http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_current.php
The PI’s Perspective
Making Old Horizons New
Alan Stern
November 1, 2006
Since I last wrote in this space, September wound up, October came and
went, and November is now upon us. Up on New Horizons, we’ve completed
another long series of important milestones as we prepare our
spacecraft
and instrument payload for the long journey ahead.
As we continue to fly outward from the Sun at 78,700 kilometers (48,600
miles) an hour, our communications time, or RTLT (round trip light
time), is increasing rapidly. In fact, it’s now approaching an hour and
a half round trip, at the speed of light! For that reason, our mission
and payload operations team has been working to complete a whole series
of activities that are best done now, before the communications time
increases still further. Since late September, they’ve completed each
of
the following activities:
- Loading and enabling the Guidance and Control flight software
build 4 (G & C 4.0), our cruise load, which I wrote about in my
previous two columns. - Completing initial calibrations and taking the first cruise
science plasma data with our particles and fields instruments,
PEPSSI and SWAP. - Calibrating Venetia, our student-built dust counter, to filter
out
noises on the spacecraft that mimic the dust particle impacts
we’re trying to study. - Loading and activating Autonomy Flight Software build 14,
containing a series of enhancements and bug fixes to our onboard
fault detection and correction software.
In addition to these activities, the spacecraft team at APL and our
navigation team at KinetX, Inc., in southern California, have carefully
compiled all available tracking data to show that our actual course is
very close to the optimal trajectory to the Pluto aim point at Jupiter.
At a meeting on Oct. 19, we concluded that we’re so close to the
optimal
course that no corrections appear to be necessary on the way to
Jupiter.
Actually, we got a formal solution telling us we should make a
mid-December course correction of about 0.4 meters/second (0.9
miles/hour), but the effort to design and test this engine burn wasn’t
worth the time it would take away from more important activities, like
planning the best possible Jupiter encounter. Moreover, the error
induced by skipping this maneuver will be small - only about 870
kilometers (550 miles) at Jupiter. That may sound like a lot, but when
you compare that to the Pluto aim point’s 2.48 million-kilometers
distance from Jupiter, you see that we’re on a bulls-eye course with a
predicted error of 0.035% (that’s 3.5 parts in ten thousand, folks,
equivalent to throwing a football two miles and still putting it within
easy reach of the receiver). So we’ll accept this tiny aim-point error
up for now, and then make it up in our planned post Jupiter trajectory
flyby clean up engine burn next spring.
Back on Earth, we’ve been putting increasing amounts of effort into
planning the long list of command sequences needed to take of the many
calibration and science observations associated with our Jupiter flyby.
Keep in mind that our mission team is very small - literally less than
a
couple of dozen operations people to plan and test all of this activity
and control the spacecraft.
Every activity we plan - each imaging session, each spectroscopic
observation, each plasma instrument activity, et cetera, et cetera
requires us to design, code, and then test a unique command sequence
two
ways: via a software simulation and a run on the spacecraft simulator,
followed by test results reviews and then uplinking to our bird out
there past 4 AU.
With more than 165 observations on the Jupiter encounter wish list and
a
record-setting fast trajectory to Jupiter, we recently found ourselves
on too tight of a schedule to get everything planned, built, tested,
and
ready for uplink to New Horizons.
So last week, as mission PI, I opted for us to go light on distant
observations beginning in January and early February. This meant
cutting
out almost 2/3 of the originally hoped for observations in January.
Most
of these involved the two instruments I am responsible for, Alice and
Ralph. Although we’re still left with more than 20 science activities
in
January (a record compared to anything we did during instrument
commissioning), including plasma monitoring on approach, imaging of
Jupiter, and some distant calibration activities, there was pain in
this, because I had to sacrifice a bevy of Io torus and Jovian auroral
observations on approach.
The reward for this sacrifice on distant approach, however, comes at
Jupiter. There, in an intensive 10-day period around our Feb. 28
closest
approach, we now have a fighting chance of designing, testing and
carrying out virtually all of the approximately 100 observation
sequences planned to occur when we’re in the heart of the Jupiter
system.
It’s no fun to cut anything out, but we’re a small team on a tight
budget and the time until we reach Jupiter is rapidly running out. In
fact, observations will begin just two months from now - on Jan. 1,
2007!
