...and 2019 and beyond

When will we get to see all that juicy data? As with the Pluto flyby, it'll take a year or so to get all the data down, but they have planned early downlinks of some of the best stuff. Two "failsafe" downlinks before closest approach will send some of the best pre-encounter content to Earth just in case New Horizons does have an unlucky meeting with a little bit of space gravel as it flies past. A successful "phone home" four hours after the flyby will inform Earth that no such meeting happened. And three early downlinks planned for January 1 and 2 will contain all the science highlights, hopefully.

Almost as soon as the encounter and earliest downlinks are over, New Horizons will go into solar conjunction from January 4 to 7. I asked Alice Bowman what implications that has for the mission, and she replied: "During solar conjunction, the downlink data rates are lower because of interference from the Sun. Generally, because New Horizons has fairly short solar conjunctions, we can stand down from science data playback (as the chances of dropping data is increased); we also make sure not to conduct any critical uplinks or sequence transitions, and we set the spacecraft’s command loss timer to allow it more time to receive commands."

Most of 2019 will be focused on spinning downlink. There will also be two periods of three-axis mode for distant observations of Kuiper belt targets. Beyond that, Kuiper belt observations will continue to alternate with hibernation periods until April 30, 2021, the end of the Kuiper extended mission.

After that, what next? One possibility is a further mission extension, during which they could potentially cooperate with the Voyager team to perform heliospheric observations. They began to lay the groundwork for such a mission extension at a collaborative meeting between the two science teams on January 16, the day before the New Horizons meeting. The conclusion of the workshop as reported to the rest of the team by Ralph McNutt: the placement of two Voyagers and one New Horizons at similar solar longitude but different latitudes and distances was not planned, and taking advantage of the coincidence would provide worthwhile science. Working together should enable new discoveries, making more from the two missions than their individual contributions. The instrument packages of the two missions complement each other. For example, New Horizons' ultraviolet spectrometer can detect lyman-alpha emissions from interplanetary hydrogen, while the Voyagers' ultraviolet instruments have been turned off for years. The Voyagers, by contrast, have working magnetometers, while the comparatively diminutive New Horizons was never planned to have one.

The Voyagers should still be able to run for at least another 5 years. Beyond that, power will be critically low. McNutt said that their goal was to try to keep the lights on until 2027, the 50th anniversary of the launch. "A bit dicey," he characterized it, but not inconceivable. New Horizons' power is also diminishing with time. Like the Voyagers and Curiosity, New Horizons is powered by the decay of a limited number of plutonium atoms in a radioisotope thermoelectric generator (RTG). As the plutonium decays, the total output of the power source also decays. You'd think the decline would be logarithmic (that is -- the more time has elapsed, the slower the rate of decay) because the decay of plutonium is logarithmic. But decay of an RTG is actually closer to linear, because the slowing of the decay of the plutonium is balanced by the speeding of the decay of the performance of the aging thermocouples in the power supply. So the output of New Horizons' RTG decreases by about 3.5 watts per year. It's currently at about 193 watts, will be at about 190 at the time of the MU69 flyby, and will be down to 150 by 2032, when the spacecraft will be 82 astronomical units away. 150 watts is supposed to be the minimum for spacecraft operation, but mission systems engineer Chris Hersman said that with some ingenuity they should be able to keep it running considerably farther than 82 AU. It will definitely not still be operable by 120 AU. The end will be somewhere in between 80 and 120, but he can't predict where yet.

Don't expect much news about New Horizons as it slumbers between now and wakeup in June. But there's plenty of action behind the scenes, as the science team prepares for the encounter, publishes papers about the Pluto system, drafts papers about Kuiper belt science, and continues delivering data to NASA's Planetary Data System. The first data release from science beyond Pluto is due out from the mission this summer. Tune in in June for extreme detail on MU69 encounter plans!