I get asked this question every once in a while and I finally got some free time over Christmas break to try and track an answer down. I quickly found a NASA brochure online titled “International Space Station Utilization Statistics Expedition 0 – 44 December 1998 – September 2015” . Seemed like a good place to start and right on the first page I found this:
Easy peasy. “Total investigations from ISS Expeditions 0-44 is 2,060.” There. Finally, an answer I can give those who ask. But is an “investigation” the same as a payload? They give the definition of investigation in the paragraph above the table, but it’s kind of loose and doesn’t really say something is specifically a physical research payload or not.
NASA also maintains a website with a list of experiments by expedition (and a few other categories) and I thought of comparing the number of payloads on that site to the number above. After some Excel wrangling I came up with the following:
Well, the total of 3,818 from the data on that page is a bit larger than the 2,060 reported in the brochure. Even if you include the three additional expeditions since that publication, it only adds up to 2,609. Also, the values I have don’t match the values they have for “Total Investigations” during each expedition increment.
A quick check for duplicate values in the data (pink in the Excel sheet above) showed that many of the experiments are repeated through expeditions. For example, Made In Space’s 3D printer is listed three times in the data (shown as “3D Printing in Zero-G”) because it ran multiple times during several expeditions. I learned from a NASA friend that they call each payload use an “interaction” and as the data above shows, there can be several interactions with a payload over multiple increments. Still don’t know why the totals don’t match up, though. NASA’s data in their brochure doesn’t even match up with the data shown on their website. Alrighty.
Curiouser and curiouser (or more like: nerdier and nerdier), I edited out the duplicates and came up with what I think is a much more reasonable number.
OK, 1,062 payloads ~16 years seems like a more reasonable estimate to me. Plotted as a bar graph,
It’s great to see a slight upward trend in new payloads since late 2013-early 2014. Since I was already in the thralls of Excel ecstasy, I was wondering what caused the spikes of new payloads, especially between March 2014-September 2015. I plotted this data along with the number of visiting payload vehicles during the increment.
The peaks correlate nicely to a very busy time on the ISS between March 2014 and September 2015 with 12 vehicles arriving: SpaceX had 4 Dragon visits, Orbital had one Cygnus visit, there was 1 ESA ATV and JAXA HTV visit each, and finally 5 Progress vehicles (see below). The other peaks seen at October 2007-April 2008, April 2009-October 2009 and March 2011-September 2011 correlate to ATV-1, HTV-1, and STS-134, STS-135 ATV-3, respectively.
This certainly shows the value of Dragon and Cygnus at being able to carry a significant amount of science payloads. Dragon is particularity useful for life sciences, not only because of its ability to return samples, but because researchers can deliver their samples to be loaded in the capsule at ~30 hours before launch.
Overall, it looks like a reasonable number for the amount of research payloads flown to the ISS in 16 years is ~1,000. I hesitate to say the number is exactly 1,062 as calculated, because I may be missing something (maybe NASA has an explanation for the numbers they published) and some of the payloads listed on the NASA site are not really research payloads (such as “Crew Earth Observation” or “Story Time From Space”).
We can also see from these data the significant value Dragon and Cygnus bring to the research community. This value will no doubt increase again hopefully in 2019 when Sierra Nevada flies their Dream Chaser cargo vehicle to the ISS, adding more late sample loading and return capabilities. Now if we could just get more crew up there to work on all of this science…