Glossary

Another interesting object in the solar system is the comet. Comets are made up of dust and ice. They also are of irregular shape and their maximum size is around 30 km. They come from the outermost part of the solar system, go around the Sun, and go back. Some of them return, some of them don’t! Some of them are destroyed on the way. As they approach the Sun, they start melting releasing gases. This produces a visible atmosphere or coma, and sometimes also a tail. The tail always points away from the Sun. We know more than 500 comets so far.

A scientist called Halley had discovered a comet which returns after every 76 years. He used the notes of earlier scientists, did some calculations and predicted that the comet will return in 1758. Unfortunately Halley died in 1742 before he could observe this himself! Find out when this comet will be seen again. Indian scientist Vainu Bappu discovered a comet when he was a Ph.D. student at Harvard University. The comet was named 'Bappu-Bok-Newkirk' comet (Bok and Newkirk are the two colleagues who worked with Vainu Bappu).

Amateur astronomers (people who are interested in astronomy but do not earn by working in astronomy) observe the sky and keep notes. Several comets are discovered by amateur astronomers. They can also access the data from different observatories, for example, the Solar and Heliospheric Observatory (SOHO), analyze it and discover a planet. An Indian student, Prafull Sharma, discovered a comet using SOHO data when he was in Grade XII.
 

Figure 5: Comets

Figure 5a: Halley’s Comet captured on 8 March 1986
(Credit: By NASA/W. Liller - NSSDC's Photo Gallery (NASA):http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-comets.htmlhttp://nssdc.gsfc.nasa.gov/image/planetary/comet/lspn_comet_halley1.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=544352)
 

Figure 5b: Nucleus of 103P/Hartley captured through a spacecraft. The nucleus is about 2 km in length.
(Credit: By NASA/JPL-Caltech/UMD - http://www.nasa.gov/mission_pages/epoxi/images/version1/IINMVUAXF_6000002_001_001_crop.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=11964349)

Apart from these objects, the Solar System is mostly a vacuum.

What is outside the Solar System? There are other stars like our Sun. Some of them have planets revolving around them. Planets revolving around other stars are known as ‘exoplanets’. We will learn more about the other stars and and the Universe in Lesson 4.