Alpha Magnetic Spectrometer

Particle detector on the International Space Station

The Alpha Magnetic Spectrometer (AMS 02) is a particle physics experiment module that is mounted on the International Space Station (ISS). The experiment is a recognized CERN experiment (RE1). The module is a detector that measures antimatter in cosmic rays; this information is needed to understand the formation of the universe and search for evidence of dark matter. The principal investigator is Nobel laureate particle physicist Samuel Ting. The launch of flight STS 134 carrying AMS 02 took place on May 16, 2011, and the spectrometer was installed on May 19, 2011. By April 15, 2015, AMS 02 had recorded over 60 billion cosmic ray events and 90 billion after five years of operation since its installation in May 2011. In March 2013, Professor Ting reported initial results, saying that AMS had observed over 400,000 positrons, with the positron to electron fraction increasing from 10 GeV to 250 GeV. (Later results have shown a decrease in positron fraction at energies over about 275 GeV). There was "no significant variation over time, or any preferred incoming direction. These results are consistent with the positrons originating from the annihilation of dark matter particles in space, but not yet sufficiently conclusive to rule out other explanations." The results have been published in Physical Review Letters. Additional data are still being collected.

The alpha magnetic spectrometer was proposed in 1995 by the Antimatter Study Group, with the stated objective to search for primordial antimatter, with a target resolution of antimatter/matter ≈10−9.

An AMS prototype designated AMS 01, a simplified version of the detector, was built by the international consortium under Ting's direction and flown into space aboard the on STS 91 in June 1998. By not detecting any antihelium, the AMS 01 established an upper limit of 1.1×10−6 for the antihelium to helium flux ratio and proved that the detector concept worked in space. This shuttle mission was the last shuttle flight to the Mir space station.

After the flight of the prototype, the group, now labelled the AMS Collaboration, began the development of a full research system designated AMS 02. This development effort involved the work of 500 scientists from 56 institutions and 16 countries organized under United States Department of Energy (DOE) sponsorship. The instrument which eventually resulted from a long evolutionary process has been called "the most sophisticated particle detector ever sent into space", rivaling very large detectors used at major particle accelerators, and has cost four times as much as any of its ground based counterparts. Its goals have also evolved and been refined over time. As built it is a more comprehensive detector which has a better chance of discovering evidence of dark matter along with other goals. The power requirements for AMS 02 were thought to be too great for a practical independent spacecraft, so AMS 02 was designed to be installed as an external module on the International Space Station and use power from the ISS. The post plan was to deliver AMS 02 to the ISS by space shuttle in 2005 on station assembly mission UF4.1, but technical difficulties and shuttle scheduling issues added more delays. AMS 02 completed final integration and operational testing at CERN in Geneva, Switzerland which included exposure to energetic proton beams generated by the CERN SPS particle accelerator. AMS 02 was then shipped by specialist hauler to ESA's European Space Research and Technology Centre (ESTEC) facility in the Netherlands, arriving February 16, 2010. Here it underwent thermal vacuum, electromagnetic compatibility, and electromagnetic interference testing. AMS 02 was scheduled for delivery to the Kennedy Space Center in Florida, United States, in late May 2010. This was, however, postponed to August 26, as AMS 02 underwent final alignment beam testing at CERN. A cryogenic, superconducting magnet system was initially installed on the AMS 02. When the Obama administration extended International Space Station operations beyond 2015, the decision was made by AMS management to exchange the AMS 02 superconducting magnet for the non superconducting magnet previously flown on AMS 01. Although the non superconducting magnet has a weaker field strength, its on orbit operational time at ISS is expected to be 10 to 18 years versus only three years for the superconducting version. In December 2018, it was announced that funding for the ISS had been extended to 2030. In 1999, after the successful flight of AMS 01, the total cost of the AMS program was estimated to be $33 million, with AMS 02 planned for flight to the ISS in 2003. After the Space Shuttle Columbia disaster in 2003, and after a number of technical difficulties with the construction of AMS 02, the cost of the program ballooned to an estimated $2 billion.

