New Horizons: Updates From the January 2014 Science Team Meeting, Part 2
Posted by Ted Stryk
24-01-2014 13:16 CST
Editor's note: This is Ted's second report from the meeting. Read the first one here. --ESL
The weather here in Baltimore has been quite Plutonian, barely breaking into the double digits Fahrenheit and with snow on the ground. It is against this backdrop that New Horizons team continued to work to prepare for the 2015 flyby.
The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument and Solar Wind Around Pluto (SWAP) instruments have been studying a region of the outer solar system that hasn't been studied since Voyager. PEPSSI operates even when New Horizons is in hibernation and the team is working to characterize the charged particle environment that Pluto encounters. This spring, they will be using NASA's Chandra X-Ray observatory to see if it can detect Pluto. What they would be measuring is the interaction between Pluto's atmosphere and the solar wind. While it is a long shot, a successful detection would greatly enhance our ability to interpret the results of PEPSSI during the encounter. SWAP has shown that the Solar Wind has been generally calm in recent years, although it got a little more active toward the end of 2013.
The Student Dust Counter is also hard at work, measuring the distribution of dust grains in the outer solar system. Basically, the distribution looks like a donut shape, with a gap in the area around Neptune.
There is a risk that New Horizons will discover that the planned trajectory through the Pluto system is too dangerous due to dust. So it will begin searching for hazards at 99 days out and repeat the search at 65, 47, 41, 29, 21, 18, and possibly 12 days. Through the 18 day search, it would be possible to switch trajectories, but early discovery is desirable because the sooner the spacecraft can maneuver, the less fuel it will use up. At 12 days, the only option would be to turn the spacecraft to that it was travelling high gain antenna first, using it as a shield, but since this would cost valuable science, this is only something that would be used in extreme circumstances.
If the trajectory is changed to a SHBOT (Safe Haven By Other Trajectory), it could mean a number of things, depending on the nature of the threat. Most are farther out, but one would be much closer, in the case that there is a dangerous dust field with a "doughnut hole" opening at the center cleared by Charon, which New Horizons can pass through. The downside of this is that most of the closest images would be badly smeared.
The fuel reason is also the reason that the team has been studying historical photos from 1930-1950 and using a diverse range of telescopes including the ALMA submillimeter telescope to refine Pluto's position. The sooner that this, along with optical navigation from New Horizons itself, can pin down the position of Pluto, the sooner it can make its final trajectory correction maneuver, meaning that fuel would be saved. And saving every drop of fuel they can is critical.
This is because of the state of the KBO search. So far, although 51 KBOs have been found, none are within reach of New Horizons. The more fuel it has, the more ability it will have to maneuver should a candidate or candidates be found. The goal would be to discover such an object early enough that we would have at least a year of orbital measurements in order to refine its position well enough to retarget the spacecraft in the fall of 2015 (before then, such maneuvers are obviously off the table, since it would mean missing Pluto). If a burn couldn't be made at this time, the area in which New Horizons has enough fuel to get to gets smaller and smaller.
There is a proposal to involve the Hubble Space Telescope in the search. This would involve a massive, 160-orbit search with Hubble using the WFC3 and ACS instruments in parallel. Such a survey has some scientific value in itself, but based on current estimates of the KBO density in the area, which is now thought to be only half as dense as when the mission was designed, the main thing is that it would give them an 89% chance of discovering at least one suitable object. Given the late notice, the massive amount of time needed, and the non-zero risk of failure, this proposal is a bit of a long shot. But, to paraphrase Alan Stern, without Hubble, we might find a KBO for New Horizons to encounter but probably won't. With Hubble, we might not find a KBO for New Horizons to encounter but probably will. Still, a deeper search with Japan's Subaru telescope is planned, using a new, improved camera, so even without Hubble, there is a chance. Should an object be found, plans are to fly within 25,000 kilometers of it, so it should be quite spectacular.
As I mentioned before, this summer will mark the 25th anniversary of the Voyager Neptune encounter. In recognition of this, on July 19, New Horizons' LORRI imager will take an image of Neptune and Triton, showing them separated by 18 pixels. Beyond this, most of what is going on is calibration and testing for the encounter. There was also going to be a calibration period at the beginning of the encounter period as well as afterward, but the one before the encounter has been canceled due to the risk that a malfunction could happen without time to fix it.
Observations are being made from the ground, including lightcurves, spectroscopy, and stellar occultation observations in a campaign going from 2014-2016. The goal is to put the observations that NH makes in context of previous and future observations. Basically, Pluto has until now been an astronomical object, then will become a place where humanity has an emissary, and then go back to being an astronomical object (in terms of how we study it). This also will give us a "ground truth" for measurements of other KBOs taken from earth and its vicinity.
Things are definitely picking up, and will only get busier until most of the team effectively camps out in Baltimore for the summer for the flyby. The encounter phase begins in January, and as it gets closer, it will begin to take lightcurve data, first of Pluto and Charon and later of the smaller moons, to begin to characterize them and, in the case of the smaller moons, determine their rotation periods. Also, the positions of Nix and Hydra will be refined. The remaining, distant outer moons, have positions less well known than Nix and Hydra, but New Horizons will not get close enough to them to need as much targeting accuracy as it needs with Nix and Hydra. Depending on who you ask (based on different assumptions of LORRI capability), New Horizons will beat Hubble resolution in May or early June. Closest approach is on July 15, and it will continue observing Pluto's thin crescent for two more weeks. This will be a truly historic time, where multiple worlds, and indeed a whole region of our solar system, come within our grasp.
Finally, a correction. While it will be far from complete, the images and data, including the LORRI images I referred to yesterday but also from other instruments, will allow us to see the some of the juiciest pictures quickly. Also, my impression was that the lossy images being downlinked from September to November were thumbnails. It turns out that they will be JPEG-like images with 20:1 compression, which means they will be quite spectacular, even if not up to the quality of the lossless images that will eventually come down over the next year. So this won't require the kind of patience I thought – I am very glad to have been wrong!
It has been a real treat to join the New Horizons Team again. We often hear of an individual or two behind a mission, but to see the multitude of dedicated people who sacrifice years of their lives to making missions like New Horizons happen is an amazing experience. Team leaders are important, but without full teams of expert, dedicated members, solar system exploration wouldn't happen.