[meteorite-list] The Search for Vulcanoids Continues in the Twilight Zone

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:08:26 2004
Message-ID: <200209150523.WAA27683_at_zagami.jpl.nasa.gov>

http://planetary.org/html/news/articlearchive/headlines/2002/VulcanoidSearchContinues.html

The Search for Vulcanoids Continues in the Twilight Zone
By A.J.S. Rayl
The Planetary Society
12 September 2002

As the autumnal equinox approaches, scientists Daniel Durda and Alan Stern
will strap into the backseat of an F/A-18B fighter jet at NASA's Dryden
Flight Research Center in California, then take to the stratosphere to
continue their airborne search for vulcanoids, the string of small asteroids
that may be circling the Sun within the orbit of Mercury.

During the early morning hours before sunrise, Stern - on September 17 and
18, and Durda -- on September 19, will rocket up to 49,000 feet to snap
images of the virtually unexplored region of the sky near the Sun with their
modified, miniature video camera known as the Southwest Ultraviolet Imaging
Systems Aircraft (SWUIS-A).

Durda and Stern, both of the Southwest Research Institute (SWRI) in Boulder,
Colorado, conducted their first dedicated airborne search for these elusive
rocks with the SWUIS-A last Spring. Basically, the imaging system is an
85-millimeter camera that shoots images at 60 frames per second, then sends
them to a recorder where they are stored in a continuous stream of video.
Initially designed at SWRI in 1996 to operate on the space shuttle, the
SWUIS-A not only allows the researchers to obtain images of objects that are
as much as 600 times fainter than what is visible to the naked eye, but,
Durda points out, allows them to take time exposures to look at objects
fainter than normally could be seen in any individual 1/60-second exposure.

Although they did not find any vulcanoids in the some 100,000 images
gathered on their first flights, this time up they'll be using a brand new
lens on the SWUIS-A that will allow them to capture even fainter objects.

The one thing they did find on their first series of exploratory flights,
however, was that the high-performance fighter jets make excellent
telescopic platforms. "Our techniques of flying to high altitudes to reduce
the brightness of the twilight sky does indeed work, giving us the distinct
advantages of range and altitude over ground-based searches," says Durda, as
he takes a break from making up the requisite star charts and conducting
last-minute equipment checks.

The vulcanoid enigma

At the heart of the search lingers some controversy as to whether or not
vulcanoids actually exist. While theoretical models indicate a few hundred
of the relatively small rocks, ranging in size from one to 20 kilometers in
diameter, could have survived the harsh environment of the inner Solar
System, skeptics argue that any vulcanoids that may have once existed would
be gone now because the harsh collisional environment would have ground them
away to dust some time ago.

But absence of evidence is not evidence of absence, as the saying goes, and
discovering a vulcanoid could provide a major breakthrough for studies of
the Solar System and of Mercury. "Since they lie so near the Sun, they could
contain traces of the first materials that formed within the inner Solar
System, and finding one, being able to study the physical properties, would
help astronomers better understand the conditions in the solar nebula from
which the planets - including our home planet Earth - formed," elaborates
Durda.

If they do exist, vulcanoids would be visible in the region inward of
Mercury, the area where it would be reasonable to find any remnants of the
little planetesimals from which the planets formed. At this point, says
Durda, "we have kind of hit the limits about what we can theorize about and
the only way to know for sure is to go up there and look."

Ground-based searches for vulcanoids have been conducted for years during
small windows of time that open during solar eclipses and in the twilight
moments, just after sunset and before sunrise, but this particular region of
the sky is extremely difficult to search from the ground because of such
factors as atmospheric hazes, turbulence and glare from the Sun. So Stern,
the director of the space studies department at SWRI who led the development
of the airborne astronomy program, and Durda, a senior research scientist,
proposed taking a look from a higher perspective, and NASA's Planetary
Astronomy program and the National Geographic Society funded the study.

Into the twilight zone

Durda and Stern will arrive at Dryden on Monday, September 16. "While Alan
is going through egress and ejection refresher training, I'll be working
with the NASA engineer to make sure the equipment is installed in the
aircraft properly," Durda explains. "Each morning, we'll take off around
05:15 hours and fly north over Edwards Air Force Base, pointing our SWUIS-A
imaging system out the right side of the cockpit to sweep our search area in
the eastern sky, about a 6-1/2 by 5-1/2 degree patch, in the morning
twilight."

