That bright beacon is particularly important for the medium gain antenna, especially just after the vehicle first wakes up after a long hibernation period. The medium gain antenna has a 10° half power beamwidth, so pointing the vehicle so the medium gain antenna points at that bright beacon means the vehicle is able to receive the command from the Earth that tells it where to point itself so the high gain antenna is pointed at the Earth.

In addition to rate gyros and star trackers, New Horizons is also equipped with bright beacon detectors, aka Sun sensors.

@david-hammen Thanks, I added the other antennas.

I highly doubt the omni is in range anymore.

@Joshua it appears not The low gain system was only intended to be used within 1AU. While there probably was some room to push it using a more powerful transmitter at its current distance or ~44AU an increase of ~2000x would be needed vs at 1AU. While Arecibo is a more powerful transmitter than any of NASAs normal radio telescopes, it's not that much more powerful. spaceflight101.com/newhorizons/spacecraft-overview

@asdfex the light from the sun will take hours to reach New Horizons, therefore any picture will give an already wrong location. How do they solve for this problem? And if they extrapolate knowing the sun’s orbit and New Horizons’s trajectory, how does New Horizons know how far away it is from the sun (and therefore how “out of date” the picture is)?

NH has no way to tell how far away it is on its own, but mission control can find out very accurately and upload that information if necessary. But it is not needed. In the 6 hours or so of travel time, the Earth does not move enough to move out of the HGA beam. This question does that math for Voyager. NH is closer, but that 0.6º beam width covers a large portion of Earth's orbit. If Venus were in conjunction with Earth, you could receive NH at Venus.

@user New Horizons moves almost straight out of the solar system, it's not in a circular orbit like the planets. The only thing that moves quickly is Earth, From the probes perspective from Pluto, it moves by 2.6° within half a year, or 0.003° during the six hours the light needs - that's only about 1% of the beam width.

Perhaps, for the benefit of lay readers who are not intimately familiar with angle measures in practice, it should be noted that the 0.6° beam angle is slightly larger in HN's view of the sky than the full moon is in our sky. So it's not like it needs to be aimed with fantastic precision that exceeds what human eyes could do.

@asdfex To be clear, when you say Torch you're referring to what North Americans would commonly call a flashlight, right?

@Wipqozn: You just saved from a lot of confusion and a funny picture with people burning themselves, while holding lenses to a (fire) torch :)

@user NH is 6 billion kilometers from the sun. The distance it travels in 6 hours is around 300,000 km. Therefore the sun moves less than 0.003 degrees in NH's sky in 6 hours. In reality (since most of that velocity is radial), it's much less than that.

@Christian you would use a holder that incorporates both a lens and the fire spot in one piece, of course. More expensive models feature multiple lens slots for higher gain short-range communication in case of heavy precipitation or magical darkness.

@user even if you were trying to track an object moving at relativistic speeds (and had an appropriately ludicrously-high-gain antenna), you would still want to point your receiver toward the transmitter's apparent location. As for the transmitter, you would have to measure the signal delay and point the antenna at where the receiver is going to be at the appropriate moment (apparent location + apparent velocity * round trip time)

Even with such a tightly focus beam pointing it at the sun would give it coverage to be received on Earth. The signal would spread out inversely with the square of the distance and after 6.3 billion kms that is a hell of a lot of signal footprint.

@hobbs Sometimes the radial velocity is larger than 0.003 - when Sun, Earth and Pluto form a line. 3 months later the apparent speed is 0.

@Old_Fossil Only during a few days per year - most of the time Earth would be outside the beam. With high gain antennas there is almost no signal outside the intended opening angle, the falloff is very steep.