by Alan Stern
Washington DC (SPX) Jul 31, 2008
New Horizons passed the orbit of Saturn on June 8th and continues on its way to exploring the last of the nine planets known prior to the exciting discovery of many new, dwarf, planets in the solar system's distant Kuiper Belt-our scientific destination.
As avid followers of New Horizons know, our spacecraft has been hibernating most of the time since February, and will continue to so do until September 2nd, when it will be awakened to begin its second annual checkout. Many of you will also recall that New Horizons passed the orbit if 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 almost 60 million miles beyond Saturn, and will cross the orbit of Uranus-about two billion kilometers from the Sun as Saturn-in March 2011.
In my last PI posting, I wrote about our plans to wake New Horizons up in late May for two weeks of onboard engineering activities. That brief wake up went well, accomplishing the antenna re-pointing, SWAP instrument software load, and other activities that I described in my May 1st posting to you. Following that, our spacecraft was put back into hibernation on June 3rd, right on schedule.
We then resumed our weekly Monday check ins with the spacecraft to verify it is in good health. And starting in June, what had been our weekly Thursday telemetry (TLM) passes for engineering monitoring during hibernation became every other Thursday events. As we gain experience in hibernation, we are weaning ourselves toward a goal of making these telemetry checks just once a month beginning 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 7th when our weekly beacon (BEA) check in revealed that New Horizons was transmitting a red (emergency) beacon instead of its familiar green (nominal flight) beacon.
This indicated 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 dumping telemetry diagnostics to the ground the next day.
By mid-week our operations team had diagnosed the problem and had planned a recovery strategy. The problem that had occurred was that our main flight computer had unexpectedly reset itself after becoming hung up in a software loop. By Friday July 11th, our operations and engineering teams had assessed this anomaly, determined that it was safe to re-enter hibernation, and commanded the spacecraft to do so.
In the three weeks since then, New Horizons has hibernated uneventfully, sending green beacons every Monday, while our spacecraft computer engineering team, led by Steve Williams of the Johns Hopkins Applied Physics Laboratory (APL), worked to diagnose why our main computer, called C and DH-1, had gotten itself hung up.
Although this investigation is not complete yet, we have held a failure review board and are using groundbased testbeds of the C and DH system to attempt to reproduce the failure. I'll update you further on this investigation when we know more.
As we continue outbound to the Kuiper Belt, our flight plan for August is pretty boring, since we plan to be hibernating throughout the month. The figure just below shows that flight plan in summary form. In it you can see the Monday beacon passes using 1 DSN station each for 1.5 hours and the every other week Thursday telemetry passes that use a DSN antenna for 8 hours.
You can also see that the Student Dust Counter (SDC) instrument will be taking data all month, as is normal in hibernation. Those of you with a sharp eye will notice there is an extra Thursday telemetry pass at the end of August. This is because we want to take some extra engineering data just before emerging from hibernation on September 2nd. Some of you way also notice the code "SDC 005" on a few days near the end of August; this indicates we'll be performing a special internal calibration of the SDC at the end of the hibernation to help with data analysis from the instrument.
While our August flight plan may be nearly catatonic, 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 annual ACO ("Annual Checkout"). ACO-1 was conducted in the fall of 2007 (in 2006 we did not conduct an ACO because we were still busy commissioning the spacecraft and instrument payload in preparation for our early-2007 Jupiter flyby).
I'll be writing more about ACO-2 in my next two updates here, but just to give you a feel for how busy the spacecraft will be by comparison, look at the September schedule just below to the August schedule, and think about this: October will be far busier still! As our spacecraft hibernates in August, our mission operations and science operations teams are planning and testing all of the detailed command sequences for ACO-2, which is without doubt an intensive, full time job.
Before I close for this time, I want to tell you about two recent science results that relate to our main flyby target-the Pluto system, and I also want give you a heads up on a cool new way to follow the progress of New Horizons.
The first of the two science results I want to tell you about is a paper that just recently appeared in the Astronomical Journal's August issue; it's by Bradley Shaffer of Louisiana State University and a host of co-workers from various institutions. What Brad and company did was to accurate reanalyze and recalibrate old photographs taken of Pluto taken in the 1930s, 1940s, and 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 this for the first time because the parts of Pluto we could see in the early 1930s and early 1950s were identical-something that hasn't occurred since.
Their more accurate analysis of the old data using modern techniques made it possible to detect what astronomers of the mid-20th century had missed in these data, 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 important role of global atmospheric change on Pluto, as had been predicted. Indeed, this may even portend other kinds of changes (like day to day or day/night frost migration) will be discovered when New Horizons arrives and makes detailed maps over a period of many weeks as it approaches Pluto in 2015.
The other scientific result that I want to share with you isn't about Pluto itself, it's about the planet's largest moon, Charon. This result, which has been submitted for publication by a team led by New Horizons Co-investigator Mike Summers of George Mason University and three co-workers (myself included), isn't based on new data, it's based on new computer models.
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.
The million outer planets of a star called Sol
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