Number 20: 24/05/2003
A scientific publication by SGF and NEODyS

NEOs' Spacemissions Special: mission GIOTTO

"The Giotto mission to Comet Halley" by Horst Uwe Keller

Introduction: why a Giotto mission?

Interest in cometary physics was strongly stimulated when it has been realized that comets are building blocks of the planets and hence remnants of the early solar system. They themselves accreted from the dust and ice grains that formed the dust disk surrounding the early sun after the collapse of the molecular cloud. Spectroscopic observations of the inner gas coma showed that the observed species (atoms and unsaturated diatomic compound so called radicals) could not be stored as such. They were products of dissociation of stable so called mother molecules mainly by solar radiation (Wurm 1939). The postulation of a solid cometary nucleus and particularly its modelling as a conglomerate of ice and dust (Whipple 1950) required that comets have seen very little alteration since their accretion from the dust and ice particles in the proto planetary disk. They should be built of material that comes the closest to the conditions of the early solar system one could possibly lay ones hands on.With the advent of space exploration it was only consequential that there was strong interest in a mission to a comet. At the end of the 1970s serious planing of a mission was triggered by the recurrence of the famous comet Halley, one of the brightest short periodic comets. Its last very bright display in 1910 was still widely remembered. Comet Halley would reach its closest distance from the sun (perihelion) in 1986. Its copious activity shedding about two orders of magnitude more gas and dust than the vast majority of all short periodic comets made it an outstanding target for a space mission. A working group of American and European scientists defined the requirements and goals of a cometary mission. On the American side the fly-by of comet Halley was considered as a stepping stone to a rendezvous mission with another much less active comet (Tempel 2). When the decision to materialize this Comet Halley Fly-by, Tempel 2 Rendezvous mission had to be taken NASA withdrew and the opportunity to encounter the great comet Halley seemed to slip away. The science directorate of ESA made a brave and fast decision to define a European only fly-by mission to comet Halley. This was the first European mission into interplanetary space (only to be followed up by Mars Express to be launched in 2003!). Prime data would be taken in only 1/2 hour during a very fast fly-by with 68 km s1. And even more scary there was only one launch window of less than a month in June 1985 (compare the fate of the Rosetta mission in 2003).The unique chance of visiting a very active comet was also realized by space agency of the the former Soviet Union and two spacecraft originally designed for Venus missions were changed and redirected to fly-by comet Halley (Vega 1 and 2).

The Giotto Mission

The ESA mission was approved in 1980 and named after the Italian painter Giotto di Bandone. His relatively realistic representation of a comet as the "Star of Bethlehem" in a frescoe dated 1304 was probably inspired by the appearance of comet Halley in 1301. The spacecraft was launched on 5 July 1985 by an Ariane 1 launch vehicle from Kourou in French Guiana. Its trajectory and the orbit of comet Halley are shown in Fig. 1.The postulated cometary nucleus was too small to be resolved by even the best earth based telescopes when the comet is not active beyond heliocentric distances of about 3 au. Closer in the nucleus is hidden in the centre of the gas and dust coma - invisible from the outside. Therefore it was one of the prime goals of any mission to a comet to detect and observe its nucleus. Requirements to determine the composition derive from the expected pristinity of the cometary material as witness from the formation of the solar system. The interaction of the cometary gas with the passing solar wind leads to unique physical processes. The investigation of these plasma processes but also of the processes that describe the activity (sublimation) of the cometary nucleus itself were of high relevance to the success of the mission.The instruments of the scientific payload could be categorized by these three main lines of investigations:

1) Camera to characterize the nucleus and its activity.

2) Neutral mass and ion mass spectrometer to analyse the composition of the gas. Dust grain analyser and counter.

3) Plasma and solar wind analyser and magnetometer.Table 1 lists the complement of instruments.

