A Precursor Balloon Mission for Venusian Astrobiology
Andreas M. Hein, Manasvi Lingam, T. Marshall Eubanks, Adam Hibberd, Dan Fries, William Paul Blase
DDraft version November 10, 2020
Typeset using L A TEX twocolumn style in AASTeX63
A Precursor Balloon Mission for Venusian Astrobiology
Andreas M. Hein,
1, 2
Manasvi Lingam,
3, 4
T. Marshall Eubanks, Adam Hibberd, Dan Fries,
6, 2 andWilliam Paul Blase Universit´e Paris-Saclay, CentraleSup´elec, Laboratoire Genie Industriel, 3 rue Joliot-Curie 91190, Gif-sur-Yvette, France Initiative for Interstellar Studies (i4is), 27/29 South Lambeth Road, London, SW8 1SZ, UK Department of Aerospace, Physics and Space Sciences, Florida Institute of Technology, 150 W. University Blvd, Melbourne, FL 32901,USA Institute for Theory and Computation, Harvard University, 60 Garden St, Cambridge, MA 02138, USA Space Initiatives Inc., 527 Burlington Ave, Palm Bay, FL 32907, USA Department of Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, 2617 Wichita St C0600, Austin,TX 78712, USA
ABSTRACTThe potential detection of phosphine in the atmosphere of Venus has reignited interest in thepossibility of life aloft in this environment. If the cloud decks of Venus are indeed an abode of life,it should reside in the “habitable zone” between ∼ −
60 km altitude, roughly coincident with themiddle cloud deck, where the temperature and pressure (but not the atmospheric composition) aresimilar to conditions at the Earth’s surface. We map out a precursor astrobiological mission to searchfor such putative lifeforms in situ with instrument balloons, which could be delivered to Venus vialaunch opportunities in 2022-2023. This mission would collect aerosol and dust samples by means ofsmall balloons floating in the Venusian cloud deck and directly scrutinize whether they include anyapparent biological materials and, if so, their shapes, sizes, and motility. Our balloon mission wouldalso be equipped with a miniature mass spectrometer that should permit the detection of complexorganic molecules. The mission is augmented by contextual cameras to search for macroscopicsignatures of life in the Venusian atmospheric habitable zone. Finally, mass and power constraintspermitting, radio interferometric determinations of the motion of the balloons in Venusian winds,together with in situ temperature and pressure measurements, will provide valuable insights into thepoorly understood meteorology of the middle cloud region. INTRODUCTIONAs the surface temperature of Venus is over 460 ◦ C, itis too extreme to permit the existence of life-as-we-know-it (Lingam & Loeb 2021). However, if one considersthe lower cloud layer of Venus at a height of ∼
50 kmabove the surface, both the temperature ( ∼ ◦ C) andpressure ( ∼ Corresponding author: Manasvi Lingammlingam@fit.edu tection, availability of liquid sulfuric acid and nutrients,and shielding against high-energy particles (Schulze-Makuch et al. 2004; Dartnell et al. 2015; Limaye et al.2018; Seager et al. 2021). Yet, in contrast, there are sig-nificant challenges that confront hypothetical Venusianbiota, such as the extremely low water activity and thehighly acidic environment due to the presence of sulfuricacid (Cockell 1999; Zhang et al. 2012).The field of Venusian astrobiology came to the fore-front in 2020 September thanks to the possible detec-tion of phosphine at a concentration of ∼
20 ppb inthe Venusian cloud decks by Greaves et al. (2020); see,however, Villanueva et al. (2020). This ostensible dis-covery is considered significant because phosphine pro-duction on Earth almost exclusively entails biological oranthropogenic processes, and it is therefore viewed as abiosignature gas (Sousa-Silva et al. 2020). It has beenargued that the observed abundance of phosphine couldnot be explained by any known abiotic process (Bainset al. 2020), implying that either novel (and unknown)abiotic mechanisms or metabolic pathways are involved.However, distinguishing between these two hypotheses is a r X i v : . [ a s t r o - ph . I M ] N ov difficult, even assuming that additional data is collectedand sophisticated models are developed.The most unambiguous method for resolving the is-sue of whether life exists in the Venusian atmosphere isto send spacecraft to Venus and carry out on-site mea-surements and experiments amid its cloud layers. Thispoint was duly appreciated by Greaves et al. (2020), whostated that: “Ultimately, a solution could come fromrevisiting Venus for in situ measurements or aerosol re-turn.” The question that immediately springs up in thiscase is: what types of biomarkers are most suggestive oflife? Naturally, this issue has attracted much debate,and many different classes of biosignatures have beenexpounded (Summons et al. 2008; Chan et al. 2019).We describe two broad examples because they pertainto this work. First, in the context of organic molecules,prospective biomarkers include polymers derived fromamino acids or nucleotides, enantiomeric excess of chi-ral amino acids and sugars, and overabundance of high-mass amino acids and other biochemical building blocks(Neveu et al. 2020). Second, at the level of organ-isms, the manifestation