What is the lowest first stage thrust for a launch reaching orbit?

What is the lowest thrust for a first stage rocket for a rocket that reached Earth orbit?

• The shuttle's 6000 lbf OMS engines circularized the orbit. Does that count as "pushing into orbit"? How about the little verniers on the first stage of old Atlases? – Organic Marble Oct 28 '20 at 14:47
• I think the question is now what the OP intended, although I could be wrong here. There has only been one first stage engine to make it to orbit that I am aware of, the Space Shuttle main engines, but I think the first stage thrust for an orbital rocket is what is actually desired here. – PearsonArtPhoto Oct 28 '20 at 15:09
• Now I realized first stage engines do not actually reach orbit. – Joe Jobs Oct 28 '20 at 15:34
• Thrust is the force what the drive creates, measured in Newton (N). The question is surely not VLQ (imho it is a pretty okay one, at least now). – peterh Oct 28 '20 at 15:53
• Is the intention of the question thrust in absolute numbers or trust to weight ratio? – lijat Oct 28 '20 at 22:32

The U.S. Vanguard rocket reached orbit three times with a first stage thrust of only 125 kN.

The first stage of the three-stage Vanguard Test vehicle was powered by a GE X-405 28,000 pound (~125,000 N) thrust liquid rocket engine.

Vanguard TV3 — NASA NSSDCA

@2012campion's answer shows that this was not the lowest thrust

The smallest rocket to reach orbit is the Japanese SS520-5. It had a peak thrust of $$185 kN$$ according to the same web page:

Firing up its first stage, SS-520-5 shot up from its launch rail at 2:03:00 p.m. local time on Saturday with its aft fins sending the climbing rocket into a spin to provide stabilization as it climbed with a thrust outweighing the vehicle’s mass by a factor of seven. The first stage, standing 6.1 meters tall and holding 1,587 Kilograms of propellant, pushed the vehicle skyward with a peak thrust of 185 Kilonewtons (18,900 Kilogram-force), averaging at 143kN (14,600kgf) over the course of a 31.7-second burn.

I can't rule out that a slightly larger rocket nevertheless had a slightly lower thrust, but this must be close to the lowest.

• That might be the smallest absolute thrust, but a launchpad TWR of 7 is far higher than most rockets have. – Mark Oct 29 '20 at 3:08
• @JoeJobs This answer came before the one about Vanguard – Steve Linton Oct 29 '20 at 18:54