[meteorite-list] NASA Completes MESSENGER Mission with Expected Impact on Mercury's Surface

[Ron Baalke]

http://messenger.jhuapl.edu/news_room/details.php?id=284

MESSENGER Mission News
April 30, 2015

NASA Completes MESSENGER Mission with Expected Impact on Mercury's Surface

Mission controllers at the Johns Hopkins University Applied Physics Laboratory 
(APL) in Laurel, Md., confirmed today that NASA's MErcury Surface, Space 
ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft impacted 
the surface of Mercury, as predicted, at 3:26 p.m. EDT this afternoon 
(3:34 p.m. ground time).

Mission controllers were able to confirm the end of operations just a 
few minutes later at 3:40 p.m., when no signal was detected by the Deep 
Space Network (DSN) station in Goldstone, California, at the time the 
spacecraft would have emerged from behind the planet had MESSENGER not 
impacted the surface. This conclusion was independently confirmed by the 
DSN's Radio Science team, who were simultaneously looking for the signal 
from MESSENGER from their posts in California.

MESSENGER was launched on August 3, 2004, and it began orbiting Mercury 
on March 18, 2011. The spacecraft completed its primary science objectives 
by March 2012. Because MESSENGER's initial discoveries raised important 
new questions and the payload remained healthy, the mission was extended 
twice, allowing the spacecraft to make observations from extraordinarily 
low altitudes and capture images and information about the planet in unprecedented 
detail.

Last month -- during a final short extension of the mission referred to 
as XM2'-- the team embarked on a hover campaign that allowed the spacecraft 
at its closest approach to operate within a narrow band of altitudes, 
5 to 35 kilometers above the planet's surface. On April 28, the team successfully 
executed the last of seven orbit-correction maneuvers (the last four of 
which were conducted entirely with helium pressurant after the remaining 
liquid hydrazine had been depleted), which kept MESSENGER aloft for the 
additional month, sufficiently long for the spacecraft's instruments to 
collect critical information that could shed light on Mercury's crustal 
magnetic anomalies and ice-filled polar craters, among other features.

With no way to increase its altitude, MESSENGER was finally unable to 
resist the perturbations to its orbit by the Sun's gravitational pull, 
and it slammed into Mercury's surface at around 8,750 miles per hour, 
creating a new crater up to 52 feet wide.

"Today we bid a fond farewell to one of the most resilient and accomplished 
spacecraft ever to have explored our neighboring planets," said Sean Solomon, 
MESSENGER's Principal Investigator and Director of Columbia University's 
Lamont-Doherty Earth Observatory. "Our craft set a record for planetary 
flybys, spent more than four years in orbit about the planet closest to 
the Sun, and survived both punishing heat and extreme doses of radiation. 
Among its other achievements, MESSENGER determined Mercury's surface composition, 
revealed its geological history, discovered that its internal magnetic 
field is offset from the planet's center, taught us about Mercury's unusual 
internal structure, followed the chemical inventory of its exosphere with 
season and time of day, discovered novel aspects of its extraordinarily 
active magnetosphere, and verified that its polar deposits are dominantly 
water ice. A resourceful and committed team of engineers, mission operators, 
scientists, and managers can be extremely proud that the MESSENGER mission 
has surpassed all expectations and delivered a stunningly long list of 
discoveries that have changed our views not only of one of Earth's sibling 
planets but of the entire inner solar system."

MESSENGER's Final Hours

MESSENGER's last orbit with real-time flight operations began at 11:15 
a.m. EDT, with initiation of the final delivery of data and images from 
Mercury via the DSN 70-m antenna in Madrid, Spain. See the last image 
delivered here.

After a planned transition to the 34-m DSS-15 antenna at Goldstone, California, 
at 2:40 p.m. EDT, mission operators later confirmed the switch to a beacon-only 
communication signal at 3:04 p.m. The mood in the Mission Operations Center 
at APL was both celebratory and somber, as team members watched MESSENGER's 
telemetry drop out for the last time after more than four years and 4,105 
orbits at Mercury.

"We then monitored MESSENGER's beacon signal for about 25 additional minutes," 
said Mission Operations Manager Andy Calloway of APL. "It was strange 
to think that for those last three minutes MESSENGER had already impacted 
onto Mercury, but we could not confirm that fact yet because of the vast 
distance across space between Mercury and Earth. MESSENGER passed behind 
Mercury (as viewed from Earth) at 3:29 p.m., however the signal from our 
intrepid spacecraft started fading prior to that and dropped out for the 
last time at 3:25 p.m."

At 3:38 p.m. EDT, at the time the spacecraft would have emerged from behind 
the planet as viewed from the Goldstone station had MESSENGER not impacted, 
mission operators began monitoring for a signal, but as expected they 
were unable to establish communications between MESSENGER and the DSN. 
This radio silence was the confirmation of the end of the MESSENGER mission.

Before impact, MESSENGER's mission design team predicted that the spacecraft 
would pass several miles over the lava-filled Shakespeare impact basin 
before striking an unnamed ridge near 54.5 degrees North latitude and 
210.1 degrees East longitude. Because the probe hit on the far side of 
the planet, no Earth-based telescope was able to observe the impact. Moreover, 
space-based telescopes are precluded from observing Mercury because of 
the planet's proximity to the Sun, exposure to which would damage sensitive 
optics and instruments.

A future Mercury mission, such as the BepiColombo mission now in development 
by the European Space Agency and Japan Aerospace Exploration Agency, might 
be able to identify the impact crater left behind by MESSENGER. The MESSENGER 
team has acquired images of the entire planet, so a future mission will 
have MESSENGER's observations of the region before the impact to use as 
a baseline for comparison with subsequent imaging data sets to help pinpoint 
MESSENGER's impact site. The impact crater should be one of the youngest 
on Mercury and should have exposed fresh material from Mercury's subsurface 
that will have been exposed to the effects of space weathering for only 
a limited and precisely known time, so multispectral observations of MESSENGER's 
crater will provide an important constraint on rates of optical maturation 
of Mercury's surface material.

"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, associate 
administrator for NASA's Science Mission Directorate in Washington. "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 unraveling the mysteries of 
Mercury."

MESSENGER's Education and Public Outreach team included the public in 
the final chapter of the MESSENGER story by sponsoring a "Name that Crater," 
competition, providing an opportunity for the public to name five impact 
craters. Thousands of submissions were received, and the winners were 
announced on April 29.

Although the MESSENGER flight mission has now officially ended, the science 
data collected by MESSENGER are archived in NASA's Planetary Data System, 
where they are preserved and remain accessible for future use by the scientific 
community for years and even decades to come. The Science Team will continue 
to use these data to pose and answer questions about Mercury's formation 
and evolution and the planet's place in our Solar System through the end 
of the MESSENGER project in May 2016.

Additional information about MESSENGER's top science findings is available 
here; the mission's technological innovations are detailed here; and videos 
of team members discussing the mission are available online here.
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 entered orbit 
about Mercury on March 17, 2011 (March 18, 2011 UTC), to begin a yearlong 
study of its target planet. Dr. Sean C. Solomon, the Director of Columbia 
University's Lamont-Doherty Earth Observatory, led 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.