arXiv:1802.06855v1 [astro-ph.EP] 19 Feb 2018 n .Goe eCastro G´omez de I. Ana h oa ytm hyaecmo netaoa plan- extrasolar of in common bodies are minor They diverse System. very Solar the and common are Introduction 1 systems planetary traviolet: Keywords Space World the project. to (WSO-UV) made Observatory-Ultraviolet is refer UV space-born special of with A prospects instruments. in and the comets comets discuss of of and observations observations (UV) review ultraviolet briefly the exper- We ad- in-situ for and iments. observations require ground-based studies spectrum. ditional complete electromagnetic need the and of range Comprehensive and UV the nuclei the in cometary study data in to and processes physical composition to chemical tests the Important determine evolution. and formation System lar Abstract E.Kanev Shustov B. UV in Comets usa cdm fSine,Ksgn 9 191Mso,Ru Moscow, 119991 19, sia Kosygina Sciences, of Academy Russian Spain Madrid, 28040 3, 3 Ciencias de Plaza Complutense, Russia 2 Moscow, 119017 48, skaya 1 nttt fGohmsr fteRS Russia RAS, the of Geochemistry of Institute Russia RAS, V.Dorofeeva the of Astronomy of Institute Spain E.Kanev Complutense, Universidad Group, Research AEGORA Vallejo C. Juan Castro G´omez de Russia I. RAS, Ana the of Astronomy of Institute M.Sachkov Shustov B. endk nttt fGohmsr n nltclChemis Analytical and Geochemistry of Institute Vernadsky Universidad Matematicas, CC. Fac. Group, Research AEGORA Pyatn Sciences, of Academy Russian Astronomy, of Institute 1 oesaeiprat“ywtess fSo- of “eyewitnesses” important are Comets • 1 oes eea,utailt eea,ul- general, ultraviolet: general, comets: V.Dorofeeva • M.Sachkov 3,1 1 2 • • unC Vallejo C. Juan 2 try, • it- s- bu wnysotpro n ogpro oescan comets long-period and short-period twenty about Sys- Solar early the of System, tem. understanding Solar our of verify information evolution can the this we of linking models By dynamic these with where formed. disk been the have of bodies of to area help about model could information disk) chemical restore (protoplanetary adequate System Solar with early comets of position (2015)). Meierhenrich (2015); al. et system Dones planetary the e.g. of (see processes formation the with the models associated accompany many is that In populations these comets. considered In of main-belt also ). origin for is AU 000 source belt 100 a to as (up Main cloud the Oort years, (30 recent the disc scattered and region, the AU) 100 and trans-Neptunian – belt the Kuiper the of comets: consisting observed sources be the to considered of are bodies small of populations ones) changes. period significant long any the of undergone composi- least not chemical (at has the comets time of cores this of all tion over billion i.e. 4.5 which approximately ago, from formed years was the cloud System represent protoplanetary Solar the basically the to of believed of forma- composition is composition its chemical cores of are The cometary comets “witnesses” evolution. itself early major and System the tion Solar be the to to considered As around stars. watching planets other by and analogues, disks protoplanetary System circumstellar Solar the of evolution the on also but space understand- in . only us not help phenomena many that theming of laboratories make space coma chemical cometary excellent and in physical place of taking variety processes objects wide interesting A very themselves. are in Comets too. systems etary tpeet aao h oe opsto of composition the on data present, At com- chemical the of analysis careful that believe We two System Solar the of part outer the in present, At the of stages early and origin the about learn can We 2 be found in the literature (Bockel´ee-Morvan (2011); clues on the comets thermal history. But they have not Mumma and Charnley (2011); Ootsubo et al. (2012); yet been detected by any UV instrument in cometary Paganini et al. (2014); Cochran et al. (2015); Roth et al. comas. (2017)). This data are presented in Fig.1. Analysis of Delsemme (1980) reminded that the atmospheric this data shows that there are no fundamental differ- bands of ozone absorb strongly at wavelengths less than ences between the volatiles content in comets of differ- 300 nm and still absorb irregularly up to wavelengths ent dynamic types. Of course, we should bear in mind longer than 320nm. The 300 – 320nm range (where that only the very deep interiors (> 50m) of cometary the famous OH line at λ309 nm) lies is a very impor- nuclei would have information on the primary compo- tant “interface domain” typically covered by space-born sition of cometesimals. UV instruments and still reachable by groud-based tele- Observations of cometary interiors are challenging. scopes located in mountains. This makes possible vari- Since we are limited by the distant observations, a crit- ous tests and calibrations. ical problem is distinguishing between the coma and the In this paper we discuss findings from UV (far UV nucleus signatures. Chemical composition of the matter (FUV) and near UV (NUV)) observations of comets evaporated from the surface of the comet (i.e. coma) is with space born instruments (Section 2). Special at- typicaly far from primordial. The upper layers of the tention is paid to the prospects of the World Space Ob- core of short period comets are intensively reproccessed servatory – Ultraviolet (WSO-UV) project (Section 3). during previous encounters with the . This is con- Concluding remarks are given in Section 4. firmed by the data obtained for comet 67P/C-G in the Rosetta experiment (Le Roy et al. (2015); Rubin et al. (2015); Balsiger et al. (2016); Altwegg et al. (2016); 2 Half a century of UV-observations of comets Mall et al. (2016)). This data are also represented in from space Fig. 1: asterisks (before perihelion) and by encircled asterisks (after perihelium). All the volatile compo- In an interesting review by Delsemme (1980) of early nents are stronger after perihelion passage, when the UV observations of comets, all spectral first-time iden- upper shielding layer is destroyed in the southern hemi- tifications of comets are classified in three periods: sphere of the comet. From the analysis of the data and • From the early times until 1911 (named by Delsemme considering the propagation velocity of heat within the (1980) as Dark Ages of Cometary Spectra), when the nucleus, it can be concluded that observational data two major features of the neutral coma spectrum, obtained 2 – 3 weeks after the perihelion comet is of CN and C2, had been already identified as well as particular value, since they will most adequately corre- + the two major ions of the ion tail (CO+ and N ). spond to the composition of the cometary nucleus. 