The New Horizons science mission to the Pluto-Charon system is about to begin

It's been a long journey, but it's nearly over: New Horizons is just about ready to begin its science mission to Pluto, Charon, Styx, Nix, Kerberos, and Hydra. The spacecraft has spent most of its decade-long trip napping in a hibernation mode that required little energy from either the spacecraft or its human controllers, but this snoozy phase ends this weekend. On December 7 at 02:30 UT ( December 6 at 18:30 PT), New Horizons will wake up for the final time; it will remain awake and alert for two years. The Planetary Society's Mat Kaplan will be hosting a video event during the wakeup, so you can watch with us as we wait for that all-important beep that signals that New Horizons is ready to begin work.

New Horizons' flyby of Pluto happens on July 14, 2015 at 11:50 UT, but it will begin gathering science data in January and will not finish returning all of the data until late 2016. Let's take a look at the spacecraft we have sent on this journey, and at the work it's about to begin.

New Horizons' Instruments

What New Horizons can tell us about the worlds of the Pluto system depends upon the capabilities of its instruments. New Horizons is by far the smallest spacecraft ever sent to the outer solar system, and its instrument package is sized to match, weighing only 30.4 kilograms. For comparison, Cassini's camera instrument alone weighs 57.8 kilograms. In general, mass serves as a proxy for instrument capability, but the New Horizons engineers took advantage of every opportunity to miniaturize components and made careful (and sometimes painful) selections of instrument capabilities in order to make the most of the mission's relatively tiny payload.

New Horizons has seven science instruments. Spacecraft instruments come in two main flavors: remote sensing (which look at things from far away using various parts of the electromagnetic spectrum) and in situ instruments (which measure things directly sensed at the position of the spacecraft, including magnetic fields and ions, dust, or other particles).

Remote sensing instruments include:

• Alice, an ultraviolet spectrometer seeing wavelengths from 46.5 to 188 nanometers and a spatial resolution of 5 milliradians per pixel. Primarily for studying Pluto's atmosphere, it is similar to the instrument of the same name on Rosetta.
• Ralph, which consists of two sub-instruments, MVIC and LEISA. MVIC is a multispectral imager with 5 channels in visible and near-infrared wavelengths from 400 to 975 nanometers and a spatial resolution of 20 microradians per pixel. It has a wide field of view and color imaging capability. There are panchromatic, blue, red, methane, and near-infrared channels (note the lack of a green channel, which means that some kind of simulation and/or manipulation of color channels is necessary to produce "true" color images from New Horizons data). LEISA is an infrared imaging spectrometer spanning 1.25 to 2.5 microns, for measuring composition and temperatures of surfaces.
• LORRI is a camera sensitive to wavelengths of 350 to 850 nanometers. It has very high resolution of 5 microradians per pixel so is used for detailed imaging as well as long-range optical navigation. It is monochrome, so the best images from New Horizons will be black-and-white, but LORRI images can readily be colorized with lower-resolution MVIC data.
• REX can perform 4.2-centimeter radiometry and will perform uplink radio occultations of Pluto's atmosphere (that is, it will send a radio signal through the atmosphere to giant radio dishes on Earth).

In-situ instruments include:

• PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation) has 12 channels spanning higher energies of 1 to 1000 keV, designed to study pickup ions from Pluto's escaping atmosphere.
• SWAP (Solar Wind at Pluto) is a solar wind analyzer sensitive to lower-energy particles, from 25 eV to 7.5 keV.
• The Venetia Burney Student Dust Counter measures size distribution and density of particles with masses greater than a picogram; it has been active throughout the long cruise.

The New Horizons mission is also shaped by the exigencies of deep-space communications. New Horizons has a huge dish, relative to its size, but a 2.1-meter dish is not very big when you are more than 4 billion kilometers away. Data transfer is slow, and it's not possible at all when the spacecraft is pointing its instruments at specific targets -- which is what it will be doing throughout the closest approach. There will be long, scary periods during the two weeks around the closest approach where New Horizons will be silent, and all we can do is hope that it will start talking to us again when we expect it to.

