Thursday, January 3, 2019

Hats off to New Horizons

A few years ago, around the time of the New Horizons encounter with Pluto (or if you're really serious about the demotion thing, minor planet 134340 Pluto), I gave the team a bit of grief over the probe having to go into "safe mode" with only days left before the flyby, though I also tried to make clear that this was still engineering of a very high order.

Early on New Year's Day (US Eastern time), New Horizons flew by a Kuiper Belt object nicknamed Ultima Thule (two syllables in Thule: THOO-lay).  I'm posting to recognize the accomplishment, and this post will be grief-free.

The Ultima Thule encounter was much like the Pluto encounter with a few minor differences:
  • Ultima Thule is much smaller.  Its long axis is about 1-2% of Pluto's diameter
  • Ultima Thule is darker, reflecting about 10% of light that reaches, compared to around 50% for Pluto. Ultima Thule is about as dark as potting soil.  Pluto is more like old snow.
  • Ultima Thule is considerably further away (about 43 AU from the sun as opposed to about 33 AU for Pluto at the time of encounter -- an AU is the average distance from the Sun to the Earth)
  • New Horizons passed much closer to Ultima Thule than it did to Pluto (3,500 km vs. 12,500 km).  This requires more accurate navigation and to some extent increased the chances of a disastrous collision with either Ultima Thule or, more likely, something near it that there was no way to know about.  At 50,000 km/h, even a gravel-sized chunk would cause major if not fatal damage.
  • Because Ultima Thule is further away, radio signals take proportionally longer to travel between Earth and the probe, about six hours vs about four hours.
  • Because Ultima Thule is much smaller, much darker and significantly further away, it's much harder to spot from Earth.  Before New Horizons, Pluto itself was basically a dot, with a little bit of surface light/dark variation inferred by taking measurements as it rotated.  Ultima Thule was nothing more than a dot, and a hard-to-spot dot at that.
  • We've had decades to work out exactly where Pluto's orbit goes and where its moons are.  Ultima Thule wasn't even discovered until after New Horizons was launched.  Until a couple of days ago we didn't even know whether it had moons, rings or an atmosphere (it appears to have none).  [Neither Pluto nor Ultima Thule is a stationary object, just to add that little additional degree of difficulty.  The Pluto flyby might be considered a bit more difficult in that respect, though.  Pluto's orbital speed at the time of the flyby was around 20,000 km/h, while Ultima Thule's is closer to 16,500 km/h.  I'd think this would mainly affect the calculations for rotating to keep the cameras pointed, so it probably doesn't make much practical difference.]
In both cases, New Horizons had to shift from pointing its radio antenna at Earth to pointing its cameras at the target.  As it passes by the target at around 50,000 km/h, it has to rotate to keep the cameras pointed correctly, while still out of contact with Earth (which is light-hours away in any case).  It then needs to rotate its antenna back toward Earth, "phone home" and start downloading data at around 1,000 bits per second.  Using a 15-watt transmitter slightly more powerful than a CB radio.  Since this is in space, rotating means firing small rockets attached to the probe in a precise sequence (there are also gyroscopes on New Horizons, but they're not useful for attitude changes).

So, a piece of cake, really.

Seriously, though, this is amazing engineering and it just gets more amazing the more you look at it.  The Pluto encounter was a major achievement, and this was significantly more difficult in nearly every possible way.

So far there don't seem to be any close-range images of Ultima Thule on the mission's web site (see, this post is actually about the web after all), but the team seems satisfied that the flyby went as planned and more detailed images will be forthcoming over the next 20 months or so.  As I write this, New Horizons is out of communication, behind the Sun from Earth's point of view for a few days, but downloads are set to resume after that.

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