2 Metallic lines had already been seen in a sun-grazing Many problems of determining the chemical compo- comet, and the first UV features below 340 nm had sition of comets can only be solved by using observa- just been observed as a fuzzy unresolved band. Next tional data in the ultraviolet (UV) range of the electro- thirty years was a period with no bright achievements magnetic spectrum. Cometary spectra in the UV and in the field, visible range include lines formed by scattering of the • Period 1941 – 1970 started with the first definite de- on solid dust particles and emission lines from tection by Swings et al. (1941) of OH(0-0) at 309nm the gas in coma. UV spectroscopy provides important and NH(0-0) near 336nm in the spectrum of comet information about the composition of a cometary coma. Cunningham (1941I). The discovery of the UV line of In general, cometary coma are dominated by emission OH demonstrated that comets contain water. This features that originate from the dissociation products was strongly confirmed by the first rocket and satel- of water: OH, H and O, or correspond to secondary lite experiments running UV observations of comets. atomic, molecular, and ionic species, such as C, C+, The “space era” of UV studies of comets began about CO, CO+, CO+, S, and CS. Also, the spectra and im- 2 half a century ago. ages contain information on structure and dynamical • Since then, we are in the “modern” in UV features of coma. The spatial structure and time varia- studies. A number of space-born instruments (few tions of the outflows inferred from the images and spec- to 240cm aperture, both spectroscopic and imaging tra help to reveal the structure and other properties of ones) were (and are) used for numerous UV observa- cores. There are ways to restore the thermal history of tions of comets. We think this period will continue cores too. For instance, N2 and noble gases are both up to the appearance of giant UV telescopes with chemically inert and highly volatile (evaporate at very aperture of 4 – 30m. This future epoch will come low temperatures), so they are particularly interesting not earlier than 20 years from now and currently, the 3

Fig. 1 Values of relative contents of gas components in the coma of long-period and short-period comets by percent of water mass. For 67P/Churyumov-Gerasimenko comet the gas contents are shown by black asterisks (before perihelion) and by encircled asterisks after perihelium passage. See text for references.

most powerful tools are telescopes of the 2m class (1970)), by the OAO-2 and the (HST, WSO-UV). OGO-5 satellite (Bertaux et al. (1973)). (1976VI) was observed with an Aerobee rocket payload In the article by Delsemme (1980), the first two pe- (instruments): spectra (Feldman and Brune (1976)) riods are described in some detail. Here we briefly and imagery of the Lyα halo (Opal and Carruthers discuss the most important scientific and technological (1977)) were obtained. These are some of the first achievemets of the third epoch as well as some prospects observations of comets in UV. for the coming decade. We think that the major di- Rocket experiments remain important in our days. rections for the UV observation of comets mentioned FUV imagery and objective grating spectroscopy of by Delsemme (1980) remain important in our days. comet C/2012 S1 (ISON) were acquired from NASA These are: observations of the Lyman-α (Lyα) halos sounding rocket 36.296 UG launched in 2013 (McCandliss et al. of comets, OH observations and observations of C, 0, S (2015)). The comet was 0.1◦ below ground horizon, atoms, ions, and molecules. Both imaging and spectro- 0.44AU from the Sun. The payload reached an apogee scopic technologies are used. of 279 km and the total time pointed at the comet 2.1 Imagery and low resolution spectroscopy was 353 s. A wide-field multi-object spectro-telescope called FORTIS (Far-UV Off Rowland-circle Telescope Imagery and low resolution spectroscopy were histori- for Imaging and Spectroscopy) observed the Lyα emis- cally the most popular technologies at the dawn of UV sion making use of an objective grating mode through “space era”. an open microshutter array over a 0.5 square degree field of view. Hydrogen production rate was estimated ∼ × 29 −1 2.1.1 Rocket and balloon UV experiments to be 5 10 atoms s . They also acquired a broad- band image of the comet in the 128 – 190 nm bandpass. First rocket and satellite experiments on UV obser- An obvious disadvantage of rocket experiments is vation of comets immediately brought the very im- the short exposure time. This is overcome by balloon pressive discovery of the immense (up to 1.5 mil- missions. The UV window has been scarcely explored lion km across) Lyα halo of comets: Tago-Sato-Kosaka through balloons. An UV-VIS (Ultra-Violet/Visible) 1969g (C/1969 T1) by the OAO-2 satellite (Code et al. instrument was designed for the BOPPS mission (Bal- (1970)) and an Aerobee sounding rocket (Jenkins and Wingertloon Observation Platform for Planetary Science) (1972)); (1969i) (Bertaux and Blamont Young et al. (2014). BOPPS was launched in 2014 and 4 observed comets, using a 0.8m aperture telescope, a by dissociation of H2O and OH, though a broader pro- pointing system that achieved better than 1’ point- file far from the coma suggests a contribution from hy- ing stability, and an imaging instrument suite covering drogen atoms produced by charge transfer with solar from the near-ultraviolet to the mid-infrared. BOPPS wind protons. observed two comets: C/2013A1 (Siding The relative paucity of solar UV photons has lim- Spring) and C/2014E2 (Jacques). The major results ited the observations from orbiting observatories. The were received with the IR instruments (Cheng