From the May 2015 issue

If my dog Pluto were sitting on that planet July 14, would he be likely to catch a glimpse of New Horizons as it zooms by?

John Cawley III, Goodview, Virginia
By | Published: May 25, 2015 | Last updated on May 18, 2023
New Horizons’ closest approach to Pluto just before 8 a.m. EDT on July 14 puts it some 7,770 miles (12,500 kilometers) away from that world’s surface. Because the Sun is so faint in the outer solar system, a skywatcher on that world would still have a tough time noticing the flyby.
Astronomy: Roen Kelly
The closest New Horizons will get to Pluto is about 7,770 miles (12,500 kilometers) above the surface. At this distance, the spacecraft would be only 45 arcseconds across, around the size of Jupiter as viewed from Earth. New Horizons will be traveling at 8.6 miles per second (13.8 km/s) relative to Pluto, meaning it will zip through the sky at 3 arcminutes per second. That’s much slower than an International Space Station pass on Earth, but still fast enough that the spacecraft will go from horizon to horizon in a couple of hours.

The real trick is how bright the spacecraft will be. New Horizons is covered in very reflective material, but Pluto is extremely far from the Sun. At its brightest, New Horizons only will be about magnitude 18, and then for only 15 minutes at closest approach. For comparison, Pluto is about magnitude 14 now, and its largest moon, Charon, is magnitude 17.

So unless he has some serious glass, Pluto the dog will really strain to see New Horizons fly past. However, in addition to taking images and spectra of Pluto, New Horizons will probe the atmosphere of Pluto by transmitting a radio signal as it briefly passes behind the dwarf planet. Scientists back on Earth will need to use NASA’s giant 70-meter Deep Space Network antennas to hear New Horizons’ signal, but Pluto the dog could easily hear it with a ham radio set to the 3-centimeter band. So buy Pluto a radio set, and let him hear New Horizons call back to Earth as it flies past.

Simon Porter
Southwest Research Institute, Boulder, Colorado