Overview
The baseline mission foresees a launch in April 2018. The spacecraft driven by Solar-Electric Propulsion (SEP) will fly-by Earth in 2019 and arrive at the target comet 79P/du Toit–Hartley in mid-2023. During approximately 6 months of operations at the comet the spacecraft will get samples from three surface locations of the cometary nucleus by touch-and-go sampling. To prepare sampling and to enhance the scientific return, the comet will be investigated by remote sensing instruments and in situ dust and gas measurements. The sampling devices are corers that will be driven into the cometary surface once the spacecraft touches the ground. Ground contact will be for a few seconds only. After retrieving the samples, return travel to Earth will begin in early 2024. The samples are cooled down to 133 K during the complete return travel, except for 2 h at re-entry into Earth’s atmosphere when the temperature will increase to 163 K. The spacecraft will return to Earth and deliver the re-entry capsule in April 2028, after a total mission duration of 10 years.
The target comet: 79P/duToit-Hartley
A mission to Comet 79P/du Toit–Hartley (hereafter 79P) was selected as the baseline because of the relatively low ΔV of approximately 10.3 km/s required for the round trip and because it offers a launch opportunity at the beginning of the Cosmic Vision timeframe. Five other comets with ΔV between 10.0 km/s and 11.3 km/s were identified.
Comet 79P was detected by D. du Toit from South Africa in 1945. After its first observed perihelion passage it was lost for several decades. Finally it was recovered by M. Hartley in 1982, after a probable splitting event in 1976. 79P was observed again by professional astronomers during its last two orbits in 1995 and 2003. From observations of its inactive nucleus at large distance from the Sun the radius of 79P is estimated to be 1.4 ± 0.3 km [39].
The perihelion distance of 79P is currently 1.23 AU, but it will decrease to 1.12 AU by its perihelion in 2023 when it will be visited by Triple F. Its aphelion distance of 4.8 AU and orbital inclination of 3° are both relatively low, making 79P a good target for a sample return mission.
Launcher
For the Triple F mission a Soyuz launch from Kourou is considered. For Soyuz both, direct escape and escape from Geostationary Transfer Orbit (GTO) using a propulsion module are attractive. Various escape scenarios are possible. As the baseline, we consider use of a propulsion module and a lunar gravity assist which results in an escape mass of 2,000 kg and an excess velocity at escape of 1,100 m/s.
The interplanetary trajectory
The baseline trajectory for the Triple F mission is outlined in Fig. 1. An optimal solution was found for a launch date in April 2018. The round trip mission duration for this option is 10 years. The stay time at the comet is 7 months which provides sufficient time for a characterization of the comet and the sampling.
Transfer
Even with a state of the art electric propulsion system a direct transfer to Jupiter-family comets such as 79/P is not possible within reasonable time. A generically applicable strategy to reduce the ΔV for the transfer to a comet is the return to Earth for a gravity assist. The basic sequence for the transfer to the comet is the following:
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Launch into GTO
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Raise apogee to lunar crossing orbit by chemical propulsion stage (requires ~700 m/s from GTO)
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Perform a Lunar Gravity Assist
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Leave Earth and thrust for increasing eccentricity of heliocentric orbit and for targeting Earth
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Come back to Earth after little more than 1 year and perform an Earth Gravity Assist.
For the return trajectory it needs to be taken into account that the re-entry velocity into the Earths atmosphere shall not exceed a certain value. For the proposal a maximum reentry velocity of 13 km/s is chosen that lies within the range that was already demonstrated by previous missions (e.g. Stardust, Genesis). Hence the re-entry velocity is limited to 13 km/s. In order to comply with this limit, the return trajectory requires a braking maneuver that adds to the roundtrip ΔV.
Comet approach
Due to the low-thrust transfer the spacecraft will approach the comet at a very low relative velocity. The rendezvous will be achieved by a series of small maneuvers. In order to ensure collision avoidance during this phase (with the comet’s gravity field still badly known), the maneuvers will never directly target the comet but always some point outside of its cross section. This strategy is also foreseen for Rosetta. However, the rendezvous maneuver sequence for Triple F is less risky due to the much lower relative velocity.
Re-entry
For the re-entry the Triple F spacecraft targets a hyperbola with its perigee at the Earth’s surface. Approximately 5 h before the perigee the re-entry capsule is deployed.
The re-entry capsule will follow the desired re-entry trajectory entirely passively and without maneuver capability. After the deployment of the re-entry capsule the spacecraft conducts an orbit maneuver that puts itself on an Earth fly-by trajectory in order to avoid destruction.
Spacecraft
The basic concept of Triple F is simple: A single spacecraft will be sent to the comet, acquire the sample, and return. Due to the low gravity environment, the propellant penalty for this concept compared to a mothercraft–lander system is minimal and one can avoid the complexity of having to rendezvous the mothercraft with the lander in order to transfer the sample. The preferred sampling concept is that of a touch-and-go sampling. The whole maneuver will typically take about 1 h. Limiting factors are the rotational period of the comet and the need that the whole approach sampling and take-off sequence has to be carried out in daylight. The conclusion that touch-and-go sampling with the whole spacecraft is the preferred operational scenario had already been reached for the Hayabusa asteroid sample return mission [22] and also during the Asteroid Sample Return Technology Reference Study [1] for ESA. A major advantage is that the thermal control of the spacecraft does not have to be adjusted to the “hot” surface and that all resources of the main spacecraft are available.
A major challenge is to reconcile the large wingspan of the solar arrays of approximately 15 m each, that is required by the electric propulsion system, with the need to avoid simultaneous ground contact of both solar panels (with the spacecraft continuing to move downwards) when landing. Our concept uses a 1 degrees-of-freedom (DOF) rotational mechanism so that when moving, the solar array follows an imaginary cone. To maximize ground clearance, the solar array will have to be turned in the morning/evening orientation when landing which is compatible with the desired landing scenario. The option yields optimal ground clearance and makes use of an existing solar array drive mechanism. The disadvantage of the concept is the motion of the centre of mass of the spacecraft introduced by the solar panel movement. However, this appears not critical, since it will be compensated by the thruster pointing mechanism of the electric propulsion system.