For several years it was uncertain if AMS 02 would ever be launched because it was not manifested to fly on any of the remaining Space Shuttle flights. After the 2003 Columbia disaster, NASA decided to reduce shuttle flights and retire the remaining shuttles by 2010. A number of flights were removed from the remaining manifest, including the flight for AMS 02. was proposed to launch AMS 02 to the ISS on an additional shuttle flight in 2010 or 2011. The bill was passed by the full U. S. House of Representatives on June 11, 2008. The bill then went before the Senate Commerce, Science and Transportation Committee where it also passed. It was then amended and passed by the full Senate on September 25, 2008, and was passed again by the House on September 27, 2008. It was signed into law by President George W. Bush on October 15, 2008. The bill authorized NASA to add another space shuttle flight to the schedule before the space shuttle program was discontinued. In January 2009, NASA restored AMS 02 to the shuttle manifest. On August 26, 2010, AMS 02 was delivered from CERN to the Kennedy Space Center by a Lockheed C 5 Galaxy jet. It was delivered to the International Space Station on May 19, 2011, as part of station assembly flight ULF6 on shuttle flight STS 134, commanded by Mark Kelly. It was removed from the shuttle cargo bay using the shuttle's robotic arm and handed off to the station's robotic arm for installation. AMS 02 is mounted on top of the Integrated Truss Structure, on USS 02, the zenith side of the S3 element of the truss.

By April 2017, only one of the 4 redundant coolant pumps for the silicon trackers was fully working, and repairs were being planned, despite AMS 02 not being designed to be serviced in space. In November 2019, after four years of planning, Liquid carbon dioxide coolant was also replenished. The entire spacewalk campaign was a central feature of the Disney+ docuseries Among The Stars.

The first spacewalk was conducted on November 15, 2019. The spacewalk began with the removal of the debris shield covering AMS, which was jettisoned to burn up in the atmosphere. The next task was to install three handrails in the vicinity of AMS to prepare for the next spacewalks and remove zip ties on the AMS' vertical support strut. This was followed by the "get ahead" tasks: Parmitano removed the screws from a carbon fibre cover under the insulation and passed the cover to Morgan to jettison. The spacewalkers also removed the vertical support beam cover. The duration of the spacewalk was 6 hours and 39 minutes.

The second spacewalk was conducted on November 22, 2019. Parmitano and Morgan cut a total of eight stainless steel tubes, including one that vented the remaining carbon dioxide from the old cooling pump. The crew members also prepared a power cable and installed a mechanical attachment device in advance of installing the new cooling system. The duration of the spacewalk was 6 hours and 33 minutes.

The AMS 02 uses the unique environment of space to advance knowledge of the Universe and lead to the understanding of its origin by searching for antimatter, dark matter and measuring cosmic rays. The first physics results were published in Physical Review Letters on April 3, 2013. said that "The first results from the space borne Alpha Magnetic Spectrometer confirm an unexplained excess of high energy positrons in Earth bound cosmic rays." These results are consistent with the positrons originating from the annihilation of dark matter particles in space, but not yet sufficiently conclusive to rule out other explanations. Ting said "Over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin." On September 18, 2014, new results with almost twice as much data were presented in a talk at CERN and published in Physical Review Letters. A new measurement of positron fraction up to 500 GeV was reported, showing that positron fraction peaks at a maximum of about 16% of total electron+positron events, around an energy of 275 ± 32 GeV. At higher energies, up to 500 GeV, the ratio of positrons to electrons begins to fall again. The AMS team presented for 3 days at CERN in April 2015, covering new data on 300 million proton events and helium flux. It revealed in December 2016 that it had discovered a few signals consistent with antihelium nuclei amidst several billion helium nuclei. The result remains to be verified, and the team is currently trying to rule out possible contamination. A study from 2019, using data from NASA's Fermi Gamma ray Space Telescope discovered a halo around the nearby pulsar Geminga. The accelerated electrons and positrons collide with nearby starlight. The collision boosts the light to much higher energies. Geminga alone could be responsible for as much as 20% of the high energy positrons seen by the AMS 02 experiment. The AMS 02 on the ISS has, as of 2021, recorded eight events that seem to indicate the detection of antihelium 3. Over a twelve year period aboard the ISS, the AMS has accumulated a dataset of more than 230 billion cosmic rays, spanning energies reaching multi TeV levels. The precise measurements obtained by the magnetic spectrometer enable data presentation with an accuracy approaching ~1%. Particularly significant is the high energy data regarding elementary particles such as electrons, positrons, protons, and antiprotons, which presents challenges to theoretical frameworks. Additionally, observations of nuclei and isotopes reveal energy dependencies that deviate from theoretical predictions. The extensive dataset collected by AMS necessitates a reevaluation of existing models of the cosmos, as discussed at the APS April meeting in 2024.