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(CNN) —  

Former astronaut Mark Kelly this week returned $55,000 he was paid for a speech in the United Arab Emirates last year at an event sponsored by the country’s crown prince and attended by leaders of the country.

Kelly delivered the speech along with his twin brother and fellow astronaut Scott Kelly as part of a lecture series sponsored by the Crown Prince of Abu Dhabi, Sheikh Mohamed bin Zayed Al Nahyan.

Kelly is now a Democratic candidate in the 2020 US Senate race for the seat held by Republican Sen. Martha McSally of Arizona. Kelly has pledged to refuse corporate political action committee donations and to fight the influence of big money in politics as cornerstones of the early days of his campaign. The appearance could open Kelly to criticism of foreign influence now that he’s running for the Senate.

CNN’s KFile, which was reviewing Kelly’s time on the paid speaking circuit, first reached out to Kelly’s campaign last Thursday to inquire about his appearance in the UAE. An aide to Kelly’s Senate campaign said he received a fee of approximately $55,000 from his speakers bureau, Keppler Speakers, who booked the appearance and paid Kelly through a consulting firm. The aide said Kelly decided to return the money this week to Keppler Speakers.

In early March, The Intercept reported Kelly made at least 19 paid appearances at corporate events across the US.

According to local reports from the United Arab Emirates, Kelly and his brother spoke at the June 2018 event in Abu Dhabi about human space exploration and the future of the UAE space program. The lecture was titled “The Sky is not the Limit: Life Lessons from NASA’s Kelly Brothers” and his appearance was promoted by the UAE embassy in the US. NASA and the UAE have formal agreements to cooperate on space exploration and research.

In a statement to CNN, Kelly’s campaign spokesperson, Jacob Peters, said, “Like many other former astronauts, Mark has given speeches to a variety of groups about his experiences in space, the United States Navy, and how he and (his wife, former Arizona Rep. Gabby Giffords) worked together to overcome tragedy. This engagement was focused entirely on Mark sharing his experiences in space and discussing our countries’ space programs.”

In a tweet tagging his brother, Scott Kelly said he broke the Ramadan fast with the crown prince.

The paid speaking circuit is not uncommon in the world of politics. But there have been some controversies over the nature of some speeches, with critics and ethics officials charging the paid speeches can be used to direct influence. Hillary Clinton was criticized by the political left during the 2016 Democratic presidential primary and then by Republicans during the general election for paid corporate speeches and foreign donations to the Clinton Foundation.

President Donald Trump’s former national security adviser, Michael Flynn, was also criticized in 2017 for giving a $45,000 speech in 2015 in Moscow to RT, a Russian state-owned television company.

Kelly’s speech was delivered as part of a lecture series sponsored by the crown prince, who is also the deputy supreme commander of the UAE’s military. In this position, he has also played a leading role in the Saudi-led military coalition in Yemen, which has been accused of committing war crimes against civilians. Both Saudi Arabia and the UAE have denied the allegations.

The UAE and its authoritarian government have been accused routinely of violating basic human rights, according to numerous international human rights organizations, and reports from the United Nations and the US State Department.

Kelly isn’t alone in giving speeches to the lecture series. Others include former astronaut Buzz Aldrin, former British Prime Minister Tony Blair and Bill and Melinda Gates, the Microsoft founder and his wife who co-chair the world’s largest private charitable organization.

CORRECTION: This story has been updated to correctly reflect that it was Scott Kelly who tweeted about a meal with the crown prince and that it was the UAE embassy in the US that tweeted about Kelly’s speech.