January 23, 2018

European probe blasts off on gravitational wave demonstration

Credit: ESA/CNES/Arianespace – Optique Video du CSG – S. Martin
Credit: ESA/CNES/Arianespace – Optique Video du CSG – S. Martin

KOUROU, French Guiana — A solid-fueled Vega launcher fired through a low cloud deck and sped into orbit from the South American jungle early Thursday with a pioneering European spacecraft that marks a key step in a century-long quest to detect elusive gravitational waves predicted to permeate the cosmos.

Fastened on top of the 98-foot-tall (30-meter) Vega launcher was LISA Pathfinder, an experimental spacecraft that took 10 years to build and cost more than $630 million, a price divided between the European Space Agency, European governments and NASA.

But the idea has been around for decades, rooted in a prediction from Albert Einstein’s general theory of relativity that the movement of immense objects in the universe, such as galaxies and black holes, send out low-frequency waves of gravitational energy rippling across spacetime.

LISA Pathfinder’s job is to test the technologies, some of which had to be invented and refined for the mission, required for a future project to deploy two or three satellites to measure gravitational waves, a detection that would give astronomers a new way to study the universe.

“They are a direct prediction of the general theory of relativity, but they are yet to be directly detected,” said Alvaro Gimenez, ESA’s science director. “Gravitational waves are ripples in the curvature of spacetime, but they are very weak and one needs a very quiet sensor.”

Engineers crammed a laser ranging sensor, ultra-fine accelerometers, and novel micro-thrusters aboard LISA Pathfinder, which also carries two gold-platinum cubes procured from a precious metals workshop in Germany.

LISA Pathfinder, which weighed about 4,200 pounds — 1.9 metric tons — at liftoff, tows along a propulsion module to give it a boost toward the L1 Lagrange point, a gravitationally stable position nearly a million miles — 1.5 million kilometers — from Earth in the direction of the sun.

A test burn of the propulsion section is due Dec. 6, followed by six maneuvers through Dec. 12 to raise LISA Pathfinder’s altitude, according to Cesar Garcia Marirrodriga, ESA’s project manager for the mission. The final firing of the main engine will send the probe toward L1, save for a few potential course-correction burns along the way.

Artist’s concept of LISA Pathfinder with its propulsion module. Credit: ESA

The spacecraft was manufactured by Airbus Defense and Space’s UK division, with the orbit-raising module based on propulsion systems developed for Airbus’ line of Eurostar telecommunications satellites.

The LISA Pathfinder science module, a six-sided, solar-powered craft housing the mission’s instrumentation, will separate from the expendable engine section in late January, then begins a final commissioning phase before starting a six-month prime science mission.

The satellite must first release the two gold-platinum test cubes from their launch restraints, a complicated procedure involving needle-like appendages that must carefully pull away from the cubes — 1.8 inches or 46 millimeters on a side — to avoid disturbing them with electrostatic forces.

Lasers bouncing between the cubes will track their movement to a precision of one picometer, or one one-hundredth the size of an atom, according to Paul McNamara, ESA’s LISA Pathfinder project scientist.

Each test mass will be suspended in separate vacuum chambers lined with electrodes to help keep the cubes in near perfect free-fall.

LISA Pathfinder’s control software will essentially fly the spacecraft around the test masses, keeping them centered in their housings without contacting anything.

The journey began when the Vega rocket took off from the Guiana Space Center at 0404 GMT Thursday (11:04 p.m. EST Wednesday), or 1:04 a.m. local time in French Guiana, darting through clouds and sending a rumble across the French-run spaceport on the northeastern coast of South America.

Less than two hours later, after a pair of burns by the Vega’s upper stage engine, news reached launch controllers that LISA Pathfinder was safely in its preliminary orbit.

The Italian-developed Vega rocket, flying its sixth mission, aimed to deploy LISA Pathfinder in an elliptical orbit ranging from a high point of 957 miles (1,540 kilometers) to a low point of 128 miles (207 kilometers), with an inclination of 5.96 degrees.

Telemetry streaming to the ground from the Vega rocket and LISA Pathfinder showed it was released on the proper trajectory, officials said, and engineers at the European Space Operations Center in Germany confirmed the craft was operating normally after the launch.

Stephane Israel, chairman and CEO of Arianespace, the Vega’s commercial operator, declared the launch a success in remarks after the flight, adding that LISA Pathfinder was placed into its targeted orbit.

“We have a mission,” McNamara said.