This thruster pointing mechanism is mandatory anyway in order to minimize the angular momentum build-up that would otherwise result from shifts in the spacecraft’s center of mass due to fuel consumption and antenna pointing. Landing legs are foreseen to guarantee clearance of the attitude control thrusters from the ground.
A 2 DOF articulated high gain antenna (HGA) is proposed to ensure permanent radio link to the spacecraft during thrust phases and to allow simultaneous remote observations and data downlink at the comet.
The re-entry capsule needs to be placed on a panel of the spacecraft that is permanently in shadow. This panel also accommodates the sampling mechanism. The resulting spacecraft configuration is shown in Fig. 2.
Solar electric propulsion
The mission is based on the simultaneous operation of two RIT22 or T6 thrusters. The total thrust time of nearly 20,000 h per thruster is well within the 25,000 h expected lifetime capability of the BepiColombo thruster. In addition a third thruster is foreseen for redundancy. The architecture of the propulsion system corresponds to that of BepiColombo hence extensive use of the BepiColombo heritage could be made.
Power, telecommunications and data handling
For the Triple F mission a solar array with an area of 60 m2 is foreseen. Considering aging and losses in the power processing units this corresponds to an available power of approximately 12,650 W at 1 AU. Of this power typically 500 W (+100 W margin) will be required for the bus leaving approximately 12,050 W to the electric propulsion system enabling a thrust level of 350 mN at 1 AU. The available power will decay with heliocentric distance, R, roughly as R
− 1.7 due to the improved solar cell efficiency at lower temperatures. Ample power is available during coast phases of the transfer and during the comet proximity operations. In particular, an off-pointing of the solar arrays from the Sun direction that may be necessary to achieve sufficient ground clearance of the solar array during the touch-and-go operations is fully compatible with the available power. The minimal thrust level of the RIT22 and T6 ion engines of 30 mN is reached at 3.2 AU heliocentric distance. Beyond this distance the electric propulsion system needs to be switched off.
The solar array sizing is entirely driven by the power demand of the electric propulsion system. Also the power system design is determined by the requirements of the electric propulsion system: A 50 V bus is needed for the propulsion system while the rest of the spacecraft is powered by a 28 V bus derived from the 50 V bus. The power system architecture can be based on the concept of BepiColombo.
An X-band telecommunication system based on BepiColombo with a transmit power of 27.5 W is foreseen for telemetry, tracking and telecommand and science data downlink. An additional Ka-band link for Doppler measurements is proposed in order to allow a precise determination of the comet’s gravity field even at low angles between the Earth and the Sun as seen from the spacecraft. For the HGA a design similar to that of Mars Express and Rosetta with a diameter of 1.6 m is foreseen. This system will provide a data rate of 2 kbps over a distance of 5 AU.
The two drivers for the data handling system are on the one hand the Guidance, Navigation and Control (GNC) during the touch-and-go sequence and on the other hand the demands of the remote sensing instruments, e.g. data compression. For the GNC the requirements are reduced by the fact that the gravitational acceleration due to the comet is small, and hence, the approach will be slow. Consequently processors like the LEON III that will be available in the Cosmic Vision timeframe will be more than sufficient for the GNC. Probably even a current ERC32 with 14 Mips would fulfill the requirements. The requirements for the handling of the instrument data will depend on the processing capabilities that are already incorporated within the instruments.
Guidance, navigation and control
Three-axis stabilisation is the only viable control concept in order to fulfill the pointing needs during thrust phases and proximity operations. The attitude control system consists of sensors and actuators. A sun sensor is foreseen for initial attitude acquisition after launch and safe mode. Star trackers and an inertial measurement unit are used for attitude determination. Standard reaction wheels of 12 Nms capacity and a set of 12 (+12 redundant) 10 N thrusters are foreseen as actuators. In all nominal modes the attitude control of Triple F relies on reaction wheels. For safe mode the attitude control will rely on the thrusters. The thrusters are also used for wheel desaturation and orbit control.
The major challenge for the guidance navigation and control (GNC) are the comet proximity operations, including the touch-and-go on the comet. Years before the launch of the Triple F mission, extended experience of operating in the near nucleus cometary environment will have been accumulated by Rosetta.
The rendezvous with the comet takes place at heliocentric distances below 2 AU. Hence, the spacecraft will be confronted with significant outgassing from the comet during its proximity operations. Due to the gas and dust flux from the comet, stable orbits may not exist and a different strategy must be devised. Attractive options are hovering, which was the nominal mode of Hayabusa, or an eclipse-free terminator orbit (low cometary activity), the latter being the baseline in Agnolon [1]. For the terminator orbit, regular eccentricity control must be conducted in order to compensate disturbances by the cometary environment. These correction maneuvers will have to rely on optical navigation based on landmark tracking or limb recognition. Due to the high operational effort of a ground controlled GNC during proximity operations, the implementation of autonomous GNC seems preferable. Also for the hovering strategy, a certain level of autonomous GNC for the spacecraft is required in order to avoid a strong deviation from the nominal position due to changes in the cometary environment.
Hazards due to the activity of the comet need to be considered. The gas streaming from the nucleus exerts a pressure on the solar panels that pushes the spacecraft away from the comet. However, even for active Jupiter-family comets the average gas pressure at heliocentric distances larger than 1 AU is of the order of the gravity attraction. When the samples will be collected at a heliocentric distance of about 2 AU it will be considerably less. This can easily be compensated by the SEP. The second hazard is fine dust leaving the nucleus; the importance of that hazard depends on the sticking properties of the dust particles and the ill-constrained dust size distribution. Our strategy to avoid high dust fluxes on the solar panels is twofold. Firstly, the solar panels will be directed in a way that the area pointing towards the comet is minimized (orbit near terminator). Secondly, when operating close to the comet, times (local noon) and regions of high dust activity will be avoided. The dust flux will be continuously monitored by the cameras and the in-situ dust instrument. Due to the experience from Rosetta we will have a good understanding of the expected dust environment and the knowledge how to operate close to a comet. There is ample time (7 months) to characterize the inhomogeneous activity of 79P and to devise a safe strategy before touch down.
