US astronaut Buzz Aldrin is suing two of his children and his former business manager alleging they stole money from him and are slandering his legacy.
The lawsuit, which also claims they are stopping him from getting married, was filed after his children petitioned to take control of his finances.
They asked a judge to name them as his legal guardians because he is suffering from memory loss and confusion.
Mr Aldrin, 88, was the second man to walk on the surface of the Moon.
He did so as part of the Apollo 11 mission in 1969 and was accompanied by Neil Armstrong, who took the first steps on the surface and was followed minutes later by Mr Aldrin.
In a Wall Street Journal interview last week, the former US Air Force colonel said: "Nobody is going to come close to thinking I should be under a guardianship."
In his lawsuit, he claims that his son, daughter and former manager Christina Korp also undermined his "personal romantic relationships" by forbidding him from getting married.
At issue is the management of his private company, Buzz Aldrin Enterprises, and his non-profit organisation, the ShareSpace Foundation.
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In a legal filing earlier this month, his children Andrew Aldrin, 60, and Janice Aldrin, 60, contended that their father had recently begun associating with new friends who were trying to alienate him from his family.
CFal, Thanks for understanding that sometimes life gets tricky https://t.co/0bIIpVLlfy— Buzz Aldrin (@TheRealBuzz) June 24, 2018
They also said he has been spending money at "an alarming rate".
Mr Aldrin's lawsuit claims that his son and Ms Korp had improperly taken control of his finances - including millions of dollars worth of "space memorabilia" and "space artifacts".
It adds that they did so "for their own self-dealing and enrichment".
The legal action also claims the pair had been slandering him for years by claiming he suffers from dementia and has Alzheimer's disease.
He accuses his son of exploiting the elderly by "knowingly and through deception or intimidation" depriving him of his property, and his daughter of fraud and of not acting in his financial interest.
His eldest son is not involved in the lawsuit.
A court-appointed mental health expert is planning to evaluate Mr Aldrin later this week.
In a statement, his children say they are saddened by the "unjustifiable" lawsuit.
"If nothing else, our family is resilient and our ability to work together to solve problems and accomplish great things is strong," the Aldrin children said.
"We love and respect our father very much and remain hopeful that we can rise above this situation and recover the strong relationship that built this foundation in the first place."
Mr Aldrin has remained in the limelight in recent years, delivering cameo appearances in hit American television shows such as the Big Bang Theory, The Simpsons and 30 Rock.
In 2002, he escaped assault charges after punching a man who demanded he swear on a Bible that the Moon landing was not staged.
He has also become a strong advocate for human habitation of Mars, and last year launched a virtual reality movie detailing his plan to get humans there.
Last week, he appeared at a National Space Council event at the White House, where US President Donald Trump announced the formation of a Space Force to serve as a sixth branch of the US military.