Because small worlds are scattered all over the solar system, we use different approaches to explore them.
Asteroids—in particular, near-Earth asteroids—are relatively easy to reach with a spacecraft, but their low
gravities make orbiting them a challenge. Many asteroids are close enough for us to image with Earth-based
radar, which helps us figure out their shapes.
Comets fly through the inner solar system at a variety of angles. Halley’s comet, for instance, enters the inner
solar system clockwise, from beneath our orbital plane. This means a spacecraft trying to rendezvous with the
comet must cancel out Earth's counter-clockwise momentum, pick up speed in the opposite direction, and tilt the
plane of its orbit significantly. This requires significant amounts of fuel, which is why solar sail missions
have been proposed for reaching certain comets.
The Oort cloud is too distant to visit for now, but the Kuiper Belt is within reach—providing you have some
extra time on your hands. New Horizons was the fastest object to ever leave Earth at the time of its launch, and
still took 9 years to reach Pluto.
Exploring asteroids
NASA’s Jupiter-bound Galileo spacecraft made the first asteroid flyby in 1991, coming within 1600 kilometers of
Gaspra in the main asteroid belt. The pictures revealed an irregularly shaped world formed by debris from a
collision between other objects. Two years later in 1993, Galileo gave us a close look at another main belt
asteroid, Ida, which turned out to have a tiny moon now named Dactyl.
NASA’s NEAR Shoemaker spacecraft entered orbit around near-Earth asteroid Eros in 2000 and later touched down on
its surface—both firsts for any spacecraft. The agency in 2007 launched the Dawn spacecraft to orbit Vesta and
then Ceres, the two largest worlds in the main asteroid belt. Under the IAU’s planet classification system,
Vesta is an asteroid and Ceres is a dwarf planet.
Dawn found water-bearing minerals on Vesta’s surface and abundant water ice and organics on Ceres. Because these
materials evaporate close to the Sun, Vesta and Ceres either formed farther away or were struck by objects
carrying water and organics that formed farther away. Ceres has hundreds of bright spots formed by briny (salty)
water bubbling up from a subsurface reservoir. While icy moons like Enceladus and Europa have subsurface oceans
that are warmed by the pull of nearby giant planets, Ceres appears to have enough internal heat to keep its
subsurface water from freezing.
In 2010, Japan’s Hayabusa spacecraft returned samples from Itokawa, a peanut-shaped near-Earth asteroid a
half-kilometer long. Water found in the samples was remarkably similar to that of Earth’s oceans, indicating
much of our water could have come from similar asteroids.
Sample return missions from two more near Earth asteroids are in progress. Hayabusa2 is on its way back to Earth
now with samples from the diamond-shaped, rubble-pile asteroid Ryugu, while NASA’s OSIRIS-REx spacecraft is
preparing to collect samples from a similar asteroid Bennu.
