/Halley acquired on March 14th 1986. We are able to show that at Borrelly the dust brightness dependence as a function of radial distance is different to that of Halley. At large distances both comets show constant values as the size of the concentric envelopes increases (as one would expect for force free radial outflow). For Halley the integral decreases as one gets closer to the nucleus. Borrelly shows opposite behaviour. The main cause for Halley's intensity distribution is either high optical thickness or particle fragmentation. For Borrelly, we have constructed a simple model of the brightness distribution near the nucleus. This indicates that the influence of deviations from point source geometry is insufficient to explain the observed steepening of the intensity profile close to the nucleus. Dust acceleration or fragmentation into submicron particles appear to be required. We also estimate the dust production rate of Borrelly with respect to Halley and compare their dust to gas ratios.

The inner dust coma of Comet 26P/Grigg-Skjellerup: multiple jets and nucleus fragments?

On 1992 July 10 the European Space Agency's spaceprobe Giotto passed the nucleus of the comparatively inactive comet 26P/Grigg-Skjellerup at a relative velocity of 14 km s(-1). This Giotto Extended Mission (GEM) followed a highly successful encounter in 1986 with Comet 1P/Halley. We present results returned from the Optical Probe Experiment (OPE) and in particular consider data gathered by the channels sensitive to the scattering of solar light by cometary dust grains, in emission-free continuum bands. Owing to the demise of the Halley Multicolour Camera (HMC) during the Halley encounter, and the low number of impacts registered by the Dust Impact Detection System (DIDSY), OPE data offer the best indication of the actual encounter geometry. We find that it is likely that Giotto was on the sunward side of the shadow terminator plane at closest approach, with our modelling results suggesting that Giotto passed similar to 100 km from the nucleus (although distances of up to 300 km cannot be ruled out). We investigate possible causes of the striking 'spike' features, or 'events', in the OPE data. While scattering of sunlight from ejecta particles as a result of dust impacts on the spacecraft body cannot be ruled out, considerations of the hypervelocity impact mechanisms and impact geometry show that this explanation is not without problems, and more investigation is needed before it can be conclusively accepted. As an alternative solution, we find that the complex data profiles can be fitted by jet activity in the innermost coma (which was not resolvable by ground-based observers). One particular event occurring at least 1000 km from the nucleus can be fitted if the OPE line of sight passes close to a nucleus fragment of radius 10-100 m which is situated around 50 km from the spacecraft and which is producing a small dust coma.

INEO—Imaging of near-Earth objects

INEO is a program of four explorer flyby missions for imaging near-Earth objects by means of small spin stabilized spacecrafts. A low-cost approach is achieved by using existing hardware and available scientific instrumentation. For the first mission the payload consists of the flight space model of the Halley multicolour camera and three Munich dust counters. The imaging instrument will determine the physical properties of the target. The dust environment of the asteroid or comet will be investigated with the dust detectors. For the follow-on missions different cameras and spectrometers and a magnetometer are proposed. In this paper the outline of INEO is presented which will either be launched as an auxiliary passenger on large launch systems, like ARIANE IV, or as the main payload on medium sized launch vehicles, like TAURUS, M-3S-II or CZ-1D, without changing the configuration of the space probe. First results of mission analysis will be presented.

Collimation of cometary dust jets and filaments

It has been suggested that cometary dust jets, as for example imaged by the Halley Multicolour Camera (HMC), possibly originate from vents, or crater-like surface features. Dust flow emitted from such indentations is collimated if compared to emission from a flat surface. Dust liberated from the bottom of a cometary “crater” emerges at the surface level (top of the crater) with a finite velocity. As a consequence these dust particles have a larger outward (radial) momentum than particles leaving the surrounding surface with zero initial velocity. The resultant collimation of dust trajectories (reduced opening angle of the dust jet) is calculated as a function of crater depth and physical parameters of the dust grains applying axisymmetric gas dynamic code. Fine structures observed by HMC in cometary dust jets can be modelled by emission from active regions with inactive centres. The decrease of pressure above the non-sublimating surface leads to a converging gas flow that concentrates larger particles in a radial filament.

Large dust particles along the Giotto trajectory

Images of Comet Halley taken by the Halley Multicolour Camera (HMC) onboard ESA's Giotto spacecraft provide highly accurate information on the viewing direction. As a part of the spacecraft dynamical system, HMC was very sensitive to attitude variations that occurred during the final approach phase. HMC identified several discrete events that were caused by dust particles in the 1–50 mg range. Together with impacts reported from the Japanese Suisei spacecraft by Uesugi (1986, 20th ESLAB Symp., ESA SP-250 II, 219–222), this is the first independent observation of particles in this mass range. The well-calibrated measurements unequivocally set firm lower limits of the mass of large dust particles. These particles dominate the impact history of the spacecraft and, therefore, the dust mass distribution. The data suggest the existence of a halo of heavy old dust grains surrounding the cometary nucleus. The statistics of the observed impacts are discussed in the context of other dust observations and models. The HMC measurements support models that assume a dust mass distribution dominated by large grains.

