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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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