Miniature lightweight x-ray optics (MiXO) for surface elemental composition mapping of asteroids and comets

2.3 MB·

by  Jaesub Hong

DraftversionJanuary7,2016
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
MINIATURELIGHTWEIGHTX-RAYOPTICS(MIXO)FORSURFACEELEMENTALCOMPOSITIONMAPPINGOF
ASTEROIDSANDCOMETS
JaesubHong1∗,SuzanneRomaine2andtheMiXOteam
1HarvardUniversity,60GardenStreet,Cambridge,MA02138,USA,([email protected])
2SmithsonianAstrophysicsObservatory,60GardenStreet,Cambridge,MA02138,USA([email protected])
∗SendrequeststoJ.Hong.
DraftversionJanuary7,2016
ABSTRACT
6 The compositions of diverse planetary bodies are of fundamental interest to planetary science,
1 providingcluestotheformationandevolutionaryhistoryofthetargetbodiesandtheSolarsystemas
0 awhole. UtilizingtheX-rayfluorescenceuniquetoeachatomicelement,X-rayimagingspectroscopy
2 is a powerful diagnostic tool of the chemical and mineralogical compositions of diverse planetary
n bodies. Until now the mass and volume of focusing X-ray optics have been too large for resource-
a limitedin-situmissions, sonear-targetX-rayobservationsofplanetarybodieshavebeenlimitedto
J simplecollimator-typeX-rayinstruments.
6 WeintroduceanewMiniaturelightweightWolter-IfocusingX-rayOptics(MiXO)usingmetal-
ceramic hybrid X-ray mirrors based on electroformed nickel replication and plasma thermal spray

] processes. MiXOcanenablecompact, powerfulimagingX-raytelescopessuitableforfutureplane-
P tary missions. We illustrate the need for focusing X-ray optics in observing relatively small plane-
E tary bodies such as asteroids and comet nuclei. We present a few example configurations of MiXO
. telescopes and demonstrate their superior performance in comparison to an alternative approach,
h
micro-poreoptics,whichisbeingemployedforthefirstplanetaryfocusingX-raytelescope,theMer-
p
curyImagingX-raySpectrometer-T(MIXS-T)onboardBepicolumbo. X-rayimagingspectroscopyus-
-
o ing MiXO will open a large new discovery space in planetary science and will greatly enhance our
r understandingofthenatureandoriginofdiverseplanetarybodies.
t
s Subjectheadings:X-rayfluorescenceelementalabundanceX-rayimaging
a
[

1 1. INTRODUCTION observed by such a telescope will be more or less the
v ROSAT,theChandraX-rayObservatoryandotherX-ray same regardless of the distance as long as the FoV is
0 observatories have revealed that most planetary bod- fully occupied by the emitting surface. On the other
0 ies in our Solar System emit X-rays via various phys- hand,iftheasteroidof∼1kmsizeisobservedatadis-
3 tance of roughly 1000 km, only a tiny fraction of the
ical processes including X-ray fluorescence (XRF) and
1 charge exchange with plasma. In XRF, the observed XRFflux(10−5)willreachthetelescopesincetheemis-
0 energies are unique to the atomic levels of each ele- sionoftheXRFismoreorlessisotropic.
1. ment. ThispropertyofXRFenablesapowerfulwayto X-ray optics in the past are massive, so X-ray instru-
0 probethesurfacedistributionofelementalcomposition ments flown on near-target missions in the past have
6 in the emitting bodies. Fig. 1 shows a simulated X-ray foregonetrueimagingcapabilities. Instead,theyhadto
1 spectrumfromanasteroidofC1chondritecomposition employsimplecollimator-typeinstrumentsasintheX-
: located at 1 AU during a typical quiet sun state. X- rayspectrometers(XRS)onNEAR-Shoemaker(Trombka
v
rayemissionfromtheasteroidcontainsnumerousXRF et al. 2000), Hayabusa (Okada et al. 2006) or Messen-
i
X emissionlinestiedtovariousatomicelements(Nittleret ger (Nittler et al. 2011). This is largely because a large
r al.2004). Fig.1alsoshowshowtherelativeabundance portion of the resources in a typical planetary mission
a ofMg/Sivs.S/Sicanbeusedtoidentifymatchingme- oftenhastobereservedforthespacecrafttomakepos-
teoritespecimentypes. sible the long journey to the target and the remaining
While Earth-orbiting X-ray observatories are capa- resources are shared by a collection of onboard instru-
ble of high angular resolution X-ray imaging (e.g. sub- ments required to unveil the mysteries of the targets.
