Giotto

European Space Agency Probe to Halley's Comet
Courtesy of ESA and NASA

Table of Contents

• Introduction
• The Giotto Spacecraft
• Scientific Payload
• Halley Encounter
  • Nucleus
  • Neutral Gas and Ion Composition
  • Plasma
  • Dust
• Grigg-Skjellerup Encounter
• References

Introduction

The Giotto mission was designed to study Comet P/Halley, and also studied Comet P/Grigg-Skjellerup during its extended mission. The major objectives of the mission were to:

• obtain color photographs of the nucleus;
• determine the elemental and isotopic composition of volatile components in the cometary coma, particularly parent molecules;
• characterize the physical and chemical processes that occur in the cometary atmosphere and ionosphere;
• determine the elemental and isotopic composition of dust particles;
• measure the total gas-production rate and dust flux and size/mass distribution and derive the dust-to-gas ratio; and,
• investigate the macroscopic systems of plasma flows resulting from the cometary-solar wind interaction.

The Giotto Spacecraft

Giotto is a dual spin configuration spacecraft with the main spacecraft structure spinning, and carrying all the subsystem equipment and the experiments, and the large paraboloid reflector of the high-gain antenna, the beam of which is inclined 44.3° with respect to the spacecraft's spin axis, kept pointing towards Earth.

A central structure surrounds the transfer propulsion system (solid propellant boost motor or TPS) and provides structural location and support for the dust protection shields, the nozzle closure mechanism (NCM), the despin mechanism and the three equipment platforms.

The top platform supports the high-gain antenna tripod which carries the X- and S-band feeds and the magnetometer sensors. Various subsystem units (mainly telemetry, tracking and command subsystem) plus the optical probe experiment are also located on the top platform.

Most of the other units and subsystems (including the attitude control subsystem propellant tanks and thrusters) are located on the main spacecraft platform, while the majority of experiments are accommodated on a lower platform. The sensitive spacecraft elements are protected from dust impacts by a two stage protection system consisting of the bumper and rear shields.

The general integrity of the dust protection system is completed after transfer propulsion system firing when the nozzle closure mechanism closes off the TPS nozzle aperture.

The cylindrical solar array (four identical cell panels) together with the skirts and the top radiator plate complete the thermal enclosure of the spacecraft body.

The Scientific Payload

The scientific payload for the Giotto spacecraft was selected in January 1981. Ten hardware experiments were chosen. In addition. the radio signals from Giotto were to be studied to monitor the dynamical behavior of the spacecraft. The experiments fall into four categories: imaging, mass spectroscopy, dust experiments, and plasma experiments. The camera (HMC) was designed to track on and image the nucleus to determine its shape and size and additionally, could monitor structures in the dust coma. The composition and energy of neutral gas in the comet Halley's coma was studied by the neutral mass spectrometer (NMS) while the ion composition was determined by an ion mass spectrometer (IMS) which comprises two detectors (called HIS and HERS) for studying ions with mass to charge ratios between 1 and 57 amu/q.

The mass and flux of dust particles was determined by the dust instrument detector (DID), which detected the high velocity impacts on the protective shield of Giotto. A dust mass spectrometer (PIA) also contributed to the study of the dust flux and in addition provided data on the composition of the particles. The optical probe experiment (OPE) was used to monitor the brightness of dust and gas, in the coma and derive dust densities along the spacecraft trajectory. The radio science experiment (GRE) measured the decrease in velocity of Giotto caused by the momentum imparted by the impacts of dust particles on the shield.

Four experiments studied the plasma environment. The magnetometer (MAG) measured the local magnetic field to determine the influence of the comet on the magnetized solar wind. JPA also studied this interaction by recording fluxes and velocity distributions for low to medium energy ions. High energy ions were detected by the energetic particle experiment, EPA. Finally, the RPA experiment studied the velocity distribution of electrons and the composition of cold massive (up to 200 amu) ions.

The Halley Encounter

The spacecraft encountered Halley on March 13, 1986, at a distance of 0.89 AU from the Sun and 0.98 AU from the Earth and an angle of 107° from the comet-sun line. The goal was to come within 500 kilometers (310 miles) of Halley's comet at closest encounter. The actual closest approach was measured at 596 kilometers (370 miles). The spacecraft had a dust shield consisting of a front sheet of aluminum (1-millimeter thick) and a 12-millimeter (.4-inch) kelvar near sheet separated by 25 centimeters (10 inches), which could withstand impacts of particles up to 0.1 g.

