.composition {
    position: relative;
    width: 600px;
    height: 400px;

    background-image: url("img/bg.jpg");    
}

.composition img {
    position: absolute;
}

.asteroid {
    top: 80px;
    left: 450px;
}

.planet-1 {
    top: 20px;
    left: 200px;
}

.planet-2 {
    z-index: 1;
    top: 80px;
    left: 90px;
}

.planet-3 {
    z-index: 1;
    top: 140px;
    left: 180px;
}

.spaceship {
    z-index: 0;
    top: 200px;
    left: 170px;
}

.ufo {
    z-index: 2;
    top: 100px;
    left: 450px;
}

.superman {
    z-index: 0;
    top: 70px;
    left: 100px;
}