December 14, 2018

Pioneering probe for gravitational wave observatory ends mission


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Artist’s illustration of the LISA Pathfinder spacecraft. Credit: ESA

The European Space Agency’s LISA Pathfinder spacecraft, now sailing around the sun on a trajectory away from Earth, was deactivated Tuesday after a nearly 18-month mission testing previously-untried lasers, vacuum enclosures, exotic gold-platinum cubes and micro-thrusters needed for a trio of gravitational wave observatories set for launch in the 2030s.

Stefano Vitale, principal investigator of the LISA Pathfinder mission’s core instruments, sent the long-planned command to “passivate” the probe at 1800 GMT (2 p.m. EDT) Tuesday from the European Space Operations Center in Darmstadt, Germany.

The end of LISA Pathfinder’s mission Tuesday marked another turning point in gravitational wave research, a field of astrophysics reinvigorated in the last two years by two major advances, according to Paul McNamara, the mission’s project scientist at ESA.

First came the launch of LISA Pathfinder on Dec. 3, 2015. Three months later, scientists announced the first confirmed detection of gravitational waves, ripples in the fabric of spacetime produced by the movement of massive objects in space, such as immense supermassive black holes at the centers of galaxies.

The gravitational waves, first predicted more than a century ago by Albert Einstein, were discovered by scientists crunching data gathered in September 2015 from a ground-based observatory called LIGO, which has antennas positioned 1,800 miles (3,000 kilometers) apart in Hanford, Washington, and Livingston, Louisiana.

Gravitational wave research requires huge detectors spread of thousands or millions of miles because the ripples are observed at very low frequencies as they travel through the universe at the speed of light. Astronomers say the waves, which can be triggered by violent phenomena such as black hole mergers, reveal a new way of studying the cosmos impossible with conventional optical telescopes.

The back-to-back breakthroughs catapulted gravitational waves to the forefront of astronomical journals and space mission planning.

“Was it a big step forward? Absolutely, because up to this point there were two doubts,” McNamara said in an interview this week with Spaceflight Now. “One doubt was gravitational waves don’t exist, and then LIGO comes along and detects them.

“Then we launched LISA Pathfinder, and we demonstrated the hardware in space,” he said. “So the two big questions — do they exist and can we detect them? — both were answered within three months of each other.”

LISA Pathfinder was named for a follow-on mission dubbed the Laser Interferometer Space Antenna, which was formally selected by ESA’s science planning board June 20 to move into the next phase of mission planning after decades of starts and stops.

“With the astonishing success of LISA Pathfinder, we now know how to build a mission like LISA,” said Vitale, a researcher at the University of Trento and the National Institute for Nuclear Physics in Italy.

Launched from French Guiana aboard a Vega rocket, the hexagonal space probe is about the size of a small car. LISA Pathfinder reached an operating point at the L1 Lagrange point nearly a million miles (1.5 million kilometers) from Earth in January 2016, lurking near a gravitational balance point between in a direction toward the sun.

In March 2016, on the first day LISA Pathfinder was in full science mode, ground controllers confirmed the mission had already met its minimum success requirements.

Two gold-platinum test cubes launched inside the LISA Pathfinder spacecraft were released from their launch restraints, a complicated procedure involving needle-like appendages that carefully pulled away from the cubes — each 1.8 inches (46 millimeters) on a side and with a mass of 4.4 pounds (2 kilograms) — to avoid disturbing them with electrostatic forces.

At the core of LISA Pathfinder are the two test masses: a pair of identical 46-millimeter gold–platinum cubes, floating freely, several millimeters from the walls of their housings. The cubes are separated by 38 centimeters and linked only by laser beams to measure their position continuously. Credit: ESA/ATG medialab

The crux of the mission was to prove the test cubes could be kept in a constant state of nearly perfect free fall during LISA Pathfinder’s mission.

Two sets of low-impulse thrusters essentially steered the spacecraft around the free-floating test masses suspended inside two vacuum enclosures placed 15 inches (38 centimeters) apart on the satellite.

Accelerometers aboard LISA Pathfinder precisely tracked its movements, and a control computer sent signals to the low-thrust rocket packs outside the probe to continuously correct to keep the test cubes from contacting the walls of their chambers.

A high-precision laser interferometer constantly measured the range between the two test cubes, and that device also exceeded requirements, measuring the relative motion of the test masses with a precision of a femtometer, or one quadrillionth of a meter.

LISA Pathfinder is 10,000 times more stable than any satellite flown on a previous science mission, officials said, demonstrating that it was possible for the test masses to remain virtually motionless with respect to each other.

ESA said the test masses had a relative acceleration of only ten billionths of a billionth of Earth’s gravity, an achievement made possible by a tedious accounting of every component of the spacecraft that could influence the floating metallic cubes.

Many of the lessons learned from LISA Pathfinder were not in how to build a space-rated gravitational wave detector, but how to operate it, McNamara said. Even the switch-on of a transponder or star tracker added noise to the instrument beyond acceptable limits.

