LISA Pathfinder

LISA Pathfinder placed two test masses in a nearly perfect gravitational free-fall, and controlled and measured their relative motion with unprecedented accuracy. The laser interferometer measured the relative position and orientation of the masses to an accuracy of less than 0.01 nanometres,{{cite news |url=https://spacenews.com/lisa-pathfinder-ready-for-launch-from-kourou/ |title=LISA Pathfinder Ready for Launch from Kourou |via=SpaceRef |publisher=Space Ref |date=November 30, 2015|access-date=March 1, 2025}} a technology estimated to be sensitive enough to detect gravitational waves by the follow-on mission, the Laser Interferometer Space Antenna (LISA).

The interferometer was a model of one arm of the final LISA interferometer, but reduced from millions of kilometers long to 40 cm. The reduction did not change the accuracy of the relative position measurement, nor did it affect the various technical disturbances produced by the spacecraft surrounding the experiment, whose measurement was the main goal of LISA Pathfinder. The sensitivity to gravitational waves, however, is proportional to the arm length, and this is reduced several billion-fold compared to the planned LISA experiment.

LISA Pathfinder was an ESA-led mission. It involved European space companies and research institutes from France, Germany, Italy, The Netherlands, Spain, Switzerland, UK, and the US space agency NASA.{{Cite web | title = LISA Pathfinder international partners | work = eLISAscience.org | url = https://www.elisascience.org/articles/lisa-pathfinder/lpf-partners | access-date = 7 September 2015 | archive-url = https://web.archive.org/web/20150926180758/https://www.elisascience.org/articles/lisa-pathfinder/lpf-partners | archive-date = 26 September 2015 | url-status = dead }}

LISA Pathfinder was a proof-of-concept mission to prove that the two masses can fly through space, untouched but shielded by the spacecraft, and maintain their relative positions to the precision needed to realise a full gravitational wave observatory planned for launch in 2035. The primary objective was to measure deviations from geodesic motion. Much of the experimentation in gravitational physics requires measuring the relative acceleration between free-falling, geodesic reference test particles.[https://www.elisascience.org/articles/lisa-pathfinder/lpf-science science objective of LISA Pathfinder] {{Webarchive|url=https://web.archive.org/web/20141021163912/https://www.elisascience.org/articles/lisa-pathfinder/lpf-science |date=2014-10-21 }}.

In LISA Pathfinder, precise inter-test-mass tracking by optical interferometry allowed scientists to assess the relative acceleration of the two test masses, situated about 38 cm apart in a single spacecraft. The science of LISA Pathfinder consisted of measuring and creating an experimentally-anchored physical model for all the spurious effects – including stray forces and optical measurement limits – that limit the ability to create, and measure, the perfect constellation of free-falling test particles that would be ideal for the LISA follow-up mission.{{Cite web | title = LISA Pathfinder Science | work = eLISAscience.org | url = https://www.elisascience.org/articles/lisa-pathfinder/lpf-science | access-date = 9 July 2014 | archive-url = https://web.archive.org/web/20141021163912/https://www.elisascience.org/articles/lisa-pathfinder/lpf-science | archive-date = 21 October 2014 | url-status = dead }}

In particular, it verified:

• Drag-free attitude control of a spacecraft with two proof masses,
• The feasibility of laser interferometry in the desired frequency band (which is not possible on the surface of Earth), and
• The reliability and longevity of the various components—capacitive sensors, microthrusters, lasers and optics.

For the follow-up mission, LISA,{{cite web|url=https://www.elisascience.org/|title=LISA Gravitational Wave Observatory - We will observe gravitational waves in space - New Astronomy - LISA Pathfinder}} the test masses will be pairs of 2 kg gold/platinum cubes housed in each of three separate spacecraft 2.5 million kilometers apart.Official design proposal at https://www.elisascience.org/files/publications/LISA_L3_20170120.pdf