et al. highest spatial resolution has been obtained with the (2017)). Space Telescope Imaging Spectrograph (STIS) on HST, but is typically limited to tens of kilometers for the 2.1.2 UV observations onboard interplanetary comets passing the closest to the Earth. The ALICE missions FUV spectrograph onboard the Rosetta mission made possible observations on scales of tens to hundreds of Many planeatry missions have been equipped with UV meters near the nucleus of the comet. ALICE is an instruments that were capable to observe comets. We imaging spectrograph in the λλ70 – 205nm range, with ◦ mention few of them. resolution 0.8 – 1.2 nm. The slit is 5.5 long with a ◦ ◦ The Lyα halo of was repeatedly ob- width of 0.05 – 0.10 . ALICE is an off-axis telescope served by the UV spectrometer of the Mariner10 space- feeding a 0.15-m normal incidence Rowland circle spec- craft during its transit to planet Venus (Kumar et al. trograph with a concave holographic reflection grating. (1979)) and by different instruments on board Sky- The MCP detector used solar-blind opaque photocath- lab, namely, the electrographic Schmidt camera (110– odes (KBr and CsI). Feldman et al. (2015) reported the 150 nm) (Carruthers et al. (1974)), the high resolution results of the observations beginning in August 2014, spectrograph (Keller et al. (1975)). when the spacecraft entered an orbit about 100km Crismani et al. (2015) reported that NASA’s Imag- above the nucleus. These observations were an unique ing Ultraviolet Spectrograph (IUVS) ( λλ110 – 340 opportunity to probe the coma – nucleus interaction re- nm, resolution 0.5 nm) onboard the Mars Atmosphere gion that is not accessible to the in situ instruments on and Volatile EvolutioN (MAVEN) orbiting spacecraft Rosetta or to observations from Earth orbit. obtained images of the hydrogen coma of the planet- grazing comet C/2013 A1 (Siding Spring) 2 days before 2.1.3 UV observations of comets onboard general its close encounter with Mars. Observations by IUVS purpose (astrophysical) missions brought interesting results: the water production rate 28 α was 1.1 ± 0.5 × 10 molecules/s, the total impacting Observations of the Ly halos of comets continued in fluence of atoms and molecules corresponding to the forhcoming experiments. We briefly describe some ex- photodissociation of water and its daughter species was amples. 2.4 ± 1.2 × 104kg. These observations were used to con- The Copernicus satellite spectrometer scanned the α firm predictions that the mass of delivered hydrogen is Ly halo of comets Kohoutek and Kobayashi-Berger- Milon (Festou et al. (1979)). comparable to the existing reservoir above 150 km. Kaneda et al. (1986)) presented results of Lyα imag- Spectral observations of Comets C/2012S1 (ISON) ing of Lyα Halley comet performed with S/C Suisei. and 2P/Encke were acquired by the MESSENGER The strong “breathing” of comet Halley coma with a spacecraft during the close passes of both comets by period of 2.2 days was monitored throughout all phases Mercury (Vervack et al. (2014)). in the 1985 – 1986 perihelium passage. Concerning to Several campaigns of cometary observations have the water production, postperihelion rates exceeded the been performed by the UV spectrometer SPICAV from preperihelion ones by factor of 2 or 3. This fact suggests 2012 to 2014. SPICAV is one of the Venera-Express a postperihelion heating of the cometary nucleus. Some instruments. The UV channel of SPICAV is a full UV fine structures in the Lyα photometry data taken at the imaging 40mm aperture spectrometer for 118 – 320nm, encounter indicate spatial variations of the atomic hy- ∆λ ∼ 0.5 nm. For the 6 observed comets water subli- drogen density in the coma. These may correspond to mation rates as a function of the Sun distance were the shell structures in the images so far obtained and estimated (Chaufray and Bertaux (2015)). be ascribed to the presence of a hydrogen source other Comet 2P/Encke was observed with UVCS/SOHO than H2O in the cometary gas or dust particles. This near perihelion (2000 September 9 and 11) in the Ly- second hydrogen source is expected to have a larger man lines of hydrogen. Raymond et al. (2002) reported photo-dissociation rate than H20. that the narrow Lyα profile indicates that the observed Molecules of OH were repeatedly observed in comets. photons are scattered from hydrogen atoms produced For instance, the OH bands in spectra of on comets 5

Fig. 2 Fragment of comet Halley UV spectra obtained by the mission ”Astron” in 1986 (Boyarchuk (1994)). Photometic cuts in OH made in direction shown by arroware shown either.

Bennett, Kohoutek and periodic comet Encke were ob- band. Seven broadband filters allow color discrimi- served with the NASA Convair 990 aircraft especially nation, and two grisms provide low-resolution spec- equipped for this purpose. The OH bands are very troscopy at UV (170 – 520nm) and optical (290 – strong in all UV spectrograms of comets that covered 650nm) wavelengths. These grisms provide a resolv- the proper range (Harvey (1974)). ing power ∼ 100 for point sources. UVOT and X-Ray Comet Halley was observed by the Soviet UV obser- Telescope were used to provide the first ever simultane- vatory “Astron” with the 80cm telescope “Spika” on- ous X-ray and UV image of a (Carter et al. board (the largest space telescope at that time). The (2012)). The UV and X-ray emission are on opposite telescope was equipped with a French scanning spec- sides because comet Lulin has two oppositely directed trometer (Boyarchuk (1986, 1994)). The very strong tails. Lim et al. (2014) presented observation of both lines of OH at 309 nm and NH at 336 nm seen in the spectra and images of comet C/2001Q4 (NEAT) cre- spectrograms are shown in Fig.2. ated from the observations with Far-Ultraviolet Imag- Comet Seargent (1978m), the first comet to be ob- ing Spectrograph (FIMS), onboard the Korean satellite served by the International Ultraviolet Explorer (IUE) STSAT-1, (launched in 2003). FIMS is a dual-channel satellite, resolved the OH (0-0) band into a rotational FUV imaging spectrograph (S-channel 90 – 115 nm, structure of at least ten different branches completely L-channel 135 – 171nm , and resolving power ∼550 identified (Jackson (1980)). for both channels) with large imaged fields of view (S- These first observations and the models developed to channel 4◦×4.6’, L-channel 7.5◦×4.3’, and angular res- explain the structure and isophotes of the Lyα emission olution 5’– 10’) optimized for the observation of the brought to the conclusion that the bulk of the produc- FUV radiation produced by hot gases in our Galaxy. tion rate of hydrogen and OH comes from the photodis- The Galaxy Evolution Explorer (GALEX) has ob- sociation of H2O, which could be the major volatile served 6 comets since 2005 (C/2004Q2 (Machholz), constituent of comets. 