New Horizons: Journeying Beyond Saturn
- From: baalke@xxxxxxxxxxxxx
- Date: Mon, 4 Aug 2008 15:17:03 -0700 (PDT)
http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_current.php
The PI's Perspective
Journeying Beyond Saturn
Alan Stern
July 29, 2008
As avid followers of New Horizons know, our spacecraft has been mostly
hibernating since February, and will continue to so do until Sept. 2,
when we will wake it to begin its second annual checkout. Many of you
will also recall that New Horizons passed the orbit of Saturn in early
June. New Horizons is the first spacecraft to venture this far (a
billion kilometers from the Sun!) since the last of the Voyagers
accomplished the same milestone in the summer of 1981. We are now
nearly
96 million kilometers (60 million miles) beyond Saturn, and will cross
the orbit of Uranus - about 2 billion kilometers from the Sun - in
March
2011.
My last PI Perspective
<http://pluto.jhuapl.edu/overview/piPerspectives/
piPerspective_05_01_2008.php>
detailed our plans to wake New Horizons up in late May for two weeks
of
onboard engineering activities. That brief wake-up went well; we
re-pointed the antenna and loaded SWAP instrument software, among
other
accomplishments. We then put our spacecraft back into hibernation on
June 3, right on schedule.
After that, we resumed our weekly Monday check-ins with the spacecraft
to verify its health. In June, what had been our weekly Thursday
telemetry ("TLM" on the mission calendar) contacts to monitor
engineering during hibernation became every-other-Thursday events. As
we
gain experience in hibernation, our goal is to check this telemetry
just
once a month beginning in 2009 (though weekly beacon health-status
checks will continue whenever we are in hibernation, right through
2015).
Of course, spaceflight isn't as routine as other forms of flight, and
that was re-emphasized to us on Monday, July 7, when our weekly beacon
check-in revealed that New Horizons was transmitting a
"red' (emergency)
beacon instead of its familiar "green" (nominal flight) beacon. This
told us that the spacecraft had experienced a significant anomaly in
the
past week. With the help of NASA's Deep Space Network of tracking
stations, our mission operations team immediately swung into action,
contacting the spacecraft that evening and downloading telemetry
diagnostics the next day. By mid-week our operations team had
diagnosed
the problem and had devised a recovery strategy. Our main flight
computer had unexpectedly reset itself after becoming hung up in a
software loop. By Friday, July 11, our operations and engineering
teams
had assessed this anomaly, determined that it was safe for the
spacecraft to re-enter hibernation, and commanded New Horizons to do
so.
In the three weeks since, New Horizons has hibernated uneventfully,
sending green beacons every Monday while our spacecraft computer
engineering team, led by Steve Williams of the Johns Hopkins
University
Applied Physics Laboratory, worked to diagnose why our main computer
(called "C&DH-1") had gotten itself hpretty boring, since we plan to
hibernate throughout the month. The
figure below summarizes that plan; in it you can see the Monday beacon
passes ("Bea") using one DSN station each for 1.5 hours, and the
bi-weekly Thursday telemetry contacts that use a DSN antenna for eight
hours. You can also see that the Venetia Burney Student Dust Counter
(SDC) instrument will be taking data all month, as is normal during
hibernation. Those with a sharp eye will notice an extra Thursday
telemetry pass at the end of August; this is because we want to gather
extra engineering data just before emerging from hibernation on Sept.
2.
Some of you may also notice the code "SDC 005" on a few days near the
end of August; this indicates a special internal calibration of the
SDC
we'll perform when hibernation ends, to help with data analysis from
the
instrument.
Like a kid returning to school in the fall, New Horizons gets busier
after August. The plan for September-October-November involves a zoo
of
checkout activities, spacecraft tests, software updates, and
instrument
calibrations and tests that comprise our second ACO ("Annual
Checkout").
ACO-1 was conducted last fall; we didn't conduct one in 2006 because
we
were still commissioning the spacecraft and instrument payload in
preparation for our early-2007 Jupiter flyby.
I'll write more about ACO-2 in my next two updates, but just to give
you
a feel for how much more active the spacecraft will be, compare the
September schedule (below) to the August schedule, and think about
this:
October will be busier still! As our spacecraft hibernates in August,
our mission operations and science operations teams are planning and
testing the detailed command sequences for ACO-2, which is, without
doubt, an intensive, full-time job.
Pluto Science
Before I close, I want to tell you about two recent science results
that
relate to our main flyby target - the Pluto system - and give you a
heads up on a cool new way to follow the progress of New Horizons.
The first of the two science results appears in the August
Astronomical
Journal, in a paper by Bradley Shaffer of Louisiana State University
and
a host of co-workers from various institutions. Brad and company
accurately reanalyzed and recalibrated photographs of Pluto taken from
the 1930s through the early 1950s. The really neat thing they found is
that Pluto's surface appearance changed a good amount during that
time,
indicating that frost deposits are migrating around the surface on a
global scale due to seasonal and/or orbital distance changes. This has
long been suspected, but Brad and his team could conclusively prove it
because the parts of Pluto we could see in the early 1930s and early
1950s were identical - something that hasn't occurred since. Their
analysis of the old data using modern techniques made it possible to
detect what astronomers of the mid-20th century had missed, and
therefore allowed Shaffer's team to rule out the competing theory -
that
Pluto's changing photometric properties were just due to our seeing
Pluto from differing angles over the decades. This result also
confirms
the prediction that global atmospheric change is important on Pluto.
Indeed, this may even portend other kinds of changes (like day-to-day
or
day/night frost migration) will be discovered when New Horizons makes
detailed surface maps as it approaches Pluto in 2015.
The other scientific result is about the planet's largest moon,
Charon.
Submitted for publication by New Horizons co-investigator Mike Summers
of George Mason University and three co-workers (myself included), the
result is based on new computer models, rather than new data. These
models show that some of Pluto's escaping atmosphere is captured by
Charon's gravity, creating a tenuous atmosphere around Charon which -
if
confirmed by New Horizons - can be used to help diagnose the escape
rate
from Pluto.
As I close, I want to tell you that the New Horizons project is
"Twittering." If you don't know about Twitter, go to www.Twitter.com
<http://www.twitter.com/> or look up Twitter on Wikipedia; it's a
micro-blogging service. Most weeks we will post a one-sentence update
on
what New Horizons is doing or where it is, or write something else
interesting about the project. To get these updates, check out
www.twitter.com/NewHorizons2015 <http://www.twitter.com/
NewHorizons2015>
from time to time, or create your own Twitter account and check the
box
to follow "NewHorizons2015" so you get updates whenever we post them.
Pretty sweet - especially if my posts and the other occasional news
items
on the New Horizons Web page aren't enough for you!
Well, that catches you up with where New Horizons is and what the
spacecraft and project team have been doing. I'll be back with more
news
in September. In the meantime, keep on exploring, just like we do.
ung up. Although this investigation
is ongoing, we have held a review board and we are using test versions
of the C & DH (Command and Data Handling) system to reproduce the
failure here on the ground. I'll update you on this when we know more.
As we continue out toward the Kuiper Belt, our flight plan for August
is
.
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