of cell-like morphological struc-tures, motility, and biofabrics are believed to constitutepromising indicators of life (Nadeau et al. 2016).In this Letter, we map out a precursor astrobiologi-cal mission to search for putative biosignatures in situ with a fleet of instrument balloons that could surveythe Venusian atmosphere as early as 2022–2023. Theoutline of this Letter is as follows. We discuss the sci-ence objectives in Section 2, followed by a descriptionof the mission architecture and stages in Sections 3 and4, respectively. We summarize our findings in Section 5,while the Appendices provide the technical information. SCIENCE OBJECTIVES AND INSTRUMENTSThe chief science objectives of the mission are de-scribed as follows.1. Objective 1: Collect aerosol and dust samples, anddetermine the shape, size, and motility of putativemicroorganisms (if they exist).2. Objective 2: Search for macroscopic signs of life,potentially analogous to the Jovian lifeforms envi-sioned by Sagan & Salpeter (1976).3. Objective 3: Search for complex organic com-pounds, especially polymers composed of aminoacids, nucleotides, and repeating charges.4. Objective 4 (optional): To better understand themeteorological dynamics, including zonal windsand ground-linked gravity waves, of the possibleVenusian habitable zone that represents the tar-get. Measurement of the water activity in this re-gion could also be undertaken using a hygrometer.The instrumentation for the science objectives com-prises the following elements. 1. A combined collection plate (for gathering aerosolsand dust particles) and mini-microscope with anaccompanying light source (Objective 1).2. A camera to take contextual images of the Venu-sian atmosphere (Objective 2).3. A miniature mass spectrometer (MS) equippedwith a separation stage that permits the identifi-cation of complex organics (Objective 3). In prin-ciple, the MS would also permit the characteri-zation of inorganic compounds in the atmospherearising from either abiotic processes or potentiallymetabolic pathways.4. Very Long Baseline Interferometry observations ofradio emissions of each balloon together with ap-propriate meteorological instruments (Objective4). As this objective is not geared toward thesearch for life, and as the Vega balloon missionconducted similar observations (Sagdeyev et al.1992), these measurements are not described here.We will begin by tackling Objectives 1 and 2. Ghoshet al. (2011) presented a fluorescence microscope with aresolution down to 1 . µ m and a mass of 1 . . µ m range. Otheropen source designs such as FinchScope, UCLA Scope,and CHEndoscope exist with masses between 1 . .
5g (Aharoni & Hoogland 2019). In tandem, miniaturizedcollection plates and petri dishes have been delineatedin the literature. For example, Ingham et al. (2007)described a 36 × . ×
224 pixel resolution and anintegrated light source for illumination. A megapixelcamera of mass ∼ .
25 kg was employed by the Mars
Curiosity rover, but much smaller cameras are realiz-able. We estimate that the total mass of scientific in-strumentation for the first two objectives can be kept to ∼ . (cid:46) https://nexis.gsfc.nasa.gov/rrm phase2vipir.html https://mars.nasa.gov/msl/spacecraft/rover/cameras/ In contrast to Objectives 1 and 2, Objective 3 entailsa larger payload. For instance, the miniature MS de-scribed in Yang et al. (2008) has a mass of ∼ . . PRECURSOR VENUS BALLOON MISSIONARCHITECTUREOur objective is to identify a mission architecture,which can be developed at minimal mass and launchedas quickly as possible, but is nevertheless capable of re-turning a significant amount of in situ data from theVenusian cloud decks. We will elucidate the details ofone such architecture, without claiming optimality.Concepts proposed for surveying the Venusian atmo-sphere span various types of balloons, paragliders, kites,fold-out wing gliders, solar powered aircraft, and air-ships (Dorrington 2010). We focus on balloons, asthey have already been successfully operated withinthe Venusian atmosphere and have withstood extensivescrutiny (Babu & Pant 2020). Venus balloons were pre-viously utilized during the Vega 1/2 program with atotal balloon mass of 21.5 kg (Sagdeev et al. 1986); anotable point of difference, however, is that the entryprobe also contained a lander. Several Venusian balloonstudies were subsequently conducted, such as the Euro-pean Space Agency’s European Venus Explorer concept(Chassefi`ere et al. 2009). Overviews of balloon investi-gations have been presented in Dorrington (2010) andBabu & Pant (2020). Current challenges for Venus mis-sions are summarized in Glaze et al. (2018): key obsta-cles include atmospheric entry with complex deploymentand sufficient power generation.Although numerous missions to analyze the Venusianatmosphere have been proposed in the past, an exclu-sively astrobiology-oriented mission comprising a fleetof balloons appears to be missing in the literature. Wewill describe two different types of balloon probes thatcould be put into operation in the mission, with a cer-tain degree of redundancy built in. Figure 1 depicts theballoon fleet in the Venusian atmosphere. 3.1.
