The answer to this is that we have the technology right now to modify this object's orbit to guarantee its collision with earth.
What it would require is a major, probably unconcealable, launch of one or more multi-tonne unmanned spacecraft which would rendezvous with this object and attach itself (or more likely several sub-packages) to the object. The sub-packages would then fire their own engines to subtly alter the orbit of this object over a period of days or weeks to ensure its capture by earth's gravity well and its inevitable collision with earth. With continued operation, it would be possible to hit pretty much any desired target on Earth. It might even be possible to cause a glancing impact where the object would enter the atmosphere, scrape a city off the map, then bounce back out into space, doing far less than the maximum possible amount of damage.
This would take a lot of planning, money, labour and computer time to achieve, and would not be a one-man job, though it is possible that a handful of people (educated lunatics?) with enough money and the right skills could achieve this.
While the launch(es) necessary to divert the object would be unconcealable, since undoubtedly the US and Russia amongst others are still looking out for potential ICBM launches, there is nothing preventing a commercially funded overt launch from being a cover for the launch of this project. The commercial launch could then be declared a failure if necessary. The modification of the body's orbit is fairly likely to go unnoticed long enough for an operation to readjust its path to be unfeasible in the remaining time after discovery of its new path and destination.
Yes, humanity would probably survive an 860 GT impact, but not much of it at all. This would probably be an extinction-level event, causing major ecological damage similar to the event which finished off the dinosaurs 65 million years ago. A direct impact in an ocean would cause giant tsunamis (as opposed to the normal kind of tsunami), and throw up uncountable tons of dust and rock that would fall to earth as meteorite impacts right around the globe, raising the air temperature significantly for days, before an impact winter set in.
An impact in the pacific would wipe out the American western coast cities, as well as Japan, SE Asia and Australia's east coast from the tsunami effects alone, which would likely be a hundred meters or so high when it hit land. Collateral projectiles re-entering the atmosphere would cause global damage. It would actually be better for Earth if such an impact occurred on land.