Dust Tail Comet Siding Spring's dust tail will completely engulf the entire planet when Mars travels through it. Hubble images show Comet Siding Spring has passed the snow and water lines, the points at which the Sun's warmth activates or releases gases and water ice to form the coma and tail. Gas and dust in the comet's nucleus and coma often separate into two parts of the comet's tail. A comet's dust tail is the trail of dust and gas illuminated by the Sun. It is blown away from the comet's coma by the solar wind, and follows the curve of the comet's orbit. When comets are traveling through the outer solar system, they are frozen and do not have tails. Far away and extremely small, they are almost impossible to detect. As they approach the Sun in the inner solar system, radiation from the Sun turns some comet materials like water ice into a gas. As gases leave the nucleus, they carry comet dust with them. When sunlight illuminates the coma and tail as the comet approaches the Sun, astronomers have a better chance of detecting it.

Nucleus Though early estimates suggested the size of comet Siding Spring's nucleus could be anywhere from 0.62 to 31 miles (1 to 50 kilometers) across, new data from NASA's Swift Satellite indicates that the icy nucleus of comet Siding Spring is only about 2,300 feet (700 meters) across. Hubble images of Comet Siding Spring show two jets coming from the icy nucleus in opposite directions. Measuring them can help us understand how the comet is rotating and what it might look like when it flies by Mars. The nucleus is the solid, frozen core of a comet. It is made of rock, dust, water ice, and frozen gases (e.g., carbon dioxide, carbon monoxide, methane, and ammonia). The nucleus of a comet is usually small in size, usually about a few miles across. The surface of a comet's nucleus is often dry, dusty, rocky, and dark. Dark materials may contain organic compounds, the chemical building blocks of life. When a comet absorbs heat from the Sun, the nucleus releases water ice and frozen gases. Sometimes, dazzling jets of gas can erupt from inside the nucleus when the Sun heats some parts of its surface more than others. That can cause the nucleus to spin or even break up into smaller pieces. Such events can change a comet's trajectory and, ultimately, its fate.

Coma Comet Siding Spring has crossed the snow and water lines, key points when it is close enough for the Sun's warmth to activate it. Now that scientists can see the coma, they estimate it is roughly 12,000 miles (19,000 kilometers) across. A comet's coma is the atmosphere of gas and dust around the nucleus of the comet. It can be hundreds of thousands of kilometers wide. The coma of a comet is made largely of water and dust. When a comet approaches the Sun, it warms up. Heat from the Sun changes the comet's icy materials to gases. The comet releases these gases, forming the coma. Radiation from the Sun and the solar wind then push a lot of this material away from the nucleus, forming the comet's tail. When a comet is about the same distance from the Sun as Mars (about 1.5 AU), its coma can shrink, even though it is producing more gas as it warms. That's because the solar wind becomes forceful enough to push more coma material into the tail, making the tail a lot bigger. That can be a big deal for planets like Mars, which is crossing through the debris field of Comet Siding Spring's coma and tail.