arcsec angular resolution for Chandra), the large dis- Hence the resources allowed for each instrument are
tance to relatively X-ray faint rocky airless bodies limited,andconventionalX-rayfocusingopticsusedin
severely limits the photon statistics and often makes it thepastaresimplytoolargeandtooheavytobeapart
difficult to identify interesting surface features or sub- ofplanetarymissions.
structuresofX-rayemission. Forinstance,theobserved Despitethelackofimagingcapability,X-rayobserva-
X-rayfluxoftheCrabnebula,oneofthebrightestX-ray tionsofthetargetshavebeenproventoprovideunique
sources in the sky is a few photons cm−2 s−1 keV−1 at insights. Forinstance,XRSobservationsof433Erosby
NEAR-Shoemaker revealed that the elemental composi-
around1keV.ThisiscomparabletotheXRFfluxfrom
tion of the asteroid from the X-ray spectra is different
anasteroidlocatedat1AUfromtheSunobservedbya
telescope with a FoV of (cid:38) 10 deg × 10 deg at the sur- from the asteroid classification acquired by the optical
and infrared spectra (Trombka et al. 2000). It is sus-
face. Due to the Lambertian reflectance, the XRF flux
2
Figure1. (Top)SimulatedX-rayfluorescencespectrumofanasteroid Figure2. Optical image of Comet 67P/Churyumov-Gerasimenko
of C1 chondrite composition at 1 AU during the quiet sun state il- takenbyNAVCAMontheRosettamissionrevealstheoutgassingjets
lustratingdiverseelementalcomposition.(Bottom)Abundanceratios (CourtesyofEuropeanSpaceAgency-ESA).
(Mg/Sivs.S/Si)asanidentifierofawiderangeofmeteoritespecimen
types(Nittleretal.2004).
contextoftheatomiccompositionforthereturnedsam-
plesaswellasassistingthesamplesiteselectionduring
pectedthatongoingspaceweatheringaltersthesurface themission.
of the asteroids up to a few µm depth, which modi- InSec2,wereviewthemajorchallengesencountered
fies the optical and infrared spectra, whereas the XRF in near-target X-ray observations using collimator in-
probesmorethan20µmbelowthesurfaceandcanre- struments, and we illustrate how focusing X-ray optics
veal the true, unweathered, composition (Binzel et al. can overcome these challenges and enhance a chance
2010). In the case of Itokawa, definitive evidence of of success and new discoveries especially in observing
spaceweatheringontheasteroidwaslaterfoundinthe relativelysmallbodiessuchasasteroidsandcometnu-
returnedsample(Noguchietal.2011,2014). clei. InSec3,webrieflyreviewtheadvancesofWolter-I
From recent images of Comet 67P/ Churyumov- X-ray optics, and introduce our new approach to build
Gerasimenko taken by NAVCAM (∼ 18(cid:48)(cid:48)resolution) on compact, lightweight X-ray optics by merging Electro-
the Rosetta mission one can imagine exciting poten- formed Nickel Replication (ENR) and Plasma Thermal
tialoffutureX-rayobservationsofasteroidsandcomets Spray(PTS)processes. InSec4,weshowexampleMiXO
whenX-rayspectroscopyiscombinedwithhighresolu- configurationssuitableforplanetarymissionsandesti-
tionimaging.TheopticalimageofComet67Pshowsdi- matetheperformancesincomparisontoanalternative
verseterrainandvolatileactivity(Fig.2). Inparticular, approach,micro-poreoptics(MPO).
