| I am giving two presentations in Ventura County, CA, in March. See below for more detail. On January 1, 2019, NASA’s New Horizons spacecraft flew by a tiny remote Kuiper Belt object (KBO) named Arrokoth and captured stunning images. This is the story of how New Horizons was able to come within 2200 miles of something 15 miles wide and over 4 billion miles from Earth. New Horizons launched in January 2006. Its primary mission was a close flyby of Pluto, which it achieved in July 2015, taking spectacular images of the former ninth planet (see my newsletter of that month). This image shows New Horizons’ flight plan from its launch to Pluto and through the Kuiper Belt. With a working spacecraft careening through the Kuiper Belt, NASA searched for other interesting targets to explore with New Horizons’ limited remaining fuel. Using the Hubble Space Telescope, the team found 5 candidates, with Arrokoth being the most accessible. Arrokoth is so small and remote that even our largest ground-based telescopes cannot image it, due to atmospheric disturbance. Hubble’s relatively small 95-inch (2.4m) mirror can determine its celestial coordinates to an uncertainty of ±0.05 arc-seconds, which corresponds to ±1000 miles. That might seem AOK, but it’s not. Telescopes can measure two angular coordinates — like latitude and longitude — but not distance. For New Horizons (moving at 28,000 mph) to rendezvous with Arrokoth (moving at 10,000 mph), 4 years after passing Pluto, requires precise knowledge of the KBO’s orbit. New Horizons formed a special KBO_Chasers team, led by Marc Buie, to observe stellar occultations by Arrokoth — events in which the KBO eclipses a distant star. A single occultation can define its orbit 100 times more precisely than a single Hubble observation. The trick is knowing which star to observe, and when and where to observe it. Like a total solar eclipse, the “shadow” of a KBO stellar occultation rapidly sweeps across Earth’s surface in a narrow band, but is much harder to observe. Arrokoth’s shadow is 20 miles wide, lasts 2 seconds at each location, and eliminates the light of only one of many thousand nighttime stars. These, like other eclipses, require placing mobile telescopes wherever the heavens dictate. With initially only rough orbital estimates from Hubble, Buie et. al. identified 4 stars that Arrokoth would likely occult from June 2017 to August 2018. The team launched campaigns for all 4 occultations, from Africa to South America to the west Pacific. Each event refined Arrokoth’s orbit, and made the next campaign more effective. Included in the second event was NASA’s airborne telescope SOFIA, flying from Christchurch, New Zealand. Its skilled pilot was able to bring the plane within 1000 feet of the target point within 2 seconds of the target time. The team’s primary tool was an array of 22 telescopes with 16-inch apertures and high-performance cameras. Amazingly, each of the 22 sets cost only $5000. The third event crossed southern Argentina. The city and university of Comodoro Rivadavia, nicknamed “Capital of the Wind”, provided invaluable assistance, setting up large trucks as windbreaks and eliminating headlight glare by closing a nearby highway for two hours. After its orbit was refined, Arrokoth was promoted to Minor Planet # 486958. With precision astrometry, New Horizons did rendezvous with Arrokoth and captured this and other images. Arrokoth is comprised of two planetesimals — “Ultima” and the smaller “Thule” — that fused together. Arrokoth’s overall dimensions are 22 by 14 by 9 miles. It takes 16 hours to spin on its axis, and 298 years to traverse its solar orbit, which is 45 times larger than Earth’s. It is the most distant object ever explored by spacecraft, and the reddest solar system object yet seen. Spectral data reveal Arrokoth has methanol, water, and organic compounds. Assuming Its density is similar to comets, its gravity is so weak that if I were standing on its surface, I would weigh less than a third of an ounce, and if a coin fell out of my pocket, it would take over 30 seconds to hit my shoe. Due to the paucity of bodies in the Kuiper Belt, and their low orbital speeds, impacts are rare. Arrokoth’s surface is nearly pristine — only one significant crater in the upper part of Thule. Arrokoth is so far away that it takes 6 hours for radio signals from New Horizons to reach Earth. If you think your internet connection is slow, the complete download of all Arrokoth data will take 21 months. Best Regards, Robert
March, 2020
Note: Previous newsletters can be found on my website.
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