Comet Frenzy

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After the international armada of spacecraft provided exceptional and unexpected results at Halley which indirectly showed just how little we knew about comets, the various space agencies proposed a variety of new and more complex missions to comets. There were four major themes: (1) spacecraft which were to investigate a number of nuclei in order to establish a statistical database of their characteristics; (2) spacecraft to collect samples of dust and gas from the coma and return this to Earth; (3) spacecraft to rendezvous with a single comet and accompany it along its orbit to make a detailed study; and (4) spacecraft that would return to Earth with samples from the very surface (and interior) of the target nucleus.

The first type is represented by Vesta, which was a Soviet-French proposal that will be described in relation to asteroid missions (see below).

Cometary dust had been collected in the Earth's upper atmosphere and in Earth orbit. In the 1980s an interesting experiment to collect dust was undertaken jointly by French and Soviet scientists by installing extremely pure metallic targets on the exterior of Space Station Mir and exposing them to sample the most active meteor showers. Similar experiments were planned for flights by the Space Shuttle and for Space Station Freedom.335 As related earlier, one of the objections against trying to return samples during a flyby of Halley in the style of a HER-like mission was that the material would be 'atomized' by the high-speed impact, with the result that the original chemistry would be lost. A sample-return mission should return a sample in a close approximation to its pristine state. Of course, the obvious advantage of a coma sample-return mission would be that the material would be analyzed not by onboard instruments of limited capabilities but in a laboratory using state of the art methods. And in contrast to complex sample-return missions involving landing on the nucleus, sampling the coma could be done with a less sophisticated (and hence cheaper) spacecraft. Interest in such a mission was revived by JPL researcher Peter Tsou, who showed in 1984 that 'underdense' materials like polymer foam or the exotic material aerogel, which is a silicon-based foam of an extremely low density, could halt a particle arriving at 10 km/s without imposing a thermal load on it that would modify its chemistry. This triggered a number of new studies of missions to return a coma sample.336 On the basis of the work by Tsou, the US-European Joint Working Group devised an Atomized Sample Return mission in the style of HER to gain samples of dust from the coma of a comet by using either a metallic target or a low-density material that would capture the dust intact. The samples would be either delivered directly to the Earth or aerobraked into orbit for later recovery by a

Shuttle.337 In 1984 a Giotto 2 mission was proposed as a joint NASA/ESA venture. ESA would provide the bus, which would use the basic structure as its predecessor but with a sample collection mechanism and entry capsule replacing the solid-fuel kick motor. The capsule would be based on the one developed by the US Air Force for its highly successful CORONA reconnaissance satellite. A camera would locate the comet early on and enable the spacecraft to refine its trajectory to make a flyby at a range of 80 km from the nucleus, where 'parent molecules' would be collected before they could be chemically modified by their environment. Several potential targets were identified for interception in the 1990s, but the agencies were so slow to consider the issue that ESA's 'comet interceptor' team dispersed, and in the end the mission did not advance beyond the study phase.338

Many of the scientists who supplied experiments for Giotto set out to build upon the Giotto 2 concept by proposing CAESAR, an acronym which, depending on the source, stood for Comet Atmosphere Encounter and Sample Return or for Comet Atmosphere and Earth Sample Return. The name referred to one of history's most famous comets - the one that appeared in July of 44 B.C. several months after the assassination of Julius Caesar. The spacecraft would present an array of deployable collectors of up to 5 m2 in area to sample the gas and dust in the coma. As in the case of Giotto 2, the large particles would be brought to a halt by a soft medium such as polystyrene foam and remain relatively intact and unchanged. The smaller particles would be atomized on impact, and their remains recovered from the walls of the collector cells. Gases would be collected by chemically inert surfaces similar to the 'Swiss flag' of extremely pure aluminum foil deployed by Apollo astronauts on the lunar surface to trap solar wind ions then returned to Earth for analysis. No fewer than 38 sampling opportunities were identified in the 1990s. One option was for CAESAR to be launched by an Ariane 3 in December 1989 in order to sample comet 73P/Schwassmann-Wachmann 3 in May 1990. It would then return to Earth and release its capsule. In the initial design, this would use a retrorocket to enter orbit for later collection by a Shuttle, but as the design progressed the capsule was revised to make a direct atmospheric entry.339,340

