LISA Pathfinder

LISA Pathfinder
Type:	Research satellite
Operator:	ESA
COSPAR-ID :	2015-070A
Mission dates
Dimensions:	1.9 t (475 kg payload)
Size:	2.1 x 1.0 m
Begin:	December 3, 2015, 04:04 UTC
Starting place:	CSG , ELV
Launcher:	Vega VV06
Status:	in orbit (disabled)
Orbit data

LISA Pathfinder ( SMART-2 ) is a research and test satellite of the ESA to test the measuring devices and technologies for the planned mission eLISA / NGO (Evolved Laser Interferometry Space Antenna / New Gravitational wave Observatory). Techniques for detecting gravitational waves were tested with the satellite . To do this, it was necessary for test masses to be immobile in relation to one another to a previously unknown extent. The main objective was to demonstrate that the technology and measurement method used works in practice. This result could not be achieved through tests and simulations on the ground or in a near-earth orbit, but only through a space mission to a point where the earth's gravity field is canceled.

At an early stage as SMART-2, technology for the Darwin space telescope was also to be tested with the satellite . However, the Darwin project did not go beyond planning, so that no technical tests were necessary.

Technology and instruments

The satellite was built by EADS Astrium and tested by IABG in Ottobrunn . In addition to ESA, several national space agencies were involved financially and with technology contributions, especially Italy ( ASI ); Germany ( DLR ); The United Kingdom ( UKSA ); France ( CNES ); Spain ( CDTI ); Switzerland ( SSO ); and the Netherlands ( SRON ).

In order to be able to detect gravitational waves with a gravitational wave detector , the instruments must be able to detect gravitational changes in the order of magnitude of 10 ⁻¹⁶  g and changes in distance in the order of magnitude of 10 ⁻¹²  m in a frequency range of 0.001 to 0.1 Hz. LISA Pathfinder was satisfied with an order of magnitude lower measurement accuracy. While eLISA / NGO is supposed to carry out distance measurements between satellites that are about 5 million kilometers apart, LISA Pathfinder measured the distance between two reference bodies within the satellite. The function of the measuring devices and the measuring principle was checked - due to the small distance between the test masses, detection of gravitational waves for LISA Pathfinder was not expected.

A 64 × 38 × 38 cm and 150 kg heavy technology test device on board the satellite, which essentially consists of a special optical bench and various measurement, control and monitoring systems, served as the measuring instrument . Before launch, the entire spacecraft was precisely weighed to know the mass, center of gravity and moments of inertia. The probe contains two vacuum containers and each a cube-shaped test mass made of a gold-platinum alloy from Heraeus with an edge length of 46 mm and a mass of 1.96 kg, which float freely in it at a distance of about 40 cm during the measurement. In addition to the test masses, the main payload is a laser interferometer , which determines the distance between the two cubes. It consists of a 20 × 20 cm block made of Zerodur glass ceramic with 22 mirrors and beam splitters . The laser light is fed into the bank via two glass fibers .

Electric engines (Field Emission Electric Propulsion) are used to precisely control the position of the satellite . In this case there are weak ion thrusters with cesium as the propulsion medium with a thrust of 0.1 to 150 µN, which, however, could only be used outside of the measurement times.

mission

The launch was initially planned for 2008, was then postponed several times and finally took place on December 3, 2015 with a Vega rocket .

After the successful launch and reaching the final position, the rocket motor was dropped on January 22, 2016. The rocket motor with the remaining fuel could have falsified the measurement results due to its own weight. For the test, LISA Pathfinder was in a 500,000 km × 800,000 km Lissajous orbit around the sun-earth libration point L1 at a distance of approx. 1.5 million km from the earth. Since February 22, 2016, the two test masses have been in "free fall" within the measuring apparatus, without any external influence. After the mechanical connections had been loosened, they had been held in position using electrostatic forces. Scientific research began on March 8, 2016.

Two different propulsion technologies were used and tested during the free fall: the cold gas micronewton propulsion and NASA's colloidal micronewton propulsion. Both systems only emit minimal thrust forces which guide the satellites according to the cubes inside. The test also included various experiments to artificially deflect the two test masses from one another. The test system should react accordingly and calculate the strength and direction of the acting forces. As a final experiment, the mechanism for capturing and releasing the test masses was subjected to extreme tests in order to test the technological limits.