thehighratioofdeuteriuminthewatervaporofthejets
questions our understanding of the origin of the water 2. ADVANTAGESOFFOCUSINGX-RAYOPTICS
on Earth (Altwegg et al. 2015). While soft X-ray obser- X-rayobservationsinnear-targetmissionsinthepast
vations(0.1–15keV)areinsensitivetoisotopevariation, havebeenchallenging. Someofthechallengesoriginate
X-ray imaging spectroscopy can separate the elemental from incomplete prelaunch ground calibration of the
compositionofgaseousjetsfromthecomet’ssurfaceas instrumentsandinadequateonboardcalibrationsystem
wellasthecompositionalvariationofthesurfacedueto (usuallyduetotightassemblyscheduleandbudgetcon-
impactsandvolatileactivitiesinthepast. straints). As a result, the data analysis was complex
Astheneedforsampleretrievalofvariousplanetary and often led to somewhat ambiguous interpretation.
bodies rises for in-depth analysis in attempt to unam- On the other hand, a cause of many challenges in the
biguously unveil their nature, origin and evolutionary pastplanetaryX-rayobservationliesintheverynature
history, many future sample return planetary missions ofnon-focusingopticsofcollimatorinstrumentsasde-
are being developed. The retrievable size of samples, scribedbelow.
however, is very small: e.g. sub-gram in Hayabusa 2 REgolith X-ray Imaging Spectrometer (REXIS) on-
(Tachibana et al. 2013), less than 1 kg in OSIRIS-REx board OSIRIS-REx, which is scheduled to launch in
(LaurettaandO.-R.Team2012). Therefore,X-rayimag- 2016 to collect samples from Bennu, was designed to
ing spectroscopy onboard future near-target missions take into account lessons learned from the previous
becomes an indispensable tool in acquiring a global missions(Inamdaretal.2014). Forinstance,REXISwill
3
Figure3. (a)Forlargesphericalplanetarybodies(radiusRT>∼1000km),observationcanbeconductedataclosestableorbit(RO)(i.e. RT∼
RO),whereasimplecollimatorcanbesufficientforidentifyinglargesurfacevariation. (b)&(c)Forsmalltargets(RT<∼100km),lowgravity
orirregularshapepreventsobservationataclosesteadyorbit(i.e. RT <<RO),andthesizeoftheimagingfootprintonthesurfacecanchange
dramaticallyovertimeoreveninasingleorbit(HongandRomaine2014).
be equipped with one of the most comprehensive on-
boardcalibrationsystemsinspaceX-rayinstruments: a
setof55Feradioactivesourcesarestrategicallymounted
to monitor any potential drifts or changes of spectral
gain and resolution of every node of 4 X-ray CCDs on-
board. Inaddition,theteamplanstoconductaseriesof
extensive ground and flight calibrations to understand
thedetectorresponseandtotrackanyvariationduring
theobservation.
REXIS utilizes coded-aperture imaging to identify ∼
50 m scale surface variation of the elemental composi-
tionofBennu. Theadditionofimagingcapabilityisthe Figure4. AllskysoftX-raybackgroundimageinGalacticcoordinates
first of its kind (coded-aperture imaging) in planetary by ROSAT/PSPC after removing bright point sources (red: 0.1-0.4
X-ray observations (see below about the Bepicolumbo keV,green: 0.5-0.9keV,blue: 0.9-2.0keV,CourtesyofMax-Planck-
InstitutforextraterrestrischePhysik).
mission). Coded-apertureimagingisbasicallyshadow-
gram imaging using a mask coded with a pattern of
open and opaque elements in front of the detector. It
Galacticplane glowsbrightly inX-ray, whichis known
is a novel technique to enable imaging without focus-
as Galactic ridge X-ray emission from the early days of
ing optics, and thus allows a relatively compact, sim-
pletelescopedesign(REXISfitsinanenvelopeof∼10 theX-rayastronomy. TheskyathighGalacticlatitudeis
cm × 10 cm × 20 cm). However, coded-aperture imag- also filled with X-ray emission from unresolved Active
Galactic Nuclei (Fig. 4). These components contribute
ingstilloperatesunderthebasicprincipleofcollimator
to bright Cosmic X-ray background (CXB), which can
instruments,facingthesamechallenges. Thusitsimag-
often dominate X-ray emission from the target in the
ingsensitivityisrelativelylowcomparedtofocusingin-
collimator-typeinstrumentswhentheirFoVisexposed
strumentsandithasbeenmainlyemployedtoidentify
to the background sky. The continuum component of
pointsourcesinastrophysicsinthepast.
