diff --git a/downloadImage.go b/downloadImage.go
index d5c6f86..80fb8aa 100644
--- a/downloadImage.go
+++ b/downloadImage.go
@@ -6,8 +6,14 @@ import (
 	"os"
 )
 
+type ImageDownloader interface {
+	DownloadImage(url, dir, filename string) error
+}
+
+type RealImageDownloader struct{}
+
 // DownloadImage downloads an image from a URL and saves it to a file in a subdirectory
-func DownloadImage(url string, subdirectory string, filename string) error {
+func (d RealImageDownloader) DownloadImage(url string, subdirectory string, filename string) error {
 	// Create the subdirectory if it doesn't exist
 	if _, err := os.Stat(subdirectory); os.IsNotExist(err) {
 		os.Mkdir(subdirectory, 0755)
diff --git a/fetchAPOD.go b/fetchAPOD.go
index 116ac18..187bfdb 100644
--- a/fetchAPOD.go
+++ b/fetchAPOD.go
@@ -2,15 +2,20 @@ package main
 
 import (
 	"encoding/json"
+	"fmt"
 	"io"
 	"net/http"
 	"os"
 )
 
 // FetchAPOD fetches the Astronomy Picture of the Day (APOD) from the NASA API
-func FetchAPOD() (Response, error) {
+func FetchAPOD(apiURL string) (Response, error) {
 	apiKey := os.Getenv("NASA_KEY")
-	url := "https://api.nasa.gov/planetary/apod?api_key=" + apiKey
+	if apiKey == "" {
+		return Response{}, fmt.Errorf("NASA_KEY environment variable not set")
+	}
+
+	url := apiURL + "?api_key=" + apiKey
 
 	resp, err := http.Get(url)
 	if err != nil {
diff --git a/fetchAPOD_test.go b/fetchAPOD_test.go
new file mode 100644
index 0000000..3699876
--- /dev/null
+++ b/fetchAPOD_test.go
@@ -0,0 +1,40 @@
+package main
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strings"
+	"testing"
+)
+
+func MockResponse(statusCode int, body string) *http.Response {
+	return &http.Response{
+		StatusCode: statusCode,
+		Body:       io.NopCloser(strings.NewReader(body)),
+	}
+}
+
+func TestFetchAPOD(t *testing.T) {
+	os.Unsetenv("NASA_KEY")
+	_, err := FetchAPOD("https://api.nasa.gov/planetary/apod")
+	if err == nil || err.Error() != "NASA_KEY environment variable not set" {
+		t.Errorf("expected error 'NASA_KEY environment variable not set', got %v", err)
+	}
+
+	os.Setenv("NASA_KEY", "DEMO_KEY")
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"date":"2023-10-01","explanation":"Test explanation","media_type":"image","service_version":"v1","title":"Test Title","url":"https://example.com/test.jpg"}`))
+	}))
+	defer server.Close()
+
+	resp, err := FetchAPOD(server.URL)
+	if err != nil {
+		t.Errorf("expected no error, got %v", err)
+	}
+	if resp.Title != "Test Title" {
+		t.Errorf("expected title 'Test Title', got %v", resp.Title)
+	}
+}
diff --git a/main.go b/main.go
index 189d254..ec9254d 100644
--- a/main.go
+++ b/main.go
@@ -17,10 +17,16 @@ type Response struct {
 }
 
 func main() {
+	downloader := RealImageDownloader{}
+	if err := run(downloader); err != nil {
+		panic(err)
+	}
+}
 
-	result, err := FetchAPOD()
+func run(downloader ImageDownloader) error {
+	result, err := FetchAPOD("https://api.nasa.gov/planetary/apod")
 	if err != nil {
-		panic(err)
+		return err
 	}
 
 	fmt.Println("Downloading image: " + result.Title)
@@ -28,6 +34,5 @@ func main() {
 	filename := result.Title + ".jpg"
 	noSpaceFilename := strings.ReplaceAll(filename, " ", "_")
 
-	DownloadImage(result.Hdurl, "images", noSpaceFilename)
-
+	return downloader.DownloadImage(result.Hdurl, "images", noSpaceFilename)
 }
diff --git a/main_test.go b/main_test.go
new file mode 100644
index 0000000..a9d07b9
--- /dev/null
+++ b/main_test.go
@@ -0,0 +1,36 @@
+package main
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+)
+
+type MockImageDownloader struct{}
+
+func (d MockImageDownloader) DownloadImage(url, dir, filename string) error {
+	return nil
+}
+
+func TestRun(t *testing.T) {
+	downloader := MockImageDownloader{}
+
+	os.Unsetenv("NASA_KEY")
+	err := run(downloader)
+	if err == nil || err.Error() != "NASA_KEY environment variable not set" {
+		t.Errorf("expected error 'NASA_KEY environment variable not set', got %v", err)
+	}
+
+	os.Setenv("NASA_KEY", "DEMO_KEY")
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"date":"2023-10-01","explanation":"Test explanation","media_type":"image","service_version":"v1","title":"Test Title","url":"https://example.com/test.jpg"}`))
+	}))
+	defer server.Close()
+
+	err = run(downloader)
+	if err != nil {
+		t.Errorf("expected no error, got %v", err)
+	}
+}