The Giotto spacecraft was derived from a design of a spinning earth satellite and had to be oriented in such a way that its spin axis would be pointing in direction of the relative velocity vector (at larger distances directly towards the comet nucleus) so that the spacecraft body could hide behind the shield (Fig. 2). The spin rate was 15 rpm. The Giotto spaceprobe had to penetrate the coma deeply so that the in situ instruments (gas and dust analysers) could collect enough material during the fast fly-by and also detect the at that time mostly inferred mother molecules such as water. The lifetime of the expected mother molecules could be so short that an approach to about 1000 km seemed necessary even though it was clear that tiniest dust grains hitting the spacecraft at the relative speed of 68 km s1 could destroy it. A bumper shield was designed that would protect the spacecraft against particles smaller than 1 g. The orientation of spacecraft (spin axis) would, however, be affected already by smaller masses depending where they hit.The close encounter with comet Halley started about 4 h before closest approach (CA) on 13 March 1986 around 20 h UT. At that time the distance to the nucleus was a little less than 106 km. The parameters of the fly-by are listed in Table 2. Shortly before CA the spacecraft was hit by dust particles at a distance from the nucleus of ca. 2000 km. It started to tumble and lost communication with the earth for a few minutes. Several instruments were damaged and stopped sending data back. Specifically the Halley Multicolour Camera (HMC) could not acquire the nucleus after CA on the way out of the coma. Later it turned out that its baffle was torn off and obscured its aperture.

Figure 1. The interplanetary trajectory of Giotto from launch on 2 July 1985 until encounter with comet Halley on 14 March 1986. Comet Halley's orbit is retrograde and inclined by 162_ with respect to the ecliptic plane.

Figure 2. The Giotto spacecraft design was derived from the Geos concept. The camera is shown looking downward, which is the direction towards the comet during the fly-by.

Figure 3. Principle of operation of the Halley Multicolour Camera on board the rotating spin-stabilised Giotto spacecraft.

Scientific Results

Damage of the spacecraft and of its payload was a risk that had to be taken to come close enough to the nucleus and investigate its immediate environment - a region never observed and no other spacecraft had penetrated. This dare devil approach payed - the new results were bountiful. 

Solar wind interaction

Cometary atoms and ions could be detected at millions of kilometres upstream (toward the sun) where cometary atoms are ionised and then rapidly accelerated to join the solar wind (pick-up ions). This interaction of the solar wind with cometary ions is responsible for the often tens of millions of kilometre long blue ion tails of comets. At about 106 km the bow shock was detected where the speed of the onstreaming solar wind becomes subsonic due to being loaded by the relatively heavy cometary ions. A similar bow shock is observed near planets (e. g. the earth) and was predicted for comets. Many other structures were found that shed light on the details of the interactions of the strong cometary source of molecules, atoms, and ions with the solar wind. Giotto was able to penetrate the magnetic cavity around the nucleus with a radius of about 4700 km. The dynamic pressure of the cometary molecules, atoms, and ions balances the pressure of the solar wind at this boundary and no solar wind ions and therefore the magnetic field carried with the wind - neither can penetrate. To no ones surprise no magnetic field of the nucleus itself could be detected. 

