NEW
YORK – A NASA spacecraft aimed at Mercury has already returned valuable observations
from the planet closest to our sun, despite still being months away from
entering orbit around the small, rocky world, the mission's lead scientist said.
Sean
Solomon, principal investigator for NASA's MESSENGER mission, said the spacecraft
is poised to enter orbit around Mercury next March to build the most detailed
maps ever made of the planet.
"Mercury
is not what we thought it was even 2 1/2 years ago," Solomon said during a
public lecture this week at the American Museum of Natural History here as part
of the 73rd Annual Meeting of the Meteoritical Society.
After three flybys of Mercury, MESSENGER has beamed home stunning views of the cratered world and provided a fresh look into its volcanic past and tenuous atmosphere. But the best is still to come, Solomon said.
NASA's
MESSENGER probe, short for MErcury Surface, Space ENvironment, GEochemistry,
and Ranging, is the first spacecraft to examine Mercury up close since the
space agency's Mariner 10 mission in the mid-1970s. In
addition to the new global maps, MESSENGER scientists hope the mission will
find new clues on how Mercury was formed, how it evolved, and how it generated its magnetic
field.
"We're
trying to learn how a planet very near its host star differs from others that
are more far out and more massive," Solomon said. "Exploring the
inner part of our solar system is to understand our place in the solar
system."
Not
a direct flight
While
MESSENGER's main mission is to orbit Mercury, the probe had to perform six different flybys that took it past Earth once, Venus twice and Mercury three times.
Each time, the spacecraft used the gravity of each planet flyby to refine its
flight path through space.
The
gravitational tugs from the flybys also work to slow the spacecraft's speed
enough so that MESSENGER's propulsion system can successfully execute its
scheduled "orbital insertion" on March 18, 2011, putting the
spacecraft in an elliptical orbit around Mercury.
"We
could have gotten to Mercury in four or five months, as Mariner 10 did, but we
can't get into orbit flying that fast," Solomon said. "We have to be
going by Mercury slowly enough so that the propulsion system can perform the
orbit insertion."
Three
of the six flybys had MESSENGER swooping by Mercury itself – in January and
October of 2008, then again in September 2009.
The
flyby events allowed the probe to snap pictures of Mercury's surface, enabling scientists to map the planet in
unprecedented detail. The first flyby alone returned 1,317 images.
Magnetic
fields and volcanic processes
In
one of the flybys, MESSENGER was also able to take measurements of the planet's
magnetic field over one hemisphere. Mercury was found to have a dominantly
dipolar magnetic field – with opposite magnetic poles – with the dipole being
closely aligned with the planet's spin axis, said Solomon.
This
seems to suggest that Mercury's magnetic field is similar to Earth, and was
possibly generated in the same way. Once MESSENGER enters the orbital stage of
its mission, researchers will be able to study this in greater detail.
Scientists will also be interested in looking for evidence of volcanic
processes on Mercury, said Solomon.
MESSENGER
is expected to enter Mercury's orbit on March 18, 2011. The propulsion system
will fire for 15 minutes, placing the spacecraft in an initial orbit around
Mercury at an altitude of 124 miles (200 km).
Each
elliptical orbit will last 12 hours, meaning the probe will circle the planet
twice per Earth day. At the time of orbit insertion, MESSENGER will be 28.7
million miles (46.1 million km) from the sun, and 96.4 million miles (155.1
million km) away from Earth.
Studying
Mercury in the future
The
$446 million MESSENGER spacecraft launched in August 2004 and is fully funded
to orbit Mercury for one Earth year, which is equivalent to just over four of
Mercury's 88-day years. Solomon is currently writing a proposal to extend the
mission beyond the initial timeframe.
"We
think we have ample propellant not only for that one year, but probably for a
year or two after that," Solomon said. "But, we'll eventually run out
of propellant or NASA dollars, and that will cause the probe to impact the
surface."
Still,
the MESSENGER probe has already yielded important results, and has paved the
way for future missions to our innermost planet – including a joint European
Space Agency and Japanese space agency (JAXA) mission that will launch two
spacecraft in 2014 to study Mercury.
And
beyond that, Solomon has his sights set on a future sample-return mission to Mercury.
"It'll
probably not happen nearly as soon as one to an asteroid and eventually Mars,
but I think it's the next thing to do," Solomon said.
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