the CXB is about 10 ph cm−2 s−1 sr−1 keV−1 at 1 keV,
When observing large planetary bodies such as the
which is about 100 – 1000 times brighter than the ex-
Moon, a simple collimator based X-ray instrument can
efficientlycollecttheX-raysfromthesurfaceandiden- ampleinFig.1.
X-rayinstrumentsinspacealsoexperienceadditional
tify spatial variations as the instrument orbits around
internal X-ray background which occurs due to inter-
the target. The large gravity and the spherical geome-
actions between cosmic-ray particles and the instru-
try of the target with a small eccentricity allows stable
ments. TheinternalbackgroundcancontainXRFlines
circularorbitsveryclosetothesurface(Fig.3a). Inad-
originated from the instrument, which can complicate
dition,thefieldofview(FoV)oftheinstrumentisfully
the elemental identification of the target. The sensi-
occupied by the target, so that no external background
tivityofatypicalcollimatorinstrumentisproportional
cancontaminatethedata.
to a square root of the detector area (instead of being
In the case of relatively small planetary bodies such
linearly proportional to the detector area) because the
asasteroidsandcometnuclei,thesmallgravityandthe
internal background increases as the detector area in-
irregular shape of the target often limit stable orbital
creases. Ontheotherhand,focusingtelescopesallowsa
configurationsnearthetarget(Fig.3b&c). Inorderto
smallfocalplanewithalargeeffectivearea,sothatthe
collect faint X-ray emission from the target, the FoV of
internal background can be 10-100 fold smaller than
theinstrumentisusuallydesignedtofillthetarget,and
thus the collimator type instrument cannot efficiently thecollimatorinstrumentsofsimilarcollectingarea.
DuetohighCXB(andthelargeinternalbackground),
identifythesurfacefeatureofsmallscales. Atthesame
observations with collimator instruments are suscepti-
time, the exposure to the background sky in the FoV
bletochangesinobservingconfigurations(e.g.changes
should be limited because the X-ray sky is not dark,
butitshineswithmanydiffuseandpointX-raysources, in pointing directions or observing distances), which
can severely narrow the observing windows. Focusing
often brighter than the target itself. For instance, our
telescopes,ontheotherhand,canstartmeaningfulob-
4
majorelements(e.g. O-K,Fe-L)overtheentiresurface.
Theadvantagesoffocusingopticsextendtoobserva-
tionsoflargeplanetarybodiesaswell. Forinstance, at
Mercury, the high thermal loads force spacecraft to be
at highly elliptical orbits, limiting the observing duty
cycle of collimator instruments. With focusing optics
lesssusceptibletochangesinobservingconditions,the
observing duty cycle can be maintained high. In the
caseoftheJoviansystem,highradiationenvironment(1
kradperanorbit,dependingonshielding,etc)severely
limitsthedetectorandshieldsize,andthus, thefocus-
ing optics is required to achieve large light-collection
powerwithasmalldetector.