9P/, 73P/Schwassmann-Wachmann3 Frag- The UV image of comet C/2009 P1 (Garradd) in ments B and C, 8P/Tuttle and C/2007 N3 (Lulin)). the OH band was taken in April 1, 2012, when the GALEX is a NASA Small Explorer (SMEX) mis- comet was 229 million kilometers away (Bodewits et al. sion designed to map the history of star formation in (2012)). The imagery was made by the Ultraviolet the Universe. GALEX’s telescopes aperture is 50 cm. and (UVOT). UVOT has a 30 cm Field cameras were equipped with grism providing the aperture that provides a 17′ × 17′ field of view with FUV (134 – 179 nm) channel and the NUV (175 – a spatial resolution of 0.5’/pixel in the optical/UV 310 nm) channels at resolution 1 – 2 nm. It is also 6 well suited to cometary coma studies because of its UV studies of comets with the The Hubble Space Tele- high sensitivity and large field of view (1.2 degrees). scope (HST). HST is currently the only option for ob- Morgenthaler et al. (2009) reported that OH and CS taining UV cometary spectra in the 115 – 300nm range. in the NUV were clearly detected in all of the comet 44 comets were observed with the HST (till 2016). data. The FUV channel recorded data during three The Space Telescope Imaging Spectrograph (STIS) of the comet observations and detected the bright which allows registration of spectra in a long-slit mode CI 156.1 and 165.7 nm multiplets. There is an evidence is a powerful tool for studying the gas in the inner parts of SI 147.5 nm emission in the FUV. The GALEX data of the coma with a high spatial resolution. This area is were recorded with photon counting detectors, so it has essential for cometary research, as the gas in these parts been possible to reconstruct direct-mode and objective of comets might not be in the radiative equilibrium grism images in the reference frame of the comet. state; additionally, complex and little studied chemi- cal reactions can take place there. Like the IUE the 2.2 Mid and high resolution spectroscopy HST has a significant limitation for cometary research. This is the Sun avoidance angle which does not allow Higher resolution spectroscopy brings most detailed in- observations of objects closer than 50◦ from the Sun, formation on astronomical objects. But it requires i.e., approximately 0.8 AU. rather large apertures and spectrographs with stable Feldman et al. (2016) summarised the results ob- structures. We briefly describe some findings and tech- tained with HST for four comets observed in the nical details of the larger UV observatories. FUV with the Cosmic Origins Spectrograph (COS): 103P/Hartley2, C/2009P1 (Garradd), C/2012S1 (ISON) UV observations of comets with the International Ul- and C/2014 Q2 (Lovejoy). The principal objective was traviolet Explorer (IUE). The operation of the IUE to determine the relative CO abundance from measure- satellite in 1978 – 1996 was a real breakthrough in UV ments of the CO Fourth Positive system in the spectral observations, in general, and in comets, in particular. range of 140 – 170nm. In the two brightest comets, The IUE was able to observe objects no closer than nineteen bands of this system were clearly identified. 45◦ from the Sun, i.e., the comets that approached the The water production rate was derived from observa- Sun to about 0.7 AU were accessible. The stabiliza- tions of the OH (0,0) band at 308.5 nm by the STIS. The tion system of the satellite allowed the observation of derived CO/H2O production rate ratio ranged from moving objects (comets) with an accuracy of several 0.3% for comet Hartley to 20% for comet Garradd. arcseconds for a 30min exposure. UV spectrographs Still most enigmatic cometary objects studied with of the IUE satellite were practically the only option the HST in last decade are the main belt comets for systematic research and monitoring of the spatial (MBC). MBCs exhibit comet-like mass loss resulting distribution of some of the most common elements in from the sublimation of volatile ice even though they oc- cometary comas, which are the key to understanding cupy orbits in the main asteroid belt. Till now there is the production mechanism of these elements. Practi- no definite understanding on how these cometary bod- cally all comets that reached the brightness of stellar ies appeared in the main asteroid belt. magnitude 6 were observed with the IUE satellite. In Of course field cameras of the HST were used for the total, 55 comets were systematically studied. observation of comets though the small field of view was Very briefly, the results of 18.5 years of UV studies a serious limitation. can be formulated as follows (Festou (1998)): comets are composed of dust, water, CO, and CO2. The con- Comets with Far Ultraviolet Spectroscopic Explorer tent of other molecules is a few per cent of the water (FUSE). The FUV region shortward of 200nm, con- content (from 0.5% to 1% in comet Hale-Bopp). Chemi- tains the resonance transitions of the cosmically abun- cal composition of periodic and aperiodic comets differs dant elements, as well as the electronic transitions of by no more than a factor of two and also differs by the the most abundant simple molecules such as CO and gas-to- dust mass ratio (see however more recent data H2. The principal excitation mechanism in the ultravi- presented in Fig.1). New species were discovered in the olet is resonance fluorescence of solar radiation. coma, and some of the most abundant nuclear species FUSE, launched in 1999 June, provided an orbit- have been studied through the spatial distribution of ing capability for the temperature and density diagnos- their