Category 1 probes
What we label the Category 1 probes are intended totheoretically fulfill Objectives 1 and 2. These probescomprise not only the scientific payload but also a num-ber of other key subsystems such as communications,power, harness, balloon (with fuel), and parachute. Thebreakdown of the various masses is delineated in Ap-pendix A.1, where it is determined that each probe maynecessitate a total mass of M t ∼ . ∼ bps to Earth is not entirely unreasonable overa mission duration of ∆ t ∼
48 hr. In other words, overthis interval, the transfer of ∼
22 MB is potentially re-alizable. If we take the camera associated with VIPIRfrom Section 2 as an example, and employ a conversionfactor of 3 bytes per pixel, it would be possible to trans-mit a total of ∼
143 such images per each Category1 probe. At higher resolution, however, fewer imagesare transmittable to Earth. This data limitation is in-evitable when it comes to small spacecraft like Cubesats(Selva & Krejci 2012; Poghosyan & Golkar 2017).Second, we ask ourselves how many microbes wouldbe collected over ∆ t . To do this, we need an estimatefor the biomass number density n bio , which remainshighly indeterminate. However, for the sake of argu-ment, we consider ρ bio ∼ − g m − , which is ∼ ∼ µ m and a massof ∼ − g for the microbes (Milo & Phillips 2016, pg.10), we obtain n bio ∼ m − . If these microbes pos-sess a settling velocity of ¯ v , then the amount of microbes N m collected over the area A is N m ∼ n bio ¯ vA ∆ t . Wechoose ¯ v ∼ − m s − based on the estimate for 1 µ maerosols (Junge et al. 1961, pg. 2178). Note that themass of Category 1 probes is roughly equivalent to thatof a standard Cubesat with cross-sectional area of 10 cmby 10 cm (Selva & Krejci 2012), of which ∼
1% can beset aside for microbe collection; moreover, the resultantarea of A ∼ − m is comparable to the total area ofthe petri dish mentioned in Section 2.By substituting these fiducial values, we obtain N m ∼
170 microbes. Thus, even if the biomass density istwo orders of magnitude smaller than the value adoptedhere, it seems conceivable that each Category 1 probemight stumble across a Venusian microbe.3.2.
Category 2 probes
The mass of the Category 2 probe is higher relative toCategory 1 probes because the former is equipped withthe MS to carry out Objective 3. We invoke the tandemMS that was described in Section 2 for our purposes.As with the Category 1 probes, the analysis of varioussubsystems is undertaken in Appendix A.2. Based onthese estimates, we calculate a mass of M t ∼ . Figure 1.
