[meteorite-list] Stardust On Way Back To Tempel 1

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 8 Aug 2007 11:36:01 -0700 (PDT)
Message-ID: <200708081836.LAA05497_at_zagami.jpl.nasa.gov>

http://www.spaceblogger.com/reports/Stardust_On_Way_To_Tempel_1_999.html

Stardust On Way Back To Tempel 1
by Bruce Moomaw
SpaceDaily
August 8, 2007

Cameron Park CA (SPX) - The reuse of Deep Impact for a second comet flyby,
while sensible, was always probable -- it's still a nice fresh spacecraft
that took only six months to get from Earth to Tempel 1 and will be only
four years old when it flies by Comet Boethin. The reuse of the Stardust
comet-sampling probe is a lot more surprising.

In order to set itself up to fly past a comet at an acceptable low speed
(a mere 22,000 km/hour) so that the comet's dust grains wouldn't be
seriously damaged when they smacked into the craft's aerogel fielder's
glove, Stardust had to make a convoluted five-year cruise through the
inner Solar System -- and then it spent another two years returning to
Earth to drop off its sample capsule.

Moreover, at that point it had much less midcourse maneuvering fuel left
than Deep Impact had at the end of its Tempel flyby. Nevertheless, a few
years ago CONTOUR's principal investigator, Joseph Veverka of Cornell,
discovered that the craft could still arrange to make a flyby of Earth
in January 2009 that would put it into an orbit allowing it to fly by
Tempel 1 during the comet's next visit to the inner Solar System on Feb.
14, 2011. And since Stardust -- even after 8 1/2 years in space -- still
seems to be in good shape, spending a mere $25 million for such a return
visit was once again judged by NASA to be a worthwhile bargain.

There are several good reasons for paying another call on Tempel 1,
rather than trying to aim Stardust for still another new comet. The
biggest is that Deep Impact was completely unable to fulfill one of its
larger scientific goals: photographing the actual crater left by its
Impactor after it crashed into Tempel, thanks to the surprisingly thick
cloud of ejecta dust that shrouded the scene.

The resolution of Stardust's camera is admittedly a lot less than that
of Deep Impact -- even given its focusing problem, Deep Impact's HRI
camera could have viewed the crater with a resolution of only 2 or 3
meters per pixel. Stardust's camera does have one extra advantage -- a
swivel mirror that will allow it to hunker down behind the craft's dust
shields and continue viewing the comet's surface while Stardust whizzes
past Tempel at only 200 km distance -- but even with that, it can't view
the crater with a resolution better than 12 meters per pixel.

Since the crater must be only a few hundred meters wide, this means that
its views of any internal layering in the crater will be quite fuzzy.
And on top of that, the multiple filter wheel on Stardust's camera stuck
at the very beginning of the mission, allowing it to obtain only
black-and-white views as opposed to the multispectral data that the 18
filters on Deep Impact's two cameras can provide.

Still, an actual look at the fresh crater left by the Impactor would be
very valuable. It could allow us to tell whether the material on
tempel's surface really is as loose as talcum powder -- or whether (as
some researchers now think) the surface may be a bit firmer, so that the
huge cloud of ejecta from the crash may have been largely kicked up not
just by loose dust being hurled away by the crash's force in the comet's
extremely weak gravity, but instead by gas flowing from the vaporization
of subsurface ices freshly exposed to sunlight by the impact.

A look at a provably fresh impact crater on the comet could also help us
tell whether the natural craters on the surfaces of Wild and Tempel are
actually unmodified meteor impact craters whose strange shapes stem from
the fact that their impactors hit very loose surface powder, or whether
they are instead "sublimation pits" produced when much smaller impact
craters or gas vents on the comets were steadily widened and changed in
shape by ice evaporation driven by sunlight on the holes' slopes.

