function checkPlanetNav()
{
    var element     = document.getElementById('planetselect');
    var button_prev = document.getElementById('prevplanet');
    var button_next = document.getElementById('nextplanet');

    if(element.selectedIndex == element.options.length-1)
        button_next.disabled = true;

    if(element.selectedIndex == 0)
        button_prev.disabled = true;
}


function gotoNextPlanet(urlPrefix, urlSuffix)
{
    var element     = document.getElementById('planetselect');
    var button_prev = document.getElementById('prevplanet');
    var button_next = document.getElementById('nextplanet');

    button_prev.disabled = false;
    element.selectedIndex++;

    if(element.selectedIndex == element.options.length-1)
        button_next.disabled = true;

    document.location.href=urlPrefix+'&__setPlanet='+element.options[element.selectedIndex].value+urlSuffix;
}


function gotoPreviousPlanet(urlPrefix, urlSuffix)
{
    var element     = document.getElementById('planetselect');
    var button_prev = document.getElementById('prevplanet');
    var button_next = document.getElementById('nextplanet');

    button_next.disabled = false;
    element.selectedIndex--;

    if(element.selectedIndex == 0)
        button_prev.disabled = true;

    document.location.href=urlPrefix+'&__setPlanet='+element.options[element.selectedIndex].value+urlSuffix;
}