diff --git a/astronaut b/astronaut
index ea4c5ab..4ad460f 100755
--- a/astronaut
+++ b/astronaut
@@ -13,7 +13,7 @@ Just describe what he's got to do in a satellite file.
 Usage: $0 [-s <dir>] [-b <dir>] [-i <dir>] <satellite>
 
 -s  Set source directory
--b  Set build directory
+-b  Set build directory [WARNING: Will be deleted before build]
 -i  Set install directory"
 }
 
@@ -51,12 +51,16 @@ msg() {
     echo "=> $@"
 }
 
+mksum() {
+    echo $(md5sum "$@" 2> /dev/null | cut -d' ' -f1)
+}
+
 download() {
     local name=$(basename "$1")
     local path="$dir_source/$name"
     local checksum=""
     if [ "$2" -a -f "$path" ]; then
-        checksum="$(md5sum ""$path"" 2> /dev/null | cut -d' ' -f1)"
+        checksum=$(mksum "$path")
     fi
     if [ ! -f "$path" -o "$checksum" != "$2" ]; then
         msg "Downloading $name"
@@ -66,6 +70,12 @@ download() {
         else
             curl -#L -o "$path" "$1"
         fi
+        checksum=$(mksum "$path")
+        if [ "$2" -a "$checksum" != "$2" ]; then
+            msg "Houston, we have a problem: Checksum failed."
+            msg "Checksum: $checksum"
+            exit 1
+        fi
     fi
 }
 
@@ -89,6 +99,7 @@ extrafile() {
 }
 
 # Create the satellite
+rm -rf "$dir_build"
 mkdir -p "$dir_source"
 mkdir -p "$dir_build"
 mkdir -p "$dir_install"