The briefing will be held at NASA Headquarters' and broadcast live on NASA Television and the agency's website.

Comet C/2013 A1 Siding Spring will miss Mars by only about 88,000 miles (139,500 kilometers). That is less than half the distance between Earth and its moon and less than one-tenth the distance of any known comet flyby of Earth. The comet's nucleus will come closest to Mars at about 11:27 a.m. PDT (2:27 p.m. EDT), hurtling at about 126,000 mph (56 kilometers per second), relative to Mars.

The concerted campaign of observations by multiple spacecraft at Mars and by numerous NASA assets is directed at the comet and its effect on the Martian atmosphere. The observations of the comet may yield fresh clues to our solar system's earliest days more than four billion years ago.

Panelists include:

- Jim Green, director, Planetary Science Division (PSD), NASA Headquarters, Washington

- Kelly Fast, program scientist, PSD

- Carey Lisse, senior astrophysicist, Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland

- Padma Yanamandra-Fisher, senior research scientist, Space Science Institute, Rancho Cucamonga Branch, California

The public may ask questions on social media using the hashtag #askNASA.

The news conference will be available live and archived on: http://www.ustream.tv/nasajpl

For more about the comet, visit: http://mars.nasa.gov/comets/sidingspring

For NASA Television downlink information, scheduling information and streaming video, visit: http://www.nasa.gov/nasatv