Billionaire Elon Musk said it is “impossible” to take EV maker Tesla Inc. private now even though he would have liked to spend more time on innovation.

“Tesla public company duties are a much bigger factor, but going private is impossible now (sigh),” Musk said in response to a tweet saying he should optimise his time in areas such as innovation.

“Engineering, design & general company operations absorb vast majority of my mind & are the fundamental limitation on doing more.”

Tesla shares, which were included in the S&P 500 index this week, have surged eightfold this year ahead of the addition to the benchmark index’s measure.

The gain is twice the advance of the next best performer on the gauge.

The share price jump also created millionaires among its investors, and propelled Musk’s net worth by $132.2 billion to $159.7 billion, making him the world’s second-richest person, according to the Bloomberg Billionaires Index.

Musk also said Starlink, SpaceX’s space-internet business, would likely be a candidate in his group to go public once its revenue growth becomes “reasonably predictable”.

SpaceX has already launched more than 240 satellites to build out Starlink.

So far, investors have had limited ways to own a piece of SpaceX, which has become one of the most richly valued venture-backed companies in the US by dominating the commercial rocket industry.