dissociation products. Time variations from few tics of molecular species. FUSE spectral resolution was moths to hours were investigated in detail. better than 0.04 nm in the wavelength range of 90 – 118.7 nm together with very high sensitivity to weak emissions, making possible both the search for minor 7 coma species and extensive temperature and density telescope (large 170 cm diameter primary mirror, UV diagnostics for the dominant species. Feldman et al. optimized coating of the optical surfaces, high-accuracy (2001, 2002) reported observations of comet C/2001A2 guidance and stabilization system, etc.) were chosen to (LINEAR) with FUSE. Several new cometary emissions fit requirements of high angular resolution and maxi- were identified. mum effective area in the 110nm – 320nm range. Con- struction of the telescope provides the solar avoidance angle about 40◦. This is important for observations of 3 Future observations of comets with World comets at low angular distance from the Sun. Space Observatory – Ultraviolet (WSO-UV). • high-resolution spectroscopic observations of point- like objects in the 110 – 320nm spectral range. Re- Observations of comets from space have several specific solving power of the two high resolution spectro- features. As per Sachkov (2016) these are: graphs designed for observations in the NUV (180 • Comets are moving objects. Their velocities can – 320nm) and FUV (110 – 180nm) is R> 50000; reach tens of arcseconds per hour. For example, • low-resolution (resolving power R ∼ 1000) spectral comet Halley moved at a velocity of 11”/hour, rel- observations. A long slit spectrograph will operate in ative to the stars. For obtaining the UV spectra, it the 110 – 320 nm range too. It is especially suitable is necessary to support the spacecraft stabilization for faint and extended objects such as comets; during long exposures, having a priori knowledge of • solar-blind FUV imaging with angular resolution this velocity. (0.08”) for direct imaging with capabilities for low • The presence of solar lines in the spectrum signifi- dispersion slit less spectroscopy; cantly complicates the observation for some cometary • wide field NUV-visible imaging with field of view velocities since both solar and cometary spectra, are 10’×7.5’ and angular resolution 0.15”; recorded together. The observed solar spectrum is Broadly speaking, the spectroscopic capabilities of shifted by the comet’s heliocentric velocity, the so- WSO-UV and HST are rather similar; in the NUV called Swings effect. range, WSO-UV spectrographs will be more efficient • Comets are variable objects. Cometary nuclei pro- than the HST/STIS spectrograph however, the sen- duce short-lived tails and comas. Monitoring of sitivity of HST/COS in the FUV range will not be this temporal changes requires continuous or quasi- matched by WSO-UV. To evaluate the impact of the continuous observations. design on the scientific objectives of the mission, a sim- • Comets are extended objects. Larger fields of view ulation software tool has been developed for the WUVS of cameras onboard space observatory are preferable. (Marcos-Arenal et al. (2017)). This simulator builds For (imaging) spectrography long-slit spectrographs on the development made for the PLATO space mis- are the most suitable. sion and it is designed to generate synthetic time-series • Comets are faint objects. Therefore, high resolu- of images by including models of all important noise tion spectroscopic observations require for sufficiently sources. large aperture telescopes. Some characteristics of the Field Camera Unit • An important restriction in cometary studies from (FCU) channels significantly differ from those of HST the near-Earth orbit is the significant sun avoidance as shown in Tabl.1 (Hubble data are taken from HST angle which limits the observations during perihelium Instrument Handbooks). In Fig.3 we illustrate the dif- passage. ference between the field of view of the NUV imager The World Space Observatory – Ultraviolet (WSO- and the high resolution camera PC1 WFPC-2 on board UV) fits most of the requirements listed above. WSO- HST. UV is a multi-purpose international space mission born Similar to Weaver (2002) we consider that the main as a response to the growing up demand for UV fa- scientific objective is to obtain accurate abundance cilities by the astronomical community. WSO-UV was measurements for all known UV-emitting cometary described in previous publications in great detail (see species CO from the CO4PG bands, C2 from the CO Shustov et al. (2011, 2014); Sachkov et al. (2014, 2016), Cameron bands, S2 from the S2 B-X bands, CS2 from for this reason only basic information on the project, CS emissions, and water from OH emissions and to relevant to the observation of comets, is briefly pre- perform a deep search for any previously undetected sented here. species. The long slit capability of the WSO-UV will The main scientific purpose of WSO-UV is the spec- allow us to characterize the spatial distribution of the troscopic observation of faint UV sources and high res- coma species, so that we can identify those derived olution UV imaging. The parameters of the T-170M from an extended source e.g. CO, study the decay of 8

Table 1 Comparative characteristics of the NUV and FUV channels of the FCU with those of HST/ACS/SBC and HST/WFC3/UVIS Parameters FUV Channel NUV Channel HST/ACS/SBC HST/WFC3/UVIS Detector MCP CCD MCP, MAMA CCD Spectral range, nm 115 170 174 310 (ext. to 1000) 115 170 200 1000 D (primary mirror), m 1.7 1.7 2.4 2.4 Field of view, arcsec 163 × 163 597 × 451 35 × 31 162 × 162 a Scale, arcsec/pixel 0.08[ ] 0.146 0.033 × 0.030 0.0395 Detector size, mm 30 49× 37 25 61× 61 Detector format 2k×2k 4k×3k 1k×1k 1k×1k Number of filters up to 10+2 prims upto 15 6+2 prism 62 [a] Angular resolution.

Fig. 3 Comparison between the field of view (FoV) of the WSO-UV NUV imager and the FoV of the plan- etary camera PC1 WFC-2 on HST. The Hubble image and field of comet 17P/Holmeshas been taken from https://apod.nasa.gov/apod/ap071128.html. Each red rectangle in the lower left inset represents the FoV of the WSO-UV NUV imager. Notice that just 9 exposures are require to cover the inner coma at full.