Cometary composition

A wealth of data was collected pertaining to the composition of the cometary material.3.2.1 The volatilesVolatile compounds are stored in the cometary nucleus as ices that sublime setting free the mother molecules. Molecules do not emit light in the visible wavelength range and are therefore difficult to observe (in the infrared and radio regime). The molecules are readily dissociated by the UV radiation of the sun. Their dissociation products - radicals and atoms - emit the spectra that can be observed with large telescopes. Therefore only indirect information existed. For example hydrogen (H), oxygen (O), and hydroxil (OH) are observed and correctly it was concluded that their mother molecule is water (H2O). The measurements of Giotto confirmed that indeed water is the dominating volatile material of comets. In addition CO2, CO, CH4 (methane), NH3, HCN, and H2CO (alcohol) were detected. In the meantime many more molecules could be found in comets confirming their pristine character. Many of these molecules have also been observed in the interstellar medium strengthening the close relation of cometary material with the composition of the pre-solar nebula. Of particular importance for the quest of cometary origin are isotopes. The ratio of deuterium (a heavy isotope of hydrogen) and hydrogen (D/H) in cometary water was found to be much higher than the typical value of material in the solar system and in particularly in the oceans of the earth. Therefore, the notion that the terrestrial oceans and atmosphere are the result of a bombardment with comets in the early stages of the solar system can be refuted. Not more than 10 % of our water could be of cometary origin. Cometary water has to come from ice that condensed on grains. During the formation of water molecules on the surfaces of grains in space the heavy deuterium atoms are slowly enriched compared to their concentration in the gas because the lighter hydrogen atoms can more easily leave the grains again.3.2.2 The dustThe volatile compounds found in the coma essentially confirmed our conception, with the possible exception of the D/H ratio. The dust revealed more surprises. First of all it was found that the cometary nucleus contained much more dust than previously assumed and determined from remote observations with telescopes. The ratio of volatiles (ices) to dust (non-volatiles) was found to be one or even smaller. With other words most of the material in a cometary nucleus is non-volatile (less volatile than water). Dust grains as small as a few hundred molecules to centimetre size were detected and counted. An even bigger surprise was revealed by the analysis of the composition of the grains. About 30 % of the dust grain material consisted C, H, O, and N the building blocks of organic compounds, some particles (so called CHON particles) were mainly composed of organic compounds. A considerable fraction of the cometary organics are found probably as polycyclic compounds (large molecules) that sublime only at high temperatures and count therefore to the non-volatile fraction. 

The nucleus

An artist's impression of a cometary nucleus in the 1970s reflecting the contemporary conception of a "dirty snowball"

The detection and characterisation of the nucleus was a mandatory goal of any mission to comet Halley. In addition the image sequence of the camera was needed to determine the details of the fly-by trajectory. This task was particularly challenging on board the Giotto spacecraft because of its continuous spinning motion. As can be seen on Fig. 3 the field of view (FOV) of the camera would sweep across the target every 4 s (15 rpm) for a few milliseconds. The picture taking had to be synchronized with the rotation of the spacecraft. This is the more difficult the better the resolution and hence smaller the FOV would be. The Halley Multicolour Camera (HMC) was designed with a scale of 20 m px1. The telemetry rate limited the data volume per spin for HMC to an image with 75 x 75 pixel (clear filter) and 3 images with half the resolution in colour. It was considered to be too risky to transmit images over several spin periods while the spacecraft was in the inner coma. The nucleus itself could be detected from a distance of 124,000 km (see Fig. 4). The last good image centering around the brightest area of activity was taken from a distance of ca. 1500 km with a resolution (two pixels) of better than 50 m.The phase angle (angle sun - comet - camera) hardly changed during approach. Only during the last few images the camera hat to begin swinging around. Therefore a composite image could be created where smaller and smaller parts of the image are replaced by pictures taken from closer and closer distances. One has to keep in mind that the resolution within the "best" image (Fig. 5) changes from 500 m to 50 m. Unfortunately most of the cometary nucleus is not illuminated by the sun because the camera had to look into the sun during approach (phase angle 107°).

Fortunately the outline of the dark limb can be seen against the faintly illuminated dust in the background. This made it possible to determine the shape of the nucleus accurately seen from one direction.The shape can be fit with an ellipse of 14.2 km length and 7.2 km across. Comparison with images taken by the VEGA spacecraft showed that the nucleus is slightly longer (15.2 km) and that its third dimension was also about 7.2 km. The closeup images of the nucleus do not provide the perception of a snowball spewing gas and dust from its bright surface (c.f. the perception depict in an artist's picture on the left). Only a small part of the surface was active during the Giotto fly-by, about 20 % of the illuminated hemisphere. The elongated nucleus shows pronounced topography with a mountain, ridges, flat planes, chains of hills (Fig. 6). It is obvious that the nucleus is formed by the inert non-volatile dust and not by the icy compounds. It looks much more like a "Snowy Dirtball" than a "Dirty Snowball". In agreement with above mentioned results the non-volatile component dominates.The very vicinity of the nucleus (up to several times it dimension) shows interaction of dusty gas streams emanating from the active areas. Fine filament-like dust structures, only several hundred meters across, strongly collimated, can be found and made more clearly visible by image processing (Fig. 7). They criss-cross each other reflecting the topography of their source areas.