LISA Pathfinder was assembled by Airbus Defence and Space in Stevenage (UK), under contract to the European Space Agency. It carried a European "LISA Technology Package" comprising inertial sensors, interferometer and associated instrumentation as well as two drag-free control systems: a European one using cold gas micro-thrusters (similar to those used on Gaia), and a US-built "Disturbance Reduction System" using the European sensors and an electric propulsion system that uses ionised droplets of a colloid accelerated in an electric field.Ziemer, J.K.; and Merkowitz, S.M.: “Microthrust Propulsion of the LISA Mission,” AIAA–2004–3439, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Fort Lauderdale FL, July 11–14, 2004. The colloid thruster (or "electrospray thruster") system was built by Busek and delivered to JPL for integration with the spacecraft.{{cite web|last=Rovey, J.|title=Propulsion and Energy: Electric Propulsion (Year in Review, 2009)|url=http://www.aerospaceamerica.org/Documents/Aerospace%20America%20PDFs%20(2009)/Aerospace%20America_DEC2009.pdf|publisher=Aerospace America, December 2009, p. 44|access-date=2012-10-26|archive-url=https://web.archive.org/web/20151208101354/http://www.aerospaceamerica.org/Documents/Aerospace%20America%20PDFs%20(2009)/Aerospace%20America_DEC2009.pdf|archive-date=2015-12-08|url-status=dead}}

File:LISA Pathfinder exploded view.jpg

The LISA Technology Package (LTP) was integrated by Airbus Defence and Space Germany, but the instruments and components were supplied by contributing institutions across Europe. The noise rejection technical requirements on the interferometer were very stringent, which means that the physical response of the interferometer to changing environmental conditions, such as temperature, must be minimised.

On the follow-up mission, eLISA, environmental factors will influence the measurements the interferometer takes. These environmental influences include stray electromagnetic fields and temperature gradients, which could be caused by the Sun heating the spacecraft unevenly, or even by warm instrumentation inside the spacecraft itself. Therefore, LISA Pathfinder was designed to find out how such environmental influences change the behaviour of the inertial sensors and the other instruments. LISA Pathfinder flew with an extensive instrument package which can measure temperature and magnetic fields at the test masses and at the optical bench. The spacecraft was even equipped to stimulate the system artificially: it carried heating elements which can warm the spacecraft's structure unevenly, causing the optical bench to distort and enabling scientists to see how the measurements change with varying temperatures.{{Cite web| title = LISA Pathfinder Technology| work = eLISAscience.org| url = https://www.elisascience.org/articles/lisa-pathfinder/lpf-technology| access-date = 9 July 2014| archive-url = https://web.archive.org/web/20141021204854/https://www.elisascience.org/articles/lisa-pathfinder/lpf-technology| archive-date = 21 October 2014| url-status = dead}}

Mission control for LISA Pathfinder was at ESOC in Darmstadt, Germany with science and technology operations controlled from ESAC in Madrid, Spain.{{Cite web | title = LISA Pathfinder: Fact sheet | publisher = ESA | url = http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31714 | access-date = 20 April 2009}}

The spacecraft was first launched by Vega flight VV06 into an elliptical LEO parking orbit. From there it executed a short burn each time perigee was passed, slowly raising the apogee closer to the intended halo orbit around the Earth–Sun {{L1}} point.{{Cite web | title = LISA Pathfinder: Mission home | publisher = ESA | url = http://sci.esa.int/science-e/www/area/index.cfm?fareaid=40 | access-date = 5 February 2011}}{{cite web| title=ESA's new vision to study the invisible universe| url=http://www.esa.int/Our_Activities/Space_Science/ESA_s_new_vision_to_study_the_invisible_Universe |website=www.esa.int| access-date=26 June 2014}}