To do so, of course, Stardust must fly past Tempel at the right time and
angle to see Deep Impact's crater. This could be troublesome, since
comets are notorious for changing even their solar orbits (let alone
their rate and tilt of rotation) due to the powerful gas jets that erupt
unpredictably from them -- and this may explain why re-viewing D.I.'s
crater was not listed among the goals of "Stardust-NExT" in the press
release announcing its official selection.

Tempel 1, however, is a placid comet whose gas jets up to now have been
very gentle. Dr. Veverka thinks that, if Tempel continues to act the
same way, it's a virtual certainty that Stardust can be aimed to view
the crater from only 200 km away -- and that, even if Tempel suddenly
turns contrary, the odds are still over two to one that we can estimate
Tempel's new rotation well enough in advance to allow a last-minute
course correction by Stardust to hit that goal.

Stardust could also image large areas of Tempel's surface that Deep
Impact missed, and its second look at other areas six years after Deep
Impact viewed them for the first time could allow us to determine how
fast the comet's surface features are being changed by the continuing
slow loss of surface ice and dust during its close solar encounters.

Finally, Stardust does carry another of CONTOUR's four instruments that
isn't on Deep Impact: a dust-impact mass spectrometer that can directly
analyze in detail the elements and light ions contained in the puffs of
vapor released when the comet's dust and ice grains slam into a metal
target on the instrument. Even if Stardust had failed to return its
samples to Earth, this instrument had already provided useful data on
the composition of ices and organic compounds in the dust grains that
were too volatile to survive a crash into the aerogel pad that would
allow them to be returned to Earth.

Similar instruments were carried on the European and Soviet probes that
visited Halley in 1986, suggesting general similarities with the
composition of Wild's grain, but also some interesting differences. We
would naturally like to make another comparative study of this at
Tempel. (Alas, neither Stardust nor Deep Impact carries a copy of
CONTOUR's other mass spectrometer for directly analyzing the gases given
off by comets.)

So -- even given the shortage of instruments on these two craft compared
to CONTOUR -- their additional two low-cost comet visits would seem very
cost-effective. On top of that, however, Goddard SFC's extrasolar-planet
expert Drake Deming devised a particularly unlikely and clever scheme to
turn a lemon into lemonade, by also utilizing Deep Impact's powerful HRI
camera to make observations of planets orbiting other suns -- and by
actually using its mild focusing problem into an advantage.

In his scheme Deep Impact, while crusing though interplanetary space
toward its ultimate encounter with Comet Boethin, will spend January
though May of next year turning its HRI camera on at least three
different stars already known to possess at least one planet each.

These planets are all "hot Jupiters" --giant planets like Jupiter or
Saturn (or bigger) which, after their formation, have migrated inwards
to within only a few million kilometers of their suns (vastly closer
than Mercury is to our own Sun), and so have had their outer atmospheres
raised to near-incandescent temperatures as they whirl around their suns
with a period of mere days.

We have nothing like them in our own System, and so the discovery last
decade that hot Jupiters are actually very frequent at other stars (they
were, in fact, the first type of extrasolar planet discovered) came as a
real shock. They suggest that the nicely orderly arrangement of our own
Solar System's planets may be a lot more unusual than we had thought --
or, alternatively, that our own Sun may have had one or two other giant
planets to start with that spiraled all the way into the Sun very early
in its career, leaving Jupiter and the others at a greater distance
while the four small inner rocky planets later formed out of the
stirred-up Sun-orbiting debris left behind in the the wake of the
suicidal big planets. We simply don't know yet.

Most extrasolar planets so far have been discovered entirely because of
their slight gravitational tuggings on their far bigger parent stars,
which causes the spectral lines of the light from the stars to slightly
Doppler-shift rhythmically up and down in wavelength as the planet drags
its star first slightly toward us and then slightly away from us.

However, we have now begun discovering (or confirming) extrasolar
planets another way: the "transit" technique, in which -- if a planet's
orbit around its star happens to be edge-on to us here on Earth -- we
can detect mild rhythmic dips in its light level as the planet passes
repeatedly between its sun and us.