Meanwhile, the success of the ICE mission prompted scientists of the Goddard Space Flight Center to devise a Multi-comet Sample Return mission which would make extensive use of Space Station Freedom. It would require a 770-kg spacecraft of the Planetary Observer class and a pair of 250-kg coma probes, which would be launched together by a Shuttle in 1992 but fly independently. They would observe the Sun during their interplanetary cruise. One of the probes would collect samples during a close flyby of the nucleus of comet d'Arrest in July 1995 and on returning to Earth would fire a retrorocket to enter an orbit from which it would be retrieved by an orbital tug that would take it to the space station. Meanwhile the other two spacecraft would fly by Honda-Mrkos-Pajdusakova, and the sample collected in its coma returned to Earth in a similar manner. The main spacecraft would then go on to encounter Giacobini-Zinner in 1998 and Tempel 2 in 1999. In the meantime, astronauts on the space station would equip the coma probes with clean collectors and then relaunch them to sample the latter two comets within a few weeks of the main spacecraft performing its encounters.341 However, this ambitious idea (which was considered to be competitive with rather than complementary to JPL's CRAF) remained at the study stage. Undeterred, the Goddard team joined forces with their JPL counterparts and proposed the Comet Intercept and Sample Return as a Planetary Observer-class mission. As it had been recognized that a direct entry was the simplest and least expensive solution to returning a sample to Earth, this mission envisaged releasing a CORONA capsule. Comets Kopff, Honda-Mrkos-Pajdusakova and Giacobini-Zinner were all candidates for interception between 1995 and 1998.342,343

Some of the Americans who had worked on the HER and Comet Intercept and Sample Return proposals then teamed up with Japanese researchers from ISAS to propose the Sample of Comet Coma Earth Return (SOCCER) mission. This was to encounter one of the many Jupiter-family comets, the leading candidates including Finlay, Churyumov-Gerasimenko, Wirtanen, du Toit-Hartley, Kopff and Wild 2. The baseline scenario called for a launch in 2000, an encounter with 15P/Finlay in 2002 at a range of under 100 km (and possibly as small as 10 km in order to ensure that pristine 'parent molecules' would be collected) and a return to Earth precisely 4 years after it was launched. Finlay was discovered in 1896, and is a small comet in a 7-year orbit. Although its brightness dramatically decreased during the 20th century, as did its rates of gas and dust production, it was considered to be a good candidate for collecting cometary particles. On returning to Earth, SOCCER would fire its engine to enter an eccentric orbit that would be circularized by aerobraking, then be collected by a Shuttle. ISAS modified its MUSES-A (Hiten) engineering lunar spacecraft to test using a series of passes through the rarefied upper atmosphere to progressively lower the apogee of an orbit. To protect SOCCER during this phase of its mission, it would face a cone of refractory material in the forward direction. A Shuttle would detach the sample canister and abandon the spacecraft in orbit. As a rehearsal, ISAS planned for its Space Flyer Unit satellite to be launched in 1995 by a Japanese rocket and recovered by a Shuttle. SOCCER was seen from the start as a low-cost mission, and its appeal was further increased by the fact that even this would be shared. ISAS, exploiting Japan's experience in making digital imaging chips, was to provide the imager, various instruments and the bus. NASA would provide JPL's sample cell system and in-orbit retrieval. The spacecraft itself would be fairly mundane, consisting of an octagonal bus with the dust collection system on one end (and doubling as a dust shield during the encounter) and the high-gain antenna on the other end. Six solar panels would extend from the bus for the cruise, but be retracted during the comet encounter. A bipropellant engine would make various maneuvers, including course corrections during the cruise, two final comet targeting burns, a large deep-space maneuver, Earth-capture, aerobraking 'walk-in' and 'walk-out', and the final orbit circularization. The engine would therefore have to be rated for a major total change in velocity. Although SOCCER would be spin-stabilized most of the time, it would be able to adopt 3-axis stabilization when this was required. The CCD camera would be used both to obtain navigation images of the comet for precise targeting and for science - the latter including distant studies of the coma and close-in views of the nucleus at much better resolution and quality than had been possible in the case of Halley. The spacecraft would be launched by a