{{multiple image | align =center| direction = horizontal| width =

| header = Animation of LISA Pathfinder {{'s}} trajectory

| image1 = Animation of LISA Pathfinder trajectory - Polar view.gif

| caption1 = Polar view

| image2 = Animation of LISA Pathfinder trajectory - Equatorial view.gif

| caption2 = Equatorial view

| image3 = Animation of LISA Pathfinder trajectory viewed from the Sun.gif

| caption3 = Viewed from the Sun

| footer ={{legend2| RoyalBlue| Earth}}{{·}}{{legend2|Magenta|LISA Pathfinder}}

}}

File:LISA Pathfinder final results.png

The spacecraft reached its operational location in orbit around the Lagrange point L1 on 22 January 2016, where it underwent payload commissioning.{{cite news |url=http://sci.esa.int/jump.cfm?oid=57290 |title=First locks released from LISA Pathfinder's cubes |work=ESA |publisher=ESA Press Release |date=February 3, 2016 |access-date=2016-02-12 }} The testing started on 1 March 2016.{{cite news |last=Amos |first=Jonathan |url=https://www.bbc.com/news/science-environment-35689814 |title=Gravitational waves: Tests begin for future space observatory |work=BBC News |date=1 March 2016 |access-date=2016-03-01 }} In April 2016 ESA announced that LISA Pathfinder demonstrated that the LISA mission is feasible.{{cite conference |title=The ESA–L3 Gravitational Wave Mission - Final Report |url=http://www.cosmos.esa.int/documents/427239/653121/goat-final-submitted1.pdf/10eaf634-e7df-4db2-921b-43d930993101 |format=PDF |conference=ESA–L3 Final Report |editor-last=Gravitational Observatory Advisory Team |date=28 March 2016 |page=4 }}

On 7 June 2016, ESA presented the first results of two months' worth of science operation showing that the technology developed for a space-based gravitational wave observatory was exceeding expectations. The two cubes at the heart of the spacecraft are falling freely through space under the influence of gravity alone, unperturbed by other external forces, to a factor of 5 better than requirements for LISA Pathfinder.{{cite journal |title=Sub-Femto-g Free Fall for Space-Based Gravitational Wave Observatories: LISA Pathfinder Results| author=M. Armano| journal=Physical Review Letters|year=2016|volume=116| issue=23| doi=10.1103/PhysRevLett.116.231101|display-authors=etal| bibcode=2016PhRvL.116w1101A| pmid=27341221| page=231101| doi-access=free| hdl=2117/102419| hdl-access=free}}{{cite web| title=LISA Pathfinder exceeds expectations| publisher=ESA| url=http://www.esa.int/Our_Activities/Space_Science/LISA_Pathfinder_exceeds_expectations| date=7 June 2016| access-date=7 June 2016}}{{cite web| title=LISA Pathfinder exceeds expectations| publisher=elisascience.org| work=Benjamin Knispel| url=https://www.elisascience.org/news/top-news/lisa-pathfinder-exceeds-expectations| date=7 June 2016| access-date=7 June 2016| url-status=dead| archive-url=https://web.archive.org/web/20160803124722/https://www.elisascience.org/news/top-news/lisa-pathfinder-exceeds-expectations| archive-date=3 August 2016}} In February 2017, BBC News reported that the gravity probe had exceeded its performance goals.{{cite web| title=Gravity probe exceeds performance goals |work=Jonathan Amos, BBC Science Correspondent, Boston | url=https://www.bbc.com/news/science-environment-38897697?SThisFB| date=18 February 2017| access-date=20 February 2017}}

LISA Pathfinder was deactivated on 30 June 2017.{{cite web

| url =http://sci.esa.int/lisa-pathfinder/59238-lisa-pathfinder-to-conclude-trailblazing-mission/

| title =LISA Pathfinder Will Concludee Trailblazing Mission

| date =20 June 2017

| website =ESA Science and Technology

| publisher =ESA

| access-date =17 August 2017

}}

On 5 February 2018, ESA published the final results. Precision of measurements could be improved further, beyond current goals for the future LISA mission, due to venting of residue air molecules and better understanding of disturbances.{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/ESA_creates_quietest_place_in_space |language=en |title=ESA creates quietest place in space |date=2018-02-05 |access-date=2018-02-07}}

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• Einstein Telescope, a European gravitational wave detector
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• Taiji 1, a Chinese technology demonstrator for gravitational wave observation launched in 2019
• Virgo interferometer, an interferometer located close to Pisa, Italy

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{{Reflist|30em}}

{{commons category|LISA Pathfinder}}

• [https://www.elisascience.org/ LISA and LISA Pathfinder's Homepage]
• [http://sci.esa.int/lisa-pathfinder/ LISA Pathfinder mission home at ESA]
• [http://directory.eoportal.org/web/eoportal/satellite-missions/l/lisa-pathfinder LISA Pathfinder at eoPortal] {{Webarchive|url=https://web.archive.org/web/20151017030659/https://directory.eoportal.org/web/eoportal/satellite-missions/l/lisa-pathfinder |date=2015-10-17 }}
• [http://www.aei.mpg.de/14026/AEI_Hannover Max Planck Institute for Gravitational Physics (Albert Einstein Institute Hannover)] {{Webarchive|url=https://web.archive.org/web/20160517102903/http://www.aei.mpg.de/14026/AEI_Hannover |date=2016-05-17 }}

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