This technique allows one to estimate the diameter of the planet,
whereas the Doppler technique provides data on its mass -- and if one is
lucky enough to detect a planet using both techniques, we can thus
calculate its density and thus its general composition.

Moreover, when you put even a quite small telescope into space -- free
from the confusing blurring and twinkling produced by the Earth's air --
the sensitivity of the transit technique is tremendously increased.
COROT, the little French astronomy satellite launched last December, is
using a telescope with a mirror only 27 cm wide to survey 120,000 stars
for planets only a few times more massive than Earth that have wandered
into such close orbits around their stars.

Kepler, the American mission planned for 2009, will do a much more
sensitive and ambitious census with the same transit technique,
examining 100,000 distant stars for four years straight to try and give
us our very first estimate of just how common small Earthlike planets
are in close orbits around other stars that could allow them to exist at
temperatures that could support life.

We hope to find at least a few dozen such planets properly lined up for
visible transits among all those stars. And Kepler's coming estimate of
the commonness of potentially life-supporting planets around other stars
will be crucial in picking our later strategy to use vastly huger space
telescopes to actually try and analyze the atmospheres of some such
planets orbiting nearby stars, to look for gases that indicate the
presence of life there. (In fact, Deep Impact will also observe Earth's
IR spectrum from great distances in order to help calibrate future
instruments for that latter goal.)

In the case of Deep Impact, however, the goals are more modest -- but
still worthwhile. The craft will examine three (or maybe four) stars
where hot Jupiters have already been discovered using the transit
technique with ground telescopes, squinting at them continuously for
periods of up to 50 days straight as the planets transit them repeatedly.

Even with such a small 30-cm telescopic mirror, doing this in the
clarity of outer space will allow far sharper profiles than we now have
of the size and duration of the dips in their stars' visible light
produced by those planetary transits -- allowing us to get unprecedented
sharp data on the precise size and orbits of the hot Jupiters.

In fact, it's hoped that Deep Impact's observations will be sensitive
enough to allow us to look for rings -- or perhaps even big moons --
around those planets. And -- by allowing very precise timing of the
changes in the beginning and end times of the transits -- it may also be
able to detect slight orbital fluctuations produced in the hot Jupiters
by smaller Earth-sized inner planets, whose size and orbit could thus be
indirectly calculated.

Finally, D.I. will also observe the slight rhythmic increase in the
total light we see from each star just before its hot Jupiter disappears
behind it and just after it reemerges from behind it, thanks to the
additional light reflected from the planet's illuminated dayside toward us.

This could give us information on the albedo (the shading) of the
planet's cloud decks -- and, by observing subtle changes in the spectra
of the total light from the star as the planet's reflected light is
added to it (or when some of its sunlight flows through the outer layers
of the planet's air as it passes in front of the star), we may also get
new data on the makeup of their atmospheric gases. While there is
precisely no chance that any of these planets or their moons could
support life so close to their suns, every new piece of data we can get
about these amazingly far-away worlds will be invaluable.

Ironically, the HRI's focusing problem actually improves its sensitivity
and accuracy for this very specialized task of precise light-intensity
measurement. It causes more light from the star to be focused on each
individual pixel in the camera's CCD chip than would otherwise be the case.

So this new possible task for an already-existing spacecraft -- one even
more unexpected and ingenious than the idea of using Stardust for a
revisit to Tempel 1 has once again been accepted by NASA as a way of
milking useful additional science out of such craft for a very low
price. The total cost of Deep Impact's astronomical observations and its
visit to Comet Boethin is only $30 million.

Alan Stern -- the mastermind of the New Horizons Pluto mission, who has
just become the latest director of NASA's overall space science division
-- has declared his eagerness to accept such ingenious economizing
measures from whoever can come up with them. Given NASA's continuing
need to pinch pennies (especially for space science), it seems likely
that we will be seeing more such extended missions for already-flying
spacecraft, many of them perhaps very surprising.
Received on Wed 08 Aug 2007 02:36:01 PM PDT