In April 2017, a preliminary deorbit maneuver was carried out that transferred the probe into a heliocentric cemetery orbit . LISA Pathfinder was still collecting data through June 30, 2017 and was deactivated on July 18, 2017.

Results

On June 7, 2016, ESA announced the results of the first two months of scientific operation - the measurement accuracy achieved exceeded the requirements by five times, and is already close to the planned requirements for eLISA / NGO.

On December 7, 2016, after tests of a new type of drive, a second phase of scientific operation began, which served to better understand the system and thus to reduce interference signals. The primary concern was two sources of noise. Below 0.6 mHz, not yet understood secondary effects of engine maneuvers dominate, between 0.6 and 50 mHz, air molecules trapped in the measuring chamber that collide with the test masses dominate. A clear improvement could be achieved through a better understanding of the probe and the escape of air molecules into space. Sensor noise dominated above 50 mHz, but this is already a factor of 100 below the requirements and therefore no longer played a role.

On February 5, 2018, ESA published the final results; The measurement accuracy could be increased again by the intermittent escape of interfering air molecules and the better understanding of sources of interference and now also exceeds the measurement accuracy targeted for the actual LISA mission.

In the test operation, the influence of cosmic particle radiation could be measured, mainly of electrically charged particles such as protons and electrons. A NASA team used LISA Pathfinder as a micrometeorite detector . Any impact on the probe causes the satellite to move slightly in relation to the cubes inside. The electronics now compensate for this shift by using the engines. The momentum and direction of the dust particle can in turn be obtained from this data. Through longer observation, statistical data can be obtained about the dust distribution at the L1 point, which in turn allows predictions about the pollution by micrometeorites of future missions to the L1 point. It is hoped that this will lead to new insights into the distribution of dust particles in the entire solar system. Since L1 is approx. 1.5 million km from the earth, it is assumed that there are considerably fewer micrometeorites there than near the earth.

The results were incorporated into the decision-making processes for the planned eLisa / NGO mission. The start of eLisa / NGO is currently (2017) planned for 2034 .

Web links

• Current information on LISA Pathfinder from ESA
• EADS Astrium - LISA Pathfinder
• Max Planck Institute for Gravitational Physics (Albert Einstein Institute Hannover)

Individual evidence

1. ↑ Manfred Lindinger: Europe's new research satellite started perfectly after a breakdown. FAZ, December 3, 2015, accessed on December 3, 2015 . 
2. ↑ Status report LISA and LISA Pathfinder, January 2008 (PDF; 748 kB), page 18
3. ↑ Astrium delivers LISA Pathfinder ( memento of the original from September 14, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
4. ↑ FliegerRevue June 2009, pp. 46–47, LISA Pathfinder - On the trail of gravitational waves
5. ↑ LISA Pathfinder overview. ESA, January 10, 2013, accessed May 7, 2013 . 
6. ↑ LISA Pathfinder arrives at its worksite. ESA, January 22, 2016, accessed February 12, 2016 . 
7. ↑ Freefall achieved by Lisa Pathfinder. ESA, February 24, 2016, accessed February 24, 2016 . 
8. ↑ A perfectly still laboratory in space. ESA, March 8, 2016, accessed March 10, 2016 . 
9. ^ ESA: LISA Pathfinder to conclude trailblazing mission. June 20, 2017, accessed June 21, 2017 . 
10. ↑ Sub-Femto-g Free Fall for Space-Based Gravitational Wave Observatories: LISA Pathfinder Results. June 7, 2016, accessed July 1, 2016 . 
11. ↑ ESA: LISA Pathfinder's pioneering mission continues. December 13, 2016, accessed June 29, 2017 . 
12. ↑ ESA creates quietest place in space. February 5, 2018, accessed February 7, 2018 . 
13. ↑ News: Top News | LISA Gravitational Wave Observatory. (No longer available online.) Archived from the original on August 5, 2017 ; accessed on June 4, 2017 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.