NASA's Mercury-orbiting MESSENGER spacecraft will capture images of Earth on July 19 and 20. The images will be taken at 7:49 a.m., 8:38 a.m. and 9:41 a.m. EDT on both days.

Nearly half of the Earth, including all the Americas, Africa, and Europe, will be illuminated and facing MESSENGER, according to Hari Nair, the Johns Hopkins University Applied Physics Laboratory planetary scientist who designed and is implementing the campaign. The images on the second day will also include pictures of the Moon, where all six of the Apollo landing sites will be illuminated, 44 years to the day after Apollo 11 landed on the Moon's rocky surface.

"It's important to note that the Earth and Moon are going to be less than a pixel in size, and so no details will be seen," Nair cautioned. "In practice, all we're going to see are two bright dots."

These images of Earth and the Moon are coincidental, taken as part of a search for natural satellites around Mercury. Of all the planets, only Venus and Mercury have no known moons. Earth has one, and Mars has two. Jupiter and Saturn have dozens each; and even tiny Pluto is now known to have five moons.

Mercury's Moon Mystery
Scientists do not entirely understand how moons are formed, but there are several theories. Some of the moons around the giant planets probably formed from the disks of gas and dust that encircled those planets in the early solar system.

Others, like Earth's Moon and Pluto's largest moon, Charon, were likely assembled from material ejected during collisions between the planet and a slightly smaller body. In still other cases, moons are thought to have originally been asteroids that passed sufficiently close to their host planet to be gravitationally captured.

"We don't know why Mercury does not have a moon," said William Merline, of the Southwest Research Institute (SwRI) in Boulder, Colorado, who, along with SwRI's Clark Chapman, leads MESSENGER's investigation of small bodies orbiting Mercury and in the inner solar system.

"It may have been just unfortunate in not having the right history, in terms of collisions," Merline continued. "Or it may at one time have had a moon in an orbital trajectory that was disrupted by the strong gravitational pull of the Sun, in combination with Mercury's highly eccentric (oblong) orbit around the Sun.

"Such an orbit makes the effect of the Sun's gravity highly variable with time, and may degrade the conditions for stability of a moon's orbit. But these possibilities are only speculations, based on theoretical ideas. To complete the picture, we must search for the existence of satellites to validate any of these suggestions."

During Mariner 10's first flyby of Mercury in 1974, its instruments detected an anomalous ultraviolet signal off the edge of Mercury's surface, leading a scientist to conclude that the signal was the result of a satellite. But the source of the signal was soon found to be a bright star with a high ultraviolet emission. Later, Mariner 10 performed a dedicated search for satellites and found none larger than 5 kilometers in diameter.

In February, MESSENGER, which began orbiting Mercury in March 2011, conducted its first search for satellites, and those data are still being studied. This search takes place during Mercury's aphelion, the planet's farthest point from the Sun. "This location has the advantage that the camera will be as cold as it ever gets," Nair explained.

"Since we are looking for very faint objects, having a warm camera introduces thermal noise. The downside of being farther from the Sun is that any satellites will be dimmer at this time, as the Sun is their light source. So it was a tradeoff between brighter targets or a quieter detector, and we opted to go for a time when the detector would be quieter."

The team has also optimized the search pattern, taking many images at varying time intervals to spot faster-moving objects, according to Chapman. "The camera can potentially see objects as small as 100 meters, or 328 feet, in size, about the length of an American football field," he said.

MESSENGER's images of Earth and the Moon will be released next week. NASA's Cassini spacecraft, in orbit around Saturn, will also be acquiring images of Earth on July 19, between 5:27 and 5:42 p.m. EDT. The Cassini Earth portrait is part of a more extensive mosaic - or multi-image picture - of the Saturn system as it is backlit by the Sun.

"That images of our planet can be acquired on a single day from two distant outposts in the solar system provides a wonderful reminder of the vigor and excitement of this nation's ongoing program of planetary exploration," added MESSENGER Principal Investigator Sean Solomon, of Columbia University's Lamont-Doherty Earth Observatory.

"The Saturn system and the innermost planet are two very different outcomes of planetary formation and evolution, so these two sets of images also prompt a sustained appreciation of the special attributes of Earth. There's no place like home."