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[NASA] 15 Years Ago: MESSENGER Launched to Orbit Mercury
« dnia: Sierpień 03, 2019, 06:19 »
15 Years Ago: MESSENGER Launched to Orbit Mercury
Aug. 2, 2019

Launch of MESSENGER.

Mercury, the innermost planet in our solar system, is difficult to study from Earth because of its proximity to the Sun. Mariner 10 made the first closeup observations of Mercury during three flybys of the planet in 1974 and 1975, leaving scientists yearning for a longer and more detailed investigation to answer the many questions raised by those first observations. In 1999, NASA selected for flight the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission under the leadership of Principal Investigator Sean C. Solomon, Director of Lamont-Doherty Earth Observatory, Columbia University, to place a spacecraft in orbit around Mercury for an in-depth study of the small planet. The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, designed, built, and operated the spacecraft for NASA. APL had to overcome numerous challenges to successfully accomplish the first orbital mission at Mercury, including the difficulty of reaching the innermost planet and operating a spacecraft in the much hotter environment much nearer to the Sun than most spacecraft had experienced. The first was solved by using a series of planetary gravity assist maneuvers to slow the spacecraft without using too much propellant so it could be inserted into orbit around Mercury and the second by the installation of a sunshade and radiators to maintain normal operating temperatures inside the spacecraft.

Illustration of MESSENGER in orbit around Mercury.

The MESSENGER spacecraft was launched from Cape Canaveral, Florida, on Aug. 3, 2004, aboard a Delta II rocket, beginning a 4.9-billion-mile journey that took more than six and a half years. MESSENGER used several gravity-assists and course-correction maneuvers to lower its velocity so it could be propulsively inserted into orbit around Mercury. On Aug. 2, 2005, MESSENGER returned to Earth’s vicinity for the first gravity-assist flyby, passing within 1,459 miles of its home planet. This maneuver sent the spacecraft toward Venus, setting up two flybys of the cloud-shrouded planet, on Oct. 24, 2006, at a distance of 1,856 miles, and again on June 5, 2007, at a distance of just 210 miles. Then it was on to Mercury for three flybys, providing the first close-up looks at Mercury in more than 30 years: on Jan. 14, 2008, at a distance of 125 miles; on Oct. 6, 2008, at 124 miles; and finally on Sep. 29, 2009, at 124 miles. During the flybys, MESSENGER operated all of its instruments including its cameras and photographed areas of Mercury not imaged during the Mariner 10 encounters. In November 2010, MESSENGER turned its cameras toward the other planets and took a family portrait of the solar system, reminiscent of the one taken by Voyager 1 in February 1990 from the outer reaches of the solar system.

Illustration of MESSENGER’s complex series of gravity assists before entering orbit around Mercury.

Illustration showing MESSENGER’s science instruments.

MESSENGER carried seven instruments and a radio science experiment to complete its comprehensive study of Mercury and its environs:

- The Mercury Dual Imaging System (MDIS) consisted of wide-angle multi-spectral and narrow-angle monochrome cameras to image the surface of Mercury and investigated the diversity of geological terrains on the planet’s surface. 

- Images taken with the narrow-angle camera resolved objects as small as 60 feet across.

- The Gamma-Ray and Neutron Spectrometer (GRNS) contained two sensors to collect data on the elemental composition of Mercury’s crust.

- The X-Ray Spectrometer (XRS) studied the elements in the top tens of millimeters of Mercury’s crust and monitored X-rays from the Sun bombarding the planet.

- The Magnetometer (MAG), mounted on a 12-foot boom to distance it from the spacecraft’s own magnetic field, characterized Mercury’s magnetic field.

- The Mercury Laser Altimeter (MLA) used laser pulses to determine the planet’s topography.

- The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) combined an ultraviolet-visible spectrometer and a visible-infrared spectrograph to detect gases in Mercury’s tenuous atmosphere and minerals on its surface.

