Encyclopedia of Astrobiology

Living Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James Cleaves, Daniele Pinti, José Cernicharo Quintanilla, Michel Viso

Giotto Spacecraft

  • Anny-Chantal Levasseur-RegourdEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_648-3

Keywords

Comet Nucleus Coma Water Dust Organics CHON Giotto Giotto extended mission Spacecraft ESA Halley Grigg-Skjellerup 

Definition

The Giotto spacecraft (Fig. 1), the first ESA (European Space Agency) interplanetary probe, was designed to flyby comet Halley. Launched on 2 July 1985 by an Ariane-1 rocket from Kourou, Giotto succeeded in approaching the cometary nucleus to within 600 km on 14 March 1986. Through its first accurate images of a nucleus and in situ studies of gases and dust particles within a coma, the mission has revealed the complexity of comets. Afterwards, the Giotto spacecraft was re-oriented in order to study comet Grigg-Skjellerup, which was flown by on 10 July 1992, at a nucleus distance in the 150–200 km range.
Fig. 1

Giotto spacecraft

History

Giotto was named after the painter Giotto di Bondone, who, in 1301, depicted a comet as the star of Bethlehem in his fresco Adoration of the Magi in Padua. The comet, easily visible from Europe at that time, was later called 1P/Halley.

Overview

Although comets had been observed for centuries, their knowledge remained limited until the 1986 flybys of Halley by an armada of spacecraft (see below). Giotto, the first independent European mission, provided a close-up view of the cometary nucleus, afterwards described as an icy dirt ball, together with evidence for previously unsuspected properties of its dust particles, found to be under-dense and rich in refractory organics. Giotto was also the first mission to be reactivated after hibernation and to return from interplanetary space for an Earth swingby, which oriented it toward a second comet.

Basic Methodology

At the end of the 1970s, a mission to a comet was mandatory to assess the existence of the nucleus, the properties of the coma, and the solar wind interactions. The expected return in 1985–1986 of the active short-period comet 1P/Halley, the trajectory of which was well known, made it an obvious target for a ballistic flyby. Its retrograde orbit (with respect to that of the Earth and of a space probe) nevertheless implied a huge relative velocity between the comet and any spacecraft, close to 70 km s−1. In July 1980, once it had become clear that a sophisticated NASA mission with a flyby of Halley and a rendezvous with another comet would not take place, the European Space Agency approved a mission specifically dedicated to the study of Halley during its passage in the ecliptic at the orbital node closest to its perihelion, i.e., in March 1986.

The onboard experiments (Fig. 2) were selected in January 1981. They consisted of ten hardware experiments with a total mass of about 60 kg, i.e., one camera, three mass spectrometers (neutral, ion, dust), one dust impact detector, one optical probe, two plasma analyzers, one energetic particles analyzer, and one magnetometer, plus one radio-science experiment. The spacecraft had a cylindrical shape (diameter 1.86 m, height 2.85 m, dry mass 574 kg), with its main cylinder covered by a solar array. It was spin stabilized (period of 4 s), with the spin axis aligned with the relative velocity vector during Halley encounter. A two-stage aluminum-kevlar dust shield, perpendicular to the spin axis, protected Giotto against high-velocity dust impacts; the despun (to counteract the effect of the spin) parabolic reflector of the high-gain antenna was mounted on a tripod at the other end, in order to be oriented toward the Earth during the flyby.
Fig. 2

Giotto onboard experiments

After 5 years of development and tests, Giotto was launch by an Ariane-1 rocket from Kourou, French Guyana, on July 2, 1985, into a highly eccentric geosynchronous transfer orbit. About 1 day and a half later, its solid-propellant motor was fired at perigee to inject the spacecraft into a solar ballistic orbit that allowed it, after a cruise of about 685 million kilometers, to flyby the cometary target without major trajectory adjustments. The encounter took place on March 13–14, 1986, with a relative velocity of 68.4 km s−1, at 0.89 AU heliocentric distance and 0.98 AU geocentric distance and for a phase angle of 107.05°. The closest approach took place immediately after midnight UT, at (596 ± 2) km distance from the nucleus (Fig. 3).
Fig. 3

Geometry of Comet Halley flyby

To allow Giotto to approach as close as possible to the nucleus without being destroyed, a unique international cooperation had been organized. An armada of spacecraft had been launched toward Halley, including besides Giotto the Japanese Suisei with a nucleus miss distance of 151,000 km and the Russian Vega 1 and Vega 2, respectively, at 8,890 and 8,030 km. The pathfinder concept, developed in cooperation between ESA, IKI (USSR), and NASA (USA) was used to define the photometric center (presumed to correspond to the nucleus) on the images collected by the Vegas, the positions of which were determined by the American Deep Space Network. The distance of closest approach of Giotto was accurately controlled through this unique effort, allowing the spacecraft to approach the nucleus without being totally destroyed by impacts from dust particles hitting it with a high relative velocity.

