SpaceX aims to launch a new communications satellite into high-altitude orbit during a launch window that opens at 5:40 p.m., but all eyes will be on what happens next—another attempt to bring its speeding rocket booster in for a gentle landing on a robotic ship at sea.

You’ll be able to watch the countdown here once it begins:

The company’s primary mission is to put a satellite called Thaicom 8 on a path to reach an altitude of 36,000 kilometers above Earth. There, it will enter a “geosynchronous orbit,” appearing to hang over India, east Africa and Thailand by moving at the same speed as the earth’s rotation. You can get a sense of where that is compared to the 1,300 or more satellites already orbiting Earth by checking out our handy orbital map.

Launching satellites to such great heights is a lucrative business—launch providers earned $8.1 billion in 2014, according to the consulting firm Eurospace, with $3.28 billion coming from cargo headed for geosynchronous orbit.

SpaceX CEO Elon Musk wants to dominate that market, and one way the company hopes to do so is by reusing its rockets rather than discarding them, allowing it to undercut its competitors on price by tens of millions of dollars. That’s what today’s experiment in landing its rocket booster at sea is all about.

Landing a 70-meter rocket returning from space is always a challenge, but returns from geosynchronous launches are especially tricky because the rocket is going faster than when it carries cargo to lower altitudes. The rocket stage must carefully conserve fuel as it brakes and steers itself toward the landing ship, and SpaceX engineers say this attempt may not succeed. Previous attempts have ended in big explosions, but earlier this month SpaceX managed the feat after launching a Japanese communications satellite.

If the launch itself is successful, it will be the fifth for SpaceX this year; the company hopes to launch as many as 18 rockets in 2016, including test flights of its forthcoming Falcon Heavy rocket. Displaying its ability to routinely launch payloads on time is important if the company is to establish itself as the leading provider of space access.

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