[meteorite-list] MESSENGER Detects Comets ISON and Encke, Prepares For Closer Encounters

[Ron Baalke]

The Johns Hopkins University Applied Physics Laboratory
Office of Communications and Public Affairs
Laurel, Maryland
Media Contact: Paulette W. Campbell
240-228-6792
Paulette.Campbell at jhuapl.edu
November 14, 2013

FOR IMMEDIATE RELEASE

Note: Images of the comets can be seen at 
http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=1296

MESSENGER DETECTS COMETS ISON AND ENCKE, PREPARES FOR CLOSER ENCOUNTERS

NASA's Mercury-orbiting MESSENGER spacecraft has captured images of 
two comets -- 2P/Encke and C/2012 S1 (ISON) -- setting the stage for 
observations later this month when both comets will be substantially 
brighter and much closer to Mercury and the sun.

ISON was discovered in September 2012 by amateur Russian astronomers, 
who observed with a 16-inch telescope that is part of the 
International Scientific Optical Network (ISON), after which the 
comet was named. On Nov. 28, ISON will fly within 700,000 miles (1.2 
million kilometers) of the sun's photosphere, at which time it is 
expected either to flare brilliantly or disintegrate.

As part of an ISON observation campaign involving ground- and 
space-based NASA observatories, as well as many other observatories 
around the world, MESSENGER has been poised for several weeks to 
collect observations of ISON. From Nov. 9 through Nov. 11, the 
probe's Mercury Dual Instrument System (MDIS) captured its first 
images of the comet.

"We are thrilled to see that we've detected ISON," said Ron Vervack 
of the Johns Hopkins University Applied Physics Laboratory, who is 
leading MESSENGER's role in the ISON observation campaign. "The comet 
hasn't brightened as quickly as originally predicted, so we wondered 
how well we would do. Seeing it this early bodes well for our later 
observations."

A few days earlier, from Nov. 6 through Nov. 8, MESSENGER's imagers 
picked up its first snapshots of Encke. Unlike ISON, Encke has been 
known for quite a while. It was discovered in 1786 and recognized as 
a periodic comet in 1819. Its orbital period is 3.3 years -- the 
shortest period of any known comet -- and Nov. 21 will mark its 62nd 
recorded perihelion.

"Encke has been on our radar for a long time because we've realized 
that it would be crossing MESSENGER's path in mid-November of this 
year," Vervack explained. "And not only crossing it, but coming very 
close to Mercury." The early images of both comets are little more 
than a few pixels across, Vervack said, but he expects improved 
images next week when the comets make their closest approaches to 
MESSENGER and Mercury.

On Nov. 18, just a few days shy of its perihelion on Nov. 21, Encke 
will travel within 2.3 million miles (3.7 million kilometers) of 
Mercury. According to the Minor Planet Center, if Encke came this 
close to Earth, it would rank as the third-closest known approach of 
a comet to our planet. On Nov. 19, ISON will pass within 22.5 million 
miles (36.2 million kilometers) of Mercury while at a distance of 44 
million (71 million kilometers) from the sun.

"By next week, we expect Encke to brighten by approximately a factor 
of 200 as seen from Mercury, and ISON by a factor of 15 or more," 
Vervack said. "So we have high hopes for better images and data." 
Three of MESSENGER's instruments -- MDIS, the Mercury Atmospheric and 
Surface Composition Spectrometer, and the X-Ray Spectrometer -- will 
be trained on the two comets and will collect as many observations as 
payload operational constraints will allow.

There are complicating issues that could impact the volume of data 
the team gathers, Vervack explained.

"Closest approach occurs during what we call a 'hot season,'" he 
said. "So, for the health of the spacecraft, portions of each orbit 
must be spent in a thermally safe mode, which precludes gathering 
data over the entire orbit."

The critical observations also happen during a low-downlink period 
for MESSENGER.

"We can't fill up the spacecraft recorder with comet data because 
doing so could cause a backlog that impacts our primary mission of 
collecting observations from Mercury," he said.

But the team is optimistic that all will go as planned, he said. "We 
just need the comets to hold up their end of the bargain."

###

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 18, 2011 (UTC), to 
begin its primary mission - a yearlong study of its target planet. 
MESSENGER's first extended mission began on March 18, 2012, and ended 
one year later. MESSENGER is now in a second extended mission, which 
is scheduled to conclude in March 2015. Sean C. Solomon, the director 
of Columbia University's Lamont-Doherty Earth Observatory, 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.