Elon Musk's estimated net worth has now surpassed $400 billion, according to an estimate from Bloomberg News — the first person ever to reach such a milestone.

While the value of shares in Musk's Tesla electric car company and robotics group have soared in recent months, it is Musk's privately held stake in SpaceX, the rocket company he helped found, that has catapulted his wealth beyond the latest threshold.

This week, Bloomberg reported that a private sale of SpaceX shares increased the market cap of the firm to approximately $350 billion, making it the most valuable private startup in the world. Musk is said to own 42% of the firm.