Photometric calibration of the Halley Multicolour Camera

The absolute (photometric) calibration of the Halley Multicolour Camera (HMC) is described. HMC, prior to its observations of comet Halley on 13-14 Mar. 1986, imaged several stars and planets. The fluxes from the objects and the observed signals in digital units (DU) have been computed. A spectrally independent measure of the response of the instrument has been determined. This value has been used to derive conversion factors from DU to mW m(-2) sr(-1) and reflectivities for all HMC configurations.

Identification of surface features on comet P/Halley

The Giotto Halley Multicolour Camera obtained images of P/Halley that resolved the nucleus, showing active regions and other features on the surface. At least twenty-one discrete bright features can be identified in the images, and eight distinct regions associated with morphological characteristics can be recognized. The projected limb is approximated with a two-to-one axial ratio ellipse, establishing a reference coordinate system for the body based on the center of gravity and moments of inertia of the profile. A catalog of surface features is presented giving size, shape and location in the reference coordinate system.

A model for intensity profiles of dust jets near the nucleus of comet Halley

Intensity profiles of jet-like dust features, associated with active regions on the nucleus, have been analyzed in the Halley Multicolour Camera (HMC) images from the Giotto spacecraft. The asymptotic behavior of an intensity profile at large distance from the nucleus ( R > 100 km) can be modeled as if arising from a source located at a hypothetical point below the active source region. In this limit, the intensity decreases proportionally to 1 R , as expected for free outflow, but reveals a strong flattening relative to an 1 z profile near the nucleus, where z is the vertical distance above an extended, dust-producing source region. This flattening can be explained by geometrical effects based on the extended size and nonuniformity of an active region on the surface. A critical height, z c, is introduced to describe the geometrical effects that cause the flattening. This critical height is determined by the vertical distance of the hypothetical point source below the surface, but reflected outward from it. The model is consistent with the observations without invoking other mechanisms.

On the rotation axis of comet Halley

The observations of comet Halley by the Halley Multicolour Camera (HMC) on board the European Space Agency's Giotto spacecraft were supported by an extensive ground-based observational programme using the same filter set1. The intention was to place the transitory but detailed observations of the encounter in the frame of ground-based observations, thereby yielding the evolution of the comet. The importance of this connection has increased because of the limited information on the three-dimensional extent of the cometary nucleus and its environment retrievable from HMC observations, caused by the communication loss just before closest approach. Here we model dust jets visible on the ground-based images, assuming active sources on the rotating nucleus. We found no fit using the rotation axes so far derived from nucleus observations. A mean instantaneous spin axis, at the time of the Giotto encounter, pointing towards α = 328°, δ = −63° is consistent with both ground-based and HMC observations of the dust. We have confirmed a rotation period of about two days.

Surface features on the nucleus of comet Halley

Images taken by the Halley Multicolour Camera on board the ESA spacecraft Giotto during its fly-by of comet Halley in March 1986 have been improved by careful calibration and image processing methods. Surface features a few hundred metres in dimension are now visible. Several surface structures are presented in detail and briefly described.

Image restoration using the point spread function of the Halley Multicolour Camera

The optical resolution of the Halley Multicolour Camera (HMC) is limited by its finite aperture, by minute mechanical degradations within the instrument, and by the high speed time delay and integration (TDI) illumination method adapted to the spin of the spacecraft. The sum of all implies a visible loss of image sharpness. The smearing effect has been calibrated using images of a star which can be considered as a point source. The images were superimposed in the frequency domain with subpixel accuracy. With these means the point spread function (PSF) has been determined and applied to the received images using Fourier methods. The effectiveness of the scheme is demonstrated on images of the earth and comet Halley.

The Halley multicolour camera

The Halley multicolour camera is a high-resolution imaging system on board Giotto. The fast spin of this spacecraft (15 RPM) required a unique and highly specialised design complex fully autonomous operation. Design criteria and trade-offs and operational aspects are emphasised in this instrument description. Actual performance numbers are given whenever available. The imaging quality is demonstrated by sample images taken from the Earth and the comet.

Specialized image processing technique applied to Halley multicolour camera images of the Earth

A special image processing technique has been applied to earth images taken by the Halley Multicolour Camera (HMC) on‐board ESA's space probe Giotto on its way to a close encounter with comet Halley on March 14, 1986. The method depends on the knowledge of the point spread function of the optical system. Deconvolution has been achieved by a direct inversion of the convolution process subject to boundary conditions that would correspond to non‐linear filter processes. Experimental evidence is presented that under the prevailing conditions the technique can provide pixel resolution. Comparison with weather satellite images allow verification of the results for the earth observations.

First Halley Multicolour Camera imaging results from Giotto

The first imaging results from the Halley Multicolour Camera (HMC) during the Giotto fly-by of comet Halley provide images centred on the brightest part of the inner coma which show the silhouette of a large, solid and irregularly shaped cometary nucleus and jet-like dust activity visible in reflected sunlight. A first assessment of the data yields information on the dimensions and shape of the nucleus and dust emission activity. The nucleus is at least 15 km long and ∼10 km wide; its geometrical albedo is very low (<4%). Only minor parts of the surface are active, most of it beirig covered by non-volatile material. Dust jets dominate the inner coma and are restricted to the sub-solar hemisphere. Preliminary interpretations of these observations are presented. The parameters of the fly-by trajectory relative to the nucleus are given.