A day after its successful flyby of Mercury, the MESSENGER spacecraft turned toward Earth on Tuesday and began downloading the 500 megabytes of data that had been stored on the solid-state recorder during the encounter. All of those data, including 1,213 images from the Mercury Dual Imaging System (MDIS) cameras, have now been received by the Science Operations Center at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. Preliminary analysis of these data by the MESSENGER Science Team has confirmed that all seven MESSENGER instruments are healthy and operated as planned during the flyby.
As MESSENGER flew by the planet, it missed its targeted aim point by only 8.25 kilometers (5.12 miles), affording the critical gravity assist needed to continue on a course to become - in 2011 - the first spacecraft ever to orbit Mercury. During this first encounter, the payload successfully conducted a carefully orchestrated sequence of observations designed to take full advantage of the geometry of the flyby trajectory and to optimize the science return from each instrument.
In addition to images of the previously unseen portion of the planet's surface, measurements were made that will contribute to the characterization of all aspects of Mercury and its environment, from its metallic core to the far reaches of its magnetosphere. "We have one excited Science Team," says MESSENGER Project Manager, Peter D. Bedini, of APL, "and their enthusiasm is contagious."
The analysis of these data is just beginning, but there are already indications that new discoveries are at hand.
Two New Images from MESSENGER Team Reveal Overview of Mercury and the Planet's Cratering History
Overview of Mercury as MESSENGER Approached
As MESSENGER neared Mercury on January 14, 2008, the spacecraft's Wide Angle Camera on the Mercury Dual Imaging System (MDIS) took images of the planet through each of its 11 filters. This image of the planet's full crescent was taken using the seventh filter, in light near the far-red end of the visible spectrum (750 nm). The image shows portions of Mercury previously seen by Mariner 10, but when Mariner 10 flew by the planet at each of its encounters the Sun was nearly overhead.
For this MESSENGER flyby, in contrast, the Sun is shining obliquely on regions near the day/night boundary (called the terminator) on the right-hand side of the crescent, revealing the surface topography in sharp relief. This image illustrates how MESSENGER, during its future flybys and subsequent orbital mission, will teach us much about the portion of Mercury already imaged by Mariner 10, and not just because of its superior camera and close proximity to the planet. The solar lighting geometry makes an enormous difference.
This picture provides a global context for the MDIS Narrow Angle Camera (NAC) images taken while MESSENGER was inbound. For example, the NAC image of the crater Vivaldi, released earlier this week, would fit as a small patch on the terminator just above the center of the crescent. The already released image that includes the crater Sholem Aleichem shows a part of Mercury near the top of the crescent. More NAC images of the incoming crescent will be released in the future.
This image was taken about 80 minutes before closest approach from a distance of about 27,000 kilometers (17,000 miles) and shows features as small as 10 kilometers (6 miles).
Mercury's Complex Cratering History
On January 14, 2008, MESSENGER observed about half of the hemisphere not seen by Mariner 10. These images, mosaicked together by the MESSENGER team, were taken by the Narrow Angle Camera (NAC), part of the Mercury Dual Imaging System (MDIS) instrument, about 20 minutes after MESSENGER's closest approach to Mercury (2:04 pm EST), when the spacecraft was at a distance of about 5,000 kilometers (about 3100 miles). The image shows features as small as 400 meters (0.25 miles) in size and is about 370 kilometers (230 miles) across.
The image shows part of a large, fresh crater with secondary crater chains located near Mercury's equator on the side of the planet newly imaged by MESSENGER. Large, flat-floored craters often have terraced rims from post-impact collapse of their newly formed walls. The hundreds of secondary impactors that are excavated from the planet's surface by the incoming object create long, linear crater chains radial to the main crater. These chains, in addition to the rest of the ejecta blanket, create the complicated, hilly terrain surrounding the primary crater. By counting craters on the ejecta blanket that have formed since the impact event, the age of the crater can be estimated. This count can then be compared with a similar count for the crater floor to determine whether any material has partially filled the crater since its formation. With their large size and production of abundant secondary craters, these flat-floored craters both illuminate and confound the study of the geological history of Mercury.
Additional information and features from MESSENGER's first flyby of Mercury will be available online at http://messenger.jhuapl.edu/mer_flyby1.html. Following the flyby, be sure to check for the latest released images and science results!
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.