The best time to search for vulcanoids is at the equinox - the first day of
Spring, the first day of Fall, Durda informs. "These are the times when the
ecliptic, the plane of the Solar System in which objects tend to orbit, is
as vertical as possible to the horizon."

The SWUIS-A system they use for these vulcanoid searches is, Durda notes,
"essentially the same system designed at SWRI for the shuttle, but we'll use
a slightly different camera that is optimized from broadband visible to a
little bit into the near-infrared, to about 800 nanometers."

Durda and Stern had hoped to soar up to 80,000 feet in one of the USAF's U-2
trainers, but the aircraft is busy, so they are returning to the fighter
jets. Not surprisingly, neither is complaining. "The F/A-18B is a rocket
with wings and for civilian scientists to get the opportunity to actually
fly in these jets is an amazing experience," says Durda. So amazing in fact
that they're balancing the number of times each gets to go aloft. Stern will
fly the first two flights with NASA pilots Frank Batteas and Gordon
Fullerton, a former shuttle commander, with Durda taking off on the third
flight with pilot Craig Bomben. "The last time, I got to fly two flights,
and this time Alan gets to fly two," explains Durda.

Going up to get down (in magnitude)

During the series of flights last Spring, the duo completed nine search
fields over the area around the twilight near the Sun per flight, all of
which were compared. "We were able to see stars in our images down to
astronomical magnitude V= 9.5 or so - about a magnitude fainter than people
have reported for previous ground-based vulcanoid searches and comparable to
the results we obtained using data gathered by the Solar and Heliospheric
Observatory (SOHO) spacecraft while it was monitoring the solar corona and
solar activity," says Durda.

Despite the fact that no candidate vulcanoids popped into view, Durda and
Stern did see "lots" of faint stars. "We weren't really all that surprised
that we didn't find any vulcanoids on that trip considering that we've
already searched down to nearly that magnitude limit in previous searches,"
Durda admits. "You really do need to get down into an 11, 12, 13 magnitude.
Now the instrument we have is capable of getting down to 12 or 13 if the sky
is dark enough. (Remember each magnitude is about a factor of two and a half
in brightness.) So if we could make the sky darker, we could do an even
better job."

That is exactly what they're planning on doing this time out.

Since the availability of the U-2 had been in question, particularly in
light of the events of 9-11, Durda and Stern began strategizing about how to
get the background sky as dark as possible right after they returned from
their flights last April. "While one way is to go higher - which could be
done in the U-2, the other option was to find a different lens for the
camera system," Durda says. So even as they tried to secure a cruise in a
U-2, they obtained a new lens that will 'zoom in,' using a longer focal
length for slightly deeper images of the sky in the search area. "When we
zoom in, we'll be looking at a smaller patch of the sky, but what we'll be
doing, in effect, is spreading what sky brightness there is there out over
more pixels in the imagers, while the stars, which are just point sources,
stay, per pixel, the same brightness essentially."

Durda tested out the new lens a few weeks back during the Perseid meteor
shower and found that it should allow them to go about two magnitudes
fainter than the previous lens. "If you extrapolate, that means we'll be
able to go from a magnitude of about 9.5 down to somewhere between 11 and
11.5," he says. "That will help a lot."

Once the flights are over and the images are collected, Durda and Stern will
process and analyze the data just like before. "Basically, we will digitize
and clean up the images to eliminate any movement created by the motion of
the aircraft, and to get rid of variations in sky brightness background due
to imperfections in the system - what astronomers refer to as flat fielding
the images," Durda explains.

Then, an individual star in those images will be chosen and identified for
the imaging system's software, which, in turn, will reposition all the
individual frames so that the chosen star is in the same position in every
frame or in other words reconvert them back into one long time exposure. The
hope is that they'll find one or more of the elusive vulcanoids. Meanwhile,
they're still working to line-up a ride in the U-2 for at least one more
search next Spring before this study is completed.
Received on Sun 15 Sep 2002 01:23:18 AM PDT