All experiments performed well and returned a wealth of new scientific results, of which perhaps the most important was the clear identification of the cometary nucleus. Fourteen seconds before closest approach, Giotto was hit by a "large" dust particle. The impact caused the spacecraft angular momentum vector to shift 0.9 degrees. Scientific data were received intermittently for the next 32 minutes. Some experiment sensors suffered damage during this 32-minute interval. Other experiments (the camera baffle and deflecting mirror, the dust detector sensors on the front sheet of the bumper shield, and most experiment apertures) were exposed to dust particles regardless of the accident and also suffered damage. Many of the sensors survived the encounter with little or no damage. Questionable or partially damaged sensors included the camera (later proved to not be functional) and one of the plasma analyzers (RPA). Inoperable experiments included the neutral and ion mass spectrometers and one sensor each on the dust detector and the other plasma analyzer (JPA).

Nucleus

For the first time images of a comet nucleus could be taken during the encounters with comet Halley. The nucleus was larger then expected at 16 by 7.5 by 8 kilometers (10 by 4.7 by 5 miles), implying a lower albedo (~0.04) than predicted. It was confirmed that the nucleus can be described as a dirty snowball or even as an icy dirt ball. The nucleus is probably very fluffy and might have a bulk density as low as 0.3 g cm-3. The discovery of gas and dust jets being emitted from the nucleus suggested that only about 10% of the surface was active. The structures clearly identified in the images, such as hills and depressions, were obviously related to a continuous variation in the surface morphology.

Neutral Gas and Ion Composition

The chemical composition of the gas changes with distance from the nucleus in a complex chain of chemical reactions. The 'parent molecules' at the beginning of this chain reflect the composition of the nucleus. As expected, water (H₂0) was found to be the dominant (80%) parent molecule. Other parent molecules are carbon monoxide (CO) at 10%, carbon dioxide (CO₂) at 2.5%, CH₄ (7%), NH₃ and at the 0.1% level HCN and various hydrocarbon (C₂Hn, C₃Hm). Iron (Fe) and Sodium (Na) were also found, and S₂ and H₂S are strongly suspected to be other parent molecules.

The cometary gas is ionized by solar ultraviolet radiation, by electrons and by charge exchange with the solar-wind plasma. A large number of ionic species were identified, among them H30+ (the most dominant ion near the nucleus), H₂0+, OH+, C+. CH+, 0+, Na+, C₂+, S+ and Fe+.

At the time of the Giotto encounter on 14 March 1986 at about 0.9AU heliocentric distance, comet P/Halley emitted about 18 metric tons (19.8 tons) of gas every second, together with the emission of dust, which was of nearly the same order, an estimated 30 metric tons (33 tons) of material was released every second by the nucleus.

Plasma

As the supersonic solar wind is mass-loaded by the heavy cometary ions, it is slowed down to subsonic speeds and a bow shock forms. Its distance from the nucleus varies in proportion to the comet's gas production rate. During the Halley encounter, Giotto crossed the bow shock at 1.15 million kilometers (715,000 miles) from the nucleus. Closer to the nucleus there is another major plasma physical boundary, the ionopause, which separates the region of smoothly ('cold') outward flowing cometary ions from a disturbed ('hot') transition region containing both cometary and solar-wind ions. A strong decrease in ion temperature across the ionopause was observed. The ionopause was crossed on the inward-bound leg of the journey by Giotto at a distance of 4650 kilometers (2,890 miles), and during the outward bound leg at a distance of 3,940 kilometers (2,450 miles) from the nucleus.

Inside the ionopause the magnetic field strength reached values of nearly 60 nT compared to values of 10 nT typically found elsewhere in the coma.

In addition to the bowshock and the ionopause several more boundaries were found, identified by sharp transitions in the plasma parameters, giving the impression of a complicated multilayered intersection region between the solar-wind plasma and the cometary ionopause.

Pick-up ions were detected at a distance of 7.5 million kilometers (4.7 million miles) from the nucleus. These ions are cometary particles which travel large distances from the nucleus as neutral atoms or molecules before they are ionized and 'picked-up' by the solar wind. Some particles were also observed at comparatively high energies. The pick-up process alone is insufficient to account for these energies and an additional acceleration process is required.

Dust

The elemental abundances measured for comet Halley demonstrated that except for nitrogen, all of the light elements show the same relative abundance as for the Sun. Compared with the values for the Earth and the meteorites there are distinctive deviations. This means that comet Halley consists of the most primitive material known to us in the Solar System.

The chemical composition of several thousand dust particles was determined in-situ and showed that there were two major classes of particles, those dominated by the light elements hydrogen, carbon, nitrogen, oxygen ('organic grains') and others, that are rich in the mineral forming elements sodium, magnesium, silicon, calcium and iron ('mineral grains'). The fact that the light elements were still present at large distances from the nucleus rules out the possibility that they were cometary ice particles.

