7
Dec 10

Messenger's countdown for Mercury orbit insertion

Source: Messenger Mission

One hundred days from now, MESSENGER will execute a 15-minute maneuver that will place the spacecraft into orbit about Mercury, making it the first craft ever to do so, and initiating a one-year science campaign to understand the innermost planet.


Artist's impression about the Mercury orbit insertion.
Image credits: NASA/JPL/Messenger

It has already been 14 years since this mission was first proposed to NASA, more than 10 years since the project officially began, and over six years since the spacecraft was launched.

A multitude of milestones have been passed on the way toward the primary science phase of the mission, including six planetary flybys and five deep-space maneuvers. This week the team has completed a milestone of a different sort: the orbital readiness review.

Today’s review was the culmination of more than one year of major reviews designed to confirm the readiness of all mission elements to achieve orbit about Mercury next March and to begin orbital operations shortly thereafter.

“For this and many reviews before it we have called on a number of experts outside the MESSENGER project, from both APL and outside institutions, to review our plans to see where there are gaps or weak spots,” explains MESSENGER Project Manager Peter Bedini of the Johns Hopkins University Applied Physics Laboratory. “The intent is to tap the knowledge-base of those who have lived through similar challenges, and to make any adjustments that promise to improve the chances of success in our prime mission.”

“There is still work to do in preparation for orbit insertion next March, and those preparations will also be reviewed, but today’s review was the last in a long series laid out more than a year ago,” Bedini adds.

“MESSENGER has been on a long journey,” adds MESSENGER Principal Investigator Sean Solomon of the Carnegie Institution of Washington, “but the promised land lies ahead.  All of the preparations for orbit insertion and orbital operations by the project team and the mission’s many review panels have served to maximize the likelihood that the intensive exploration of the innermost planet will begin smoothly and efficiently 100 days from now.”

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7
Dec 10

Geminid Meteors not yet completely explained

Source: NASA Science News

The Geminid meteor shower, which peaks this year on Dec. 13th and 14th, is the most intense meteor shower of the year. It lasts for days, is rich in fireballs, and can be seen from almost any point on Earth.


Geminids.

It's also NASA astronomer Bill Cooke's favorite meteor shower—but not for any of the reasons listed above.

"The Geminids are my favorite," he explains, "because they defy explanation."

Most meteor showers come from comets, which spew ample meteoroids for a night of 'shooting stars.' The Geminids are different. The parent is not a comet but a weird rocky object named 3200 Phaethon that sheds very little dusty debris—not nearly enough to explain the Geminids.


A geminid fireball over Bratislava.
Image credit: Roman Piffl

"Of all the debris streams Earth passes through every year, the Geminids' is by far the most massive," says Cooke. "When we add up the amount of dust in the Geminid stream, it outweighs other streams by factors of 5 to 500."

This makes the Geminids the 900-lb gorilla of meteor showers. Yet 3200 Phaethon is more of a 98-lb weakling.

3200 Phaethon was discovered in 1983 by NASA's IRAS satellite and promptly classified as an asteroid. What else could it be? It did not have a tail; its orbit intersected the main asteroid belt; and its colors strongly resembled that of other asteroids. Indeed, 3200 Phaethon resembles main belt asteroid Pallas so much, it might be a 5-kilometer chip off that 544 km block.

"If 3200 Phaethon broke apart from asteroid Pallas, as some researchers believe, then Geminid meteoroids might be debris from the breakup," speculates Cooke. "But that doesn't agree with other things we know."

Researchers have looked carefully at the orbits of Geminid meteoroids and concluded that they were ejected from 3200 Phaethon when Phaethon was close to the sun—not when it was out in the asteroid belt breaking up with Pallas. The eccentric orbit of 3200 Phaethon brings it well inside the orbit of Mercury every 1.4 years. The rocky body thus receives a regular blast of solar heating that might boil jets of dust into the Geminid stream.

Could this be the answer?


A Geminid fireball over the Mojave Desert.
Image credit: Wally Pacholka.

To test the hypothesis, researchers turned to NASA's twin STEREO spacecraft, which are designed to study solar activity. Coronagraphs onboard STEREO can detect sungrazing asteroids and comets, and in June 2009 they detected 3200 Phaethon only 15 solar diameters from the sun's surface.

What happened next surprised UCLA planetary scientists David Jewitt and Jing Li, who analyzed the data. "3200 Phaethon unexpectedly brightened by a factor of two," they wrote. "The most likely explanation is that Phaethon ejected dust, perhaps in response to a break-down of surface rocks (through thermal fracture and decomposition cracking of hydrated minerals) in the intense heat of the Sun."

Jewett and Li's "rock comet" hypothesis is compelling, but they point out a problem: The amount of dust 3200 Phaethon ejected during its 2009 sun-encounter added a mere 0.01% to the mass of the Geminid debris stream—not nearly enough to keep the stream replenished over time. Perhaps the rock comet was more active in the past …?

"We just don't know," says Cooke. "Every new thing we learn about the Geminids seems to deepen the mystery."

This month Earth will pass through the Geminid debris stream, producing as many as 120 meteors per hour over dark-sky sites. The best time to look is probably between local midnight and sunrise on Tuesday, Dec. 14th, when the Moon is low and the constellation Gemini is high overhead, spitting bright Geminids across a sparkling starry sky.

Bundle up, go outside, and savor the mystery.

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