Radio Telescope Pinpoints Pluto for Spacecraft Buddy
The cold surface of Pluto and its largest moon Charon as seen with ALMA on July 15, 2014.
The Atacama Large Millimeter/Submillimeter Array, or ALMA, is nearing completion after over 30 years of planning and collaborating among astronomers and engineers from several nations. The completed array will consist of 66 antennas and two supercomputers, called correlators, at the backend to collect the signals and make the array function as one large telescope.
On March 12th and 13th, 2013, over a hundred journalists descended upon the array along with politicians, scientists, engineers, and other VIPs to celebrate the official inauguration of the array and tour its facilities. I was there along with 11 other journalists from around North America as guests of the National Radio Astronomy Observatory.
Overview of the telescope's technical capabilities, science goals, and history were given by (from left to right) Michael Thorburn, Head of the ALMA Department of Engineering, Pierre Cox, incoming ALMA director, Thijs de Graauw, current ALMA director, Al Wootten, ALMA Program Scientist for North America, and Ewine van Dishoeck, Professor at Leiden University and former ALMA board member.
The ALMA control room is the hub of activity at the "low site" or Operations Support Facility. From here, telescope operators manage and control all array operations and conduct observing runs. Astronomers who win time on the telescope through a merit-based proposal process do not actually travel to the site or control the telescope in real time. Instead, as with other radio interferometers, they prepare observing scripts based on what science they want to achieve, and their scripts are then scheduled and run by ALMA staff. The data is then made available to the astronomer for a proprietary period of one year, when it is then released to the public.
Everyone gets into the spirit of ALMA, which in Spanish actually means "soul," as an art contest was held for local children. Featured here are some of the submissions and several winners in the six to nine year old group.
54 of the final 66 antennas were at the "high site" or Array Operations Site, at an altitude of 16,500 feet. The 12-meter dishes are contributed by partners in Europe, North America, and East Asia, whereas several smaller, 7-meter dishes from East Asia make up a compact array in the center. The completed array will eventually have the capability of expansion by moving the antennas to different concrete "pads" spread around the Chajnantor Plateau. With a baseline, or distance between antennas, of up to 14 kilometers, ALMA will be able to obtain resolution ten times better than that of the Hubble Space Telescope.
Reporters were allowed a peek at the massive correlator behind glass in the second highest building in the world. The correlator is the computer that brings together signals from all the antennas so that the astronomer can make an image using the full array. We received a thorough explanation of its working from correlator engineer Alejandro Saez who has spent time constructing in in Charlottesville, Virginia, and at the high site in Chile. The correlator has the processing power of 3 million laptops as it has to make calculations for up to 1,125 antenna pairs billions of times per second.
The antennas actually put on a show for us during our three hour tour of the high site, slewing, or moving, back and forth. These antennas are built for perfection, or as close to it as one can come. These sturdy, yet flexible machines must maintain a dish surface accuracy of the width of a human hair and pivot back and forth between a target source and calibrator source on the sky every ten seconds.
Taking a hundred or so journalists to a site at 16,500 feet (5000 meters) altitude is quite risky business. Everyone must pass a basic fitness exam to be cleared for access, though employees must endure a much more rigorous process involving a stress test and heart monitoring to work on site. A team of diligent paramedics were there to hand out oxygen bottles and check for signs of altitude sickness. Here, I got my pulse checked in an ambulance after running around like an excited child a bit too much.
The "front end" of a radio telescope is the place where radio light that is collected by the dish is received, amplified, and transferred to the next stage for digital processing. The large blue drum is a cryostat and holds the most sensitive electronics in the telescope at a cold 4 Kelvin using liquid helium. The silver circles are the actual windows into the feedhorms, the first stage where radio light passes into the front end. Don't think those metal plates are transparent? They are to the radio light that ALMA receives.
Just one of ten sets of electronics that can fit inside those blue drums are on display here. Various stages of the system are cooled to difference temperatures, and engineers needs to wear special gloves and shirts when handling these so that they do not impart a spark of static electricity that could ruin a sensitive (and expensive) piece of equipment. The superconducting receivers used by ALMA are state-of-the-art and were developed specifically for this purpose.
How do you move a giant antenna? You build a giant antenna mover. The on-site guests were treated to an amazing site when one of the two antenna crawlers rumbled down a huge dirt road to pick up one of the dishes that is still under testing. Otto and Lore, as the crawlers are named, have to pick up each 100-ton antenna and place them down on their pads with millimeter accuracy. They move the antennas from the Operations Support Facility up to the high site and back and will be used to change array configurations to change the telescopes resolving power.
Space is a big place. And things move, fast. So imagine if you’re a spaceship on a decade-long journey to reach a tiny world billions of miles away. Sure, orbital dynamicists have a very accurate view about where the planets will be at any given time, but it helps to have a precision lock on a celestial body’s location so you don’t miss your flyby encounter… or even hit it.
As NASA’s New Horizons mission is set to make its historic flyby mission of Pluto on July 14, 2015, a powerful ground-based observatory is currently refining the dwarf planet’s location. Why? Well, as it orbits so far away (at least 40 times the distance the Earth orbits the sun) and its orbital period is so long (it takes 248 Earth years for Pluto to complete one orbit), astronomers’ best measurements of Pluto’s location could still be thousands of miles out.
“With these limited observational data, our knowledge of Pluto’s position could be wrong by several thousand kilometers, which compromises our ability to calculate efficient targeting maneuvers for the New Horizons spacecraft,” said New Horizons Project Scientist Hal Weaver, from the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., in an NRAO press release.
A margin for error this large could become problematic for a space probe that is currently flying through interplanetary space at 10 miles per second (36,000 miles per hour), so it needs to make fine adjustments as it gets closer to its target.
To provide a more precise view on Pluto’s location ahead of a scheduled New Horizons course correction in July, mission scientists have enlisted the help of the Atacama Large Millimeter/submillimeter Array (ALMA), located in Chile.
To measure distances throughout the Cosmos, astronomers traditionally use the method of astrometry to track the motions of the planets, for example, relative to the “fixed” stars many light-years beyond. But when we’re talking about threading an interplanetary needle 40 AU away, a more precise form of astrometry is needed as even stars drift regardless of their distance from us.
Using quasars — ancient active galaxies that can be spied over 10 billion light-years away by the most sensitive radio observatories on the planet — ALMA can provide this mindblowing precision (as these objects are truly fixed in the Universal landscape), adding a new dimension of positioning data to back up decades of optical observations of the distant world.
“The ALMA astrometry used a bright quasar named J1911-2006 with the goal to cut in half the uncertainty of Pluto’s position,” said Ed Fomalont, an astronomer with the National Radio Astronomy Observatory in Charlottesville, Virginia.
During this campaign, ALMA has been able to detect the radio emissions being generated by the frozen Pluto and moon Charon and has recorded the pair’s orbital locations several times since November 2013. As these observations occurred at different times in the Earth’s orbit around the sun, the tried and tested method of parallax can be applied to this high-precision method of observation.
“By taking multiple observations at different dates, we allow Earth to move along its orbit, offering different vantage points in relation to the sun,” said Fomalont. “Astronomers can then better determine Pluto’s distance and orbit.”
It’s nice to know that even when you’re a spacecraft barreling through the solar system, when communications take 4 hours to traverse interplanetary space, telescopes back on Earth have your back, making sure you arrive in the right place at the right time.