Giotto was actually hit by a rather large dust particle 14 s before closest approach, leading by nutation to a shift of the spacecraft angular momentum vector of 0.9° and to an intermittent Earth data link for 32 min. Two weeks after the encounter, the spacecraft was put into hibernation. After its reactivation in February 1990, it could be established that it had survived with minor degradation of the Halley flyby. On July 2, 1992, after six orbits around the Sun, Giotto was back in the vicinity of our planet at 23,000 km altitude. The Giotto Extended Mission (GEM) to a second cometary target had already been approved by ESA, and the orbit of the spacecraft was retargeted through an Earth gravity assist toward 26P/Grigg-Skjellerup, a Jupiter family comet.

This last flyby took place on July 10, 1992, with a relative velocity of 14 km s−1, at 1.01 AU heliocentric distance and 1.43 AU geocentric distance. While the Giotto scientific payload was fully operational for the Halley flyby, it was only 50 % operational for Grigg-Skjellerup and no images were provided by the damaged HMC camera. Nevertheless, the OPE (Optical Probe Experiment), undamaged at the rear of the spacecraft, derived a nucleus miss distance below 200 km, most likely in the 150–200 km range, from the monitoring of the evolution of the brightness under the observational geometry.

Among the huge wealth of unique results, many are relevant from an astrobiology perspective.

Key Research Findings

The Giotto HMC camera provided the first accurate images of a cometary nucleus. They revealed a slightly reddish irregular object, significantly darker and bigger (about 15 km long and 7.5 km by 8 km wide) than anticipated, with various topographic features (hills, ridges, craters, cliffs, ridge, central depression). Well-defined dust structures with the appearance of narrow filaments were noticed in the inner coma, possibly originating from active regions covering 10 % of the surface. The mass of the nucleus was too low to be derived from the spacecraft trajectory perturbations near closest approach. Nevertheless, estimations of the mass from non-gravitational effects (later validated through the Deep Impact mission) and of the shape from imaging yield a density of 550 ± 250 kg.m−3, typical of a highly porous object (Fig. 4).
Fig. 4

Image of Comet Halley nucleus obtained by Giotto

The icy component of Comet Halley mostly consisted of water. Independent analyses of the D/H ratio from NMS and IMS (Neutral and Ion Mass Spectrometers) lead to a value in cometary water of (3.08 ± 0.3) × 10−4, i.e., twice that of seawater on Earth, corresponding to an enrichment by a factor of 15 relative to the protosolar cloud. With similar results later obtained for a couple of other comets, it may indicate that comets only marginally contributed to Earth’s water. Nevertheless, these comets are not originating in the Kuiper Belt, and water on Earth might also come from icy small bodies formed in the outer asteroid belt.

One of the most unexpected discoveries of Halley’s missions was that the dust mass spectrometers (PIA on board Giotto, PUMA on Vega) not only detected rock-forming elements (e.g., Mg, Si, Ca, Fe) but also revealed light elements, so-called CHON material, most likely consisting of complex polymeric organic molecules, composed of carbon, hydrogen, oxygen, and nitrogen (CHON), in agreement with data from one plasma analyzer. Such refractory organics, the presence of which was later confirmed for comet Wild 2 by the Stardust mission, had quite likely been formed in the interstellar medium. Besides, the properties of dust particles were varying with distance to the nucleus and with time after ejection, most likely under evaporation and fragmentation processes. Comparisons between the results of OPE and DID (Dust Impact Detector) have shown that, on the average, the geometric albedo of the dust particles was very low (about 0.04) and their density extremely reduced (about 100 kg m−3), suggesting that they mostly consisted of fluffy aggregates.