Japanese M-V (sometime also called the Mu-5; a rocket that was to replace the Mu-3SII and be capable of boosting about 500 kg to escape velocity). When it was realized that a start in 2000 would be impractical owing to constraints on ISAS's launch schedules, the launch was postponed to 2001. The plan was to make a flyby of 22P/Kopff in November 2002, with 81P/Wild 2 as a backup. Because it had also been selected for NASA's CRAF mission, Kopff was well observed. When it was realised that Kopff was beyond the scope of the M-V, the launch was switched to an American Delta.344,345,346 The mission study continued until 1993, at which time ISAS opted instead for MUSES-C, which was intended to return a sample from a near-Earth asteroid.347

Meanwhile, building on the study for the unfunded International Comet Mission which had envisaged a Halley flyby and a Tempel 2 rendezvous, JPL had set out to recover some of the intended science with the Comet Rendezvous/Asteroid Flyby (CRAF; unofficially named Newton). This would require the spacecraft to orbit the nucleus of one of the Jupiter-family comets and deliver a penetrator hard-lander to its surface. By the end of 1983, a scientific working group had been established to define the objectives and nominal payload for this mission, which was to be the first of the Mariner Mark II program.

The original plan was for CRAF to be launched in 1990 and perform a flyby of asteroid (772) Tanete on its way to rendezvous with comet Kopff. When the start-up funding was denied, the launch was rescheduled to 1991, the flyby reassigned to asteroid (476) Hedwig and the rendezvous to comet Wild 2; but again funding was denied.348,349,350,351 After a redesign, two options were available: the first setting off in September 1992 for 10P/Tempel 2, and the second in 1993 for 6P/d'Arrest. The Shuttle would deliver the spacecraft and its Centaur stage into low Earth orbit. In fact, a direct trajectory to Tempel 2 would be prohibitively expensive because the plane of the comet's orbit was inclined at 11 degrees to the ecliptic. The spacecraft was therefore to be inserted into a heliocentric orbit that would produce an Earth slingshot which would simultaneously increase the aphelion of CRAF's orbit and match its inclination to that of the comet. Meanwhile, in July 1985 NASA invited scientists to submit proposals to participate in the mission. The scientific objectives were to characterize the geology, morphology and composition of the nucleus and determine how it changed as a function of heliocentric distance; to investigate the chemistry of the coma; and to study the dynamics of the tail and its interaction with the solar wind. The trajectory to Tempel 2 would provide an opportunity to image and attempt to measure the mass of the main belt asteroid (45) Hestia. This was a reddish object about 214 km in size whose spectral characteristics resembled some carbonaceous meteorites. If the propellant margin allowed, a second flyby might be attempted, one candidate being 17-km-sized (1415) Malautra.352 One hundred days before the rendezvous, the spacecraft's camera would start to search the sky for the nucleus. By this time, Tempel 2 would be at aphelion near the orbit of Jupiter, and, because its orbital period was only 5 years, about 1,000 days from perihelion. The uncertainties in the ephemeris for a nucleus at aphelion could easily add up to tens of thousands of kilometers, so it was vital to spot it in time to make the maneuver that would place the spacecraft 5,000 km sunward of the nucleus. As CRAF slowly

The initial concept of the CRAF spacecraft exploiting Voyager technology. (JPL/ NASA/Caltech)

The Mariner Mark II version of the CRAF spacecraft as envisaged prior to the Challenger disaster. (JPL/NASA/Caltech)

