Venus is a terrestrial planet with a diameter of about 12,104 kilometers, making it almost the same size as Earth. It has a thick atmosphere made mostly of carbon dioxide, with clouds of sulfuric acid that reflect sunlight, giving Venus its bright appearance. The extreme heat on Venus is due to a runaway greenhouse effect caused by the thick atmosphere that traps heat, leading to surface temperatures hot enough to melt lead. Venus rotates very slowly, so its day is longer than its year, and its rotation is retrograde, meaning it spins in the opposite direction of most planets. Despite its close proximity to Earth, Venus has a vastly different environment and is not suitable for human habitation.