Ballistic descent from a low orbit is our baseline strategy for the GNC during the touch and go, following the conclusion of Agnolon [1]. He proposes to have the comet detection as well as the approach and orbit capture controlled by the ground, using radio navigation. The spacecraft would autonomously control the orbit and perform the landing and ascent maneuver, based on visual navigation with wide angle cameras. The duration of descent is estimated to range between 30 and 90 min, the landing precision on the order of 10 m.
The only major difference between the study by Agnolon [1] and Triple F as proposed here is the much larger size of the solar array due to the electric propulsion system. Avoiding the danger of ground contact with the solar arrays has already been discussed above. With this issue under control the GNC of Triple F can be based on the findings of Agnolon [1] and does not pose challenges beyond those identified there.
Thermal control
The Rosetta spacecraft is currently demonstrating the thermal control of a spacecraft over a range of heliocentric distances from 1 to approximately 6 AU. The thermal control of Rosetta is achieved by louvers which open at high temperatures and close at low temperatures. This simple concept is also applicable to the Triple F mission. More refined concepts could allow mass savings in the thermal control subsystems if required. In particular the louvers could be replaced by radiators and heat switches, which are currently being developed for the Exomars Rover in ESA’s Aurora Programme.
A particular challenge of the Triple F thermal control is to ensure a cryogenic temperature of the sample. In Section 3.6.1 it is argued that a temperature of at most 135 K during transfer and 170 K for the reentry is desirable to maximize the science return. We designed a simple thermal control that will perform reliably during the 4 years return transfer and keeps the temperature below 133 K in interplanetary space and 163 K during re-entry.
The sample container will be transferred to the reentry capsule soon after sample acquisition. The re-entry capsule will be kept in the shadow of the spacecraft during the complete return transfer. The capsule will have three zones at different temperatures which are well thermally decoupled from each other.
The desired temperature of the sample container can be maintained with a modest radiator size of 0.4 × 0.4 m2 and a conductivity coefficient between the cold and the intermediate zone of 0.03 W/K. This is challenging due to the requirement to have a safe mechanical interface between the zones, but well within reach.
After the detachment of the re-entry capsule from the spacecraft the cooling of the sample container can no longer rely on the radiator of the return capsule because it may be exposed to sunlight due to the approach trajectory of the capsule towards Earth. Hence, during the final approach, the reentry and on ground before recovery of the capsule the sample container will be cooled from the heat capacitor to which it is coupled. This concept is feasible because the temperature on the backside of the ablative heat shield will be quite modest. For a preliminary assessment a conservative value of 523 K was assumed taking into account the values for Stardust. Allowing for a temperature rise of the sample container of 30 K to 163 K and assuming a re-entry duration of 15 min (14 min for Stardust), followed by 2 h on ground before recovery, a heat capacitor of 5 kg water and a conductivity coefficient of 0.26 W/K between the backside of the heat shield and the cold zone is sufficient. Hence, the cooling of the sample during reentry is well compatible with the structural requirement of the re-entry capsule.
Re-entry capsule
There is a trade-off between the re-entry velocity of the return capsule and the braking ΔV of the Triple F spacecraft on its return transfer. We have taken a conservative approach in limiting the entry velocity to 13 km/s. A detailed analysis may well reveal that a higher re-entry velocity is feasible and more mass efficient and could hence lead to a better overall system performance. In any case an ablative heat shield is considered mandatory. It is the most lightweight option and it facilitates the cooling of the sample because it leads to a rather low temperature of the backside of the heat shield. For the final stage of the descent a parachute is foreseen in order to avoid high mechanical loads on the sample at touchdown.
Operations concept
The mission operations concept for Triple F shall ensure that the long mission duration of 10 years does not become a driver for the operational cost. Hence, a minimal number of ground contacts and a high level of spacecraft autonomy during transfer are desirable. For the mission the operational experience of the BepiColombo electric propulsion system will be available and hence it is assumed that also during thrust phase the spacecraft operations can be largely autonomous and ground contact can be reduced to once every week.
During the coast arcs of the transfer, ground contact will be infrequent—typically once per fortnight—because no specific telecommand or navigation needs arise during this phase. For the period around the Earth gravity assist and before the Earth re-entry permanent ground coverage and the use of delta-differential one-way ranging is desirable to achieve the best possible targeting accuracy.
For the comet rendezvous one ground station is sufficient and no large baseline tracking techniques are required. This low level of operational activity is possible due to the low approach velocity relative to the comet.
During the comet proximity operations the use of two or three ground stations is desirable in order to maximize the science return of this phase. However, the GNC strategy will be used to ensure safe operations of the spacecraft when no ground contact is possible and hence a permanent operational attention during the comet proximity phase is not mandatory.
The mission will use ESA 15 m ground stations for launch and early orbit operations as well as for apogee raising sequence for escape from GTO. ESA 35 m ground stations will be used for deep space communications during all other phases of the mission.
Sampling, storage and analysis of samples
Requirements on sampling
Number of samples and sampling locations on the comet
The purpose of the mission is to return at least three samples from different locations on the surface of the comet. Samples will be taken from places with different levels of activity and at different “geographical” locations (with respect to the spin axis and, therefore, insolation), as surveyed by the payload instruments. Highest priority will be given to a sample from an area where ice is visible, or where activity has been observed. Then, an inactive area will be sampled, to compare active versus inactive regions. Finally, one sample will be taken from a (polar) region that sees little sunlight and therefore has experienced relatively little heating since the comet reached the inner solar system. Once the topography of the comet is known from the monitoring phase, additional criteria for the choice of the sampling site may become important. For example, it would be interesting to have a sample from a smooth area similar to the ones imaged on Tempel 1 [61], because there is the possibility that this material could have erupted from the subsurface.