The Mariner Mark II version of the CRAF spacecraft to be launched by a Titan IV, shown deploying the penetrator. (JPL/NASA/Caltech)

The final Mariner Mark II version of the CRAF spacecraft closely resembled how the Cassini Saturn orbiter was then intended to be built. (JPL/NASA/Caltech)

approached over the next 45 days, it would send the imagery and other data needed to plan the remainder of the mission. In addition to determining the size, shape and rotation of the nucleus, this early characterization phase would include a number of close passes to enable the mass of the nucleus to be estimated with some accuracy.

The ensuing 18 months would be spent orbiting the comet at altitudes as low as a few tens of kilometers, each circuit taking about a month. During this phase, the various instruments would fully document the state of the still-dormant nucleus. As the comet approached perihelion and started to develop a coma of gas and dust, the spacecraft would withdraw several thousand kilometers to safely observe the onset of activity on the nucleus and gain a sense of perspective of the coma, which would be studied at various wavelengths. If necessary, some of the instruments and other systems would close 'dustcaps' for protection. As the comet reached perihelion, which would be July 1999 in the case of Tempel 2, the spacecraft was to make a distant excursion into the tail to investigate its structure and the plasma phenomena within it. Although this would end the primary mission, there was the possibility of an extension in which the spacecraft would study how the activity subsided as the comet receded from perihelion, and determine the changes to the landscape of the nucleus. As the supply of propellant neared exhaustion, the spacecraft would very likely be steered to a slow-speed impact with the nucleus.

At the time of planning the CRAF mission, Mariner Mark II was to consist of a modified Voyager bus that would host most of the electronics and subsystems and support several booms to carry a magnetometer, RTGs and two scan platforms, one of which was for instruments that needed only low-accuracy pointing and the other for instruments requiring highly accurate pointing. In the case of CRAF, the power supply would be augmented by a circular solar panel which would take advantage of the fact that at certain times in the mission the Sun-comet-Earth angle would be quite narrow, meaning that when the high-gain antenna (itself inherited from the Viking orbiters) was pointed at Earth, the solar panel would be more or less facing the Sun. The spacecraft would have a dry mass of about 1,450 kg, and most of its structure would comprise a large propulsion module using a left-over bipropellant engine from Galileo and tankage for over 4 tonnes of monomethyl hydrazine fuel and nitrogen tetroxide oxidizer.

The wide-angle and narrow-angle CCD cameras were each to have a carousel of filters selected to enable the composition of the comet to be identified. The typical imaging resolution would be about 50 cm, but by allocating one carousel slot to a magnifying lens it would be possible to image small areas at a resolution of about 5 cm. It was also proposed that if the cameras could be operated continuously as the spacecraft sailed through the asteroid belt, they could obtain interesting data on the population of meter-sized objects there.353 The other instruments were to include a visible/infrared spectrometer to map the composition of the surface of the nucleus in 320 different wavelengths, and an infrared radiometer with which to investigate its thermal structure. Another instrument would expose sample collection surfaces to the dust and then examine the grains using an electron microscope. A German dust analyzer of the type used by the Vegas and Giotto would be complemented by an instrument to determine the elemental chemistry of dust grains and the molecular

The trajectory of CRAF was to employ a Venus gravity-assist to reach Tempel 2, as intended just prior to the mission's cancellation in 1991. (JPL/NASA/Caltech)

composition of ice and gas. A dust sensor would monitor the flux of dust and the distribution of the masses, velocities and electric charges of the particles. It would also provide a timely hazard alert to close the covers on other instruments. Two ion spectrometers would measure the composition of neutral and ionized gases in the coma and ionosphere, while a plasma instrument measured how their fluxes varied with the activity of the nucleus. A magnetometer would characterize the ambient magnetic field, and a radio-science experiment would measure the electron density and temperature in the cometary 'atmosphere'. The CRAF results were expected to be excellent science.