Figure 4. Six examples of HMC images (original frame sizes) that are filtered, calibrated and deconvolved by the point spread function (PSF) (except image 3502). The Sun is on the left side, 7_ below the horizontal in mage 3502.

Figure 5. A composite image of the nucleus of comet Halley composed of 68 images at a resolution varying from 500 m px-1 to 50 m px-1 near the active region.

Figure 6. A tableau of features. Sections of the composite image (centre bottom) have been extracted and expanded by a factor of 3 to show, in detail, notable features on the nucleus. The position of each expanded section is marked as a box on the composite and a corner of each section is linked to its counterpart by a line. Non-linear enhancement has been applied to provide improved contrast.

Figure 7. The directions of filaments seen in the dust emission. The filaments are small inhomogeneities (£ 500 m in diameter at their source). This fine structure in the emission would have been far too faint to be seen by simultaneous ground-based observers. The filaments appear to criss-cross each other.

Conclusions

ESA had taken a large risk with the selection of Giotto as its first interplanetary mission. The outstanding success of Giotto was the reward. It took more than 15 years before the next fly-by succeeded. The NASA technology mission Deep Space 1 observed the nucleus of comet Borelly in 2002. The images revealed a somewhat smaller nucleus with properties very similar to those established by HMC. Comet Halley is the prototype of a group of comets that have elliptic orbits of intermediate periods and large inclinations. Their origin is suspected to be the Oort cloud. Comet Borelly, on the other hand, typically represents the family of comets controlled by Jupiter with short periods and small angles of inclination. Their origin is the Kuiper belt of bodies beyond the orbit of planet Neptune. The images did not reveal any major differences between these subgroup of comets. Comets are indeed composed from volatile, partly organic material, and silicate based dust. Similar compounds are found in interstellar material. Whether comets accreted directly from interstellar grains in the outer regions of the collapsing molecular cloud will remain to be determined by future space missions that will investigate cometary material and physics in more detail.
The rendezvous mission Rosetta will mark a major step in our understanding of the cometary nature. 

Table 1. The scientific payload on board Giotto

  Experiment

Acronym

Mass (kg)

Principal Investigator

Remote Observations

 

  

  

 Camera

HMC

13.51

H.U. Keller
MPI für Aeronomie, Lindau, Germany

Optical Probe Experiment

OPE

1.32

A.C. Levasseur-Regourd
Service d'Aéronomie du CNRS, Verrières-le-Buisson, France

In Situ

  

  

Neutral Mass Spetrometer

NMS

12.70

D. Krankowsky
MPI für Kernphysik, Heidelberg, Germany

Ion Mass Spectrometer

IMS

9.00

H. Balsiger
Physikalisches Institut, University of Bern, Switzerland

Dust Mass Spectrometer

PIA

9.89

J. Kissel
MPI für Kernphysik, Heidelberg, Germany

Dust Impact Detection System

DID

2.26

J.A.M. McDonnel
Space Science Laboratory, University of Kent, Canterbury, UK

Plasma

  

 

 Plasma Analysis 1

JPA

4.70

A. Johnstone
Mullard Space Science Laboratory, Holmbury St. Mary, UK

Plasma Analysis 2

RPA

3.21

H. Rème
Centre d'Etude Spatiale des Rayonnements, Toulouse, France

Energetic Particles

EPA

0.95

S.M.P. McKenna-Lawlor
St. Patrick's College, Maynooth, Ireland

Magnetometer

MAG

1.36

F.M. Neubauer
Institut für Geophysik und Meteorologie, Köln, Germany