Dimensions of the sample container
As discussed in Section 2.3, a sample depth of 50 cm will be sufficient to find water ice in an active region. The diameter of the sample container is driven by the desired sample mass and by the requirement that sampling change the sample properties as little as possible. Determination of the formation history of the comet by radiometric dating requires several hundred grams of material (Section 2.1). For a density of 500 kg/m3, a cylinder with a length of 50 cm and a diameter of 5 cm can collect 500 g of cometary material.
Any volatiles (including organic and inorganic molecules) that are present in the subsurface should be sampled in such a way as to avoid structural or compositional changes. Therefore, non-destructive sampling is important for the further analysis of the cometary material. The maximum allowed stress during sampling must not exceed the tensile, compressive, and shear strength of most of the sample. As we expect the cometary surface material to be extremely fragile, care must be taken to ensure that the sampler will not compress or otherwise alter the sample. We consider a sampling tube with a thin wall as the best non-invasive means to fulfill this criterion. Figure 3 shows an x-ray image of a cylindrical specimen of high-porosity (85% porosity) non-volatile cometary analogue [8] which was sampled from a 2.5 cm diameter body by means of a plastic tube (inner diameter 7.5 mm, wall thickness 0.25 mm). Although the outer edge shows a sawtooth shape, which stems from the manual operation of the sampler, the overall morphology of the sample is unaltered. In particular, the porosity of the sample is unchanged at distances ≥1 mm from the sampler wall. The sample itself is extremely fragile with a compressive strength of 500 Pa and a tensile strength of 1,000 Pa. For a sample diameter of 5 cm, only a small fraction of the sample will be altered by interaction with the walls of the sampling tool.
Temperature of the sample during return to Earth
The preservation of the sample micro-structure during sampling, cruising phase, re-entry into Earth’s atmosphere and landing is vitally important for part of the mission science. The storage temperature of the samples is driven by the requirement that (at least) the most abundant volatile species, i.e. water ice, will be preserved in solid form. The relation between the gas mass and the initial solid phase mass as a function of the reservoir temperature T
R is:
$$ m \mathord{\left/ {\vphantom {m {M_{\mbox{ice}} }}} \right. \kern-\nulldelimiterspace} {M_{\rm ice} }=\Psi \mathord{\left/ {\vphantom {\Psi {\left( {\Psi -1} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {\Psi -1} \right)}\ast {\left( {P_{\rm V} m_{\rm g} } \right)} \mathord{\left/ {\vphantom {{\left( {P_{\rm V} m_{\rm g} } \right)} {\left( {\rho _{\rm ice} k_{\rm B} T_{\rm R} } \right)}}} \right. \kern-\nulldelimiterspace} {\left( {\rho _{\rm ice} k_{\rm B} T_{\rm R} } \right)} $$
In the above equation Ψ is the fraction of the probe volume filled by solid material, P
V is the vapour pressure, m
g the mass of a water molecule, and ρ
ice the density of the ice. We used the Goff Gratch equation (Smithsonian Met. Tables, 5th ed., pp. 350, 1984) to calculate P
V(T). The calculations show that up to a temperature of 190 K and a porosity of 0.7, the fraction m/M
ice is always below 2 * 10 − 7. A temperature T ≤ 200 K will guarantee that the ice will stay in cubic form (if cubic ice is present close to a cometary surface) and that sintering will be inefficient in solidifying ice–ice contacts.
The equilibrium calculation shows that during the whole return trip only a negligible fraction of the water ice will be in gaseous form. However, equilibrium is maintained by a large number of sublimations and recondensations. In the porous cometary material the gas molecules may move before recondensation, resulting in structural changes. As a worst case scenario, we assume that sublimation and subsequent recondensation always cause modification to the sample. In this case, the requirement is that the sublimation timescale (into vacuum) is larger than the travel time from the comet to Earth.
Figure 4 shows that a temperature of 135 K is sufficient to preserve the ice quantitatively (at the 90% level) in solid form over a mission time of 5 years. A temperature increase to 170 K during 2 h of atmospheric re-entry is acceptable.
The sample temperatures fulfill the requirement of preservation of the micro-structure of the sample. This does not necessarily mean (and it is not required for the scientific goals of the mission) that the sample remains completely unchanged. For example, due to the much higher vapour pressures of other cometary volatiles, a quantitative retention of minor ice species in solid form may require still lower temperatures. Also, should amorphous ice be present in the samples, the temperatures may not be sufficiently low to avoid its transformation into crystalline ice. However, maintaining substantially lower temperatures would be technically challenging. Therefore, we decided to identify volatile ices and amorphous water ice without trying to maintain them in their original state. Since the sampling device is sealed, a mass-spectroscopic analysis of the gas composition will unambiguously determine the total contents of all volatile species. Thermal probes on the sampling device will detect the temperature increase associated with the exothermal transformation of amorphous ice into crystalline ice.
Strength of the sampled material
Unfortunately neither Deep Impact nor other comet observations measured the strength of cometary material. Various analyses of the impact cloud created by Deep Impact resulted in different upper limits for the strength of Comet Tempel 1 on scales of meters. The highest upper limit is 65 kPa [31]. The few observational constraints we have for comets and cometary meteoroids as well as theoretical considerations and laboratory measurements [8] for weakly bound aggregates lead us to estimate the quasi-static tensile (or shear) strength of cometary material in the dm- to m-range to be on the order of 1–10 kPa, while the compressive strength is estimated to be on the order of 10–100 kPa. We require the sampling mechanism to be able to sample materials with strength of up to 1 MPa. This corresponds to the highest values measured for sintered water ice.
Concept of the sampling mechanism
Several methods of sampling material from a small body have been discussed in Agnolon [1]. Here we show the preliminary design of a simple mechanism that allows rapid touch-and-go sampling (duration ≤2 s) for the expected conditions of low or moderate surface strength of a comet. It was developed at the Polytechnic University of Hong Kong based on heritage from Beagle 2 [47].