short-lived species e.g. S2, and investigate the impor- oceans. The D/H values in Jupiter Family (JF) comets tance of electron impact on CO for the excitation of and the ice ratios (CO/CO2/H2O) in both JF and Oort the Cameron bands. The latter issue can be defini- Cloud comets call into question the details of the dy- tively resolved with high spectral resolution observa- namical processes for populating the modern reservoirs tions of any comet having V<5. If an exceptionally for these two dynamical classes. Bodewits (2015) noted bright (V<2) comet is discovered, we would then re- that currently too few D/H ratios have been measured quest Director’s time to measure the D/H ratio. The in comets to allow for a meaningful interpretation. D/H ratio in cometary water is a key indicator of the Contemporary interpretation of the term “comets” role played by comets in the delivery of volatiles to the includes so called exocomets. We remind that exo- terrestrial planets. The deuterium abundance of water comets (cometary activity in extrasolar systems) have in comets preserves information about the formation been first detected in optical observations by the conditions of our , while also constraining temporal variations of the CA II-K line (Ferlet et al. the possible contribution of cometary water to Earth’s (1987)) and confirmed in UV observations spectra of 9 the well-studied β-Pictoris debris disk with the HST as well as the bridge between stellar and planetary as- (Vidal-Madjar et al. (1994)). In recent years many trophysics. Some ill known physical processes require interesting results were obtained by intensive optical to be addressed the acquisition of the data only at- monitoring of high-resolution optical spectra of the Ca tainable in the UV range of the electromagnetic spec- II lines in gaseous disks around selected stars. We trum. Thus, space-born facilities are required to study think that UV monitoring can be also valuable for fu- comets; complementary ground-based observations and ture studies of exocomets. Miles et al. (2016) presented in-situ experiments are also needed to get a comprehen- the analysis of time-variable Doppler-shifted absorption sive view of the problem. features in FUV spectra of the unusual 49 Ceti debris We briefly reviewed half a century of UV observa- disk. This nearly edge-on disk is one of the bright- tions of comets and argue that it was a very fruitful est known and is one of the very few containing de- period for understanding the complex nature of comets tectable amounts of circumstellar (CS) gas as well as and their role in the evolution of the Solar System. dust. Miles et al. (2016) calculated the velocity ranges Both short- and long-term experiments made an in- and apparent column densities of the 49 Cet variable valuable contribution to the development of cometary gas, which appears to have been moving at velocities science. Although research on comets in situ (such as of tens to hundreds of km/s relative to the central star. the remarkable Rosetta project) yield direct and very The velocities in the redshifted variable event showed important data, the remote study of comets will retain that the maximum distances of the infalling gas at the its significance for many decades. time of transit were about 0.05 – 0.2AU from the cen- The WSO-UV mission will guarantee the continuity tral star. A preliminary composition analysis brought of the UV observation of comets, as well as exocomets. to conclusion that the C/O ratio in the infalling gas is The large FoV and high sensitivity of the NUV imager super-solar, as it is in the bulk of the stable disk gas. together with the high Earth orbit will made of WSO- Measuring the evolution of gas and small bodies in UV the most efficient observatory ever flown to track the young planetary disks will help us understanding comets evolution. the formation of structures such as the Oort cloud and the Kuiper belt. However, this requires extensive mon- Acknowledgements B.Shustov, V.Dorofeeva and itoring programs that are very inefficient when carried E.Kanev thank the Russian Science Foundation for sup- out from low Earth orbit observatories like Hubble. porting this work by grant RSF 17-12-01441. Ana I WSO-UV should be able to run several monitoring pro- G´omez de Castro and Juan C. Vallejo thank the Span- grams simultaneously in a cost-efficient manner. ish Ministry of Economy, Industry and Competitiveness Comets aggregate from interplanetary dust and for grants: ESP2014-54243-R, ESP2015-68908-R. there are growing evidences that the large organic molecules found in the cometary samples come from the interstellar dust trapped in the pre-solar nebula (see e.g. Bertaux and Lallement (2017)). The UV spectrum is very sensitive to small particles and large molecules. WSO-UV imagers and filters sets are designed to mea- sure variations of the extinction curve in the coma and together with the long slit spectrograph will allow char- acterizing variations between the comets populations in particle size distribution and molecular bands. WSO- UV FUV imager will be operated from high Earth orbit facilitating the monitoring and minimizing the contam- ination from the Earth geocorona.