Comet P/Grigg-Skjellerup

Giotto was placed into hibernation following the Halley encounter. After the first reactivation of Giotto on 19 February 1990, the spacecraft status was analyzed from the telemetry data received. It was evident that Giotto had survived the Halley encounter and that four years of hibernation extremely well.

Engineers revived it again in 1992 for an encounter with another comet. On July 10, 1992, the Giotto spacecraft successfully completed its second close encounter with a cometary nucleus, when it passed within approximately 200 kilometers (124 miles) of Comet P/Grigg-Skjellerup at 15:30 UTC (ground receive time). At that time, the spacecraft was 214 million kilometers (133 million miles) from Earth and 150 million kilometers (93 million miles) from the Sun.

Giotto's payload had been switched on during the evening of 9 July, with eight of the original complement of 11 experiments still operable: namely the magnetometer, Johnstone plasma analyzer, energetic-particle analyzer, optical-probe experiment, Reme plasma analyzer, dust impact detection system, ion mass-spectrometer, and the radio-science experiment. Together, they provided a wealth of exciting data during this second cometary encounter.

Some 12 hours before closest approach, when Giotto was still about 600,000 kilometers (373,000 miles) from the nucleus, the Johnstone plasma analyzer detected the first cometary ions. Between 18,000 and 15,000 kilometers (11,185 and 9,320 miles) from the comet, both the Johnstone plasma analyzer and the Reme plasma analyzer reported what looked like a bow shock or a bow wave, which was much more distinct than had been predicted for such a weak comet. The magnetometer measurements carried out during this inbound leg of the trajectory could not confirm this finding, but reported exciting wave phenomena not previously seen in a natural plasma. On the outbound leg of the fly-by, the magnetometer saw clear indications of a shock.

The optical probe experiment gave the first indication of Giotto's entering the dust coma some 20,000 kilometers (12,400 miles) from the nucleus and from its data the first estimate for the encounter distance, 200 kilometers (124 miles), could be derived. Together with magnetometer data, there is good evidence that Giotto passed through the comet's tail-forming region, on the dark side of the nucleus.

At 15:30:56, the dust impact detection system reported the first impact of a fairly large particle, followed by two smaller ones.

At 15:31:02, shortly after the first impact, the spacecraft's high-gain antenna appeared to be oscillating slightly around its nominal position. A very small increase in spin rate, by 0.003 rpm, was also observed, while the solar aspect angle readings were fluctuating between 89.26 and 89.45 degrees, indicating a nutation of approximately 0.1 degrees. This was also recorded by the radio-science experiment and is awaiting further evaluation.

The energetic-particle experiment saw clear indications of the particle-acceleration regions and surprising differences in the structures between the Comet Halley and Comet Grigg-Skjellerup encounters. Last but not least, the ion mass-spectrometer also gathered good data, but its analysis is quite cumbersome and complex, due to the low encounter velocity of 14 km/s, compared with 68 kilometers per second at Comet Halley.

Thorough testing of Giotto's camera on July 7, 3 days prior to the encounter, could only confirm that its optical path was indeed blocked. On July 12, however, two days after closest approach, a number of further tests were performed with the camera's detectors, which provided very valuable engineering and calibration data on the long-term behavior of charge-coupled devices in space.

On July 21, about a week later than previously planned, a major orbit maneuver was conducted to put Giotto into an orbit that will bring it close to Earth (220,000 kilometers or 137,000 miles) again in July 1999. There are still 4 kilograms (9 pounds) of fuel left onboard for further attitude and orbit correction maneuvers, and this orbit maneuver leaves the door open for some further activities in 1999.

After a final orbit trimming maneuver on July 23, the Giotto spacecraft was put into hibernation for the third time

All Giotto experimenters are extremely enthusiastic about the quality of the data returned from the encounter with Grigg-Skjellerup, which have surpassed all expectations. The outstanding success of the Giotto Extended Mission (GEM), following upon that of the spacecraft's first encounter with Comet Halley in 1986, must be considered one of the highlights of the ESA Scientific Program to date.

References

Planetary Science Database, National Space Science Data Center (NSSDC), National Aeronautics and Space Administration, Greenbelt, Maryland. World Wide Web: http://nssdc.gsfc.nasa.gov/nssdc_home.htm.

Schwehm, Gerhard, "Giotto's Encounter with Comet Grigg-Skjellerup: The First Results," Giotto's Second Encounter: The Mission to Comet Grigg-Skjellerup, European Space Agency, Paris 1993.

Schwehm, Gerhard and Grensemann, Manfred, Giotto: The Second Encounter, European Space Agency, Paris, 1992.