Aerostats are objects that fly in the air using buoyancy, usually filled with gas lighter than air. Some common synonyms for aerostats include balloons, blimps, airships, dirigibles, and zeppelins. Balloons are typically smaller and filled with helium or hydrogen, whereas blimps and airships are larger and powered by engines. Dirigibles and zeppelins are both long, rigid airships that were popular in the early 20th century. While the use of aerostats has declined since the development of airplanes, they are still used in various applications such as advertising, surveillance, and scientific research.