A sampling mechanism with three corers is foreseen that can be activated separately in three different regions of the nucleus (Fig. 5). The device will use a tubular coring system with a length of 50 cm and a diameter of 50 mm that is propelled by a spring mechanism into the cometary surface. Since the bottom of the sampling cylinder is open during sampling, a shutter mechanism is needed that keeps the sample in the corer during its transport from the cometary surface to the spacecraft.
The acquired sample is directly delivered into a cooled transport container inside the re-entry capsule. Cross-contamination can be avoided by the complete separation of the samples in the cooling compartment. The shutter mechanism is activated by the same cable mechanism that retrieves the coring tube.
The suggested sampling tube will be a Titanium cylinder with a wall thickness of 0.5 mm. Its total weight is about 200 g. Such a cylinder can be accelerated by the springs to a velocity of approximately 12 m/s (spring system has flight heritage from the Rosetta lander eject system). Calculations show that the device can penetrate into material of strength of up to approximately 1 MPa (see Fig. 6). The penetration depth is calculated from the linear momentum balance equation taking into account the cohesive force of the material, friction along the surface of the cylinder, gravitational force, and the increase of the moving mass due to shoved material. The calculations represent a lower limit to the actual penetration depth because the cutting force of the sharp edge of the cylinder is not considered. Should the need arise to sample still stronger materials, a projectile gas generator (flight heritage from the Rosetta lander anchor subsystem) could accelerate the same tube to more than 90 m/s, allowing sampling even of compact water ice.
Sample analysis strategy
Handling of samples on Earth, distribution and preservation
The future return of extraterrestrial materials by ESA requires a European repository facility. In the present case, it will perform sample separation of gaseous volatiles, ices (including their trapped gases), organics and solid minerals. The gases in the sample containers would be transferred into reservoirs for further analyses. Organics will be present in icy and solid fractions. New techniques are required for separation of these phases under conditions similar to those on the comet. The facility will also coordinate the sample distribution. It will store the major part of the material for posterity, in anticipation of improved future analysis techniques. This facility might be associated with a scientific institution that already has required analytical techniques, experience in micro-sample handling including microtome and in situ focused-ion-beam lift-out, as well as the preparation of thin sections. Opening of the containers, sampling and storing of the different phases will be performed in a clean environment, with the sample in vacuum to avoid adsorption of terrestrial atmospheric contaminants.
Examination of the samples with state-of-the-art analytical techniques is necessary to accomplish the major mission goals outlined in Section 2. To maximise scientific output and minimise sample consumption, many techniques will be applied to the same material. Sample requests and allocation (a few percent of the recovered material within the first 5 years after return) should be controlled by a standing committee (similar to NASA’s “CAPTEM” committee or its “Meteorite Working Group”). For a fast publication of scientific results, a small fraction (<1%) of the returned material will be distributed rapidly to pre-selected research institutions.
Analyses
The amount of material returned with Triple F (100 s of grams) will exceed by far that returned with Stardust (~mg). Moreover, the sampling technique (Section 3.6.2) will not mix the material intimately with any collector material. These unique mission elements will allow a large number of examinations that were not possible with the Stardust samples.
Initial non-invasive sample inspection could be achieved with x-ray tomography, providing sub-micrometer resolution and full 3D structural information. The measurements will reveal information about porosity, grain size, pore size, stratigraphy and (to some extent) elemental distribution within the sample. Initial mineralogical and petrologic analysis of most of the mineral grains should entail Scanning Electron Microscope (SEM) imaging and Energy Dispersive X-ray (EDX) analyses on all particles, enabling sample requestors to analyse the most suitable samples. Quantitative x-ray spectrometric elemental analyses should be performed on each grain to establish a library of compositions, done by the repository facility for consistency in data. Large particles (>10 μm) should be analyzed with synchrotron X-Ray Diffraction (XRD). For smaller particles, microtome slices could be prepared for TEM analysis. Relatively non-destructive spectroscopic analyses including IR, Resonant Raman and UV spectroscopy will follow, prior to further analyses by more or completely destructive techniques, such as, e.g., Secondary Ion Mass Spectroscopy (SIMS) or conventional mass spectrometry.
A major goal of all mineralogical analyses will be the search for high temperature phases and hydrated minerals. The presence of the former would add to the evidence of large scale mixing in the nebula [10, 63], whereas the latter would potentially indicate extended warm periods on the comet in the past. Moreover, as yet unknown or fragile minerals might be present that were not preserved in primitive meteorites or Wild 2 particles due to sampling technique or more severe heating on the asteroidal parent bodies.
High-precision mass spectrometric analyses of all fractions of returned cometary material will have highest priority. Deuterium enrichments will unequivocally prove the extraterrestrial origin of the material [44]. H, C, and N isotopic anomalies in the organic fraction will localise interstellar material [11] that has survived the formation of the solar system. The O isotopic compositions in various extraterrestrial materials exhibit heterogeneities [12], implying spatial or temporal variations during the condensation of the first solar system material. Large samples to be returned with this mission will allow the investigation of refractory elements such as Mo, Zr and platinum group elements, which are important to assess heterogeneity and degree of large-scale mixing in the solar system. These analyses require high-precision isotopic measurements, obtainable with MC–ICPMS or TIMS. The samples will also be investigated for stable isotope fractionation effects, e.g. in Mg and Fe., which will provide information on the conditions during which cometary material formed.
The isotopic compositions of presolar stardust grains indicate that they formed in the dust ejecta of previous star generations [62]. Their abundances in extraterrestrial matter reflect thermal processing in the early solar system and on parent bodies. Thus, the abundance of presolar stardust in returned comet samples, identified by SIMS isotopic mapping, could be a sensitive probe of cometary thermal history. Only a few grains have been identified in comet Wild 2 dust [57], suggesting a lower abundance than in cometary IDPs [48]. However, sampling might have introduced severe bias. The more gentle collection method of this mission will allow presolar grains to be more readily preserved, providing better estimates of the presolar grain abundance in comets. Presolar grains might retain coatings acquired while present in the ISM, but that would not have survived the heating that other extraterrestrial material has experienced.
