nasa_desktop.sh

#!/bin/bash

{
    date_slug=$(date "+%y%m%d")
    image_filename="$PWD/images/$date_slug.jpg"
    cache_page_filename="$PWD/cache/ap$date_slug.html"

    function download_page() {
        if [ ! -f $cache_page_filename ]; then
            curl -# -L http://apod.nasa.gov/apod/ap$date_slug.html \
                > $cache_page_filename
        fi
    }

    function get_url() {
        cat $cache_page_filename |
            grep -oE "href=[^>]*" | \
            grep -oE "[^'\"]*.jpg"
    }

    function download_image() {
        if [ ! -f $image_filename ]; then
            image_url=$(get_url)
            echo "$image_url\n"
            curl -# $image_url > $image_filename
        fi
    }

    function set_desktop() {
        sqlite3 ~/Library/Application\ Support/Dock/desktoppicture.db \
            "update data set value = '$image_filename'" \
            && killall Dock
    }

    download_page
    download_image
    set_desktop
}

Work in progress.

More accurately: "Astronomy Picture of the Day" desktop and probably only for some specific flavor of OS X.