Artist’s impression of the balloon fleet in the upper “clear zone” at ∼
50 km above the surface of Venus (Imagecredit: Adrian Mann) for the Category 2 probe, which is nearly an order ofmagnitude higher than the Category 1 probes.As indicated in Appendix A.2, we have posited a datarate that is about 5 times higher at ∼ × bps; notethat this value is about an order of magnitude removedfrom the data rates associated with large-scale missionssuch as Cassini . This choice of the bit rate implies thatthe total amount of data transmitted to Earth is ∼ ∼
715 images could be sent back to Earth. Inthis case, however, it should be appreciated that the MSdata is not encapsulated by images as such. MISSION STAGESTo implement the mission, it is necessary to identifya suitable launcher for transporting our entry vehicle,which is composed of the Category 1 and Category 2probes outlined previously. https://solarsystem.nasa.gov/news/12976/cassinis-largest-science-instrument/ In terms of technology, we consider existing smalllaunchers, notably Rocket Lab’s Electron vehicle withan upper stage, which is theoretically capable of launch-ing payloads of 68 kg for a Venus flyby mission and aVenus entry probe with a mass of 37 kg. The spacecraftsubsystems, for the most part, should rely on off-the-shelf and commercial-off-the-shelf technologies to de-crease development duration. The mission sequence con-sists of launch and Earth escape, Venus arrival, directatmospheric entry and descent, balloon deployment, andcontinued balloon operations until the onset of balloonfailure. Previous studies have relied on prior orbit inser-tion, as it facilitates orbiter deployment (van den Berget al. 2006), but we opt for direct atmospheric entry.We have discussed balloon deployment and operationsin Section 3 and Appendix A. We will not address thelaunch and Earth escape, because these standard prob-lems have been widely investigated in the literature. Figure 2.
Porkchop plots for encounter velocities at Venus from Earth in km s − . An ideal mission would have both shorttransfer time and low entry velocity. Such missions are possible every synodic period ( ∼
584 days) with the next such launchopportunity arising in 2021-2022.
Table 1.
Breakdown of mass requirements for various subsystemsSubsystem Mass (in kg) NotesCategory 2 scientific instruments 3.2 Section 2Category 2 gondola 8.3 Appendix A.2Category 2 balloon mass 4.6 Appendix A.2
Total Category 2 probe mass
Total Category 2 probe mass
Total mass of all probes
Total mass entry vehicle without margin
Total mass entry vehicle with 30% margin
Additional notes:
The “gondola” is assumed to encompass the relevant subsystems such as power, communications,and harness. The “balloon mass” includes the mass of the fuel, balloon fabric, and parachute. Whenever a particularSection or Appendix is listed, the rationale for the mass specification is explained therein.
Figure 2 shows the range of Venus entry velocities as afunction of launch date and flight duration. The plot wasgenerated via the Optimum Interplanetary TrajectorySoftware (OITS) (https://github.com/AdamHibberd/Optimum Interplanetary Trajectory). It adopts thepatched conic assumption, and solves Lambert’s prob-lem for Earth departure and arrival at Venus. The re-sulting non-linear global optimization problem with in-equality constraints is solved by applying the NOMADsolver (Le Digabel 2011).The lowest perihelion velocity is achievable through aHohmann transfer, which yields v ∞ = 2 . − at theboundary of the Venusian sphere of influence. Togetherwith the second Venusian escape velocity and the vis-viva equation v = (cid:112) v ∞ + v esc , where v esc is the secondescape velocity of Venus, one obtains v = 10 . − as the minimum velocity for direct Venus atmosphericentry from interplanetary space. This entry velocity ishigher than the entry velocity of 8 km s − from the In-ternational Space Station (ISS), which necessitates thedissipation of ∼
80% higher kinetic energy during en-try, and is similar to entry conditions into the Earthatmosphere from the Moon.We rely on proven entry technology, such as thescaled-up version of the Reentry Breakup Recorder(REBR), which is one of the smallest proven Earth re-entry capsules to date with a total mass of 4 kg (Weaver& Ailor 2012; Feistel et al. 2013); it has been utilizedfor re-entry from the ISS. Due to similarities in theatmospheric entry conditions on Earth and Venus, thetechnology should be adaptable to Venus entry by in-corporating a proportionally larger heat shield. If thereare n probes of Category 1 and n probes of Cate-gory 2, the total payload mass for this entry probe is M t = n M t + n M t . In what follows, we work with n = 1 because of the higher mass associated with Cat-egory 2 probes, but we remark that this assumption canbe easily relaxed. There is, however, a crucial missingcomponent: heat shields. They comprise a sizable frac-tion of the mass of the entry vehicle. Other components,such as the balloons release mechanism, are anticipatedto involve lower mass constraints.The exact mass of the shields varies quite significantlydepending on atmospheric properties, entry velocities,and many others. The ratio of the heat shields to that ofthe entry vehicle mass is ∼ . . M v ), whichalso encompasses shields, scales linearly with M t (seeHirschel & Weiland 2009) and entails a conversion fac-tor of ζ ≈ .
