Deep Impact: A first of its kind mission

Illustration of Deep Impact-EPOXI spacecraft. Credit: NASA

• Launch Date: January 12, 2005
• Launch Site: Launch Complex 17-B at Cape Canaveral Florida, United States
• Launch Vehicle: Delta II (7925-9.5 variant with 9 strap-on SRBs)
• Mission Type: Flyby Impactor
• Target: (Deep Impact): Tempel 1
• Target: (EPOXI): Hartley 2 (And Exoplanets)
• Dimensions: Unavailable
• Mass: 650 Kilograms (Spacecraft) 370 Kilograms (Impactor)

Deep Impact was a revolutionary mission for NASA. It would send a probe out to a comet, and launch an impactor into it to reveal the subsurface material that lies beneath.

The comets visited by Deep Impact-EPOXI to scale. Credit: NASA/JPL-Caltech/UMD

The Mission

Deep Impact was equipped with a smart impactor probe. The probe was independent from Deep Impact once seperated, and had the battery life to last one day. After launch in 2005, Deep Impact made its journey to Tempel 1. The journey was relatively short, and Deep Impact reached the comet after only six months. On July 3, 2005, Deep Impact released the impactor, which subsequently impacted Tempel 1 the next day. The impactor had to make three course correction maneuvers as it appraoched Tempel 1, which it performed flawlessly.

Tempel 1 being impacted by Deep Impact's impactor. Credit: NASA

Scientists were able to learn much from the impact. Tempel 1 was much more "fluffy" than scientists had anticipated. It was loosely held together by gravity, and wasn't any more sturdy than a bank of powdered snow. A substantial amount of carbon rich organic compounds were also found. With its mission complete, Deep Impact was put into hibernation mode.

New Mission: EPOXI

Deep Impact was later reactivated for its new mission. The the mission name is a combination of EPOCh or the Extrasolar Planet Observation and Characterization mission, and DIXI or the Deep Impact eXtended Investigation. Together these form the acronym for the EPOXI mission. The main goal was to send the old Deep Impact spacecraft to a new target: comet Boethin. 

However, before Deep impact was able to make its maneuver to the comet, Boethin disapeared. It was believed that the comet had a destructive outburst, which blew it apart into many small pieces. The mission team then selected a new target: Hartley 2.

There wasn't a second impactor aboard Deep Impact, but it had several scientific instruments and cameras that it could use to investigate Hartley 2. Mission control "awakened" Deep Impact, on September 26, 2007. It then performed orbital maneuvers to send it to Hartley 2. On its way to the comet, scientists used Deep Impact's cameras to search for dips in starlight from exoplanets orbiting distant stars. At a distance of 700 Kilometres, Deep Impact made its flyby of Hartley 2 on November 4, 2010.

Deep Impact's views of Hartley 2 as it flew by the comet. Credit: NASA/JPL-Caltech/UMD

Hartley 2 is considered a "hyperactive" comet, and has intense jets coming off its surface. The two rough areas on either end of Hartley 2 spew out medium-sized but fluffy ice particles, while the smooth inner region lets out water vapour. Hartley also orbits in an irregular "excited state," without consistant rotation. This is likely due to the water and carbon dioxide jets. After the flyby, Deep impact's mission was over for good. NASA had redirected the probe to asteroid 163249 (2002 GT), but the probe suffered a malfunction, and was lost in 2013.

The Targets

Composite image of Tempel 1 (pre-impact). Credit: NASA/JPL/UMD

Date Discovered: April 3, 1867

Designation: 9P/Tempel 1

Class: Jupiter-Family Comet

Radius: 3 Kilometres

Rotational Period: 40.7 Hours

Orbital Period: 2039 Days (5.58 Years)

Hartley 2 as seen by EPOXI. Credit: NASA/JPL-Caltech/UMD

Date Discovered: March 15, 1986

Designation: 103P/Hartley 2

Class: Jupiter-Family Comet [NEO]

Radius: 0.8 Kilometres

Rotational Period: 18.1 Hours

Orbital Period: 2039 Days (5.58 Years)