The PI's Perspective: "Exploration at Its Greatest"
2 May 2006
(Source: Johns Hopkins University Applied Physics Laboratory)
With the rush of events surrounding launch over, I am back to writing this column about once per month. We're more than 100 days into flight now, and in every respect, New Horizons continues doing fine.
As you know, the New Horizons mission team spent the first couple of months checking out the spacecraft subsystems and making our initial post-launch trajectory correction maneuvers. All of that went exceedingly well: We have a very healthy spacecraft flying right on its intended course to the Pluto aim point it must reach at Jupiter on February 28, 2007.
April included our crossing the orbit of Mars, outbound at over 75,000 kilometers an hour (47,000 miles/hour), on April 7. That was a nice milestone, but the biggest spacecraft event of the month was a new software load for our Command and Data Handling (C&DH) system. This load, called C&DH 3.5, went up and on line a few days before we crossed the orbit of Mars - on April 5. C&DH 3.5 contained a fix for a bug that we wanted to protect against well before we update the code in a more extensive way after the summer. That code version, called C&DH 4.0, will include a variety of capability enhancements, including data-compression capabilities we'll need for downlinking Pluto data.
I'll have more to say about the C&DH 4.0 load in a few months. For now, I just want to say that the 3.5 load is up and running as expected. To invoke a new C&DH load after it is transmitted up to the bird, one has to reboot the main spacecraft computer. So you can imagine how much care, how many design reviews, how much event simulation, and how much nail biting was involved in planning for this. Of course, the Applied Physics Laboratory (APL) spacecraft and mission ops teams made it look easy on April 5, which is a real sign of the careful advance work put in over several weeks leading to that big day.
With the spacecraft doing well, most of the activities of April centered on instrument checkouts. Ralph, our main remote-sensing suite, and REX, our radio science experiment, both performed flawlessly in their initial functional checks. These occurred on March 21 and April 19, respectively.
Additionally, the SWAP solar wind detector, which opened its launch door on March 13 (the 151st anniversary of Percival Lowell's birthday, no less!), turned on its detectors on March 28 for the first time. All went well.
Meanwhile, the LORRI imaging team has been collecting pre-door-opening calibration images to characterize their detector noise in flight. They are seeing some additional, nuisance-level noise events over what was seen on the ground. This is common when you get your instrument into the space environment, and something we expected since our spacecraft is carrying an RTG that was installed after the instrument calibrations. In fact, we expect the Ralph and Alice detectors to see the same kind of elevated, but still nuisance-level, noise when they calibrate in May.
Speaking of May, both PEPSSI (on May 3) and Alice (on May 20) will soon open their detector doors. Carefully, step by step, both of these instruments will then be fully powered and have their detectors turned on for "first light" measurements shortly thereafter. Next up: Ralph's front door will open on May 29. But since Ralph's door has a see-through window, first light and some early calibrations will be made on May 10. These will each be big milestones: we are opening up our "eyes" to space!
Yet another milestone will be our first "AU crossing," which will occur on May 7 when our spacecraft crosses 2 Astronomical Units and is twice as far from the Sun as the Earth. We'll have 31 more AU to go to reach Pluto, but just 3.2 AU to go to reach Jupiter.
Where Is the Centaur Rocket?
Some of you have been asking what became of our Atlas' Centaur stage. As background, our Atlas first stage and its solid rocket booster never were intended to make it into Earth orbit, so they are resting at 1 AU, deep under the Atlantic Ocean; and our uppermost, STAR-48 stage that sent us on our way to Jupiter and Pluto, is headed to Jupiter and the Kuiper Belt, just like New Horizons. But the Centaur, which propelled us into Earth orbit and then out of it, isn't on an escape trajectory from the Sun. Instead, it's on an orbit that takes it from about 1 AU out just over halfway to Jupiter, The figure below shows the Centaur's path for the over its first two years of flight.
For you orbit mechanics aficionados, the orbital elements of our Centaur have been carefully calculated by Lockheed Martin's Brian Lathrop, the lead flight designer of our Atlas launch team. Here they are:
Semi-major axis = 3.0046694E+08 kilometers
Eccentricity = 0.51053830E+00
Inclination = 0.57429941E+01 degrees
Argument of perihelion = 1.1910526E+02 degrees
Longitude of the ascending node = 5.0774401E+01 degrees
Mean anomaly = 0.27115934E+00 degrees
True angle = 0.97319138E+00 degrees
For those interested only in the basics, the Centaur's orbit is essentially in the plane of the Sun's equator, like the nearby planets, and stretches from 1 AU to 3 AU, with an orbital period of 2.8469 years. When New Horizons reaches Pluto in July 2015, the Centaur will be on its fourth orbit of the Sun, outbound, just beyond the orbit of Mars.
A Trojan Course?
And while we're on trajectory matters, it's worth noting that we have just realized that New Horizons itself will be traversing through one of the Trojan regions of Neptune in 2014. For a long time, astronomers wondered if there were asteroids trapped in Neptune's Trojan regions, but in recent years a few have been discovered. These fascinating bodies probably represent a sample of the most primitive bodies in the solar system, like comets and Kuiper Belt objects.
Only a handful of Neptune Trojans are currently known, but more will no doubt be found in coming years. If any of those come close enough to New Horizons to be usefully studied, we want to plan observations.
To see if we can help that exciting prospect along, we've alerted our professional colleagues in the planetary astronomy community and asked for their help searching for new Neptunian Trojans in the region of space where New Horizons will fly as it crosses Neptune's orbit in the summer of 2014.
Finally, I just want to point to an exciting new prospect for New Horizons at Pluto itself: faster data rates. Our APL-based telecommunication team, led by Chris DeBoy, has worked out a way to use our redundant (opposite polarization) transmitters simultaneously to double our data rates. This "pump you up" technique will be tested later this year and used from time to time to reduce our need for downlink time on the Deep Space Network (DSN) on the way to Pluto.
When we reach Pluto, we plan to use the higher data transmission rates to cut the time required to send all of our data home in half-from what was almost 9 months, to just under 4.5 months. Even more impressively, the higher data rate will allow us to send home a "lossy compression" dataset with all of our spectra, all of our images, and all of our other data products within just two or three weeks of encounter! After all the years of delayed gratification that this mission entails, this is welcome news indeed. After all, everyone will be on the edge of their chairs in the summer of 2015 to see Pluto revealed - scientists and laypeople alike!
Well, that's all I have for now. So until next time . . .
-- Alan Stern