All cometary matter (ices, organics and minerals) is exposed at various times to irradiation. As the cometary surface will continuously be renewed, the most recent effects of space weathering and the production of cosmogenic nuclides can be examined. Structural changes due to such irradiation, e.g., amorphisation of ices, minerals and carbonaceous matter might be visible. For these studies, noble gases in various grains will be studied, the depth profiles of various elements, as well as the response to irradiation of carbonaceous material with Resonant Raman and IR spectroscopy. The latter techniques will also be used to find evidence for the irradiation of carbonaceous matter in interstellar space prior to its incorporation into the comet. Chemically, isotopically or structurally distinct rims around cometary grains would help to assess the conditions prevailing in the protosolar cloud. The composition of micron-sized grains will be measured, e.g., with the Time of Flight SIMS [58] technique, which provides high sensitivity at a very small scale. This technique also detects organic compounds in situ and provides information about the elemental distributions in mantles around cometary grains.
The volatile (noble gases, H, C, N, O, S) content of cometary ice is controlled by the conditions prevailing during its formation, either in the dense interstellar cloud or in the outer solar nebula [50]. Hence, it is of high priority to determine the relative volatile abundances in the various cometary ice grains. Moreover, the isotopic compositions of noble gases, especially Xe, should be determined, both in solid grains and in icy agglomerates, which requires particular, highly sensitive techniques including resonance ionization [26]. Gas abundances of CH4, CO, CO2, N2, NH3, etc. are extremely important, e.g., in order to assess the form in which N has been trapped in the ice, and for comparison with spectroscopic results.
Analyses of organic matter found in ice and associated with mineral grains will provide insights on the early history of our solar system and the extraterrestrial delivery of organic compounds that occurred on early Earth (see Section 2.2). Hence, most important is the search for biologically relevant compounds, such as amino acids or nucleobases, which have been detected in meteorites. In addition, the detection of chiral excesses in these compounds will be essential, as will the comparison with meteoritic organics. Analytical techniques include liquid chromatography with fluorescence detection (HPLC–FD), liquid chromatography–mass spectrometry (LC–MS), gas chromatography–mass spectroscopy (GC–MS) and ionization techniques such as Matrix Assisted Laser Desorption Ionization (MALDI), solid state 13C NMR (Nuclear Magnetic Resonance), XANES (X-ray Absorption Near-Edge Structure) and EPR (Electron Paramagnetic Resonance).
Payload
Overview
Apart from the sampling device, the mission requires a basic payload to fulfill the following objectives:
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1.
Characterize the comet, mainly its activity level and topography, with sufficient spatial resolution to identify appropriate sampling sites
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2.
Mapping of the nucleus to derive important chemical and physical properties that provide the context for the samples
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3.
Monitor the environment of the spacecraft to avoid hazards, mainly due to cometary dust
An overview of the payload is given in Table 1. The total mass of that payload is less than 20 kg.
Table 1 Summary of the payload (objective as defined in the text)
Cameras
Triple F will carry a complement of four camera systems, designed to address different scientific requirements of the mission. For increased reliability, except for the high resolution camera each of the cameras consists of two identical, cold redundant units, for a total of seven camera heads. The first camera (high resolution imager, HI) provides the global monitoring of the nucleus during comet approach and during phases when the spacecraft is further than several tens of km from the nucleus. The second camera (the Mapping Imager—MI) will consist of a moderate angular resolution framing imager. This instrument will be responsible for global mapping from distances between 5 and 20 km, corresponding to a resolution between 0.5 m and 2 m. The third imager is a miniature wide-angle Fast-Framing Camera (FFC) which will be active during the descent, and ascent phases. This instrument, by acquiring image sequences at a rate in excess of 5 Hz, will document (and possibly guide) the descent of the orbiter with increasing accuracy. In this way the FFC will also provide local context by imaging the neighborhood of the sampling site with a resolution up to about 1 mm. The fourth camera will be a Close-Up Imager (CUI) which will monitor the complete sample acquisition and storage sequence during touch-down. In order to be able to observe the sample in the shadow of the S/C, the CUI will be equipped with a miniature LED illumination device.
The field of view of the HI will be 1.5°, imaged on a 2,048 × 2,048 APS array. The specifications of MI, FFC, and CUI follow those of the cameras in Agnolon [1].
Near-IR imaging spectrometer
The main scientific objectives of the infrared spectrometer are the following:
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to support the selection of the sampling sites by searching for surface ice and monitoring the gaseous activity on the nucleus and its spatial distribution
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to determine the nature of the solids on the nucleus surface (composition and structure of ices, dust and characterisation of organic compounds)
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to identify the gaseous species in the coma
While it is highly probable that the design of the near-IR instrument will benefit from the technological progress of the future years, we can refer to heritage from instruments already existing or in development.
The proposed instrument could be a simplified version of VIHI, a hyperspectral imager being developed to fly on ESA’s BepiColombo. The VIHI channel concept is based on a collecting telescope and a diffraction grating spectrometer ideally joined at the telescope focal plane where the spectrometer entrance slit is located. The image of the slit is dispersed by the diffraction grating onto a two-dimensional detector. A single 256 × 256 thinned infrared array detector shall be used which can achieve high quantum efficiency even in the visible domain. The instantaneous acquisition on the two-dimensional detector consists of the slit image diffracted by the grating over the selected spectral range (1–3.5 μm). The complete image is built in time by subsequent acquisitions (push broom mode). The final result is a three dimensional data set, in which each pixel has a spectrum associated with it. The instantaneous field of view will be 250 μrad and the field of view 3.7°. The instrument is housed in a single box that contains the optical system (Telescope plus Spectrometer), the calibration unit, shutter, Focal Plane Assembly (housing the detector and the TE Cooler) and proximity electronics. The operating temperature will be 170 K.
