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太陽系形成初期，星際塵埃集結成如石塊般大小的「星子」，星子在吸積過程中黏合形成大行星。部分星子因欠缺有利的吸積環境，最終只能結聚成太陽系中諸如彗星和小行星的小天體，但這並不代表它們不重要。由於這些小天體從太陽系形成至今無經歷重大的化學和地質變化，仍保留太陽系誕生初期的物質狀態，為科學家提供研究太陽系的線索。
05年，深度撞擊號曾成功撞擊坦普爾一號彗星，研究彗星所含物質。07年，黎明號開展往灶神星與穀神星的旅程，希望藉此探索能為了解太陽系的起源開啟另一扇門。黎明號將於2011年首先到達灶神星，當探測器完成為期6個月的探察後，將會繼續旅程，並於2015年抵達穀神星。
Mission to the Minor Planets
At the dawn of our solar system, dust concentrated to form rocky planetesimals. The planetesimals in turn coalesced to become the major planets. The remaining crumbs formed the small bodies such as the comets and minor planets. As there had not been any geological processes going on on these small bodies, they practically remained unchanged since creation.
As early as 2005, the Deep Impact probe was directed to collide with Comet Tempel to study its composition. In 2007 Dawn was launched to explore two of the largest minor planets. The probe is due to arrive at Vesta in 2011 to begin a 6-month investigation. It will then re-embark to its next target and due to arrive at Ceres in 2015. ■甄枝強 資深天文愛好者 圖片鳴謝：NASA/McREL/UCLA
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