Artist's concept of the LISA Pathfinder science module. Credit: ESA
Artist’s concept of the LISA Pathfinder science module. Credit: ESA

Scientists are eager for instrument data to come down from LISA Pathfinder, which will help in the development of a more ambitious multi-spacecraft gravitational wave observatory, a concept previously known as LISA, set for launch in the 2030s.

“All of us would rather be launching LISA, and being ready to do the astrophysics data analyis in addition to all the instrumental work,” said Ira Thrope, a researcher at NASA’s Goddard Space Flight Center and a U.S. member of the LISA Pathfinder science team. “The prime objective of this is technology demonstration, but it’s more than just a yes/no answer. You don’t just go up and say, ‘Yep, it worked, we’re done.’ What we really want to get out of this is a model for how these kinds of precision accelerometers operate in space.”

Two sets of thrusters, made in Europe and the United States, will take turns controlling LISA Pathfinder during its mission, using the accelerometer measurements to adjust thrust from the tiny rocket jets, keeping the satellite pointed with remarkable stability.

“LISA Pathfinder is flying the quiestest most sensitive sensor of motion ever, a key building block for gravitational sensor,” Gimenez said before the launch.

“We can’t use traditional bang bang thrusters for something like this,” said Phil Barela, project manager for NASA’s contribution to the LISA Pathfinder mission.

The U.S. thrusters were developed by Busek, a Massachusetts company, which supplied “electrospray” jets that generate power by running an ionic liquid — essentially a molten salt — through an electric field, according to Nate Demmons, an engineer at Busek.

Barela said the Busek electrospray thrusters, which will steer LISA Pathfinder during half of its 180-day prime mission, can point the satellite with the precision equivalent to the width of the double helix in a strand of DNA.

ESA provided cold gas nitrogen thrusters for LISA Pathfinder to check their performance in a similar way.

“This is a rather special one for ESA, and unlike any previous spacecraft in the science program, this one will not look at the stars and galaxies, and not even explore other planets in the solar system,” Gimenez said. “This mission will do somethiong very different. It will follow a solid cube of gold-platinum while it moves under the influence of gravity, at the same time shielding it from every other disturbance possible.”

Artist's concept of the LISA mission concept for a future gravitational wave observatory. Credit: ESA
Artist’s concept of the LISA mission concept for a future gravitational wave observatory. Credit: ESA

LISA Pathfinder is purely experimental. Its modest size — about 6.9 feet (2.1 meters) in diameter — is too small to detect the low-frequency gravitational waves themselves.

“After we do LISA Pathfinder, we then take basically two or three LISA Pathfinder spacecraft, and we separate them by 5 million kilometers (about 3 million miles), and we have one cube in each spacecraft, and we measure the distance between each of the cubes,” McNamara said.

Thorpe said ground-based efforts to detect the gravitational ripples may confirm their existence first, but a space mission will get the most out of them.

“Just like there’s a whole spectrum of electromagnetic waves, there’s a whole spectrum of gravitational waves,” Thorpe said. “Sometimes people will say it’s like opening a new window on the universe, like when we opened up X-ray astronomy or infrared astronomy. It’s bigger than that. We’re opening up another spectrum, and there are many windows in that spectrum.”

Physicists believe gravitational waves pass through Earth all the time, but their weak signal makes them extraordinarily challenging to detect. That means a spacecraft has to use technologies like those on LISA Pathfinder to cancel out solar radiation pressure, magnetic forces and torques that could mask the elusive gravitational waves.

“We always like to make this analogy between hearing and sight,” Thorpe said. “In many ways, gravitational waves are a little bit more like our sense of hearing. Of course, there’s no sound in a classical sense in space because there’s no medium to transmit the sound wave. There’s no air or water or anything.

“But it turns out Einstein showed us space-time itself can vibrate,” Thorpe said. “Those vibrations are called gravitational waves, and they carry a very different kind of information. One thing is that gravitational waves are not impeded as much by other matter. One thing that’s difficult with optical astronomy is you want to see what’s going on right down there at the center of a galaxy, (and) it’s often difficult because there’s so much stuff in the way.”

Astronomers use X-rays to peer into the hearts of galaxies and study the dynamics of their supermassive black holes, probing some of the most energetic parts of the universe.

“But there’s no substitute for something like a gravitational wave because they will just pass through whole galaxies without being disturbed, or absorbed, or modified in any kind of way,” Thorpe said. “That’s really powerful.”

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