KeywordsComet Nucleus Coma Water Dust Organics CHON Giotto Giotto extended mission Spacecraft ESA Halley Grigg-Skjellerup
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.
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.
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.
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 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.
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.
References and Further Reading
- Calder N (1992) Giotto to the comets. Presswork, LondonGoogle Scholar
- Grewing M, Praderie F, Reinhard R (eds) (1987) Exploration of Halley’s comet. Springer, BerlinGoogle Scholar
- 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
- 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
- Reinhard R, Battrick B (1986) The Giotto mission, its scientific investigations. ESA Special Publication1077, NoordwijkGoogle Scholar