5, to wit, we have M v ∼ ζM t . If we select M v ≈
37 kg, we find M t ∼ . n ∼
5. In contrast, if we choose a higher vehicle mass of M v ∼ M t ∼ . n ∼ M t ∼ . . −
12 km s − and assume deceleration torest. After including the potential energy of the vehicle,using Phenolic Impregnated Carbon Ablators (PICA)as the heat shield material with an enthalpy of ablation0 .
233 GJ kg − , and adopting a ∼
20% safety factor forthe heat shield, we determine that the mass of the heatshield required spans 11 . . − and 12 km s − , respectively. If weadd this heat shield mass to M t , we notice that the totalmass is either below or close to the stipulated vehiclemass of M v ∼
37 kg as desired.However, in undertaking the above calculations, wedid not incorporate a mass margin, which is generallystandard practice. If we choose an approximate massmargin of 30%, then the relationship linking M v and M t is transformed into M v ∼ κζM t , where we have κ ≈ . M v ≈
37 kg, we obtain M t ∼
19 kg and n ∼
2. On the other hand, for M v ∼ M t ∼ . n ∼ >
10 Category 1 probeseven with a comfortable mass margin of 30%, therebyallowing for enhanced redundancy. DISCUSSIONThe putative (albeit contested) detection of phosphinein the Venusian atmosphere has reignited interest insending life-detection missions to our sister planet. Mo-tivated by the rapid technological growth and versatil-ity of small spacecraft such as Cubesats (Selva & Krejci2012; Poghosyan & Golkar 2017), we have delineateda possible template for precursor missions aiming tosearch for indicators of life in the cloud decks of Venus.This Letter has demonstrated that a low-mass, low-cost precursor vehicle to explore the Venusian cloud lay-ers of interest to astrobiology could be constructed andlaunched within the next 2 − < $20million and mass of ∼ https://ntrs.nasa.gov/citations/19970017002 Table 2.
Breakdown of costs involved in the precursor missionSystem Cost in M $ NotesLauncher cost 10 Baseline launcher plus upper stage cost; estimate in Appendix BProbe cost 2.4-9.6 Cost range of $50-200 thousand kg − (Wertz & Larson 1999, pg. 808);estimate in Appendix B Total cost scent system. We also incorporated a certain degree ofredundancy in the mission by allowing for the existenceof multiple probes. The life-detection mission might beable to collect data of significant astrobiological inter-est by way of measuring the composition of dust andaerosols via the mass spectrometer, µ m-scale particlesand structures via microscopes, and potential macro-scopic biogenic signatures via cameras.In closing, we stress that our proposal should beviewed as a preliminary template and forerunner formore comprehensive studies; therefore, it is not the onlyviable route. For instance, the mission could be scaledupward or downward in terms of mass and power, andthe choice of instrumentation for the scientific payloadsis also flexible because one can swap the designated in-struments on some probes with others of similar mass and power without altering our conclusions. By doingso, the architecture may permit a broader spectrum ofscientific objectives - extending beyond astrobiology intovarious domains of planetary science - to be fulfilled.Hence, future research along these lines, including in-depth subsystem-level engineering, is warranted.ACKNOWLEDGMENTSWe thank Cassidy Cobbs and Robert Kennedy for theinsightful comments and discussions. We are grateful toour reviewer, Chris McKay, for the helpful and meticu-lous report. ML acknowledges the support provided bythe Florida Institute of Technology.APPENDIX A. MASSES OF CATEGORY 1 AND CATEGORY 2 PROBESHere, we describe the rationale underlying the masses of the Category 1 and Category 2 probes.A.1.
