Science Floats
What a satellite can do, balloons can do cheaper.
TWO IN THE MORNING IN EASTERN New Mexico. About 20 people have gathered at an abandoned World War II bomber training base. There's nothing here except leftover hangars. Grass grows through cracks in the pavement. Visitors are warned of rattlesnakes. On the steel siding of one building, the letters "NASA" have been painted. The building serves as a weather station and payload preparation facility for scientists who send instruments to the very top of the atmosphere by hitching them to giant balloons.
On this warm, spring night, an X-ray telescope is ready for flight. A windsock dangles vertically, showing what looks like dead calm, but since the balloon with its payload will stand nearly 900 feet tall when released—by comparison, the Washington Monument is only 565 feet—surface calm is just one of the weather conditions necessary for a launch.
A technician releases a small rubber test balloon, some two feet across an inflated with helium. It rises rapidly on a cord, like a fish pulling a line, and reaches an altitude of a thousand feet, at which point it pulls its tether to a noticeable angle. The "piball," or pilot balloon, is showing winds aloft. A second piball, tracked with a theodolite—a small telescope linked to a computer—reports winds up there as high as 35 mph. Launch director Erich Klein cancels the attempt, and the small group disperses.
This is par for the course of NASA's balloon people, the meteorologists and launch crews of the National Scientific Balloon Facility, a government-owned, contractor-operated organization that launches approximately 20 science payloads a year from here in Ft. Sumner, New Mexico, as well as from its headquarters in Palestine, Texas, and launch pads in Canada, Australia, and Antarctica. "I've seen it take a month to get off one balloon," grumbles astronomer Jonathan Grindlay of the Harvard-Smithsonian Center for Astrophysics in Massachusetts. Grindlay, this morning's launch customer, needs to get his telescope to 128,000 feet in order for it to detect X-rays in the upper atmosphere. He is testing the effectiveness of two X-ray instruments.
Klein and his crew take the waiting in stride. "You learn to be patient," he says. "It does get frustrating. Sometimes we wait weeks. But eventually, a week can turn everything around."
Launching balloons has grown more demanding over the years. Much of the early work took place at Palestine, but around 1990 there was a significant expansion. New sites for launches were developed in Australia, Antarctica, and Canada, and the launch crews had to be willing to travel to those places.
"We went through a difficult transition," NSBF site manager Danny Ball recalls. "It took several years for guys to say, 'I just can't do this anymore.' So when one of those guys would leave, we'd bring in another person and explain very carefully that part of his job would be to go on the road three to five months out of the year.
"It took several years to get the crew we've got today," he continues. "People who don't mind the travel, who are really interested in the work."
At any of the locations, the launch crews are constantly at the mercy of the weather. Senior meteorologist Glenn Rosenberger, who has been helping launch balloons for almost 50 years, uses five different mathematical models to predict the weather, and he says they sometimes give him five different answers. In that case, he says, he consults with his colleagues, who may disagree with his conclusions. In the end Rosenberger relies on his many years of experience to decide which prediction to trust.
Whatever the predictions, balloons can't be launched in anything but a calm. The team members sometimes get dispirited when they have a run of bad weather, says Klein, but when the weather turns favorable, "you can get two off in three or four days. Then the world looks a lot brighter. Before then it's, 'Oh, I'm never going to go home.' [Then] boom, you get one off and you see the light at the end of the tunnel.
"There are a couple of other balloon programs in the world," he continues, "but we are the best. We fly the biggest balloons, the heaviest payloads, for the longest durations at the highest altitudes. It's an incredible high when you get one off."
The scientists have a different perspective. Jonathan Grindlay describes the long waits for a launch as "painful." "You're not able to do as much with your finite grant dollars," he says.
Fortunately, it didn't take a month to get Grindlay's X-ray telescope in the sky over Ft. Sumner. Less than a week after the first attempt, the telescope, which weighed 4,800 pounds, made it up. "High-altitude winds blew it to the east, towards Texas," NASA manager Steve Smith recounts. "Then those winds changed and blew it to the west. It crossed Interstate 25 toward evening, in clear sky and at very high altitude. A lot of people saw it and started making phone calls. They thought they were seeing a UFO from Roswell."
