SpaceX has been a marvel to watch, but does that mean the space stock belongs in your portfolio despite it being a privately funded company? 

On April 5, SpaceX reportedly authorized a $507 million fundraising round, which values the company at $25 billion. Founded by none other than Elon Musk, the CEO of Tesla Inc (TSLA - Get Report) , SpaceX has been steadily climbing the valuation ladder.

If this latest investment round goes through, "SpaceX is poised to become the third-most valuable venture-backed startup in the U.S.," according to PitchBook. It would sit right behind Uber and Airbnb.

It's worth pointing out that SpaceX now has roughly half the valuation of Tesla, which currently trades with a market cap of $50.4 billion. We're not sure who's investing, but Fidelity could be the lead investor, while the company seems to prefer its old investors vs. bringing in new ones. Musk could also be putting more money of his own into the mix. 

Because SpaceX is a private company, we can't buy shares in it. The closest we can get is via a publicly traded company that has invested in it, like Alphabet (GOOGL - Get Report)  or Fidelity funds. This also means we don't have open access to its financials. That makes evaluating the company rather difficult, although it's plain as day to see its disruptions. Its competitors include Boeing Co (BA - Get Report)  , Lockheed Martin Corporation (LMT - Get Report)  and among others, Blue Origin, a company founded by, Inc. (AMZN - Get Report) founder and CEO Jeff Bezos.  

But is SpaceX worth your investment bucks, particularly when it comes to your retirement?

Believe it or not, even though SpaceX is a private company, you may have more access than you thought. First, consider that Alphabet Inc.  (GOOGL - Get Report) (GOOG - Get Report) invested in SpaceX back in January 2015 when the company raised about $1 billion at a valuation close to $10 billion.

So far, this looks like a good investment by Alphabet, a holding in Jim Cramer's Action Alerts PLUS Charitable Trust Portfolio. Fidelity also got in on the deal. Together, the two owned 10% of the company following the deal.

For Fidelity's part, it has a few mutual funds in the mix. It should be noted that the Fidelity Blue Chip Growth Fund (FBGRX - Get Report) and the Fidelity Contrafund Fund (FCNTX - Get Report) both have small positions in SpaceX. But the second-largest holding in each fund is Alphabet, which adds just a bit of exposure for investors. 

For retirement, we don't want all of our eggs in one basket -- certainly not a basket destined for Mars with a private company. If and when SpaceX goes public, there will be another opportunity to gain exposure, should investors want it. 

But for now, these are the best alternatives. For younger retirement investors who are looking for a bit of SpaceX with a lot of diversification and exposure to different businesses, perhaps these investments are worth a look.

This article is commentary by an independent contributor. At the time of publication, the author had no positions in the stocks mentioned.