The verification of the 'dirty snowball' model for Halley prompted modeling of how such an object might have developed. One result of this work, the 'primordial rubble pile' model, proposed that a cometary nucleus was a loose accumulation of smaller snowballs left over from the formation of the solar system which were held together primarily by mutual gravitation. This could easily explain why nuclei had a tendency to split, and it also predicted that some nuclei might be accompanied by detached fragments that remain gravitationally bound to the parent body. Another study was the 'icy glue' model, according to which a comet's nucleus was made of boulders of a porous and refractory material, held together by a matrix of dust and ice. The Giotto and Vega imagery showed that activity on Halley was confined to a small number of active regions, and it was argued that these marked where the icy matrix was exposed to the Sun.354,355 An instrument developed by the University of Arizona's Lunar and Planetary Laboratory would enable CRAF to test the validity of these models. A spear-shaped penetrator was to impact a flat, relatively pristine-looking portion of the nucleus to determine the elemental chemistry of the material in situ using a gamma-ray spectrometer and to measure its thermal characteristics. Such data gained a particular importance after the infrared spectrometer of Vega 1 revealed that Halley's nucleus was considerably warmer than expected, suggesting there was a thin crust of black insulating material. In fact, the penetrator was to be able to apply heat to the ice and measure how this was diffused within the material. A protruding collector would ingest a small amount of material during the impact, and this would be analyzed by a calorimeter and a gas chromatograph. If possible, a camera would be carried to take pictures of the surface. Six accelerometers would monitor the impact to provide an indication of the surface structure. The penetrator would have a mass of 18 kg, be 1.18 meters in length and have a forebody 6 cm in diameter. The spacecraft would move to an altitude of just a few kilometers, spin up the penetrator for stability in flight and release it. The penetrator would use its own engine to ensure its emplacement in the ground. The engine was initially to be solid-fueled, but it was later changed to a liquid engine so that the duration of the burn could be tailored to the mass and density of the nucleus, as determined by the preliminary studies. Depending on the strength of the crust, it was expected to dig at least 30 cm into the underlying ice. Batteries would provide power for about a week. The spacecraft was to be able to accommodate two penetrators, with the second (if budgeted) serving initially as a backup, and if the first one succeeded the second would be sent to a riskier site such as a dust vent.356,357

When the Shuttle was declared 'operational' in 1982, the US Air Force was so concerned that the vehicle would never achieve the planned flight rate that in early 1984 the Department of Defense issued a Space Launch Strategy in which it called for the introduction of an expendable launcher capable of delivering a Shuttle-class payload to geostationary orbit.358 It was decided to upgrade the successful Titan III as the Titan IV, and make it compatible with the version of the Centaur developed for the Shuttle. NASA's decision after the loss of Challenger not to use the Centaur meant that a number of deep-space missions had to be offloaded to the as-yet-unflown Titan IV-Centaur. To enable CRAF to fit inside the payload shroud of the new launcher, the long propulsion module was replaced by a squat unit which incorporated the engine and plumbing for a cluster of four tanks. Germany agreed to supply this system under a $75 million arrangement in exchange for the flight of a dust particle analyzer. Several other modifications were made to save mass. For example, the Voyager-era structural bus was replaced by the lighter one of Galileo. However, even after these modifications the spacecraft would be too heavy for the Titan IV-Centaur to dispatch it on the intended trajectory, and so the mission was revised to include an additional circuit of the Sun and a Venus slingshot to pick up energy. The flight would take longer, but it would still be able to rendezvous with Tempel 2 within days of the comet's aphelion. And as a precaution against NASA

being banned from using plutonium-powered RTGs, a solar-powered version of the spacecraft using a large roll-out solar panel was also investigated.359