- The Energetic Particle and Plasma Spectrometer (EPPS) measured the composition, distribution, and energy of charged particles around Mercury.

- The Radio Science (RS) experiment used MESSENGER’s radio signals to track precisely the spacecraft’s trajectory, allowing the science team to determine Mercury’s gravity field.

Left: MESSENGER photograph of Earth during the Aug. 2005 flyby.
Right: MESSENGER photograph of Venus during the second flyby in July 2006.

MESSENGER photograph of Mercury during the first flyby in Jan. 2008.

MESSENGER’s family portrait of the solar system taken in Nov. 2010.

On March 18, 2011, MESSENGER fired its main engine and entered a near-polar eccentric orbit around Mercury, with the low point reaching as near to the surface as 124 miles and the high point as far as 9,420 miles, completing each orbit in about 12 hours. This highly elliptical orbit allowed the spacecraft to cool after each close pass over Mercury’s hot dayside surface. After controllers verified that all the spacecraft systems were working properly, MESSENGER began its primary 1-year science mission on April 4, 2011. On March 18, 2012, MESSENGER started its first one-year extended mission, changing its orbital period one month later to 8 hours. During this first extension, MESSENGER discovered the presence of water ice and organic compounds in permanently shaded regions of Mercury’s north pole. NASA granted a second 24-month mission extension, beginning on March 18, 2013, during which MESSENGER’s closest-approach distance progressively decreased. In November 2013, MESSENGER observed the short-term comet 2P/Encke and the hyperbolic-orbit comet 2012 S1 ISON during their close approaches to the Sun, returning more than 700 images of the comets as well as science data from the MASCS and XRS instruments. Orbit-correction maneuvers raised MESSENGER’s closest-approach distance to Mercury multiple times in 2014 and early 2015, prolonging the mission. During a final six-week extension, onboard propellant was finally exhausted, and because no further orbit-correction maneuvers were possible the spacecraft crashed onto Mercury’s surface on April 30, 2015. The hardy spacecraft returned its final photograph shortly before it ended its highly successful mission.

Left: MESSENGER’s first photograph taken from orbit around Mercury.
Right: Photograph of Crater Stevenson with perpendicular crater chains.

Water ice (shown in yellow) discovered by MESEENGER near Mercury’s north pole.

MESSENGER’s last photograph.

The MESSENGER spacecraft fundamentally changed our understanding of Mercury during its four-year orbital exploration of the planet, returning nearly 300,000 photographs and a wealth of information from its instruments. The formatted data totaling more than 10 terabytes reside in NASA’s Planetary Data System archive. Among the major findings were that Mercury harbors water ice and organic compounds at its north pole, that volcanism played a major role in shaping the planet’s surface, and that Mercury’s surface materials are more volatile-rich and chemically reduced than expected. Future robotic exploration of Mercury will have to wait until December 2025, when the joint European Space Agency and Japan Aerospace Exploration Agency BepiColombo mission delivers two spacecraft into orbit around the planet. The spacecraft were launched on Oct. 20, 2018, and similar to MESSENGER will make one flyby of Earth, two of Venus, and six of Mercury before finally reaching their target.

MESSENGER by the numbers.

John Uri
NASA Johnson Space Center


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Odp: [NASA] 15 Years Ago: MESSENGER Launched to Orbit Mercury
« Odpowiedź #1 dnia: Sierpień 03, 2019, 06:35 »
Messenger mission ends with plunge into Mercury
April 30, 2015 Stephen Clark [SFN]

Artist’s concept of the Messenger spacecraft flying over Mercury’s surface displayed in enhanced color. The crater ringed by bright orange is Calvino crater. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

NASA’s Messenger spacecraft closed out a successful four-year tour in Mercury’s orbit Thursday with a cataclysmic crash into the scorching planet after consuming its last gasps of fuel.