3. ADVANCESOFX-RAYOPTICS:ELECTROFORMEDNICKEL
REPLICATIONWITHTHERMALPLASMASPRAY
While the scientific motivation and the advantages
of X-ray focusing optics have been known, it has been
impractical to implement conventional focusing X-ray
telescopes in near-target planetary missions. As afore-
Figure5. Comparison of minimum detection time (global, non-
imaging)ofXRFin0.5–2keV(dependingontheoptical/IRfiltering mentioned, typical planetary missions are a multi-
scheme,mirrorreflectivityandotheropticsonfiguration)asafunc- instrumentdisciplinebynature,whichisnecessarybut
tionofdistancetothetargetforNEAR-Shoemaker(green),Hayabusa alsolimitsthepower,mass,volumeandotherresources
(red)andMiXO(blue)withplannedorbitalprofiles(OSIRIS-REx,yel-
low).Theorbitradiusandstandoffdistancearerescaledtothetarget allocatedforeachinstrument. Ontheotherhand,con-
size. Collimator instruments become useless beyond a certain dis- ventionalX-rayopticstendtobeheavyandlarge. This
tanceduetohighcosmicX-raybackground,whereasfocusingoptics is because of relatively low areal density (i.e. relatively
canextendtheusefulobservingconditiontoamuchwiderrangeof small collecting area per unit mass) of X-ray mirrors.
orbitprofiles,allowingaflexiblemissiondesign. Theminimumde- X-ray optics relies on grazing incidence for efficient X-
tectiontimeisthetotalexposurethatisrequiredtoacquirethesignal-
to-noiseratioaboveacertainlimit(e.g.5σ). ray reflection, and a set of barrel shape mirrors are
stackedtogetherintheWolter-Iconfigurationtoestab-
lishneededeffectivearea. Forinstance,theouterX-ray
mirrorofChandraisabout1.4mdiameterby1mlong
servations much farther out (e.g. during early phases
with10mfocallength,andweighsabout1500kg.
of approach and debris scouting) by simply resolving
Fig.6illustratestheadvancesofX-rayopticsinweight
andremovingmostofbothexternalandinternalback-
reduction over the years while maintaining high an-
grounds.
gular resolution (<15”–30”) and improving collecting
Fig.5comparesrelativeminimumexposuresforXRF
power. In Chandra, high resolution mirrors were made
detection from an asteroid by well calibrated XRS of
fromdirectlygrindingandpolishingglassceramic(ze-
NEAR-ShoemakerandHayabusawithanexampleMiXO
rodur), which enabled sub arcsec angular resolution
telescope(MiXO-70B,seeSec.4)asafunctionofstand-
off distance or orbit radius (R ) normalized to the tar- (Fig. 6a). Electroformed Nickel Replication (ENR)
O
process used to build X-ray mirrors in XMM-Newton
get size (radius, R ). It also shows a scaled example
T
(Fig. 6b) utilizes relatively thin Ni shell (1 mm, 8.9
orbital/approach profile from OSIRIS-REx (the orange
histogram). In-situornear-targetexperimentsstudying g/cm3) as a substrate for the support of mirror figure
smallasteroids(R (cid:46)100km)oftencannotstayinasta- and shape. The thin Ni shell enables reduction of the
O
bleorbitatcloseproximity(i.e. R /R >>1)duetolow overallmassbyafactorofmorethan5comparedtoX-
O T
gravity, debris, or an irregular asteroid shape. In addi- ray mirrors in Chandra and allows larger light collect-
tion,themultiplestagesofapproachtothetargetmake ing area under the same packaging volume. In ENR,
the overall orbital profiles to spread over a wide range the mirror figure and angular resolution is determined
ofdistancestothetarget. by the surface quality of mandrel, and 10”–15” can be
Imaging by focusing X-ray optics enables a more for- achieved routinely. The Astronomical Roentgen Tele-
giving orbit configuration for observation, and focus- scope (ART) (Gubarev et al. 2012) onboard Spektr-RG,
ing telescopes can continue to accumulate useful data which is scheduled to launch in 2016, employs a NiCo
throughout the mission (R /R (cid:46) 300 for global mea- alloy (250 µm thick) as a substrate instead of Ni un-
T O
surements, R /R (cid:46) 100 for mapping). As shown in dertheENRprocess, furtherreducingthemirrormass
T O
Fig. 5, the MiXO telescope would have completed the (Fig.6c).