Until then, we’re planning a quiet couple of months on the spacecraft
so
we can focus on Jupiter activity planning and testing. Other than
routine spacecraft housekeeping, we only plan a handful of cruise
science measurements with SWAP and PEPSSI, a couple more Ralph, LORRI,
and REX commissioning activities, and a test of our dual transmitter
communications that we hope will allow us to double our downlink rates
from Jupiter.
It’s getting late in the day and I still have a New Horizons talk to
prepare for the Texas Space Grant Consortium. But before I go, I have
two more things to tell you about. The first is a recap of some of the
exciting new Pluto-system science results presented at this year’s
Division of Planetary Science (DPS) meeting, which took place in
Pasadena, California, in mid-October. The second is about the New
Horizons communications blackout period in late November.
Let’s begin with the news from planet Pluto and its moons. At DPS, more
than 1,000 scientists and engineers gathered to present new results on
every aspect of solar system formation, evolution, and content you can
think of. Among the new findings I found most interesting for New
Horizons, and therefore for you on this blog, are the following:
* The spectroscopic discovery of ethane (C2H6) on Pluto's surface,
presented by Dale Cruikshank of NASA/Ames Research Center (a New
Horizons co-investigator) and colleagues. This ethane is produced
from the photolysis or radiolysis (i.e., the chemical conversion
driven by sunlight and charged particles) of frozen methane (CH4)
on Pluto's surface and suspended in its atmosphere. Ethane on
Pluto has long been predicted, but no one had actually detected
it
before.
* A series of talks by various workers from MIT, SwRI, Lowell
Observatory, and Williams College in the U.S., and from France,
reporting the results of the June 12 occultation of a star by
Pluto, giving us a new opportunity to probe Pluto’s atmosphere.
Among the findings that have emerged from that event were that
Pluto’s atmosphere remains at the higher pressure levels seen in
2002, with no sign of cooling or collapse just yet. However, an
increasing amount of turbulence is being seen in Pluto’s
atmosphere. What that means isn’t clear. A series of future
stellar occultations will occur over the next five years, giving
us a quasi-continuous capability to track Pluto’s atmospheric
evolution during most of the flight of New Horizons. This will
help keep us from being overly surprised about its atmospheric
state when we arrive.
* An observation by Jason Cook (ASU) confirming that ammonia
hydrates lie on Charon’s surface. This exotic mixture was
tentatively identified on Charon by Will Grundy of Lowell
Observatory (another New Horizons co-investigator) in 2002 and
2003. Its discovery indicates Charon isn’t just a dead water ball
the diameter of Texas, but has a more interesting story to tell,
possibly including recent cryovolcanism on its surface. We’ll be
on the lookout for that with LORRI, Ralph and Alice when New
Horizons arrives.
* Tight constraints on any extant rings at Pluto, reported both by
Jay Pasachoff (Williams College) and Andrew Steffl (SwRI).
* The first ground-based detections of Pluto’s small satellites,
Nix
and Hydra, reported by Cesar Fuentes (Harvard) and also
Christophe
Dumas (JPL). Now that Nix and Hydra can be studied from the
ground - in addition to from Hubble - I expect our knowledge of
them
to increase even faster.
Well, that wraps up the new science results I wanted to highlight for
you. I’ll close now with a little more on the communications blackout
that will occur later this month.
The reason for the upcoming blackout is that we have a solar
conjunction
coming up in late November. More specifically, from November 19 to 27,
New Horizons will be almost exactly on the opposite side of the Sun
from
Earth, with the spacecraft’s position in Earth’s sky less than 3
degrees
from the position of the Sun, making communications difficult. This
happens to most planetary spacecraft each year, and some of you will
recall that the Mars rovers and orbiters all experienced a solar
conjunction blackout in October.
As a result of the Sun’s interference, we won’t plan to communicate
with
New Horizons at all during those eight days, but we will monitor its
carrier signal to see how close to the Sun we can operate in future
solar conjunctions. We think we can successfully communicate more like
2
degrees off the Sun, rather than the 3 degrees we’re sure of. If this
proves out to be true, then each subsequent annual
conjunction/communications blackout the rest of the way to Pluto and
across the Kuiper Belt will be dramatically shorter than this one. I’ll
let you know how we did on that test in my next update, sometime in the
first part of December.
In the meantime, keep on exploring!