During the Giotto Extended Mission to Grigg-Skjellerup, fragmentation processes could be suspected, with one event noticed by OPE at about 1,000 km distance from the nucleus, tentatively interpreted by the presence of an active fragment (10–100 m size) releasing dust in the inner coma. All together, discoveries of (1) porous nuclei that may suffer fragmentation processes and (2) refractory organic molecules in low-density dust particles indicate that comets can significantly replenish the zodiacal cloud of interplanetary dust with organics. During the Late Heavy Bombardment epoch, there was certainly a huge amount of interplanetary dust particles of cometary origin. Their highly porous structure, which leads to a significant deceleration and heat transfer in planetary atmospheres, could have contributed to extraterrestrial delivery of carbonaceous compounds within the atmospheres of terrestrial planets.

Future Directions

The next rendezvous with a comet, provided in 2014–2015 by the Rosetta mission, should offer unique data on the nucleus density and structure, as well as on the composition of the dust in the inner coma and on the nucleus near-surface (including information on the chirality of the organic samples). Together with ongoing cometary flybys and remote observations, it should lead to a better understanding of the suspected link between comets and the origin of life on terrestrial planets.

See Also

References and Further Reading

  1. Balsiger H, Altwegg K, Geiss J (1995) D/H and 18O/16O ratio in the hydronium ion and in neutral water from in situ ion measurements in comet Halley. J Geophys Res 100:5827–5834CrossRefADSGoogle Scholar
  2. Calder N (1992) Giotto to the comets. Presswork, LondonGoogle Scholar
  3. Eberhard P, Reber M, Krankovsky D (1995) The D/H and 18O/16O-ratios in water from comet P/Halley. Astron Astrophys 302:301–316ADSGoogle Scholar
  4. Fomenkova MN (1999) On the organic refractory component of cometary dust. Space Sci Rev 90:109–114CrossRefADSGoogle Scholar
  5. Fulle M, Levasseur-Regourd AC, McBride N, Hadamcik E (2000) In-situ dust measurements from within the coma of 1P/Halley: first order approximation with a dust dynamical model. Astron J 119:1968–1977CrossRefADSGoogle Scholar
  6. Grewing M, Praderie F, Reinhard R (eds) (1987) Exploration of Halley’s comet. Springer, BerlinGoogle Scholar
  7. Keller HU et al (1986) First Halley multicolour camera imaging results from Giotto. Nature 321:321–326CrossRefADSGoogle Scholar
  8. Keller HU, Curdt W, Kramm JR, Thomas N (1994) Images of the nucleus of comet Halley obtained by HMC on board the Giotto spacecraft. In: Reinhard R, Longdon N, Battrick B (eds) Images of the nucleus of comet Halley, vol 1. ESA, NoordwijkGoogle Scholar
  9. Keller HU, Britt D, Buratti BJ, Thomas N (2004) In situ observations of cometary nuclei. In: Festou MC, Keller HU, Weaver HA (eds) Comets II. University of Arizona Press, TucsonGoogle Scholar
  10. Kissel J et al (1986) Composition of comet Halley dust particles from Giotto observations. Nature 321:336–337CrossRefADSzbMATHGoogle Scholar
  11. Korth A et al (1986) Mass spectra of heavy ions near comet Halley. Nature 321:335–336CrossRefADSGoogle Scholar
  12. Krüger FR, Korth A, Kissel J (1991) The organic matter of comet Halley as inferred by joint gas phase and solid phase analyses. Space Sci Rev 56:167–175ADSGoogle Scholar
  13. Levasseur-Regourd AC et al (1993) Optical probing of comet Grigg-Skjellerup dust from the Giotto spacecraft. Planet Space Sci 41:167–169CrossRefADSGoogle Scholar
  14. Levasseur-Regourd AC, McBride N, Hadamcik E, Fulle M (1999) Similarities between in situ measurements of local dust scattering and dust flux impact data within the coma of 1P/Halley. Astron Astrophys 348:636–641ADSGoogle Scholar
  15. McBride N, Green S, Levasseur-Regourd AC, Goidet-Devel B, Renard JB (1997) The inner dust coma of comet 26P/Grigg-Skjellerup: multiple jets and nucleus fragments? Mon Not R Astron Soc 289:535–553CrossRefADSGoogle Scholar
  16. Reinhard R (1986) The Giotto encounter with comet Halley. Nature 321:313–318CrossRefADSGoogle Scholar
  17. Reinhard R, Battrick B (1986) The Giotto mission, its scientific investigations. ESA Special Publication1077, NoordwijkGoogle Scholar
  18. Rickman H (1989) The nucleus of comet Halley: surface structure, mean density, gas and dust production. Adv Space Res 9(3):59–71CrossRefADSGoogle Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.UPMC Univ. Paris 6/LATMOS-IPSLParisFrance