Transmitting to ISEE-3

“I VOLUNTEER AS TRIBUTE” is what Dana must have heard.

I probably said something to the extent of, “it sounds like fun to spend 2.5 hours sweating profusely in a barely ventilated dome in a confined space with a 400 kW transmitter requiring manual switching to talk to a spacecraft launched before my parents even considered having kids” or, “that might be fun to do later this week”. Not, “sure, I’ll go spend my afternoon plugging and unplugging a cord in the name of science and archaic flight hardware”. To Dana, this meant “absolutely, yes: today”.

Lunchtime conversations at Arecibo Observatory range from the mundane (“these pork chops are awful this week, save them for your dog; I’m getting a sandwich”) to scientific (“another way to think of the “dunite dilemma” is to consider that there may be an excess of olivine on Earth and that the asteroid belt is not actually depleted in this mineral”) to political (“… and that’s how we saved planetary radar at the observatory!”) to practical (“can we schedule that meeting for a time when everyone is around?”).

Dana’s usual henchman was away at a workshop in Berkeley building radio spectrometers with our favorite SETI hardware guy, so his usual partner in transmitter switching crime was gone.

I finished the press release draft about asteroid 2014 HQ124 we’d observed the day prior, a regular old asteroid but a real humdinger of a radar opportunity: predicted signal to noise of this object was in the tens of thousands per transit/receive cycle. My phone rang.

“Phil says we can go up now,” said Dana’s pitched voice. I ran over to the control room, grabbed a hard hat, and shoved a paper towel into the rim to catch sweat. We teetered down the hill to the cable car, waiting for our ride to the platform.

Late afternoon and the sky was mostly clear. Blue patches were hazy with Sahara dust, blown thousands of miles across the Atlantic and keeping it from raining here. When it rains you can’t go up to the platform, it isn’t safe, especially with the risk of lightning.  The afternoon looked clear, so we milled around the cable car building, waiting for our ride to the telescope platform.

Unlike in Goldeneye, the cable car doesn’t take off very fast: it’s a slow, steady ride up to the platform.  James Bond could have run up and down the catwalk several times before 006 reached the telescope platform via cable car.  A system of interlocks prevents the cable car from taking off without you.  After pushing the “up” button, the motor engages, wheels begin to spin, and the car gently lurches (if that’s at all possible) up a highly-greased cable.  Away from the cable car motor housing, you can just hear the squish of the wheels on the greasy cable and the jungle sounds below.

The dome grows larger and larger as the cable car ride continues, and eventually you hear the pulse tubes cooling the receivers in the dome.  My first night riding up the cable car in the dark I saw fireflies lazily drifting in the darkness below the dish.

How planetary radar works at Arecibo Observatory

We know Arecibo Observatory for its 305-meter (1000-foot) diameter telescope and its appearances in Goldeneye and Contact.  Aside from battling Bond villains and driving red diesel Jeeps around the telescope (grousing at the site director about the funding status of projects is optional), several hundred hours a year of telescope time at Arecibo go toward radar studies of asteroids.  Tasked to “find asteroids before they find us”, a group of us four planetary radar astronomers at Arecibo (as well as collaborators and colleagues at institutions outside of Puerto Rico) observes asteroids for NASA’s Near-Earth Object Observation program. We study the orbits and surface properties of our rowdy neighbors, near-Earth asteroids.

William E. Gordon telescopeHow do we transmit and receive radio waves using the klystrons and radio receivers at the observatory, and how do we turn these into images of asteroids?  Read more on my guest post at the Planetary Society on how planetary radar at Arecibo Observatory works.

Guiding out the waves: engineering planetary radar

I spent last week observing of asteroids from the telescope: our first night was just another 8-4 workday night, where we looked at space rocks in our neighborhood and out beyond Mars for eight hours.

Arriving at the control room, I sat down with my binder full of… notes on how to observe at Arecibo, which hadn’t made much sense.  At any optical observatory you enter your coördinates into a computer and take an image.  After verifying that you pointed the telescope correctly, you then tell the telescope system to take data for the rest of the night, occasionally adjusting pointing or focus.

Hector está llamado por telefono

At Arecibo, you’re pointed in the right direction.  Alignment with the William E. Gordon Telescope is not an issue.  Even with the 305-meter dish, you’re good to a few millimeters.

Position of the platform, receiver, transmitter

Getting the signal out of the receiver and properly into the computer is the hard part of observing.  Instead of a few shiny silvered mirrors and a charge coupled device digitizing and sending your photons to a screen, here a maze of waveguides, cables, and wires brings signals from the matte metal dish, after being ushered into the receivers, along a path 1,600′ long to the control room.  Where computer monitors would display starfields at an optical observatory, wavy lines danced across oscilloscopes at Arecibo.  It felt like junior year electronics laboratory again in the physics department, so different from most of the things we were doing in astronomy, and not just because of cgs/MKS units arguments.

Ellen had considered walking me through cabling the week before our six-night-long marathon observing run, but ultimately decided that it wouldn’t make sense out of context.  Wait for the actual observing run to understand the cabling.

Cabling

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A week at Arecibo Observatory

Receiver

A little over a week ago I arrived at the largest single-dish radio telescope in the world.  My boss picked me up at the airport and drove me up into the hills of Puerto Rico’s karst country where I caught my first glimpse of the support towers of Arecibo Observatory, peeking out over the wooded hills.

8 o'clock tower

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