The Long Approach

The mission at Pluto is divided into several phases. In each phase, New Horizons will focus on the kind of science appropriate to its range from Pluto. New Horizons is traveling so fast that the actual close-approach part of the encounter happens in an incredibly short period; nearly all of the most important goals for the mission are met in the time from 2.5 hours before to 1 hour after closest approach. One exception to this is global image covering: Pluto takes about a week (6.4 days) to rotate, so the best global maps will be composed of images gathered beginning a couple of days before closest approach. The science observations for the encounter have been planned with both prime and backup observations and with redundancy among instruments to make sure the mission's goals are met if one observation or even one instrument fails. The observations focus primarily on Pluto, Charon, and Nix: rather than scatter observations across both Nix and Hydra, they chose to characterize Nix well and Hydra less well. The other two moons, Styx and Kerberos, are even smaller than Nix and Hydra (hence, more difficult to observe) and were discovered quite late in the mission planning process; they'll be imaged in group shots, optical navigation imagery and satellite system movies, but don't have science observations devoted to them specifically.

• Approach Phase 1: 180 to 100 days before closest approach (Jan 6-Apr 4; range to Pluto is 226-121 million km). SWAP and PEPSSI will measure plasma. LORRI will monitor motions of Pluto, Charon, and the smaller moons. Pluto is barely resolved.
• Approach Phase 2: 100 to 21 days before closest approach (Apr 4-Jun 23; range to Pluto is 121-26 million km). Add in color observations, and search for satellites and rings. The start of this phase is chosen to roughly coincide with the time when LORRI has better resolution than Hubble, but Pluto will still be only a few pixels across.
• Approach Phase 3: 21 to 1 days before closest approach (Jun 23-Jul 13; range to Pluto is 26-1.2 million km). Includes best, second-best, and third-best rotation coverage before closest approach, yielding the best global maps of Pluto and Charon. PEPSSI and SWAP may detect pickup ions and bow shock. LEISA and Alice can begin looking for variability in IR and UV. Search for clouds or hazes, tracking winds.
• Near Encounter Phase: -1 to +1 days (Jul 13-15, within 1.2 million km) -- sequenced in 2008 and 2009. Most of the highest-priority observations.
• Departure Phase 1: 1 to 21 days after closest approach (Jul 15-Aug 4; range to Pluto is 1.2 to 24 million km). Remote sensing of Pluto and Charon is performed for only 1 rotation. SWAP and PEPSSI study magnetotail, pickup ions. REX studies nightside temperatures. Nix and Hydra high-phase observations. Search for rings.
• Departure Phase 2: 21 to 100 days after closest approach (Aug 5-Oct 22; range to Pluto is 24 to 119 million km).
• Departure Phase 3: 100 to 180 days after (Oct 22-Jan 1, 2016; range to Pluto is 119 to 203 million km). No remote sensing observations planned.

Optical Navigation imaging

Beginning in January, New Horizons will use its sharpest LORRI camera to take images of Pluto, Charon, Nix, and Hydra for optical navigation purposes. Navigators will use the photos to refine the precision of their predictions of where each body will be, when. Of course, the images will also be useful for science. Geologists will study them and compare them to Hubble observations to look for further surface changes on Pluto, and astrophysicists will use the navigational information to determine the masses of Pluto and Charon more precisely. Late in the approach, as Pluto looms larger, the New Horizons team has done its best to command images at smooth intervals for the future creation of a movie of New Horizons' approach.

While it's very exciting that New Horizons will begin imaging the Pluto system in January, it's important to keep your anticipation aligned with the reality of the photos. LORRI is a very high-resolution camera, but Pluto is not very big (it's smaller than any of Jupiter's Galilean moons) and will be, for the most part, far away from New Horizons. Small and far away means few pixels and not a lot of surface detail, even as late as June of next year. In the composite below, I looked at actual LORRI images of Jupiter's moons, taken during the flyby in 2007, and figured out when, in New Horizons' approach to Pluto, LORRI's imaging of Pluto should have comparable resolution. It's not a perfect comparison, for a couple of reasons. We know already that Pluto has a more contrasty surface than Io or Ganymede. Also, by June, every single image New Horizons takes of Pluto will be the best image ever taken of Pluto, so it's going to be thrilling no matter what. But there won't really be enough pixels for us to begin to say anything particularly insightful about what is happening on Pluto's surface until the beginning of July.