same measurements as NEAR-Shoemaker or Hayabusa Recently we have begun to develop a new approach,
even before they reached ther observing distance. A where we combine the Plasma Thermal Spray (PTS)
longerobservingperiodenabledbyMiXOalsoenhances technologyandtheENRprocesstoformmetal-ceramic
achanceofsuccessandnewdiscoveries,sincedetection hybrid X-ray mirrors (Romaine et al. 2014). In the hy-
ofhardX-raysfromheavyelementsoftenreliesonrel- brid X-ray mirrors, the lightweight ceramic layer pro-
ativelyrarestrongsolarflares. Inadditiontotheglobal videsthestiffnessneededtomaintaintheoverallfigure
measurement,withMiXO,theaccumulateddataover∼ ofX-rayoptics,whiletheNiColayermaintainsthelow
2months(at1AU)duringthequietsunstatealonewill surfaceroughnessnecessaryforX-rayreflection. Byre-
be sufficient for detection of ∼1% spatial variation for placingthemajorityoftheNiColayer(∼8.9g/cm3)with
5
Figure6. AdvancesofX-rayoptics.(a)Agroundandpolishedglasssubstrate,(b)electroformedNishell,(c)thinelectroformedNiCoshell,and
(d)ametal-ceramichybridshell.Theweights(*)arefora70cmdia.,60cmlongmirrorshell.Forsmallmirrors(∼5cmdia.)suitableforplanetary
science,asingleshellwouldweigh∼10gorless.
Table1
ExampleMiXOTelescopes
Parameters MiXO-70B MiXO-70W
FocalLength(f) 70cm 70cm
No.ofShells 50 25×7
Size(Diameter) 4–14cm 2–5.6cm
17cmoverall
Mass(g) 920 210×7
Detector 1.5×1.5cm2 5×5cm2
FoV 1deg2 7deg2
Ang.Resolution 30” 30”
EffectiveArea 65cm2@1keV 6.4cm2@1keV
15cm2@4keV 4.6cm2@4keV
GrazingAngle 0.41–1.43deg 0.20–0.51deg
Figure7. A test single-bounce conical mandrel, a NiCo shell, two
*Assume10%reductionforalignmentfixture
NiCoceramichybridshellsfromlefttoright.Thewidthoftheimage
spansabout14cm.
ofMIXS-T,theinnersurfaceofsmallsquareporesormi-
thelightweightalumina(Al O ,2.3–2.9g/cm3),wecan crochannels(∼20µmpitch)ina0.9–2.2mmthickplate
2 3
furtherreducethemirrormassbyanotherfactorof3to is coated with Ir for X-ray reflection. Then, mosaics of
10comparedtonormalENRmirrors,dependingonthe segmentedMPOplatesareassembledtoformaWolter-
thickness of mirrors. Fig. 7 shows example modules: a Iconfiguration. Inthissection, weestimatetheperfor-
conicalmirrormandrel,aNiCoshell,andtwoNiCoce- mance of MiXO using a few example configurations in
ramic hybrid shells from left to right. The modules in comparisonwithBepicolombo/MIXS-T.
Fig.7issinglebounce,butENRprocesscanformamir- Fig. 8 and Table 1 show two example configurations
ror with two bounces in one package, simplifying the usingthehybridshellsat70cmfocallength: thebase-
alignment structure without introducing any degrada- line option (MiXO-70B) (left) and a wide field option
tioninangularresolution(e.g.seeGubarevetal.2012; (MiXO-70W)(right). Eachshellinboththeoptionscon-
Romaineetal.2014). ThehybridshellsinFig.7,which sists of 20 µm NiCo + 200 µm Al O layers. It is ∼ 10
2 3
demonstrateaproofofconcept,aremadeofan300µm cmlong(∼5cmforeachbounce),andthegapbetween
ceramic layer with 100 µm of a NiCo layer. Given the theadjacentshellsis500µm. Theopticsaloneweighs∼
propertiesofNiCoandceramiclayer,weexpectthatsub 0.9–1.5 kg, and the support structure will add another
20µmNiCo+200µmceramicX-raymirrorshellcanbe 0.3–0.5kgunderaconservativeestimate. Thebaseline
achievedforrelativelysmallshells(2–20cmdiameter). designisscalabletoasmallerorlargeropticsdepending
onavailableresourcesormissionobjectives. Forexam-
4. EXAMPLEMIXOCONFIGURATIONSANDPERFORMANCE ple,abiggertelescopesimilartoBepicolombo/MIXS-T(f
MiXO made of metal-ceramic hybrid X-ray mirrors ∼100cm)orasmallertelescopesimilartoJUXTA(f ∼
shows a tremendous promise in enabling compact X- 25 cm Ezoe et al. 2013) can be implemented. MiXO-
ray telescopes suitable for various types of planetary 70W is optimized for wide field monitoring with the
missions, which can open a new era of sensitive X- combinedFoVof7deg2 andbroadbandcoveragewith
ray imaging spectroscopy in planetary science. Until relatively higher grasp at high energies due to small
now,micro-poreoptics(MPO)wasconsideredtheonly grazinganglesofeachmodule. TheFoVofeachmodule
approach that can enable compact Wolter-I X-ray op- inMiXO-70Wisindependentofeachother, sothatthe
ticsinplanetarymissions. TheMercuryImagingX-ray alignmentbetweenthemisnotcritical.
