Meddelelse
ALMA får Pluto på kornet for at hjælpe rumsonden New Horizons
6. august 2014
Med ALMA, som står for Atacama Large Millimeter/submillimeter Array, er astronomerne ved at foretage ekstra præcise målinger af dværgplaneten Plutos position og dens bane omkring Solen. Det sker for at hjælpe NASAs rumsonde New Horizons med at sigte helt præcist på sit mål, Pluto og dens fem kendte måner, som bliver nået i juli 2015.
Selvom Pluto er blevet fulgt i årtier med teleskoper både her fra Jorden og fra rummet, arbejder astronomerne stadig på at bestemme dens præcise bane omkring Solen. Usikkerheden skyldes dels, at Pluto er meget langt fra Solen (omkring 40 gange længere væk end Jorden er) og dels at vi kun har studeret banen i omkring en tredjedel af dværgplanetens omløb. Pluto blev opdaget i 1930, og den bruger 248 år på at komme en hel gang rundt om Solen.
"Med så begrænsede observationsdata kan vores viden om Plutos position være adskillige tusinde kilometer ved siden af, og det gør, at det er meget svært at beregne de manøvrer, som vi skal foretage med New Horizons", siger Hal Weaver, som er videnskabelig medarbejder på New Horizons-projektet, og med i forskningsgruppen på Johns Hopkins Universitetets laboratorium for anvendt fysik i Laurel, Maryland, USA.
New Horizons-holdet har brugt positionerne fra ALMA sammen med nybearbejdede observationer i synligt lys helt tilbage næsten til Plutos opdagelse. De skal bruges til at bestemme, hvordan de første planlagte kurskorrektioner skal udføres, i juli.
Som forberedelse til dette vigtige trin, har astronomerne brug for at kende Plutos position med største nøjagtighed, og til det skal man bruge de fjerneste og mest stabile referencepunkter, man kan finde. Både det at finde referencepunkterne og det at beregne baner for så små objekter med tilstrækkelig nøjagtighed er en stor udfordring. Normalt bruger man fjerne stjerner som reference, fordi deres positioner kun ændrer sig meget lidt i årenes løb. For New Horizons bane, og mødet med Pluto er der brug for endnu større nøjagtighed.
De fjerneste og mest stabile objekter i universet er kvasarerne - meget fjerne galakser med klartlysende kerner. Men kvasarer er ofte meget svagtlysende i synligt lys, og så er det svært at måle præcist. Kvasarer har supertunge sorte huller i midten, og de udsender stråling fra det omgivende kosmiske støv, så de er meget klartlysende i radiobølgeområderne; især i millimeterbølgeområdet, som ALMA dækker.
"For at halvere usikkerheden i Plutos position brugte vi astrometrimålinger fra ALMA på en kvasar, som kaldes J1911-2006", sagde Ed Fomalont, som er astronom ved National Radio Astronomy Observatory i Charlottesville, Virginia. Han er i øjeblikket udstationeret ved ALMAs støtteafdeling i Chile.
ALMA kunne observere radiobølgeudstrålingen fra Plutos og Charons kolde oveflader - de er omkring -230° Celcius.
Holdet observerede de to kolde verdener for første gang i november 2013, og siden tre gange mere i 2014 - i april og to gange i juli. Der er programsat flere observationer i oktober 2014.
"Vi er meget begejstrede for de tip-top moderne muligheder, som ALMA giver os for bedre at kunne skyde os ind på vores historiske udforskning af Pluto-systemet", sagde den hovedansvarlige for missionen Alan Stern fra Southwest Research Institute. Stern arbejder i Boulder, Colorado. "Vi takker alle på ALMAholdet for deres støtte, og for de flotte data, de skaffer til New Horizons".
Baggrundsoplysninger
ALMA, Atacama Large Millimeter/submillimeter Array, er et internationalt astronomisk observatorium, hvor Europa, Nordamerika og Østasien samarbejder med Chile. ALMAs europæiske del finansieres af ESO (Det europæiske sydobservatorium), I Nordamerika er det USAs National Science Foundation i samarbejde med Canadas National Research Council og National Science Council i Taiwan, og i Østasien er det National Institutes of Natural Sciences, Japan og Academia Sinica, Taiwan, som betaler. Opbygningen og driften af ALMA varetages i Europa af ESO, i USA af National Radio Astronomy Observatory og i Østasien af National Astronomical Observatory of Japan. Organisationen Joint ALMA Observatory, JAO står for den fælles ledelse og styring af konstruktion og drift af ALMA.
Links
New Horizons hjemmesider (NASA)
Kontakter
Ole J. Knudsen
ESON Danmark
Aarhus Universitet
Tel: +45 8715 5419
Email: ojk@phys.au.dk
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