The $446 million robotic science mission was the first probe to ever enter orbit around the closest planet to the sun, where it mapped Mercury’s tortured landscape, confirmed the existence of water ice buried inside permanently dark polar craters, and found a world far more dynamic than researchers predicted.

The orbiter launched from Cape Canaveral aboard a Delta 2 rocket in August 2004 and returned to Earth a year later to reshape its trajectory toward Mercury. Two fleeting flybys of Venus and three encounters with Mercury set up for Messenger’s final approach to the innermost planet in March 2011.

Messenger was fitted with a sunshield to weather temperatures up to 600 degrees Fahrenheit during the trek toward the sun.

Designed for a one-year mission, Messenger lasted four years and spent the last 12 months gathering unprecedented data on Mercury from lower altitudes than any mission before it.

Messenger ran out of hydrazine fuel April 6, and it used leftover helium pressurant in a last-ditch effort to counteract the pull of gravity and extend the mission a few extra weeks.

Ground controllers at the Johns Hopkins University Applied Physics Laboratory in Maryland tracked Messenger’s final days, predicting the space probe would strike Mercury around 3:26 p.m. EDT (1926 GMT) Thursday.

Mission managers expected Messenger to impact Mercury at a velocity of 8,750 mph on the planet’s northern hemisphere, out of view of Earth. Messenger’s operations team confirmed the probe’s crash a few minutes later when they did not receive a signal from the spacecraft at the time it would have emerged back in range of ground-based communications antennas.

Messenger was predicted to impact near the center of this region on Mercury, located at 54.4 degrees north latitude and 210.1 degrees east longitude near the “Shakespeare” impact basin. This image includes data from Messenger’s dual imaging system and laser altimeter, with red colors representing higher elevations. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Scientists said the high-speed impact would carve fresh crater 50 feet wide on Mercury’s surface, and the BepiColombo spacecraft built in Europe and Japan will look for Messenger’s impact scar after its scheduled arrival at Mercury in 2024.

Messenger collected science data up to its last day, with the final downlink of stored observations and images streaming back to Earth on Thursday morning.

“Going out with a bang as it impacts the surface of Mercury, we are celebrating Messenger as more than a successful mission,” said John Grunsfeld, head of NASA’s science directorate. “The Messenger mission will continue to provide scientists with a bonanza of new results as we begin the next phase of this mission — analyzing the exciting data already in the archives, and unravelling the mysteries of Mercury.”

Messenger’s suite of science instruments gathered 10 terabytes of data on Mercury over its four-year mission, completing 4,105 laps of the planet.

“The spacecraft and the instruments have worked virtually flawlessly over those four years,” said Jim Green, director of NASA’s planetary science division. “The data is on Earth, and we have it now. We’re going to continue to make wonderful discoveries with it.”

Before Messenger’s mission, less than half of Mercury was mapped from images during flybys in the 1970s by NASA’s Mariner 10 spacecraft.

“I think it’s good to put this into perspective,” said Nancy Chabot, lead scientist for Messenger’s dual camera system at the Applied Physics Laboratory. “Before this mission, we had seen 45 percent of the planet. I had a globe, and more than half of it was blank. It’s just been huge, changing our understanding of the planet as a whole.”

Images of fault scarps on Mercury taken by Messenger indicate the planet is contracting — a sign that its internal core is still cooling — and measurements show Mercury’s magnetic field is surprisingly offset from the planet’s center, said Sean Solomon, Messenger’s principal investigator from Columbia University’s Lamont-Doherty Earth Observatory.

Sean Solomon, Messenger’s principal investigator, speaks at a celebration of the mission in April. Credit: NASA/Aubrey Gemignani

Messenger also revealed a tenuous atmosphere around Mercury extending into a comet-like tail behind the planet as molecules are blasted away by intense solar radiation. The mission monitored seasonal changes in the atmosphere as Mercury sped around the sun in its 88-day orbit.

Mercury’s reputation as a hellish world is well-deserved — temperatures can get up to 800 degrees Fahrenheit during daytime — but isolated pockets of the planet never see the sun.