Mass of Category 1 Probes
It was noted earlier in Section 2 that the scientific instruments may entail mass and power requirements of M s ∼ . P s ∼ ∼
10 bps, after assuming a P c ∼ . Mars Cube One
Cubesat mission was capable of reaching a gain of ∼ (Hodgeset al. 2017). If we choose a lower transmitter gain of ∼
100 instead, we obtain a data rate of ∼ bps. A majoradvantage of the antenna described in Hodges et al. (2017) is that it can fit into a Cubesat, has a power requirementof P c , and is characterized by a total mass of < M c ∼ . ×
17 mm andthickness of order 0 . P t ∼ P s + P c is required, based on which a total power of P t ∼ t (in hours), the energy expenditure is P t ∆ t . For a specificenergy of E ≈
500 W hr kg − , which is smaller than state-of-the-art experimental technologies by a factor of > M b ≈ P t ∆ t/ E . If we specify a missionduration of ∆ t ∼
48 h, we have M b ∼ . − . At 50 km,the range is between 200 and 400 W m − . For state-of-the-art solar cells with a conversion efficiency of 30%, we getan area-specific power of 60-300 W m − . A collection area of ∼ would therefore yield (cid:38)
100 W and necessitate amass of a few kg. While the use of solar panels might be cheaper in terms of mass, their usefulness will diminish asthe probes are swept away at horizontal speeds of ∼
100 m/s due to Venus’ superrotating atmosphere. In principle, acombination of battery and solar power is probably ideal, but we will not explicate such hybrid designs in this prefatoryLetter.Thus, the total mass of the payload is M p ∼ . (cid:46)
10% of the total payload mass for some past missions (Ball et al. 2007, Tables 23.1 and 26.1);see also van den Berg et al. (2006). We will proceed with this apparently reasonable premise hereafter, given thatthe mission takes place in the Venusian cloud layer, whose conditions resemble those of Earth’s atmosphere near ourplanet’s surface in many respects.We select a light gas zero-pressure balloon motivated by the simplicity of the design, with adjustments to ambientpressures implemented through vents. Although this choice limits the overall lifetime of the balloon, we have delib-erately opted for a simpler design to reduce complexity. In actuality, the Venusian atmosphere is beset by a numberof drawbacks such as the corrosive effects of sulfuric acid, high wind speeds and elevated pressures at lower altitudes.However, superpressure balloon prototypes have been constructed to bypass these issues, thus permitting survival ontimescales of several days (Hall et al. 2008), which is more stringent than our design parameters. For these balloons,the ratio of balloon mass to payload mass is approximately 0 .
38 (Hall et al. 2008), and this is close to the conversionfactor of ∼ . ∼
10 km s − ,achieving a terminal velocity of <
10 m s − is advisable. This reduction can be effectuated by means of a parachute.For a probe of mass ∼ ∼ Despite their large cross-sectional area, the mass of a parachute is very small. For instance,a parachute of ∼ (cid:46) . Thus, in line with the above considerations, ascaling factor of (cid:15) = 1 . M t of the Category 1 probe:from this linear scaling, we obtain M t ∼ (cid:15)M p ∼ . Mass of Category 2 probes
The mass and power of the scientific instruments is dominated by the tandem MS, which is taken to necessitate M s ∼ . P s ∼
35 W, respectively. Alternative designs are capable of reducing the power requirements to anextent, but may run the risk of losing the stipulated sensitivity. Even if other instruments accompanying the Category1 probe are incorporated herein, the mass is only weakly affected. The same also applies to the inclusion of auxiliarydevices such as low-mass quadcopters with masses of (cid:46) . ∼ P c ∼
25 W and M c ∼ . ceteris paribus , the data rate is enhanced to ∼ × bps. https://apogeerockets.com/education/downloads/Newsletter149.pdf The power required by the Category 2 probe is given by P t ∼ P s + P c ∼