4 Conclusions

Comets are important “eyewitnesses” of the processes involved in Solar System formation and evolution. Ob- servations of the UV spectra of comets from rockets and have brought to light some new clues on those primitive bodies that could be the link between inter- stellar molecules and the early planetary atmospheres, 10

References Delsemme, A.H.: Appl. Opt. 19, 4007 (1980) Dones, L., Brasser, R., Kaib, N., Rickman, H.: Space Sci. Altwegg, K., Balsiger, H., Bar-Nun, A., Berthelier, J.- Rev. 197, 191 (2015). doi:10.1007/s11214-015-0223-2 J., Bieler, A., Bochsler, P., Briois, C., Calmonte, U., Feldman, P.D., Brune, W.H.: Astrophys. J. Lett. 209, 45 Combi, M.R., Cottin, H., De Keyser, J., Dhooghe, F., Fi- (1976). doi:10.1086/182263 ethe, B., Fuselier, S.A., Gasc, S., Gombosi, T.I., Hansen, Feldman, P.D., Weaver, H.A., Burgh, E.B.: In: AAS/Division K.C., Haessig, M., Ja ckel, A., Kopp, E., Korth, A., Le for Planetary Sciences Meeting Abstracts 33. Bulletin Roy, L., Mall, U., Marty, B., Mousis, O., Owen, T., of the American Astronomical Society, vol. 33, p. 1120 Reme, H., Rubin, M., Semon, T., Tzou, C.-Y., Waite, (2001) J.H., Wurz, P.: Science Advances 2, 1600285 (2016). Feldman, P.D., Weaver, H.A., Burgh, E.B.: Astrophys. J. doi:10.1126/sciadv.1600285 Lett. 576, 91 (2002). doi:10.1086/343089 Balsiger, H.R., Altwegg, K., Bar-Nun, A., Berthelier, J.J., Feldman, P.D., Lupu, R.E., McCandliss, S.R., Weaver, Bieler, A.M., Bochsler, P., Briois, C., Calmonte, U., H.A.: Astrophys. J. 699, 1104 (2009). 0905.1695 Combi, M.R., De Keyser, J., Fiethe, B., Fuselier, S.A., Feldman, P.D., A’Hearn, M.F., Bertaux, J.-L., Feaga, L.M., Gasc, S., Gombosi, T.I., Hansen, K.C., H¨assig, M., Kopp, Parker, J.W., Schindhelm, E., Steffl, A.J., Stern, S.A., E., Korth, A., Le Roy, L., Mall, U., Marty, B., Mousis, Weaver, H.A., Sierks, H., Vincent, J.-B.: Astron. As- 583 O., Owen, T.C., Reme, H., Rubin, M., Semon, T., Tzou, trophys. , 8 (2015). 1506.01203. doi:10.1051/0004- C.Y., Waite, J.H. Jr., Wurz, P.: AGU Fall Meeting Ab- 6361/201525925 Feldman, P.D., Weaver, H.A., A’Hearn, M.F., Combi, M.R., stracts, 43 (2016) Bertaux, J.-L., Blamont, J.: Academie des Sciences Paris Dello Russo, N.: In: AAS/Division for Planetary Sciences Comptes Rendus Serie B Sciences Physiques 270, 1581 Meeting Abstracts. AAS/Division for Planetary Sciences (1970) Meeting Abstracts, vol. 48, p. 308 (2016) Bertaux, J.-L., Lallement, R.: Mon. Not. R. Astron. Soc. Ferlet, R., Vidal-Madjar, A., Hobbs, L.M.: Astron. Astro- 469, 646 (2017). doi:10.1093/mnras/stx2231 phys. 185, 267 (1987) Bertaux, J.L., Blamont, J.E., Festou, M.: Astron. Astro- Festou, M.C.: In: Wamsteker, W., Gonzalez Riestra, R., phys.25 (1973) Harris, B. (eds.) Ultraviolet Astrophysics Beyond the IUE Bockel´ee-Morvan, D.: In: Cernicharo, J., Bachiller, R. Final Archive. ESA Special Publication, vol. 413, p. 45 (eds.) The Molecular Universe. IAU Symposium, vol. 280, (1998) p. 261 (2011). doi:10.1017/S1743921311025038 Festou, M., Jenkins, E.B., Barker, E.S., Upson, W.L. II, Bodewits, D.: 2015 Far Uv Spectroscopic Measurements of Drake, J.F., Keller, H.U., Bertaux, J.L.: Astrophys. J. the Deuterium Abundance of Comets. HST Proposal 232, 318 (1979). doi:10.1086/157291 Bodewits, D., Farnham, T.L., A’Hearn, M.F., Landsman, Harvey, G.A.: Publ. Astron. Soc. Pac. 86, 552 (1974). W.B.: In: , Comets, Meteors 2012. LPI Contri- doi:10.1086/129643 butions, vol. 1667, p. 6084 (2012) Jackson, W.M.: Icarus 41, 147 (1980). doi:10.1016/0019- Boyarchuk, A.A.: Irish Astronomical Journal 17, 352 1035(80)90167-0 (1986) Jenkins, E.B., Wingert, D.W.: Astrophys. J. 174, 697 Boyarchuk, A.A.: Astrophysical Research with the Space (1972). doi:10.1086/151531 Station “astron” (astrofizicheskoe Issledovanie na Kos- Kaneda, E., Takagi, M., Hirao, K., Shimizu, M., Ashihara, micheckoi Stantsii “astron”), (1994) O.: In: Battrick, B., Rolfe, E.J., Reinhard, R. (eds.) ES- Carruthers, G.R., Opal, C.B., Page, T.L., Meier, R.R., LAB Symposium on the Exploration of Halley’s Comet. Prinz, D.K.: Icarus 23, 526 (1974). doi:10.1016/0019- ESA Special Publication, vol. 250 (1986) 1035(74)90015-3 Keller, H.U., Bohlin, J.D., Tousey, R.: Astron. Astrophys. Carter, J.A., Bodewits, D., Read, A.M., Immler, S.: Astron. 38, 413 (1975) Astrophys. 541, 70 (2012). 1204.3033. doi:10.1051/0004- Kumar, S., Holberg, J., Broadfoot, A.L., Belton, M.J.S.: 6361/201117950 Astrophys. J. 232, 616 (1979). doi:10.1086/157320 Chaufray, J.-Y., Bertaux, J.-L.: European Planetary Sci- Le Roy, L., Altwegg, K., Balsiger, H., Berthelier, J.- ence Congress 10, 2015 (2015) J., Bieler, A., Briois, C., Calmonte, U., Combi, M.R., Cheng, A.F., Hibbitts, C.A., Espiritu, R., McMichael, R., De Keyser, J., Dhooghe, F., Fiethe, B., Fuselier, S.A., Fletcher, Z., Bernasconi, P., Adams, J.D., Lisse, C.M., Gasc, S., Gombosi, T.I., H¨assig, M., J¨ackel, A., Ru- Sitko, M.L., Fernandes, R., Young, E.F., Kremic, T.: bin, M., Tzou, C.-Y.: Astron. Astrophys. 583, 1 (2015). Icarus 281, 404 (2017). doi:10.1016/j.icarus.2016.08.007 doi:10.1051/0004-6361/201526450 Cochran, A.L., Levasseur-Regourd, A.-C., Cordiner, M., Lim, Y.-M., Min, K.