Unfortunately I don't have any scales to compare it with the space ones I have here, but commonly such medals this size are made from bronze. Brass is also used for medals although its not as common these days (I may be wrong as I have only recently been getting in to these things and definitely aren't an expert). Unfortunately there is no easy way to determine what metals. Only really by look, weight and sound (pitch if pinged)... or if it has corroded
Although my guess from the pictures would be bronze, both Bronze and Brass have Copper as 90% of their base metal with either tin or zinc being added for the remainder. Any variation to elements added to the metal, age, exposure to the elements etc means that both these metals can look very similar.