“This is such a sensitive instrument that it responds to anything changing whatseover,” NcNamara said. “We’ve learned that, for LISA, we have to assume if you make any changes on-board its going to take you time to recover back into equilibirium. If you turn any unit on, you turn any heater on, or do anything on the spacecraft to put it in a slightly different orientation, it’ll take you a week to get back to operational status.”

Such precision is needed because gravitational waves have an amplitude of a few millionths of a millionth of a meter over a distance of a million kilometers (621,000 miles). Any larger movement of the test masses would mask the gravitational wave.

The LISA Pathfinder mission cost around $630 million, a figure that includes contributions from ESA, NASA and other institutions scattered across Europe.

LISA Pathfinder was conceived to prove a gravitational wave mission was technically feasible.

“People just didn’t think it was possible,” McNamara said. “That’s why LISA Pathfinder came into being. It was just to see could we build an instrument which was quiet enough.”

Artist’s impression of a Laser Interferometer Space Antenna (LISA) mission concept spacecraft. Credit: AEI/Milde Marketing/Exozet

The concept for the LISA mission selected by ESA last month calls for three spacecraft similar to LISA Pathfinder to launch in 2034 into an orbit around the sun that trails the Earth.

The LISA spacecraft will fly in a triangular formation more than 1.5 million miles (2.5 million kilometers) apart, linked by lasers to track the exact distances between the nodes, which will each contain two free-floating test masses. Sensors will watch for tiny variations in the range between the craft as gravitational waves pass through the solar system.

“With gravitational waves, it’s a completely new endeavor we’re taking on,” McNamara told Spaceflight Now. “This idea of flying three spacecraft separated by millions of kilometers, and you have to be able to measure the distance to a hundredth the size of an atom.”

“We have exceeded not only the requirements set for LISA Pathfinder, but also the accuracy required for LISA at all frequencies: we are definitely ready to take the next step,” said Karsten Danzmann, a LISA Pathfinder co-investigator, the lead proposer of the LISA mission, and director at the Max Planck Institute for Gravitational Physics in Germany.

ESA expects the LISA mission to cost up to $1.2 billion (more than a billion euros), not including support from NASA.

Paul Hertz, director of NASA’s astrophysics division, said Wednesday that the U.S. space agency wants to contribute technology and hardware to the LISA mission roughly equivalent to around 20 percent of the mission’s total cost.

NASA and ESA originally planned a larger, more ambitious LISA mission, but NASA dropped out of the partnership in 2011 due to budget constraints. ESA pressed on with a scaled-back gravitational wave observatory, which received prioritization from the agency in 2013 ahead of the LISA concept’s selection last month.

European officials want ESA to lead the LISA mission to avoid falling victim to another failed partnership, but NASA will still be a significant contributor. After discussions in the last few years for NASA to be a 10 percent partner, the U.S. stake in the LISA mission is now likely to be closer to 20 percent.

“We are talking about a more substantial contribution than a 10 percent share,” Hertz said. “ESA has welcomed us as a very major partner.”

NASA might develop lasers and telescopes for the LISA observatory, or the mission’s charge management system. Another potential U.S. addition to the mission could be the micro-thrusters needed to deftly control each of the LISA spacecraft, which will be assembled in Europe.

LISA is third in ESA’s “Cosmic Vision” line of large-class billion-euro space science missions.

A robotic spacecraft that will orbit Jupiter, and then circle Jupiter’s largest moon Ganymede, is on schedule for launch aboard an Ariane 5 rocket in 2022, followed by liftoff of the Athena X-ray astronomy observatory in 2028.

Then it will be LISA’s turn.

Before shutting down LISA Pathfinder, controllers fired its thrusters to nudge it out of its post at the L1 Lagrange point in April to head into a heliocentric orbit around the sun. The maneuver minimized the chance the spacecraft will return to Earth’s vicinity.

LISA Pathfinder’s science mission officially ended June 30, and engineers spent the final weeks practicing procedures to recapture the test masses inside their housings, which might be necessary if problems develop on the LISA mission. Other final tasks included monitoring the instrument’s behavior when the spacecraft’s thrusters were turned off, and tracking the test masses’ response to a coronal mass ejection from the sun.

Scientists were eager to see how the spacecraft responded when it was zapped by ionizing energy from a solar eruption last week. In particular, mission officials wanted to know whether the instrument would still provide useful science data when the test masses were hit by charged particles. Reviews of that data are still ongoing, McNamara said.

The final commands uplinked to LISA Pathfinder turned off the craft’s transponder and corrupted the memory files of the probe’s primary and redundant computers by filling the processors with the names of scientists and engineers who worked on the mission.

“This is a celebration, and it’s certainly not a sad moment,” Vitale said moments before sending the order that silenced the spacecraft.

“LISA Pathfinder has done everything and more that we could have asked of it,” McNamara said. “And it’s allowed LISA to go ahead, so yes, we’re sad that’s going away and it’s ending, but we’re very happy LISA is taking off.

“It’s another 17 years to go before that one launches, so we’ll exercise our patience.”

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Follow Stephen Clark on Twitter: @StephenClark1.


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