For Grindlay, the launch was worth the wait. His team was able to evaluate the properties of a small prototype X-ray detector, and a larger instrument provided spectra for further study of the well-known X-ray source Cygnus X-1. And the launch, supported by the Astronomy and Physics Division of NASA's Office of Space Science, cost less than $200,000. The balloon, a standard 40-million-cubic-foot model, cost about $120,000, plus another $12,000 for the helium to fill it. The cost of payload integration and launch services, about $25,000, was also absorbed by NASA, as par of the operating costs of the agency's National Scientific Balloon Facility.
Had Grindlay launched his payload on a Delta II rocket, it would have cost between $40 and $50 million for the service. Yes, the payload would have orbited and returned data for years, as opposed to hours. But the low cost of a balloon launch places it well within the budget of university research groups of modest size, and recently, a few noteworthy discoveries made by such groups suggest that scientists look again at the humblest of launch vehicles.
Not only is a balloon cheap to launch, the culture of balloon science encourages thriftiness in the assembly of payloads as well. One researcher avoided paying $30,000 for a space-related video camera and used an ordinary $200 security camera, relying on bathtub caulking when he needed additional electrical insulation. A gamma-ray telescope built by a team at the California Institute of Technology in Pasadena used home-movie video cameras to store data during the flight. One group of scientists, needing to protect photomultiplier tubes from stray light, fashioned shields from beer cans.
At Raven Industries, a balloon manufacturer in Sulphur Springs, Texas, senior engineer Mike Smith points out that in the world of balloons, "you don't see clean rooms. The instruments are built by guys wearing T-shirts and jeans. Probably every NASA flight has plywood as part of its payload," rather than the titanium honeycomb or similarly exotic materials used in satellites.
According to Mike Zimmerman, Raven's chief of quality control, payloads can also be cheaper because balloons can carry odd shapes that would have to be folded to fit onto a rocket, then unfolded in space. With balloons, he says, "you can have solar panels sticking out. You don't have to withstand G-loads" or strong vibrations of a rocket launch. Designers avoid costly test programs, since they don't have to demonstrate that their instruments can withstand such forces. In fact, balloons have served as test platforms for instruments that were later space-rated and flown on satellites. The Compton Gamma Ray Observatory, on of NASA's premier astronomical satellites, is one that benefited from balloon tests. The satellite helped astrophysicists learn about violent events occurring near quasars, neutron stars, black holes, and supernovae, or exploding stars. Such events produce gamma rays and X-rays, which the orbiting observatory was able to detect. "Every instrument on the Compton was first developed on a balloon," says Jonathan Grindlay.
Yet for all their usefulness, balloons have an ongoing problem: They don't stay up very long. Most flights last between 12 and 24 hours. Those launched from Ft. Sumner are not allowed to cross the Colorado River, the state line of California, because they would pose hazards to air traffic if they descended near Los Angeles. Those launched from the National Scientific Balloon Facility's main base in Palestine, Texas, must come down before they cross the border of Mexico, only a few hundred miles away, because that country will not allow overflights. Ground controllers send a radio command to release the payload and its parachute if winds carry it toward the border. That action tears the balloon, which, venting helium, descends. Even balloons that fly in Australia, crossing the Outback and the Indian Ocean, stay up no more than a few days.
When borders aren't the problem, the duration of a balloon flight is limited by physics.
As a balloon rises, its helium expands. By the time it reaches its targeted altitude—in excess of 22 miles—the gas has expanded to fill the volume of the balloon, typically 40 million cubic feet. (The National Scientific Balloon Facility Web site notes that you could fit two Boeing 747s back to back inside the envelope.) The balloon has enough lift to rise still higher; however, it can expand no more and would burst if it continued to ascend. To prevent that, polyethylene tubes serve as vents, permitting surplus helium to escape. The excess lift vanishes, and the balloon flies near the desired altitude.
