El Satèl·lit


 

Una màquina de descobriments

Gaia és un dels instruments més precisos i avançats tecnològicament a l’espai que s’ha construït. Els seus dos telescopis estan muntats en una estructura de carbur de silici resistent i lleugera. Els seus panells solars proporcionen l’energia necessària per al satèl·lit (poc més del que necessita un rentavaixella). La instal·lació de cada element en el satèl·lit ha de ser extremadament precís amb la finalitat d’assegurar l’èxit de la missió.

Escanejant el cel

El principi bàsic del mesurament de Gaia és el mateix que l’utilitzat en la missió Hipparcos (ESA, 1989-1993). Consisteix en dos telescopis observant dues adreces de visió molt separats en el cel. La seva separació angular (106 graus) ha de ser coneguda de una manera molt precisa en tot moment. Mentre Gaia gira sobre si mateix (una vegada cada 6 hores), recopila més i més observacions d’estels, cobrint tot el cel en 6 mesos. Comparant la posició dels estels en diferents parts del cel durant els cinc anys de missió, es pot construir un mapa global precís i consistent. Al final de la missió s’observarà cada estel de 40 a 220 vegades (70 vegades de mitjana).

Els ulls de Gaia

Les 106 càmeres CCD (amb un total de gairebé 1000 milions de píxels) són les responsables de detectar la llum dels estels que arriba als dos telescopis. Tots els sensors CCD es munten en el pla focal més gran mai construït per a una missió espacial (104 x 42 cm). Tres tipus diferents de CCD s’utilitzen amb la finalitat d’obtenir astrometría, espectrofotometria en blau i vermell i l’espectroscòpia d’alta resolució de cada objecte observat.

Gaia en xifres

Massa: 2030 Kg
Diàmetre del para-sol: 11 metres
Longitud focal: 35 metres
Miralls primaris: 1.45 x 0.5 metres
Nombre de miralls: 10
Nombre de CCD: 106
Nombre de píxels per CCD: 4500 x 1966 píxels
Nombre total de píxels: 938 Megapíxels
Grandària de píxel: 10 x 30 micròmetres
Grandària angular píxel: 59 x 177 mil·lisegons d’arc
Consum d’energia: 1720 W
Vida útil: 5 anys
Cost: 700 milions d’euros

 

El camp astromètric

En el pla focal de Gaia les primeres dues bandes de CCD es dedica a la detecció de les fonts (Sky Mapper), altres 9 bandes de CCD es dediquen a mesurar la posició i la brillantor dels estels en el cel amb molta precisió. El moviment del gir de Gaia fa que la imatge d’un estel transite a través de les diferents bandes CCD, recollint més i més llum.

Espectrofotòmetres BP (blau) i RP (vermell)

Dues columnes de CCD en el pla focal es dediquen a detectar la llum dels estels després de passar a través de dos prismes diferents (produint un espectre en el rang blau i l’altre en la rang vermell). Aquests espectres de baixa resolució s’utilitzen per derivar els paràmetres astrofísics (temperatura, gravetat superficial, composició, …) dels objectes observats per Gaia.

La llum dels estels, quan entren en els prismes, es divideix en diferents colors, produint una espècie d’arc de Sant Martí amb molts colors, anomenat espectre. Depenent dels paràmetres físics dels estels (com la seva temperatura, la gravetat i la metalicidad), aquests espectres seran més brillants en determinats colors. Això ens ajudarà, per tant, a reconèixer les propietats físiques de les observacions.

GOGspectra

Espectres BP/RP de Gaia d’estels amb diferents temperatures i gravetats simulats amb GOG per l’equip de la UB

 

Espectrògraf de velocitats radials (RVS)

En Gaia també s’inclou un espectrògraf d’alta resolució per mesurar la velocitat de un objecte observat apropant-se o allunyant-se de nosaltres (velocitat radial). Això, es pot fer utilitzant l’efecte d’aquest moviment radial en les línies espectrals (efecte Doppler). La llum d’un estel conté algunes línies fosques a causa de la presència d’alguns elements en la seva atmosfera. Les posicions en els espectres d’aquestes línies canviaran si l’estel es mou respecte a Gaia. Si la línia es desplaça cap al vermell (redshift) indica que l’objecte astronòmic s’està distanciant de nosaltres. Per contra, si l’objecte s’està movent cap a nosaltres, les línies espectrals sofriran un desplaçament cap al blau (blueshift).

 

Gestió de dades abord i comunicacions

Hi ha 7 unitats de processament de vídeo (VPU) a bord de Gaia, que contenen cadascuna 3 processadors PowerPC (per triple redundància), el que fa que la càrrega útil de Gaia sigui entre els sistemes de computació més potents a bord d’un satèl·lit científic. Es detectaran, processaran i comprimiran les imatges dels objectes mesurats a través dels sensors CCD de Gaia, generant més de 5 Mbps de mitjana.

 

 

La memòria abord (al voltant de 100 GB) emmagatzema aquestes dades fins que s’estableixi contacte amb l’estació terra, que succeeix durant 8 hores de mitjana cada dia. Els requisits d’estabilitat a bord de Gaia són tan estrictes que una antena parabòlica normal, donaria lloc a vibracions inacceptables, i per tant es requereix una antena de control de fase especial.

 

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