Even 100 meter NEOs could cause significant damage, and there are far
more of them. LSST will be able to detect objects as faint as 24.5 in
magnitude in a 30s visit, enabling it to detect 140m NEOs as far away
as the Main Belt asteroids.
[...] During its survey of the sky, LSST can find 90% of the PHAs over
140 meters in diameter.
Earth is 1 AU from the Sun, so that would be 1.2 to 2.2 AU away from us:
The Asteroid Belt is located in an area of space between the orbits of
Mars and Jupiter. That places it between 2.2 and 3.2 astronomical
units (AU) from the Sun. The belt is about 1 AU thick. The average
distance between objects in the Asteroid Belt is quite large. If you
could stand on an asteroid and look around, the next one would be too
far away to see very well.
Timewise, it would be about 8 years away:
The bottom right panel shows an example of a very different hazardous object - the 3 km large comet C/1996 B2 Hyakutake, which passed within 0.10 AU from Earth
in 1996. The apparent magnitude vs. distance diagram indicates that the current
generation of wide-angle imaging surveys, such as SDSS with V < 22,
could have provided an early warning about 3 years before the closest
approach (if they had sufficient cadence and sky coverage). With LSST,
the warning time would be ∼8 years, with over 500 observations over
Fortunately, your comet is 60 km, making the comet tail easy to spot:
While the solid nucleus of comets is generally less than 50 km across,
the coma may be larger than the Sun, and ion tails have been observed
to extend 3.8 astronomical units (570 Gm; 350×106 mi).
This table of impact risks shows that the most likely (5.8e-02) impact is between 2095 and 2113.