-W., Feldman, P.D., Han, W., Edel- Hadamcik, E., Lasue, J., Gicquel, A., Schleicher, D.G., stein, J.: Astrophys. J. 781, 80 (2014). doi:10.1088/0004- Charnley, S.B., Mumma, M.J., Paganini, L., Bockel´ee- 637X/781/2/80 Morvan, D., Biver, N., Kuan, Y.-J.: Space Sci. Rev. 197, Mall, U., Altwegg, K., Balsiger, H., Bar-Nun, A., Berthelier, 9 (2015). 1507.00761. doi:10.1007/s11214-015-0183-6 J.-J., Bieler, A., Bochsler, P., Briois, C., Calmonte, U., Code, A.D., Houck, T.E., Lillie, C.F.: IAU Circ.2201 Combi, M.R., Dabrowski, B., De Keyser, J., Dhooghe, (1970) F., Fiethe, B., Fuselier, S.A., Galli, A., Garnier, P., Gasc, Crismani, M.M.J., Schneider, N.M., Deighan, J.I., Stewart, S., Gombosi, T.I., Hansen, K.C., H¨assig, M., Hoang, M., A.I.F., Combi, M., Chaffin, M.S., Fougere, N., Jain, S.K., J¨ackel, A., Kopp, E., Korth, A., Le Roy, L., Magee, B., Stiepen, A., Yelle, R.V., McClintock, W.E., Clarke, J.T., Marty, B., Mousis, O., R`eme, H., Rubin, M., S´emon, T., Holsclaw, G.M., Montmessin, F., Jakosky, B.M.: Geo- Tzou, C.-Y., Waite, J.H., Wurz, P.: Astrophys. J. 819, phys. Res. Lett. 42, 8803 (2015) 126 (2016). doi:10.3847/0004-637X/819/2/126 11

Marcos-Arenal, P., de Castro, A.I.G., Abarca, B.P., Sachkov, Sreejith, A.G., Mathew, J., Sarpotdar, M., Nirmal, K., Am- M.: Journal of Astronomical Telescopes, Instruments, bily, S., Prakash, A., Safonova, M., Murthy, J.: ArXiv and Systems 3(2), 025002 (2017). 1706.02468 e-prints (2016). 1608.06385 McCandliss, S.R., Feldman, P.D., Weaver, H.A., Fleming, Swings, P., Elvey, C.T., Babcock, H.W.: Astrophys. J. 94, B., Redwine, K., Li, M.J., Kutyrev, A., Moseley, S.H.: 320 (1941). doi:10.1086/144336 In: American Astronomical Society Meeting Abstracts. Vervack, R.J., Merkel, A.W., McClintock, W.E., Holsclaw, American Astronomical Society Meeting Abstracts, vol. G.M., Izenberg, N.R., Nittler, L.R., Starr, R.D., Solomon, 225, p. 137 (2015) S.C.: In: Lunar and Planetary Science Conference. Lunar Meierhenrich, U.: Comets and Their Origin. The Tool to and Planetary Inst. Technical Report, vol. 45, p. 2553 Decipher a Comet. Viley-VCH, ??? (2015) (2014) Miles, B.E., Roberge, A., Welsh, B.: Astrophys. J. 824, 126 Vidal-Madjar, A., Lagrange-Henri, A.-M., Feldman, P.D., (2016). 1511.01923. doi:10.3847/0004-637X/824/2/126 Beust, H., Lissauer, J.J., Deleuil, M., Ferlet, R., Gry, Morgenthaler, J.P., Harris, W.M., Combi, M.R., Feldman, C., Hobbs, L.M., McGrath, M.A., McPhate, J.B., Moos, P.D., Weaver, H.A.: In: AAS/Division for Planetary Sci- H.W.: Astron. Astrophys. 290, 245 (1994) ences Meeting Abstracts 41. AAS/Division for Planetary Weaver, H.: 2002 UV Spectroscopic Investigation of Any Sciences Meeting Abstracts, vol. 41, p. 15 (2009) Bright, Newly Discovered Comet. HST Proposal Mumma, M.J., Charnley, S.B.: Annu. Rev. Astron. Astro- Weaver, H.: 2011 Using Hubble to Measure Volatile Abun- phys. 49, 471 (2011). doi:10.1146/annurev-astro-081309- dances and the D/h Ratio in a Bright Too Comet. HST 130811 Proposal Ootsubo, T., Kawakita, H., Hamada, S., Kobayashi, H., Ya- Young, E.F., Diller, J., Dinkel, K., Dischner, Z., Cheng, maguchi, M., Usui, F., Nakagawa, T., Ueno, M., Ishiguro, A.F., Hibbitts, C., Osterman, S.N.: AGU Fall Meeting M., Sekiguchi, T., Watanabe, J., Sakon, I., Shimonishi, Abstracts, 43 (2014) T., Onaka, T.: In: Asteroids, Comets, Meteors 2012. LPI Contributions, vol. 1667, p. 6397 (2012) Opal, C.B., Carruthers, G.R.: Icarus 31, 503 (1977). doi:10.1016/0019-1035(77)90152-X Paganini, L., Mumma, M.J., Villanueva, G.L., Keane, J.V., Blake, G.A., Bonev, B.P., DiSanti, M.A., Gibb, E.L., Meech, K.J.: Astrophys. J. 791, 122 (2014). doi:10.1088/0004-637X/791/2/122 Raymond, J.C., Uzzo, M., Ko, Y.-K., Mancuso, S., Wu, R., Gardner, L., Kohl, J.L., Marsden, B., Smith, P.L.: Astrophys. J. 564, 1054 (2002). doi:10.1086/324545 Roth, N.X., Gibb, E.L., Bonev, B.P., DiSanti, M.A., Mumma, M.J., Villanueva, G.L., Paganini, L.: Astron. J. 153, 168 (2017). doi:10.3847/1538-3881/aa5d18 Rubin, M., Altwegg, K., Balsiger, H., Bar-Nun, A., Berthe- lier, J.-J., Bieler, A., Bochsler, P., Briois, C., Calmonte, U., Combi, M., De Keyser, J., Dhooghe, F., Eberhardt, P., Fiethe, B., Fuselier, S.A., Gasc, S., Gombosi, T.I., Hansen, K.C., H¨assig, M., J¨ackel, A., Kopp, E., Korth, A., Le Roy, L., Mall, U., Marty, B., Mousis, O., Owen, T., R`eme, H., S´emon, T., Tzou, C.-Y., Waite, J.H., Wurz, P.: Science 348, 232 (2015). doi:10.1126/science.aaa6100 Sachkov, M., Shustov, B., G´omez de Castro, A.I.: In: Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray. Proc. SPIE, vol. 9905, p. 990504 (2016). doi:10.1117/12.2233085 Sachkov, M.E.: Solar System Research 50, 294 (2016). doi:10.1134/S0038094616040055 Sachkov, M., Shustov, B., Savanov, I., G´omez de Cas- tro, A.I.: Astronomische Nachrichten 335, 46 (2014). doi:10.1002/asna.201312015 Shustov, B., Sachkov, M., G´omez de Castro, A.I., Werner, K., Kappelmann, N., Moisheev, A.: Astrophys. Space Sci. 335, 273 (2011). doi:10.1007/s10509-011-0737-3 Shustov, B., G´omez de Castro, A.I., Sachkov, M., Moisheev, A., Kanev, E., L´opez-Santiago, J., Malkov, O., Nasonov, D., Bel´en Perea, G., S´anchez, N., Savanov, I., Shugarov, A., Sichevskiy, S., Vlasenko, O., Ya˜nez, J.: Astrophys. Space Sci. 354, 155 (2014). doi:10.1007/s10509-014-2119- 0 This manuscript was prepared with the AAS LATEX macros v5.2.