So you say you want to invest in the top space stocks? Get in line, buddy -- so do I!

Our problem, of course, is that most of the companies making the most exciting strides in space exploration and exploitation -- companies like SpaceX with its reusable rockets, Rocket Lab and its growing string of successful small rocket launches, and Planet with its constellation satellites snapping photos of Earth from orbit -- remain privately held.

One day soon, SpaceX might decide to launch an IPO for its Starlink satellite internet subsidiary. For the time being, however, an investor's choice of the top stocks to invest in space remains positively tiny.

Tiny, but not nonexistent. While we wait for the presently private companies to make an IPO, here are three top space stocks that you can add to your watchlist today.

4 rockets going up into the sky.

Image source: Getty Images.

Aerojet Rocketdyne: a top stock in rocket engines

Let's start with an oldie (but a goody). Established in 1915, the company that now calls itself Aerojet Rocketdyne (NYSE:AJRD) is still America's foremost maker of rocket engines -- both the kinds that send spaceships into orbit, and those that (potentially) send nuclear missiles to their targets.

Aerojet built the Space Shuttle's main engines, for example, and will use those same engines to power NASA's new Space Launch System super rocket. It provides the main engines for United Launch Alliance's Delta IV rocket, and the boosters on ULA's Atlas V. Aerojet Rocketdyne engines also power the second and third stages on America's fleet of Minuteman intercontinental ballistic missiles, as well as the Navy's Trident II D5 nuclear missiles.  

Financially, Aerojet is also a powerhouse, boasting $141 million in net earnings last year, and nearly $220 million in free cash flow. Relative to the company's $3 billion market capitalization, that works out to a price-to-free cash flow ratio of less than 14 -- and Aerojet has no net debt on its balance sheet.

With numbers like these, it's no surprise that Aerojet Rocketdyne's stock has outperformed the S&P 500 over the past year -- and it could continue to do so for years to come.

Iridium: a top stock in satellites

The main knock against Aerojet, of course, is that it's considered a slow growth company. Analysts who cover it think Aerojet will only grow earnings at about 5% annually over the next five years, and Aerojet could face increasing competition from new rocket and engine maker Blue Origin -- but that's not a problem our second company has to deal with.

Satellite communications company Iridium (NASDAQ:IRDM) is pegged for a 14% growth rate over the next five years -- nearly three times Aerojet's growth rate -- and there are two main reasons for this.

First, unlike Aerojet, Iridium faces little competition in its chosen "L-Band" spectrum for transmitting satellite phone signals. (New competitors in satellite communication, such as SpaceX's Starlink and Amazon's Kuiper, have to lease wavelengths in the shorter K-Band spectrum). Second, while SpaceX and Amazon are still building out their satellite constellations, Iridium has already completed its constellation of 75 "NEXT" satellites in orbit. For the next 12 (the satellites' rated service life) to 30 (their likely actual usability) years, the company shouldn't need to add to its constellation, allowing Iridium to generate strong free cash flow without the need to make significant new capital investments.

Although apparently pricey at 28 times trailing free cash flow today, Iridium's FCF is likely to grow significantly in coming years, giving it the potential to turn into a value stock in time.

Virgin Galactic: the top stock in space tourism

Finally, we come to Virgin Galactic (NYSE:SPCE). The exact opposite of a value stock, Virgin Galactic is a company whose value is premised upon the potential for exponential growth as it creates -- eventually -- a new industry for "space tourism."

Of course, the key word here is "eventually," because so far, Virgin Galactic has not flown a single paying customer to space -- ever. Virgin Galactic has proven that its SpaceShipTwo suborbital spaceplanes are capable of reaching space, however, and it has signed up 600 paying customers who want to fly on its spaceplanes as soon as they are approved for commercial operation. A further 9,200 potential customers have also registered interest in flying with Virgin Galactic at a future date.

Considering the cost of each ticket is $250,000, that's quite a lot of money Virgin Galactic's customers want to pay for the privilege of flying them to space and back.

And yet, space tourism isn't the end of Virgin Galactic's plans. Eventually, it believes that the same technology that permits it to fly tourists to the edge of space can be used to fly commercial airplane passengers there too -- and then to keep on flying up there, at hypersonic speeds, before landing somewhere else, dramatically increasing the speed of point-to-point commercial air travel.

How much could that kind of a business be worth to Virgin Galactic? For now, that's difficult to say, but investors think it will be worth quite a lot -- which is why over the past year, Virgin Galactic stock has outperformed the S&P 500's performance by a factor of 50 times.