MESSENGER To Capture Earth, Moon In Series Of Images This Weekend
July 19, 2013

MESSENGER To Capture Earth, Moon In Series Of Images This Weekend

Lawrence LeBlond for - Your Universe Online

We previously reported the NASA/ESA/ASI Cassini spacecraft was preparing to take an amazing picture of Earth as Saturn encountered a solar eclipse. This event is set to occur late this afternoon between 5:27 and 5:42 pm.

The Earth will pose for Cassini, which is more than 900 million miles away, and NASA is encouraging all Americans to go out and photobomb Cassini, and then share their pictures via the Internet. The Cassini images of Earth will be processed over the following days, yet the processing of the full Saturn system mosaic is expected to take weeks.

With some inspiration by the Cassini team's plans to image the Earth on July 19, scientists working with NASA's MESSENGER spacecraft orbiting Mercury are preparing to catch the Earth in a series of images taken of potential natural satellites around the Sun's closest planet.

MESSENGER will be snapping images on both Friday, July 19 and Saturday, July 20. The images will occur at the same times on both days: at 7:49 am, 8:38 am and 9:41 am EDT.

While Cassini's earthen images will be facing mainly North America, MESSENGER's photos will capture Europe, the Middle East and Central Asia, as well as the Americas, according to Hari Nair, the Johns Hopkins University Applied Physics Laboratory (JHUAPL) planetary scientist who designed and is implementing the MESSENGER imaging campaign.

On Saturday, the images will also include pictures of the Moon, where all six Apollo lunar landing sites will be illuminated. The Saturday images also come on the 44th anniversary of the Apollo 11 lunar landing.

"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 explained. "In practice, all we're going to see are two bright dots."

He noted the images of Earth and the Moon are only coincidental, as the mission is meant to search for natural satellites around Mercury. Of all the planets in the Solar System, Mercury and Venus are the only two with no known moons.

While the formation of moons is not entirely understood, several theories abound. Some theorize moons around giant planets formed from disks of gas and dust in the early Solar System. Others hypothesize some moons, such as Earth's, formed from material ejected during collisions between the planet and a smaller body. And another scenario is that some moons are originally asteroids that pass too close to a host planet and are 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, co-leader of MESSENGER's investigation of small bodies within Mercury's orbit.

"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."

When NASA's Mariner 10 spacecraft conducted its first fly-by of Mercury in 1974, its instruments picked up an anomalous ultraviolet signature bouncing off Mercury's surface. At first, scientists believed this was sign of a natural satellite. However, the source was later confirmed to be emitted from a bright star. Mariner 10 performed a more dedicated sweep at a later time and discovered no objects larger than 3 miles wide.

MESSENGER, which has been orbiting Mercury since March 2011, conducted its first search for satellites this past February; those images are still being studied.

This weekend's 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 said. "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."

Clark Chapman, also co-leader on the MESSENGER investigation of small bodies, said the team has optimized the upcoming search pattern to take images at varying times to spot faster-moving objects. "The camera can potentially see objects as small as [328 feet in size], about the length of an American football field."

As with Cassini's images, MESSENGER's will take a few days to analyze and the team expects to release them sometime next week.