Thermal IR imaging spectrometer/radiometer
Measuring the thermal emission from the cometary surface would support the selection of the sampling site and provide valuable information about physical and mineralogical characteristics of the target. The temperature of sublimating water ice on the surface is about 200 K near perihelion. A thermal infrared radiometer (TIR) could be very useful in detecting possible locations of surface ice (and therefore activity) by their low temperature. Furthermore, the TIR spectral range provides a further excellent opportunity to characterize the mineralogical state of the cometary surface.
From the discussion above we propose an integrated imaging TIR spectrometer/radiometer, which works in the 5–40 μm range, be included in the payload of the Triple F mission, an instrument similar to MERTIS on BepiColombo but with a larger FOV (11.5°) and covering a broader wavelength range for the spectrometer part. Because of the sensitivity limitations of thermal detectors, the spectrometer is designed to provide spectra in 240 spectral channels with good S/N mainly for surface temperatures above 300 K whereas the radiometer is optimized for accurate measurements of the emitted flux in two broadband channels to cover the whole range of relevant temperatures from 100–400 K.
The focal length of the instrument will be around 5 cm. The spatial resolution will be 0.7 mrad for the spectrometer (7 m from a distance of 10 km) and 5 mrad (50 m from 10 km) for the radiometer. A bolometer and a thermopile will be the detectors for the spectrometer and radiometer, respectively. The total dimension of the instrument will be about 14 × 16 × 12 cm.
Mass spectrometer
It is well known that comets contain a wealth of volatile and super-volatile material including radicals like CO, CH2, and CH4 (e.g., [3, 24]). The goal of the mission is to bring back original, unprocessed material. However, taking a sample from the comet and bringing it back to the Earth cannot be done without some disturbance to the material, in particular sublimation of volatiles. Therefore, it is mandatory to have a mass spectrometer on board, which can analyze the most volatile or reactive compounds in situ in order to get the starting condition of the sample. The measurement would be twofold: the mass spectrometer would analyze the natural outgassing of the comet in the approach and prelanding phase. When acquiring the main sample a second much smaller sample will be acquired and placed in front of the mass spectrometer. The mass spectrometer can then analyze the composition of this second sample during the journey back to the Earth over a long time (until the sample has fully evaporated). This will require two separate inlets to the ion source which can easily be achieved because the direction of the neutral gas flow is not critical. The mass spectrometer will not have the same resolution and sensitivity as laboratory mass spectrometers, but it will allow us to assess the initial condition of the sample and to deduce the chemical alterations the material has undergone from the comet to the laboratories on Earth.
We foresee using a time of flight mass spectrometer with an electron bombardment ion source. Such an instrument can easily reach a mass resolution m/dm of 1,500, high sensitivity and mass ranges up to >300 amu. The design could be based on the heritage of Rosetta-ROSINA/RTOF [4].
However, RTOF is too heavy to be included in the present mission. This can easily be solved by omitting the requirement that the mass spectrometer has to be built with an ultra-high vacuum enclosure and be launched sealed under vacuum as was done for ROSINA/RTOF. This requirement was necessary in the case of the Rosetta mission to study the comet far away from the Sun when cometary activity is minimal. RTOF has two independent channels (ion sources and detectors) that can be reduced to one for the present mission, thus saving not only detector and ion source but also electronics. Therefore, it would be possible, with current technology, to build a reflectron-time of flight instrument with a mass budget of 4 kg. The dimensions of the instrument will be about 20 × 20 × 50 cm.
Dust in-situ monitoring experiment
The S/C will have to operate deep inside the comet coma, both during the monitoring phase and during the descent for sampling. Therefore, it is important to characterize the dust distribution in the coma in order to control and guarantee safety for mission operation and S/C health. Therefore, an in situ dust instrument is essential as a “security device” to measure the dust flux on the S/C.
The GIADA experiment on board Rosetta will study the cometary dust environment of 67P and will accomplish unprecedented in situ primary scientific measurements [13]. A similar experiment on board the Triple F mission will fulfill the aforementioned security requirements and will in addition provide scientific data as it shall provide real time data on dust flux of “direct” and “reflected” grains, dust velocity distribution, dust evolution in the coma, dust changes vs. nucleus evolution and emission anisotropy, determination of dust-to-gas ratio, and identification of non homogeneous dust emission features from the surface (active areas, jets).
The instrument is designed to measure momentum, scalar velocity and mass of single grains. From the detection of particles vs. time information about dust abundance and spatial distribution vs. physical and dynamic properties is derived. Its two stages of detection form a cascade: an Optical Detection System (ODS) and an Impact Sensor (IS). When a grain crosses the ODS, it is optically (scattered/reflected signal) detected, and its physical (shape and size, mainly) and chemical (through the optical constants) properties are measured. For each grain impacting onto the IS, the momentum of the incident grain is measured. With this system it is possible to measure abundance and dynamic properties of grains present in the comet coma. The ODS gives a first estimate of the speed of each crossing grain, and the speed of the grain is measured again, but with more accuracy, from the time-of-flight between ODS and IS. In this way for each detected grain, speed, time-of-flight, momentum and, therefore, mass are measured. The field of view of the ODS + IS system is about 40°. In addition, a network of microbalances (MBSs), with field of view of about 40° each, point in different directions to monitor flux of submicron and micron particles from 10 − 10 to 10 − 4 g. The instrument is equipped with controlling electronics to drive the sensors and to transmit telecommunication to and from the spacecraft. The size of the instrument is about 23 × 25 × 30 cm.
Radio science investigation
The Radio Science Experiment RSE will use the radio signals transmitted from the onboard radio subsystem at the carrier frequencies X-band and Ka-band in the two-way radio mode (X-band uplink). The goal is to derive perturbing forces acting on the spacecraft by measuring the Doppler shift of the carrier signals caused by additional changes in relative velocity. Perturbing forces of interest may be:
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the gravity acceleration of the cometary nucleus which would reveal mass, bulk density (together with a volume estimate from the camera)
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the outgassing from the cometary nucleus which would reveal the combined gas and dust production rate (although the major contribution will come from the gas).
