Astronomy Picture of the Day 12-23-03

Explanation: It's back. Every 3.3 years, Comet Encke swoops back into our inner Solar System. First officially discovered in 1786, Comet Encke is on its 59 th documented return, making it one of the best-studied comets on the sky. Mysteriously, Comet Encke should have been discovered millennia earlier, since it likely became bright enough to see unaided many times over the past few thousand years. Comet Encke's elliptical trajectory reaches from outside the orbit of Mars to inside the orbit of Mercury. It passed relatively close to the Earth on Nov. 17 and will reach its closest to the Sun on Dec 29. Recent observations place Comet Encke as bright as visual magnitude six during early December, making it just on the verge of unaided human vision. Pictured above, the diffuse smudge of periodic Comet Encke was imaged through a small telescope on November 29 from Arkansas, USA.

2 more comets to watch for in the next few months are C/2002 T7 (LINEAR) and C/2001 q4 (NEAT).

First Mercury orbiter shipped to Goddard for tests
JHU APL NEWS RELEASE
Posted: December 21, 2003

Less than six months from its scheduled launch to Mercury, NASA's MESSENGER spacecraft is set for the next round of tests to prepare it for the first orbital study of the innermost planet.

An artist's concept of MESSENGER. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

MESSENGER was shipped Friday from the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. - where it was designed and built - to the environmental testing facilities at NASA's Goddard Space Flight Center in Greenbelt, Md. The 20-mile delivery capped nearly four years of detailed design, assembly and testing on one of the most complex spacecraft APL has ever built. With features ranging from a lightweight composite structure and miniaturized instruments to a heat-radiation system and protective ceramic-fabric sunshade, MESSENGER (short for MErcury Surface, Space ENvironment, Geochemistry, and Ranging) is well equipped for a 5-year cruise through the inner solar system and a yearlong study of Mercury starting in July 2009.

"We're sending a spacecraft to orbit a planet where the sun is 11 times brighter than what we see on Earth and temperatures can climb past 800 degrees Fahrenheit," says MESSENGER Project Manager David G. Grant, of APL. "This is an incredible engineering and scientific challenge that no one has ever tried before, and the team is doing all it can on the ground to make sure MESSENGER succeeds at Mercury."

Last week engineers finished the first of MESSENGER's "shake and bake" tests, checking the spacecraft's structural strength atop large vibration tables at APL. Over the next 10 weeks at Goddard the team will check MESSENGER's balance and alignment; put it before speakers that simulate the noise-induced vibrations of launch; and seal it in a large thermal-vacuum chamber that duplicates the extreme heat, cold and airless conditions of space. In March, MESSENGER will be sent to Kennedy Space Center/Cape Canaveral Air Force Station, Fla., and prepared for its May 2004 launch aboard a Boeing Delta II rocket.

"Each part of the spacecraft has passed individual vibration and environmental tests, and under tougher conditions than we expect they will see at Mercury," says James C. Leary, MESSENGER mission systems engineer at APL. "Now we're looking at MESSENGER as a whole system. By the time it launches MESSENGER will have been thoroughly tested."

Engineers prepare the MESSENGER spacecraft for an earlier vibration test. Credit: Johns Hopkins University Applied Physics Laboratory

Carrying seven scientific instruments - including a camera, laser altimeter, magnetometer and several spectrometers - the solar-powered MESSENGER will image Mercury globally for the first time. It also will gather data on the composition and structure of Mercury's crust, its geologic history, the nature of its thin atmosphere and active magnetosphere, and the makeup of its core and polar materials. While cruising to Mercury the spacecraft will fly past the planet twice - in 2007 and 2008 - snapping pictures and gathering data critical to planning the orbit study.

Sean C. Solomon of the Carnegie Institution of Washington leads MESSENGER as principal investigator; the Applied Physics Laboratory manages the mission for NASA's Office of Space Science and will operate the spacecraft. GenCorp Aerojet, Sacramento, Calif., and Composite Optics Inc., San Diego, provided MESSENGER's propulsion system and composite structure, respectively. APL, Goddard Space Flight Center, University of Michigan, Ann Arbor, and University of Colorado, Boulder, built the spacecraft's scientific instruments.

The Applied Physics Laboratory, a division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology.

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