Such regions at the bottoms of Mercury’s polar craters are cold enough to support deposits of water ice, and Messenger’s science team verified the ice’s presence at or just below Mercury’s surface after scanning the material with cameras, a laser altimeter and a neutron spectrometer.

Earth-based radars hinted at the ice on Mercury, but evidence from Mercury gave researchers confidence the ice slabs are there.

“Perhaps the most interesting discovery might be the polar deposits,” Solomon said.

The last leg of Messenger’s mission at low altitude gave scientists a chance to see into the shaded bottoms of the craters in more detail, according to Chabot.

“We’re actually seeing into these regions where the sun never shines on Mercury in higher resolution than has ever been possible before,” Chabot said in March.

Snapping long exposures and using sunlight scattered off crater walls, high-contrast images from Messenger’s two-eyed camera exposed sharp features that appear to be soils laid on top of ice.

“Most of these deposits don’t consist of water ice directly at the surface, but rather water ice covered by a dark layer which we think is 20 to 30 centimeters (8 to 12 inches) thick,” Solomon said. “And that dark layer is darker than anything else on Mercury.”

A view inside the permanent shadow of Fuller crater, where ice is thought to reside. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The stuff preserved on Mercury’s hidden crater floors could the same type of mixture of water and organic compounds that ripened into life on Earth.

“The team has put forward the hypothesis that this dark material is, in fact, organic carbonaceous material delivered to Mercury by the same objects that brought the water ice stored in these deep freezes at the north and south poles, giving us a record in these poles of the delivery process from the outer solar system not only of water ice, but of what — on our planet — were once the building blocks of organic chemistry and life,” Solomon said.

Mercury has a larger iron core relative to its overall size than any other planet in the solar system, so scientists assumed its other elements would be blown away by intense heat from the sun.

Messenger found a world rich in lighter elements like sodium, sulfur, potassium and chlorine. Despite Mercury’s iron interior, scientists say Messenger saw relatively little of the element in the planet’s crust.

“So the ideas for how the inner planets got assembled, and how the building blocks of planetary materials were delivered to the inner solar system and survived the process of plaentary accretion, are all being changed by Messenger’s results,” Solomon said.

Larry Nittler, the No. 2 scientist working on the Messenger mission, said the probe’s data archive will be tapped to help refine models on how the solar system’s rocky planets formed.

“One of the big science questions that Messenger set out to answer was why Mercury has this large core and this thin silicate shell, and I would argue we still don’t know the answer to that question a whole lot better than we knew before we went into orbit,” Nittler said. “Part of this is not due to a lack of data, but a lack of quantitative understanding of the different possibilities that could lead to this.

“For example, a very popular model for a long time was the idea that Mercury might have formed much larger and had a giant impact that stripped off much of its pre-existing mantle and crust,” Nittler said. “Then it formed a new crust on its small remaining mantle.

Messenger obtained this global view of Mercury during a flyby encounter in 2008. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

“I think a lot of the questions that were raised — and are still raised — are possibly still answerable with the data we obtained,” Nittler said. “That’s the big one that I really hope we eventually answer.”

Solomon singled out Mercury’s ice deposits as the most interesting discovery by Messenger’s mission, noting the finding’s implications for life.

“I don’t think anybody would count Mercury as habitable in the sense that there’s no part of the surface or the near-subsurface where water can be stable in a liquid form,” Solomon said.

But Mercury has a story to tell in the pursuit of life’s origins, he added, because like Earth, it was bombarded by organic-rich asteroids and comets that left behind water and the chemical constituents of organisms.

“Even a planet that is not in the nominal habitable zone of our own star is a witness to the delivery of the ingredients of habitability from the outer solar system to the inner solar system, including the ice and including the organic building blocks that are capable — on other bodies — of leading to pre-biotic organic chemistry,” Solomon said.

“In terms of understanding what leads to habitability, Mercury has to be included.”