The night falls. The balloon cools, contracts in volume, and sinks. To prevent it from descending, ground controllers send a command to drop ballast. When the sunlight of the following morning warms it anew, it rises again—and because it has dropped ballast and therefore lost weight, it vents still more helium. Such cycles can continue only as long as the balloon has ballast. Once the ballast is gone, it descends for good.
If there were a way to interrupt the warming and cooling cycle, a balloon flight could last longer. One alternative is to travel to a place where the sun never sets. For the past 10 years, NASA's balloonists have launched from a base in Antarctica, where during the south polar summer the sun shines continuously. The balloons stay aloft for as long as two weeks, and several years ago, one of these longer flights produced headlines.
Just after Christmas in 1998, a mission known as Boomerang (for "Balloon Observations of Millimetric Extragalactic Radiation and Geophysics"!) was launched from Williams Field, six miles from McMurdo Station in Antarctica. Boomerang was intended to further the work begun by the Cosmic Background Explorer satellite, which in 1992 made historic observations (with instruments first tested on balloons) of microwaves that fill all space with a thin electronic fog, the faint remnant of the Big Bang. COBE measured very slight differences in temperature—variations as small as 0.0001 degree—among various parts of the sky. Boomerang provided a finer measure, with high enough resolution to show the size of the "hot" and "cold" patches on the sky. According to Andrew Lange of the California Institute of Technology, one of the chief investigators on the project, "The Boomerang map shows structures that are the right size to have evolved into galactic superclusters, so for the first time there's a visible link between the embryonic universe and the present universe." The measurements also support the leading theory of origin of the universe (see "The Big Push," below).
"This was a real high point in balloon science," recalls Grindlay. "It made the cover of Nature." (That, for scientists, is what making the cover of Rolling Stone is for rock musicians.)
On reason that Boomerang could produce such phenomenal results is that it stayed up for 10 days. Moreover, Boomerang, appropriately enough, returned almost to its starting point because of circumpolar winds that stay nearly constant in latitude. Boomerang remained at or near 79 degrees south and traveled 5,000 miles. A radio signal then brought it down within 30 miles of its launch site.
If momentous discoveries can be had from a 10-day balloon mission, imagine what scientists could do with a balloon that stays aloft 10 times that long. NASA's balloon program office is at work on a new science balloon that could fly as long as 100 days. The Ultra Long Duration Balloon would be the first real alternative to satellites.
The ULDB is sealed and does not vent helium. The envelopes of conventional balloons are too delicate to with stand the pressure of expanding helium, but the ULDB is designed to be stronger. Mike Smith of Raven Industries, which is assembling ULDBs, explains that instead of expanding in sunlight and contracting at night, the ULDB is able to maintain a constant volume while the pressure increases and decreases.
The ULDB takes its strength from "tendons," long cords that run from top to bottom like lines of longitude on a globe, dividing the balloon into narrow zones. Within each zone, the plastic film relieves stress by bulging outward. Artists' renderings show only a few such tendons and give a ULDB the appearance of a pumpkin. Actually there are some 300 of them. The internal pressure is low, only around 0.03 pound per square inch, but a ULDB is as large as a football field and has a lot of square inches. "Even with the low pressure it's very tight," says NASA's ULDB project manager, Steve Smith. "The tendons are like guitar strings."
The ULDB programs started in 1997, but materials available at the time were either too heavy or lacked the necessary durability. Then in 1999 and new synthetic fiber became available: Zylon.
For decades the ultimate in high-strength materials had been Kevlar, which is used in bulletproof vests. But for large ULDBs, Smith says, "Kevlar is not strong enough. Zylon has about four times the strength. The only thing that's stronger is carbon fiber." A short length of tendon looks like braided brown cord and can hold 3,200 pounds.
