What's the significance of February 11, 1999 for Pluto? 

      On February 11, Pluto will move farther from the Sun than Neptune, regaining its status as the most distant planet in the solar system. JPL astronomers calculate that it will take place at 2:08 am Pacific Time. Pluto will maintain its title of "most distant planet" for the next 228 years. Neptune has been the farthest planet for the past 20 years (since February 7, 1979). 

Why is Pluto sometimes the farthest planet from the Sun, and other times the second-farthest planet from the Sun? 

      Unlike the other planets in our solar system, Pluto has a highly elliptical orbit, completing its journey around the Sun every 248 years. Thus, Pluto's distance from the Sun varies. Most of the time, Pluto is the farthest planet from the Sun, but for a short time during its orbit, Pluto is closer to the Sun than Neptune. 

Any chance Pluto and Neptune will collide when their orbits cross on Feb. 11? 

      No chance at all. Pluto goes around the Sun twice for every three times Neptune orbits the Sun. Because of this fact, Pluto and Neptune's positions relative to each other repeat every 497 years. They will never be close to each other when Pluto is crossing the same distance from the Sun as Neptune is, and therefore, a collision can't happen. The high inclination of Pluto's orbit relative to the other planets also contributes to keeping them apart. 

Why is there controversy about Pluto? 

      In some ways, Pluto is different. It's much smaller than the four inner planets (Mercury, Venus, Earth and Mars) and it doesn't fit in with the four gas giant outer planets (Jupiter, Saturn, Uranus, and Neptune). Pluto's diameter is 1, 430 miles, making it less than half the size of any other planet, and only two-thirds as big as Earth's Moon. Pluto's orbit is much more tilted and elliptical than the other planets. Some scientists believe Pluto should not be called a planet at all. They feel it should be put in the same category as Kuiper Disk objects, icy worlds smaller than Pluto that lie in the "same neighborhood" as Pluto and Neptune, and even beyond. These objects may be leftover debris from our solar system's early formation. But Pluto is spherical and it does orbit the Sun. Although this controversy has come up on occasion, Pluto is still classified as a planet. 

What else do we know about Pluto? 

      Pluto was discovered on February 18, 1930 by Clyde Tombaugh, who studied photographic plates taken of the night sky through a Lowell Observatory telescope. Pluto's moon Charon was first found in 1978. Pluto is made from a mixture of rocky and icy material, and it appears to have seasonal changes, but we don't know much else about it. 

Does NASA plan further studies of Pluto? 

      Observations from ground and orbiting telescopes will continue to glean some information from 4.5 billion kilometers' distance (about 2.8 billion miles). Detailed study requires a close-up look. A mission called Pluto-Kuiper Express, managed by JPL, would fly past Pluto and its moon Charon, and study Kuiper Disk objects. The mission would launch in 2004, and would take about ten years to reach its destination. 

The largest canyon in the Solar System cuts a wide swath across the face of Mars. Named Valles Marineris, the grand valley extends over 3,000 kilometers long, spans as much as 600 kilometers across, and delves as much as 8 kilometers deep.

Credits: NASA

Asteroids: 10 Need-To-Know Things

Ligeia Mare, shown in here in data obtained by NASA's Cassini spacecraft, is the second largest known body of liquid on Saturn's moon Titan. It is filled with liquid hydrocarbons, such as ethane and methane, and is one of the many seas and lakes that bejewel Titan's north polar region. Cassini has yet to observe waves on Ligeia Mare and will look again during its next encounter on May 23, 2013. Credits: NASA/JPL-Caltech/ASI/Cornell

Saturn's moon Titan may be nearly a billion miles away from Earth, but a recently published paper based on data from NASA's Cassini spacecraft reveals a new way this distant world and our own are eerily similar. Just as the surface of oceans on Earth lies at an average elevation that we call "sea level," Titan's seas also lie at an average elevation.

This is the latest finding that shows remarkable similarities between Earth and Titan, the only other world we know of in our solar system that has stable liquid on its surface. The twist at Titan is that its lakes and seas are filled with hydrocarbons rather than liquid water, and water ice overlain by a layer of solid organic material serves as the bedrock surrounding these lakes and seas.

The new paper, led by Alex Hayes at Cornell University in Ithaca, New York, and published in the journal Geophysical Research Letters, finds that Titan's seas follow a constant elevation relative to Titan's gravitational pull -- just like Earth's oceans. Smaller lakes on Titan, it turns out, appear at elevations several hundred feet, or meters, higher than Titan's sea level. Lakes at high elevation are commonly found on Earth. The highest lake navigable by large ships, Lake Titicaca, is over 12,000 feet [3,700 meters] above sea level.

The new study suggests that elevation is important because Titan's liquid bodies appear to be connected under the surface in something akin to an aquifer system at Earth. Hydrocarbons appear to be flowing underneath Titan's surface similar to the way water flows through underground porous rock or gravel on Earth, so that nearby lakes communicate with each other and share a common liquid level.

The paper was based on data obtained by Cassini's radar instrument until just months before the spacecraft burned up in the Saturn atmosphere last year. It also used a new topographical map published in the same issue of Geophysical Research Letters.

If all of the asteroids were combined into a ball, they would still be much smaller than the Earth's moon. If the sun was as tall as a typical front door, Earth would be the size of a nickel, the moon would be about as big as a green pea and Ceres (the largest asteroid) would be as small as a sesame seed.

Asteroids orbit our sun, a star, in a region of space between the orbits of Mars and Jupiter known as the Asteroid Belt.

One day on asteroid Ida, for example, takes only 4.6 hours (the time it takes for this asteroid to rotate or spin once). Ida makes a complete orbit around the sun (a year in this asteroid's time) in 4.8 Earth years.

Asteroids are solid, rocky and irregular bodies.

Asteroids do not have atmospheres.

More than 150 asteroids are known to have a small companion moon (some have two moons). The first discovery of an asteroid-moon system was of asteroid Ida and its moon Dactyl in 1993.

Asteroids can have rings. The first asteroid rings were spotted in 2014 around asteroid Chariklo.

NASA space missions have flown by and observed asteroids, and one spacecraft (NEAR Shoemaker) even landed on an asteroid (Eros). The Dawn mission is the first mission to orbit (2011) a main belt asteroid (Vesta).

Asteroids cannot support life as we know it.

Fact: Ceres, the first and largest asteroid to be discovered (1801 by Giuseppe Piazzi), encompasses over one-third of the estimated total mass of all the asteroids in the asteroid belt.       NASA