The measurements require a highly stable two-way link where the stability of the X-band uplink is derived from the ground station’s hydrogen maser. The spacecraft receives the X-band uplink and transponds it back to Earth phase-coherently at two simultaneous downlink frequencies at X-band and Ka-band. Although the Ka-band downlink seems to be much noisier than the X-band downlink, it is possible to correct for the plasma noise in order to achieve a clear and four times stronger Doppler signal at Ka-band than at X-band. This allows us the detection of very small perturbations in the <10 μm/s range. X/X and X/Ka transponders are available on the market and have been flown on ESA missions (SMART-1).
These measurements require that the spacecraft performs attitude changes using the reaction wheels only during the gravity observations. Any attitude changes by thruster firing would destroy the observations.
The mass determination shall be an iterative process starting during the approach phase. Each mass determination at a certain error level will allow manoeuvring the spacecraft closer to the nucleus.
Comet Penetrometry Experiment (CPEx)
Penetrometry is the use of a penetrating probe to measure mechanical properties of the target material. A comet nucleus sample return mission offers a rare opportunity to make contact with undisturbed cometary sub-surface material. Compared with previous missions, Triple F will provide measurements at multiple locations on the target comet. Penetrometry at the sampling locations will be useful for two main reasons:
Firstly, it can support sample collection by means of dynamics measurements performed during sampling. Questions that may be addressed include: Where did the material actually come from? What was its undisturbed volume? What is the strength and texture of the undisturbed material? To what extent was it disturbed, crushed and mixed during sampling? Secondly, penetrometry can generate unique ground-truth science. The penetration resistance of cometary material is sensitive to its origin and modification. Furthermore, measurements with depth can detect layering, e.g. the presence of low-cohesion material overlying a sintered layer at the water ice sublimation front. European heritage in this technique is strong, including sensors flown on Huygens and Philae.
Although the primary requirement for sampling limits the possibilities for dedicated geotechnical measurements, a great deal may still be learned from a set of simple measurements. The solution we propose for Triple F is to incorporate small sensors to monitor dynamics during touchdown and sampling. Assuming a short-duration (few s) touchdown and a coring tube driven quickly (10 m/s initial speed) into the ground, we propose the following:
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Displacement sensor to monitor the relative position of the sampling tube and spacecraft. This could be implemented by means of an optical bar-code technique. Stripes affixed to the core tube would be interrogated by a small light source and photodiode. Mass: ~5 g. Data: 1 bit at 50 kHz during sampling, i.e. 50 kbit/s. Heritage: HP3 DACTIL tether length sensor.
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Microphone-acoustic vibrations in the sampling mechanism should be sensitive to the texture of the sampled material and its variation with depth. A small piezoelectric or strain gauge sensor mounted on the sampling system at a location affording good acoustic coupling could fulfill this role. Mass: ~2 g. Data: 12 bits at 50 kHz during sampling, i.e. 600 kbit/s. Heritage: COTS aerospace components.
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If possible, a single-axis accelerometer mounted near the upper end of the coring tube. This would measure the deceleration, and thus force, encountered by the sampling tube. Mass ~2 g. Data: 12 bits at 50 kHz during sampling, i.e. 600 kbit/s. Heritage: COTS aerospace components, Philae MUPUS ANC-M, Huygens SSP ACC-I, Huygens HASI ACC, HP3 DACTIL.
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Harness and electronics: 100 g for the displacement, microphone and accelerometer sensors.
Permittivity probe
A permittivity probe measuring the electric properties of the sample between 0.1 Hz and 10 kHz is well suited to monitor changes in internal structure of the sample which are associated with sublimation processes of H2O. The electrical permittivity of ice is unique amongst the rock forming minerals in the low frequency range, as the dielectric constant is of the order of 100 under static conditions, thus, more than an order of magnitude higher than for rocky materials, and decreases with increasing frequency to a value of 3.1 for infinite frequency.
The permittivity probe used to monitor the sample closely follows the design of a mutual impedance probe. Ringsector-like electrodes printed onto a Kapton foil are attached at different depths to the inner wall of the sample container. A current generator is connected to two selectable transmitter electrodes, and the generated voltage is sensed as a function of frequency by a number of receiver electrodes. By using different transmitter/receiver geometries across the sample an electrical “image” of the sample can be created. The total mass of the permittivity probe including electronics and harness is estimated to be less than 300 g and the average power consumption during operation is less than 1 W.
Thermal probe on sampling device
The knowledge of the original cometary temperature profile at the time of sampling measured as a function of depth would be of considerable value for the interpretation of the sample measurements. Because the titanium sample container has a several orders of magnitude higher thermal conductivity than the cometary soil, contact measurements cannot easily be used for that purpose. A possible solution is, therefore, based on the use of radiometric sensors (e.g. thermopiles as will be used for measuring the surface temperature at the Rosetta PHILAE landing site of comet 67P/Churyumov–Gerasimenko, [56]) which are mounted in different depths at the inner wall of the sample container. The short time constant of these sensors of about 200 ms makes the measurement of the temperature profile during the first seconds after insertion of the sampling device possible. This is before the temperature of the low conductivity cometary material can effectively adjust to the wall temperature. In addition, continuing the measurements after sampling would allow inferring thermal properties of the fresh sample. Furthermore, in case that a significant amount of heat is released inside the sample in the process of crystallization of originally amorphous ice, monitoring of the sample temperature as a function of depth could prove the existence of this process. Assuming 25% amorphous ice in the lower parts of the sample, the crystallization heat would increase the temperature by approximately 20 K, which is easily detectable by the sensors.
For the reasons described above the implementation of several (around 8–16) thermopile sensors into the sampling device could provide valuable information about the initial temperature profile of the sample. The resources needed by this instrumentation are minor, basically an electronics comprising of multiplexer, amplifier and ADC (assuming that another DPU controls the data acquisition) with a mass of about 100 g (including harness) and a power consumption of less than 200 mW.