Another caution about planned optical navigation: photos focused on Pluto and Charon will use the full resolution of LORRI. But for most of the approach, photos focused on the smaller moons Nix and Hydra will use LORRI in a 4-by-4 binned mode that reduces the spatial resolution of the LORRI instrument by a factor of 4. This seems counterintuitive (you might think you'd want higher resolution to spot the smaller moons), but the tiny moons are so dim that it will be necessary to bin photons gathered from 16 LORRI pixels together to get enough signal to pick the tiny moons out of background noise. All of these approach images see objects in the Pluto system at nearly full illumination, with a phase angle of 14 to 15 degrees.

I wrote an earlier post in which I discussed the optical navigation campaign more thoroughly. The images in that post were based on a preliminary observation table. The following table is up-to-date as of yesterday and lists all of the planned optical navigation images of Pluto, Charon, Nix, and Hydra, including when they will be taken, and from what distance and with what resolution. At the outset, the best resolution is about 1000 kilometers per pixel -- barely more than 2 pixels across Pluto (which is roughly 2400 kilometers across). Even by July 1, LORRI will have a resolution of 80 kilometers per pixel, obtaining 30 pixels across Pluto's disk. It's going to be a long wait for those frame-filling photos that we all want!

One final note about the op navs: you may notice that images will be captured almost daily for a while beginning January 25. But captured is not the same as returned to Earth; it may be several days before the data come down. The New Horizons team has demonstrated a commitment to sharing information and data with the public, but they may not have systems in place yet to do so automatically. So be patient with them as they get their first Pluto data -- they know the value of sharing the LORRI images with the public and will do so as quickly as their small team is capable of it.

The Pluto Encounter

The moment of the flyby was chosen carefully to allow solar and Earth occultations by both Pluto and Charon, which will allow New Horizons to probe their atmospheres. Fortuitously, this moment was also the best for surface studies, with good views of Pluto's bright terrain, dark terrain, bright/dark transition, and carbon monoxide-rich longitudes. It was near optimal for viewing Nix and Hydra, and had Pluto/Charon separations that needed little slewing time to move between them.

On Sunday and Monday, July 12 and 13, New Horizons will downlink a set of "Fail Safe" data -- a subset of data that captures the highlights of the science data acquired up to that point, just in case the spacecraft does not survive the close approach. The last Fail Safe communication session will end at about 9:30 PDT / 12:30 EDT / 16:30 UTC on July 13, and will contain the best color (MVIC) views of Pluto and Charon and the best LORRI image of Charon (about 160 pixels across) that will be returned before the flyby. A final, brief pre-flyby communication session ending at 20:15 PDT / 23:15 EDT July 13 / 03:15 UTC July 14 is planned to return the best image of Pluto acquired to date, at a resolution of 3.8 kilometers per pixel; Pluto will be about 630 pixels across in that image.

Closest approach to Pluto is on Tuesday, July 14 at 11:50 UT, as time is measured on the spacecraft. During the close encounter, New Horizons will approach to within:

• 13,700 kilometers of Pluto;
• 29,500 kilometers of Charon;
• 22,000 kilometers of Nix; and
• 77,600 kilometers of Hydra.

At the moment of closest approach, it would take 4 hours 25 minutes for signals to get from Pluto to Earth. However, New Horizons will not be in contact with Earth at that time; it will be pointed at Pluto system targets, feverishly gathering highest-priority science. The first moment that we on Earth will hear from New Horizons after closest approach is not until July 14 at 18:09 PDT / 21:09 EDT / July 15 at 01:09 UTC! That moment is called the "Phone Home" and will essentially be a "beep" indicating that the spacecraft is healthy and still running its science program. The first data from the close approach phase -- including single-frame, black-and-white photos of Pluto, Charon, and Hydra -- will not be received for another 10 hours, on Wednesday, July 15 at about 04:00 PDT / 07:00 EDT / 11:00 UTC on Earth.

I will write another post later that contains much more detail about what images we will get on Earth from New Horizons, in what order. It's time to get excited! ...but also to be patient.

Sources for this post included:

• The Architecture of New Horizons' Pluto Fly-By Sequence, by Kimberly Ennico;
• New Horizons: Encountering Pluto and KBOs, by Leslie Young and Alan Stern (open access);
• my notes from the New Horizons science team meeting in July;
• and personal communication with Kimberly Ennico (thanks, Kim!)