Astro Glossary

Dark Energy

A term that describes the accelerating expansion of the universe. The expansion may be caused by a “repulsive” force that makes the universe expand faster as it ages, a discrepancy in the laws of gravity, or some other phenomenon.

Dark Matter

Matter that produces no detectable energy, but that reveals its presence through its gravitational pull on the visible matter around it. Dark matter makes up more than 90 percent of all the matter in the universe. There are many possible forms for dark matter, but most current theories suggest a subatomic particle created in the Big Bang.

Dark Matter and Dark Energy

DART mission

DART (Double Asteroid Redirection Test) is a spacecraft designed to test a possible strategy for protecting Earth against an asteroid on a collision course. Launched November 23, 2021, it will encounter a double asteroid on September 26, 2022. The larger member of the system, Didymos, is a half-mile in diameter. Its companion, Dimorphos, is only about a tenth of a mile across. DART will hit Dimorphos head on, at a speed of about 15,000 miles (25,000 km) per hour. The impact should change the smaller space rock’s orbit. Astronomers will use several telescopes to track the impact and its aftermath to see how much the orbit changes. The goal is to help determine if we could deflect an asteroid on a collision course with Earth. One way to do so would be to ram objects into it at high speeds. DART is the first test to see how well that might work. It is expected to shorten the length of Dimorphos’s orbit by several minutes.

Dawn Mission

Dawn is a NASA Discovery mission that targeting Vesta and Ceres, the two most massive asteroids. It entered orbit around Vesta in July 2011, then departed a year later. It will arrive at Ceres in February 2015, making it the first probe ever to orbit two astronomical bodies other than Earth.

Death of Stars

Deep Impact Mission

Deep Impact fired a probe at Comet Tempel 1, gouging a fresh crater and giving astronomers a look at material beneath the surface, which probably had remained unchanged since the comet’s birth.

Deep Space Missions

Missions that journey beyond the asteroid belt, into the realm of the outer solar system.

Delphinus, the Dolphin

A small summer constellation whose brightest stars form the outline of a dolphin.

Deneb

A blue-white supergiant star 1,600 light-years away in the constellation Cygnus. It is the constellation’s brightest star and the most-distant bright star visible to the unaided eye. It forms one point of the large Summer Triangle.