Spectrometer-T (MIXS-T) on Bepicolombo employs the The sensitivity or light collection power of an X-ray
MPO and will be the first focusing X-ray telescope in telescopeisoftendescribedbytheon-axiseffectivearea.
planetarysciencemissions(Fraseretal.2010). InMPO SinceX-rayemissionfromplanetarybodiesisoftendif-
6
♥
♣
♦
♥
♣ ♠
♦
♠
Figure9. Grasp of MiXO-70B, MiXO-70W and MiXO-100B optics
withIrcoating(10%reductionforsupportstructure, nosurfaceor
Figure8. Example MiXO telescopes: (Left) the baseline design multilayeroptimization)incomparisonwithMIXS-TonBepicolombo
(MiXO-70B) and (Right) a wide-field option (MiXO-70W). The red (Fraseretal.2010).(FL:FocalLength,OD:OpticsDiameter).
conesindicatea1.2degdia.FoVonthefocalplane.
routinely. Compared to MIXS-T, MiXO-70B, though a
fuse by nature especially when observed near-by, an- smallertelescope,improvesthedetectionsensitivity(∝
otherimportantparametertodescribetheperformance PSF area1/2) of small surface features by >10× or ex-
of X-ray telescopes is grasp, which is the product of tendstheobservingdistance(∝PSF area1/4)by>3×due
the effective area and the FoV. Fig. 9 compares the ex- to relatively lower background (∝ PSF area) of small
pected grasp of various MiXO optics with MIXS-T on
PSFsenabledbyhighresolution(<30”).
Bepicolombo. We assume that each shell of MiXO here HighangularresolutionplanetaryX-rayimagingwith
iscoatedwithIr. Withonly∼1/3ofthepackagingvol- sub100cm2 effectiveareawillbenormallyfluxlimited
ume, MiXO-70B (♣) meets the performance of MIXS-T
during the quiet Sun phases. On the other hand, the
(♠);orMiXO-100B((cid:114)inFig.9,thesamebaselineconfig-
fullpotentialofthehighangularresolutioncanbereal-
urationasMiXO-70Bbutf ∼100cmwithanopticsen-
izedduringstrongsolarX-rayflareswhichcangenerate
velopeof20cmdia.),whosepackageissimilartoMIXS-
higher solar X-ray flux by a few order of magnitudes.
Tinsize,outperformsMIXS-Tbyafactorof∼3. Inthe
GiventheangularresolutioninMiXOthatiscompara-
caseofMiXO-70W((cid:113)),smallgrazinganglesboosthigh
ble to NAVCAM on the Rosetta mission, a strong solar
energyresponse,enhancingachanceofthediscoveryof
X-rayflarecanenableanX-raysnapshotofthedetailed
heavyelements. Sinceourapproachenablesadditional
atomiccomposition,whichcanbedirectlycomparedto
optimizationssuchaspolynomialprofileofshellgeom-
thesurfacestructureofcometnucleiandoutgassingjets
etry for uniform response over the wide field (Conconi
inanopticalimage.
etal.2010)ormultilayercoatingtoenhancehardX-ray
response (e.g. NuSTAR; Harrison et al. 2013), one can
5. SUMMARY
furtherimprovetheresponseofMiXOtelescopes.