Even with this breakthrough, however, ULDBs have had their ups and downs. A June 2000 test flight of a small balloon went well. It stayed aloft for 30 hours, floating steadily at 93,000 feet, even at night. NASA balloon chief Henry Cathey was impressed by its performance. It even stayed up "when flying over a very cold thunderstorm at night, which tends to bring a balloon down in altitude," Cathey says. But during two tests in 2001 of full-size ULDBs, with inflated volumes of 18.4 million cubic feet, the envelopes sprang leaks.
NASA officials plan to continue ULDB flight tests in 2002. Although the balloons aren't ready to carry payloads, scientists are already lining up for places on the launch manifest. Cosmic ray detectors have a high priority on the flight list, and a larger version of Boomerang may also fly on ULDB.
"People are just waiting for verification that the ULDB will perform and will get those hundred-day flights," says Steve Smith. But even when the ULDB has proved that capability, Smith concedes another problem may interfere with the hundred-day span—a problem springing not from technology but from politics. ULDBs are to fly from Australia and to circle the world repeatedly, but winds will blow them slowly northward—towards countries that do not permit overflights. Diplomats must address these issues, if ULDBs are to remain aloft.
Will these long-duration balloons replace satellites? Astronomer Jonathan Grindlay says that "a 30-hour flight" using conventional balloon "is not competitive" with a spacecraft. However, "a hundred-day flight is very competitive." Indeed, plans already are afoot to us a ULDB itself as a satellite, in order to fly in the atmosphere of Mars. The idea is to deploy the Mars ULDB from a landing craft during the latter's decent. But let's say that on some future mission a need arises to launch a balloon from the Martian surface. There's a balloon facility we know of with an experienced launch team of very seasoned travelers...
Letting Go
THE FIRST STEP IN LAUNCHING a science balloon is to find a remote area with plenty of room in case the launch vehicle springs a leak. The main balloon base of the National Scientific Balloon Facility is near Palestine, Texas, in pine and farm country southeast of Dallas. The closest neighbors are prisons; a road sign warns motorists that hitchhikers may be escaping convicts.
Launching a balloon can take no more than part of a morning, if the winds are sufficiently gentle. The operator of a mobile crane, which the launch crew has nicknamed Tiny Tim, lifts a long steel boom, to which the payload has been attached. With the payload dangling, Tiny Tim trundles over to the center of the launch area, a circle blacktop a thousand feet across. Opposite the direction of the breeze, members of the launch crew lay down a long strip of canvas to protect the balloon from damage. They extend canvas beyond the asphalt across the field of mown grass, disturbing an audience of grasshoppers. At the edge of the field a semi awaits with an 80,000-pound load of cylinders filled with compressed helium.
The 20 people in the launch crew, mostly in their 30s and early 40s, all wear white hard hats. One man has decorated his with plastic Viking horns. "It's for luck," he says. "We always get a good launch when I wear them." Members of the crew talk quietly as they work, using walkie-talkies to stay in touch with the control tower.
The balloon, folded tightly in a large wooden box, arrives on a trailer pulled by a pickup truck. It takes four people to lift the balloon from the box and unfold it into a long strip of polyethylene, still folded lengthwise and protected by a red wrap. When it's stretched to its full length, technicians attach a parachute; the other end of the parachute is attached to the payload, which has thick pads of corrugated cardboard to cushion its impact when it lands.
With a loud hiss, helium begins to flow from the canisters through metal sleeves and into long plastic tubes feeding the balloon's upper area. Slowly the top of the balloon takes the shape of an inverted teardrop. Most of the balloon continues to lie along canvas until the rising action of the helium-filled end of the balloon releases it from a restraint. The filled end rises and pulls the rest of the balloon's length upward, then lifts the parachute as well. The payload is still attached to the mobile crane; its operator watches for any breeze that will waft the helium-filled teardrop overhead. Then, with his diesel engine rumbling, he drives slowly across the pavement, following the direction of the wind and keeping the balloon directly overhead. When he is satisfied that the balloon has taken up the weight of the payload, he releases it. There is no countdown and no turning back.