By 1988 CRAF and Cassini were sufficiently well advanced that NASA decided to combine them as a single budgetary item for 1990, the rationale being that the commonality of the spacecraft design, management and operations could reduce the overall cost by as much as $600 million in comparison to starting development separately. Finally, the two missions were financed! In this incarnation, CRAF was to set off in August 1995 and its target would be comet Kopff, which orbits the Sun every 6.4 years. The mission design now called for an Earth slingshot in mid-1997, a flyby of the 88-km main belt asteroid (449) Hamburga, and the rendezvous with Kopff in August 2000. There were even contacts between JPL and the McDonalds fast-food chain about the advertising opportunity of the encounter!360 In an effort to minimize the overall cost of CRAF and Cassini, the propulsion module was again redesigned, this time becoming a long cylindrical unit tailored to the requirements of CRAF (that for Cassini would be flown with its tanks only partially filled) and a common high-gain antenna was optimized to Cassini's requirements. Meanwhile, the launch slipped to 1996, and the new mission plan involved one Earth slingshot, two Venus slingshots and a flyby of 110-km asteroid (739) Mandeville on the way to a rendezvous with Tempel 2 in January 2003.361 Not unreasonably, the scientists no longer cared which comet they studied, so long as the mission finally got off the ground!

When in 1991 the Congress drastically reduced the combined budget for CRAF and Cassini, both launches had to be postponed to 1997. In 1992 the White House said that as a result of the soaring costs of the 'low cost' Mariner Mark II (it had by now reached $1.85 billion) NASA must cancel one of the missions. It was decided to sacrifice CRAF. The German space agency, the only major international partner in the project, was content, as it too was anxious to cut its budget in the wake of the expensive reunification with its eastern sister.362,363 The details of the decision were not publicized, but the Cassini mission probably benefited from heavy international involvement (ESA was to supply the atmospheric probe for Titan, and Italy part of the communication system). Another constraint on Cassini was that it could not be delayed any further, because it would require a Jupiter slingshot to reach Saturn.

The cancellation of CRAF also marked the end of the Mariner Mark II concept of 'low cost' deep-space missions. It had failed in every respect. In particular, the program had become so expensive that it would never have been able to fulfill the promise of mounting frequent missions to many targets. There were many reasons for this, not least the extensive redesign of the bus following the loss of Challenger. What emerged was the precise opposite of what had been intended: the 'flagship' concept, in which a single spacecraft would carry as many instruments as possible, and a schedule so sparse that the participants would devote much of their careers to a single mission.

Meanwhile, in 1984 a survey committee established by ESA delivered a report entitled 'Horizon 2000' recommending that four 'cornerstone' scientific missions be undertaken before the end of the century. One theme was asteroids and comets, and one option was to return to Earth a sample of cometary material believed to have remained unchanged since the formation of the solar system. Later that year ESA awarded Matra in France a contract to investigate such a mission, using either solar-electric or conventional propulsion.

NASA's Solar System Exploration Committee had also recommended a comet-sampling mission for the beginning of the new century. JPL was making a separate but parallel study of such a mission as a follow-on to its ambitious (and expensive) Mars Sample Return. One concept envisaged using solar-electric propulsion from the initial heliocentric orbit to make a slow-speed rendezvous as the comet neared perihelion, then the large solar panels used to power the electric engines would be retracted and the spacecraft would switch to chemical propulsion. It was not even considered necessary to land on the nucleus, because tethered drills could be reeled out and back as the spacecraft 'hovered' 100 meters above the surface.364

Ministers of the ESA member states met in Rome in January 1985 and endorsed the 'Horizon 2000' report. The fact that the Giotto mission was at that time being prepared for launch raised the profile of 'primitive bodies', and it was decided that one of the cornerstone missions should return a sample of a comet. But because it was clear that such a mission would be extremely expensive ($800 million at least) it was offered to NASA as a possible joint venture that would complement CRAF. In July 1986 a Comet Nucleus Sample Return workshop was held in Canterbury in England, and recommended establishing joint ESA/NASA teams on science and technology to investigate making the mission an international cooperative project. The ESA/NASA mission then became known as Rosetta after the 'Rosetta Stone', a tablet on which there were inscriptions which enabled the Egyptian hieroglyphic language to be deciphered. The analogy was the belief that a sample of cometary material would produce a comparable leap in understanding of the links between the interstellar medium and the origin of the solar system. Scheduled for launch in the early years of the new century, it was to sample one of the short-period comets of the Jupiter family. The initial candidate was 67P/Churyumov-Gerasimenko, but when a launch date in 2003 was chosen 73P/Schwassmann-Wachmann 3 became the target.