Inadditiontooutperformingonthroughput(bothon- We have introduced a new approach to build
axiseffectiveareaandoverallgrasp)byafactorof3or lightweight focusing X-ray optics with metal-ceramic
more,ourapproachexcelsMPOintwootherareas:first, hybrid X-ray mirrors through the ENR process and the
MPOcanfocusX-rayseffectivelyonlybelow2keVdue PTS technology. Hybrid X-ray mirrors are lighter by
to the difficulty in coating the pore surface on the 20 almost two orders of magnitude relative to X-ray mir-
µmscale. Forinstance,multilayercoatingcannotbeap- rorsinChandra.ThisenablescompactlightweightX-ray
pliedtosmallpores. Ontheotherhand,MiXO-70Win telescopes which can be mounted on resource-limited
Fig. 9 is designed for wide-field hard (> 2 keV) X-ray in-situornear-targetplanetarymissions.
imagingandthusoptimizedforthedetectionandlocal- We have illustrated the benefit of true imaging X-ray
izationofheavyelementssuchasrare-earthelements. spectroscopy especially for observations of small plan-
Second, the resolution of MPO is limited to 2’ for etary bodies such as asteroids and comet nuclei. With
lobster eye Angel optics and 5–8’ for Wolter-I optics. afewexampleconfigurationswehavedemonstratedef-
The additional degradation in resolution for Wolter- ficientlight-collectionpower(3×improvementrelative
I optics comes from misalignment among segmented to an alternative approach using MPO), high angular
mirror facets in MPO. In fact, difficult alignment be- resolution (>10× improvement), high detection sensi-
tween segmented mirrors requires a complex support tivity(>10×improvement)andwideenergybandcover-
structure, which mostly erases its lightweight advan- age(upto10–15keVwithmultilayercoating)ofMiXO.
tagerelativetoMiXO.Ontheotherhand,inMiXO,each With MiXO, we can open a new era of planetary X-ray
shell contains both bounces, greatly simplifying align- imagingspectroscopy.
ment structure andmass, anda 15-30” resolution (10–
16 × improvement relative to MPO) can be achieved 6. AUTHORS’CONTRIBUTION
7
JH carried out design and modeling work for MiXO, Noguchi,Tetal.(2014)MineralogyoffourItokawaparticles
investigated its feasibility and applications to various collectedfromthefirsttouchdownsite.Earth,Planets,andSpace
planetary missions, and drafted the manuscript. SR 66:124-133doi:10.1186/1880-5981-66-124
AltweggKetal.(2015)67P/Churyumov-Gerasimenko,aJupiter
demonstrated the proof of concept of MiXO by fabri-
familycometwithahighD/Hratio.Science
catingandtestingprototypeMIXOmodules. doi:10.1126/science.1261952
TachibanaSetal.(2013)TheSamplingSystemofHayabusa-2:
7. ACKNOWLEDGEMENT ImprovementsfromtheHayabusaSampler.LunarandPlanetary
ScienceConference44:1880-1881
Thetravelexpenseforthesymposiumwassupported
LaurettaD,O.-R.Team(2012)AnOverviewoftheOSIRIS-REx
by NASA Grant NNX12AG65G. We thank Nicole D. AsteroidSampleReturnMission.LunarandPlanetaryInstitute
Melsoforperformingsimulationstoestimatetheeffec- ScienceConference43:2491-2492
tiveareaandgraspforvarioustelescopeconfigurations. HongJ,RomaineS(2014)Miniaturelightweightx-rayoptics(MiXO)
forsolarsystemexploration.SPIEvol91441,p91441F
WealsothankMartinElvisfortheusefulcomments.
InamdarKNetal.(2014)ModelingtheExpectedPerformanceofthe
REgolithX-rayImagingSpectrometer(REXIS).SPIEvol9222,p
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