First, Slider is right, there is no letter 'u' in classical latin, our letter 'v' generally replaces it.
Secondly, the various meanings of 'ardua' is indicative of ancient languages in general. The longer a language has been in the mortuary, the more guesswork and deduction go into translating it, and the more it is altered over the years by translators / scribers in a centuries-long game of Chinese whispers. Specific meanings of words are lost, and one word can come to have many nebulous translations. Thus the 'per' in that motto translates as 'through', but in the Royal Marine motto can translate as 'by' (Per Mare, Per Terram = By Sea, By Land), and in other grammatical cases is used as 'over', 'because of', 'along' and so on. In the case of this motto, 'ardua' was pretty obviously used for poetic reasons because it fits with 'astra'.
The actual word 'ardua', or 'arduus' to use its' base form, I am guessing, is a late or medieval Latin word, ie used by early Christian monks and scribes throughout Europe, and thus is closer to modern English, a cousin twice removed via medieval French and Middle English. This might explain why it doesn't occur in many Latin dictionaries (they usually only list classical Latin), why it is close to our modern 'arduous' as gr4pilot said, and also most traditional mottos in this country are formed from late Latin, as they tended to have been coined in the last five centuries or so (classical Latin was only seriously ressurected in the 20th century). If this is correct, like I said only a guess, then the 'ardva' spelling would actually be wrong, and it should properly be 'ardua' ...but that's a bit pedantic.
All this I know because I took Medieval Latin as part of my English degree in order to sleep with a most attractive girl on the course! Worked, too
PS Quidquid latine dictum sit altum videtur.
(Whatever is said in Latin sounds profound)