NASA would supply the Rosetta mission with a Mariner Mark II bus equipped for attitude control, navigation, communication and RTGs for power. The lander, sampling system and return capsule would be provided by ESA. The Deep Space Network would provide tracking and data retrieval. The Titan IV-Centaur launcher would put Rosetta into a heliocentric orbit that would return the spacecraft to Earth after 2 years, at which time a slingshot would stretch the eccentricity of the orbit sufficiently to reach its target - which at that time would be close to aphelion, out near the orbit of Jupiter. Rosetta would spend about 100 days in the vicinity of the comet. The distant approach phase would determine the main geometrical and dynamical characteristics of the nucleus: its size, spin rate, axial alignment etc. The ensuing orbital observation phase would assess the patterns of activity, in particular any out-gassing, to evaluate the risks of operating close to the nucleus. During this time the landing site would be selected. After a candidate had been chosen on the basis of its morphology, a radiometer would assess its texture and roughness. Then the data from all the instruments would be evaluated to determine whether the site would satisfy the scientific objectives. Prior to attempting to land, Rosetta was to

Capsule Reentry
The joint ESA-NASA Comet Nucleus Sample Return spacecraft envisaged for the Rosetta mission. (ESA)

deploy a radio beacon to guide the descent. The lander would use a radar altimeter and a Doppler radar to control its approach. In view of the very weak gravitational attraction of the nucleus, on making contact with surface the lander would fire a harpoon-like device from each of its three foot pads to ensure that it would remain stable during the sampling operations. The sampling facility was to have a robotic manipulator with interchangeable 'end effectors'. After using a grasping tool to collect some surface samples, hopefully representing both volatile and non-volatile materials, it would switch to a coring drill to collect a stratigraphic sample as much as 3 meters deep. A drill that drew as little as 100 W of power and was capable of operating at -200°C was tested by the Italian firm Tecnospazio, one issue being to avoid heating the material.365,366 After samples were placed in the low-temperature storage bay of the sample-return capsule, the NASA ascent stage would lift off and head for Earth. The ESA lander would probably be equipped with an autonomous power system to enable it to make in-situ observations as the comet approached its perihelion. Another possibility was to include some kind of'sounder' with which to study the internal structure of the nucleus. As the return stage approached Earth it would jettison its RTG on a line that would pass by the planet, then release its capsule on an entry trajectory. In addition to providing a sample of the pristine material left over from the formation of the solar system some 4.6 billion years ago, it was hoped that the material might also contain dust and matter that predated the solar system. The distinctive elemental and isotopic composition of interstellar grains would yield insight into the history of nucleosynthesis in the galaxy. The complex carbon compounds expected to be found in a cometary sample would shed light on the processes that made complex chemistry out of simpler molecules. Scientists particularly wished to know whether comets might have 'seeded' planets with the building blocks of life.367,368,369

The cancellation of CRAF cast doubt on whether NASA would be able to give its support to Rosetta. In fact, ESA was facing financial difficulties, due in part to the cost of German reunification. Meanwhile, scientists in Europe were growing concerned that too much of the budget would be assigned to projects such as the Hermes spaceplane, which threatened ESA's science program in the same way that the development of the Space Shuttle had earlier ravaged NASA's program.370 In response to all these pressures, a study was initiated of an ESA-only version of the Rosetta mission that eliminated the sample return activity (the most expensive part of the original concept to implement) and ended up looking remarkably similar to the CRAF. This time the project survived, and was brought to fruition at the start of the new century.

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