In a press release Thursday about NASA’s purchase of a SpaceShipTwo flight for research activities, Virgin Galactic provided an update on sales for its primary market, space tourism. “Virgin Galactic has already collected more than $58 million in deposits from 455 future tourist astronauts,” the release noted. Those are the largest figures that the company has cited for both customers and deposits to date. But how has its backlog grown in recent years?

To try to answer this, I dug through several years of my own notes as well as media reports and press releases. The result is the chart below, showing the number of customers Virgin Galactic has reported since the beginning of 2008:

It’s worth noting that in many cases the numbers given by the company are approximate: “nearly 300″, “over 350″, etc. For the purposes of the chart I’ve rounded up or down accordingly; in the case of the previous examples, they would be 300 and 350, respectively, in the chart. (Ideally I’d put in some error bars, but Google doesn’t support them in this chart.)

What this shows is that after a relatively flat 2009, Virgin has seen a steady increase in customers. (2009, it should be noted, is not as flat as the chart suggests, because of the rounding described above: the company reported “nearly 300″ in early 2009 and “over 300″ late in the year.) Over the last 17 months, from mid-May 2010 to Thursday, the company had seen an increase of 120 customers, a rate of a new customer every 4.3 days. During the nearly 22 months prior to that, from late July 2008 to mid-May 2010, the company had reported an increase of only 65 new customers, or one every 10.1 days. The sluggish growth then is not surprising given the economic crisis that started in 2008. Meanwhile, as the economy slowly improves Virgin is inching closer to flight, which may increase the interest among prospective customers.

This data set is certainly incomplete, based on a limited amount of research. If you’re aware of new or more accurate data, please let me know and I’ll update this chart accordingly.