Ceres, the asteroid belt’s only dwarf planet, is about 950 km in diameter, whereas 4 Vesta, 2 Pallas, and 10 Hygiea have mean diameters of less than 600 km. The remaining bodies range down to the size of a dust particle. The asteroid material is so thinly distributed that numerous unmanned spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids do occur, and these can produce an asteroid family whose members have similar orbital characteristics and compositions. Individual asteroids within the asteroid belt are categorized by their spectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type).
Jupiter, along with its beautiful ringed sister planet, Saturn, are the gas-giant duo of our Sun's family of eight major planets. The other two giant planets--that dwell in our Solar System's outer limits--are Uranus and Neptune. Uranus and Neptune are classified as ice giants, because they carry within them larger cores than Jupiter and Saturn, as well as thinner gaseous envelopes. Jupiter and Saturn may (or may not) contain small, hidden cores, that are heavily veiled by extremely massive, dense gaseous envelopes. HST's detection of a site, which appears to show persistent, intermittent plume activity on Europa, provides a promising target for the Europa mission to investigate. Equipped with its new and sophisticated suite of science instruments, the mission can detect whatever may potentially be swimming around in the hidden global ocean sloshing around beneath its secretive crust of ice. When Jupiter was born along with the rest of our Solar System, approximately 4.56 billion years ago, it twinkled like a star. The energy that it emitted--as a result of tumbling surrounding material--made Jupiter's interior searing-hot. In fact, the larger Jupiter grew, the hotter it became. At long last, when the material that it had drawn in from the whirling, swirling surrounding protoplanetary accretion disk--made up of nurturing dust and gas--was depleted, Jupiter may well have attained the enormous diameter of over 10 times what it has today. It also may have reached a truly toasty central temperature of about 50,000 Kelvin. During that long ago era, Jupiter twinkled, glittered, and sparkled like a little star, shining ferociously with a fire that was approximately 1% that of our much more brilliant Sun today.