60 W. Under the assumption that thebattery technology in Appendix A.1 can be scaled to higher masses, we are in a position to deploy M b ≈ P t ∆ t/ E . Bysubstituting the appropriate values into this formula, we obtain M b ∼ . M p ∼ . (cid:15) introduced earlier. Thus, as per this scaling, the totalmass M t of the Category 2 probe is given by M t ∼ (cid:15)M p ∼ . B. PROGRAMMATICS: COST AND SCHEDULEWe use a simple cost and schedule model to get ballpark estimates for the proposed life-detection mission. The firstitem that we tackle is the cost of the launcher. The cost of one Electron launch vehicle with the upper stage is about$10 million, of which $6 million is the baseline cost vehicle; other sources point toward an even lower baseline costof $5 million. We have therefore added $4-5 million as a rough estimate for the cost incurred by the upper stage andother components.In order to gauge the development cost for the probe, including the entry and decent vehicle and balloons, we selecta specific cost of $50-200 thousand kg − , which corresponds to the range of values provided in Wertz & Larson (1999,pg. 808). Our choice may represent a conservative selection because some of the developmental costs have decreasedover time. For a mass budget of 48 kg delineated in Table 1, the development cost amounts to $2 . . < $10 million oughtto enable private investors and/or national agencies to finance this precursor mission.If one assumes typical privately developed small spacecraft - such as the Electron launch vehicle mentioned above -and draws upon off-the-shelf technology, a development duration of 2-3 yr appears realistic prima facie , which wouldpermit a launch in the 2022-2023 timeframe. REFERENCES Aharoni, D., & Hoogland, T. M. 2019, Front. Cell.Neurosci., 13, 141, doi: 10.3389/fncel.2019.00141Babu, K. M. K., & Pant, R. S. 2020, Prog. Aerosp. Sci.,112, 100587, doi: 10.1016/j.paerosci.2019.100587Bains, W., Petkowski, J. J., Seager, S., et al. 2020,Astrobiology, arXiv:2009.06499.https://arxiv.org/abs/2009.06499Ball, A. J., Garry, J. R. C., Lorenz, R. D., & Kerzhanovich,V. V. 2007, Planetary Landers and Entry Probes(Cambridge: Cambridge University Press)Chan, M. A., Hinman, N. W., Potter-McIntyre, S. L., et al.2019, Astrobiology, 19, 1075, doi: 10.1089/ast.2018.1903Chassefi`ere, E., Korablev, O., Imamura, T., et al. 2009,Adv. Space Res., 44, 106, doi: 10.1016/j.asr.2008.11.025Cockell, C. S. 1999, Planet. Space Sci., 47, 1487,doi: 10.1016/S0032-0633(99)00036-7Dartnell, L. R., Nordheim, T. A., Patel, M. R., et al. 2015,Icarus, 257, 396, doi: 10.1016/j.icarus.2015.05.006Djuric, S. M., Kitic, G., Dubourg, G., et al. 2017,Microelectron. Eng., 182, 1,doi: 10.1016/j.mee.2017.08.005 https://directory.eoportal.org/web/eoportal/satellite-missions/e/electron Dorrington, G. E. 2010, Adv. Space Res., 46, 310,doi: 10.1016/j.asr.2010.03.025Feistel, A. S., Weaver, M. A., & Ailor, W. H. 2013, in ESASpecial Publication, Vol. 715, Safety is Not an Option,Proceedings of the 6th IAASS Conference, 75Fr¨ohlich-Nowoisky, J., Kampf, C. J., Weber, B., et al. 2016,Atmos Res., 182, 346, doi: 10.1016/j.atmosres.2016.07.018Gao, L., Sugiarto, A., Harper, J. D., Cooks, R. G., &Ouyang, Z. 2008, Anal. Chem., 80, 7198,doi: 10.1021/ac801275xGhosh, K. K., Burns, L. D., Cocker, E. D., et al. 2011, Nat.Methods, 8, 871, doi: 10.1038/nmeth.1694Glaze, L. S., Wilson, C. F., Zasova, L. V., Nakamura, M., &Limaye, S. 2018, Space Sci. Rev., 214, 89,doi: 10.1007/s11214-018-0528-zGreaves, J. S., Richards, A. M. S., Bains, W., et al. 2020,Nat. Astron., doi: 10.1038/s41550-020-1174-4Grinias, J. P., & Kennedy, R. T. 2016, TrAC Trends Anal.Chem., 81, 110, doi: 10.1016/j.trac.2015.08.002Grinspoon, D. H. 1997, A New Look Below The Clouds OfOur Mysterious Twin Planet (Cambridge: PerseusPublishing)Grinspoon, D. H., & Bullock, M. A. 2007, Geophys.Monogr. Ser., 176, 191, doi: 10.1029/176GM12Hall, J. L., Fairbrother, D., Frederickson, T., et al. 2008,Adv. Space Res., 42, 1648, doi: 10.1016/j.asr.2007.03.017 Hirschel, E. H., & Weiland, C. 2009, SelectedAerothermodynamic Design Problems of HypersonicFlight Vehicles (Berlin: Springer-Verlag),doi: 10.1007/978-3-540-89974-7Hodges, R. E., Chahat, N., Hoppe, D. J., & Vacchione,J. D. 2017, IEEE Antennas Propag. Mag., 59, 39,doi: 10.1109/MAP.2017.2655561Horn, M., MacLeod, J., Liu, M., Webb, J., & Motta, N.2019, Econ Anal. Policy, 61, 93,doi: 10.1016/j.eap.2018.08.003Ingham, C. J., Sprenkels, A., Bomer, J., et al. 2007, Proc.Natl. Acad. Sci., 104, 18217,doi: 10.1073/pnas.0701693104Junge, C. E., Chagnon, C. W., & Manson, J. E. 1961, J.Atmos. Sci., 18, 81,doi: 10.1175/1520-0469(1961)018 (cid:104) (cid:105)(cid:105)