Immediately after liftoff, the balloon typically rises at several hundred feet per minute. Still, it seems to hang in the air. While scientists in the hangar follow the data that now is beginning to come in, other specialists head for the control tower, where they will track the balloon during the next several hours.
Assembling a Giant
Raven Industries, a maker of balloons of all types, including many that fly in the Macy's Thanksgiving Day parade, assembles the giants that carry science instruments to altitudes of 25 miles. The balloons are made in a building just off Interstate 30, about 80 miles east of Dallas. Inside, the work area is nearly a thousand feet long, with rows of tables stretching end to end. On the tables are the long gores, or segments, that form a science balloon. The largest use nearly a million square feet—20 acres—of polyethylene film, less than a thousandth of an inch thick.
Mike Zimmerman, Raven's chief of quality control, says, "We inspect every inch." Before the balloon is assembled, an inspector scans the film with polarized light to check for holes or weaknesses. Push your thumb into a sample of film; the dent is not easily seen. Yet it shows up clearly under the light.
A balloon of 40 million cubic feet has 172 gores and takes about three weeks to assemble. Raven's workers unroll sheets of polyethylene film, which comes folded lengthwise in rolls of 54 inches wide, down the length of a table. They then run a sealing machine down the edges where two gores join; the machine uses heat to fuse the film and form gas-tight seams. The panels are cut along a curve, marked on the work tables with tape. As workers join the gores, they install load-bearing tapes that run the length of the balloon. The balloon stays folded during construction and prior to launch. It expands to full size only at altitude.
Ravens build about 20 balloons a year. Most of their instrument packages tip the scales at a ton and more.
The Big Push
Minute variations in the temperature of the very early cosmos, mapped by Boomerang in 1999, support a theory known as inflation. Inflation theorists assert that in the course of the Big Bang, there was an inconceivably rapid expansion, during which the universe grew from a size a trillion times smaller than a proton to that of a grapefruit in less than 10-32 second.
Astrophysicists find this scenario compelling, for it predicts a universe with features that match observations: among them, uniformity over the universe's vast expanse. In an article about the Boomerang project, investigator Andrew Lange explains: "[T]he microwave background can be the same temperature everywhere because, before the universe inflated, all its parts were in very close contact and would naturally equilibrate to the same temperature."
But even in this uniformity—the temperature of the microwave background over the entire universe is 2.73 degrees Kelvin (about absolute zero)—there is variation on a very small scale. In one patch of sky, the temperature of the microwave background may be 2.7283. Inflation theorists believe that these "hot" and "cold" patches are traces of quantum fluctuations in the pre-inflationary universe and, as such, are also proof of instantaneous expansion.
The laws of quantum mechanics recognize that a small amount of energy is always present, even in empty space. Quantum fluctuations are jiggles in this background energy. (Evidence for the energy's existence was found in laboratory experiments in which closely spaced metal plates were found to be pushed together by quantum force.)
The process by which quantum fluctuations wink in and out of existence occurs at the speed of light and over a distance of 10-33 centimeters, known as the Planck length, for the German physicist who identified it. Had the universe expanded at the speed envisioned in the standard Big Bang theory, the expansion would have been far too slow for any trace of the quantum fluctuations to exist today. But according to the inflation theory, space itself expanded at a rate enormously faster than the speed of light. Inflation acted like stop-action photography, freezing the fluctuations in mid-jiggle, and at the same time, blowing them up from the "Planck length" to a macroscopic scale. At their post-inflationary size the fluctuations were no longer quantum—they now behaved according to the rules of classical physics. They took on the character of seismic waves within a universe filled with plasma, or ionized gas.
For the next 15 billion years, the universe expanded and cooled. Had observers been present when the cosmos was about 500,000 years old, and had they looked back towards is infancy, they would have seen these seismic features in optical wavelengths, and the seismic features appear as the minute temperature variations in the cosmic microwave background.
