How poor was our tally of objects that could produce potential extinction-level events back in 1998?

Question

A negative comment below my SciFi SE question Why did the Armageddon (1998) plot require a “rogue comet that jarred loose a Texas-sized asteroid”? says:

I don't understand the question. They needed an asteroid to hit the earth and they needed a reason for NASA to have plausibly not seen it, hence they knocked it out of its normal orbit with something.

to which I replied:

@Valorum This is 1998! That NASA might not have "seen it" is completely plausible. One of the side effects of this film was to help get NASA and ESA to step up their game and start searching for objects that may one day intercept Earth.

While a lot of asteroids were known in 1998, certainly there has been some additional effort to look at near Earth asteroids in the last few decades using existing telescopes, not to mention efforts to pu new ones in space for this purpose.

Question: How poor was our tally of objects that could produce potential extinction-level events back in 1998? Could there have been one that NASA didn't "see" at that time?

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Related questions about efforts to look for dangerous objects, most of this is post-1998:

• Why has the Earth-Sun libration point L1 been chosen over L2 for NEOCam to detect new NEOs?
• What is the “interesting story” on the limitations of NASA's current near Earth object survey network"?
• Has the National Near Earth Object Preparedness Strategy and Action Plan seen any action?
• How will the “fleet of small asteroid hunters” proposed by the B612 Foundation & York Space Systems work?
• Do all dangerous asteroids first pass through keyholes?
• Understanding gravitational keyhole analysis for Near-Earth Objects
• What is the name of the new NEO searching telescope “heavily based on” NEOCam?
• Why did the Herschel Space Telescope need helium coolant?
• How will the “fleet of small asteroid hunters” proposed by the B612 Foundation & York Space Systems work?

Answer 1

It was fairly poor. Wikipedia has a list of large, potentially hazardous asteroids, and 9 on that list were known before 1998:

• 2201 Olijato
• 1620 Geographos
• 4183 Cuno
• 1981 Midas
• 3122 Florence
• 3200 Phaethon
• 4486 Mithra
• 4197 Toutatis
• (4953) 1990 MU

The largest of these, 3200 Phaethon, is 5.8 km, so considerably smaller than the Yucatan impactor, but big enough to do plenty of damage if it does eventually get nudged into our orbit

At that time we hadn't observed (53319) 1999 JM₈ a 7 km asteroid, so it is not inconceivable that we might not have observed a 10 km asteroid (similar in size to the low end of estimates of the Yucatan impactor).

Now Armageddon has a "Texas sized asteroid", and Texas is huge, 1300 km from Brownsville to the Rita Blanca grasslands. It is considerably bigger than Ceres. There is essentially no chance that an asteroid the size of Texas would have gone unnoticed by NASA, indeed it would probably have been a naked eye object and known by the ancient civilisations.