Deep Impact (EPOXI)
Launch Date Jan. 12, 2005 | 18:47:08.574 UT
Launch Site Cape Canaveral, Florida, USA
Destination Comets, Earth, Earth’s Moon, Beyond Our Solar System
Type Flyby, Impact
Status Successful
Nation United States
Alternate Names 2005-001A, EPOXI, Deep Impact, DIF, Deep Impact Flyby Spacecraft, 28517, DIXI

Goals

Deep Impact's primary mission was to probe beneath the surface of a comet. The spacecraft delivered a special impactor into the path of Tempel 1 to reveal never before seen materials and provide clues about the internal composition and structure of a comet.

Accomplishments

After almost nine years in space that included an unprecedented 4th of July impact and subsequent flyby of a comet, an additional comet flyby, and the return of approximately 500,000 images of celestial objects, NASA's Deep Impact mission ended in September 2013.

Deep Impact, history's most traveled deep-space comet hunter, provided many significant results for the science community. Here are the mission team's top five:

  1. First determination that a comet's surface layer (few to 10 meters or so) is very porous (greater than 75 percent empty space)
  2. First direct evidence showing chemical diversity of outgassing associated with different parts of the cometary nucleus
  3. Discovered that hyperactive comets (5-10 percent of all comets) are driven by carbon dioxide and that the observed excess water is from icy grains in the coma. The processes of hyperactive comets are very different from those in normal comets.
  4. Observations led to re-thinking where in the solar system comets formed. Contrary to all thinking for the last half century, the Jupiter family comets must have formed closer to the sun than did the Oort cloud comets.
  5. Enabled the subsequent exciting results from the Stardust NExT mission that changes theories on how comets evolve.

Key Dates

Jan. 12, 2005 | 18:47:08.574 UT: Launch

July 1, 2005: Comet P/Tempel 1 Rendezvous

July 4, 2005 | 5:52 UT: Comet Impact

August 2005: End of Primary Mission

Nov. 4, 2010: 103P/Hartley 2 Flyby

Spacecraft

Launch Vehicle: Delta II

Spacecraft Mass: 1,433 pounds (650 kilograms)

Spacecraft Instruments:

The High-Resolution Instrument is the main scientific instrument on the Deep Impact flyby spacecraft. It features a 30-centimeter-diameter (11.8-inch) telescope that delivers light simultaneously to both a multispectral camera and an infrared spectrometer. When the flyby spacecraft comes within 700 kilometers (420 miles) of the comet's nucleus, the camera will image parts of the comet with a scale better than 2 meters (about 6 feet) per pixel. This camera is one of the largest instruments flown to date on a planetary mission.

The Medium-Resolution Instrument is the other scientific instrument on the flyby spacecraft. It is a smaller telescope with a diameter of 12 centimeters (4.7 inches). Due to its wider field-of-view, it can observe more of the field of ejected material as well as the crater created by the impact event. It can also observe more stars around the comet and is therefore slightly better at navigation during the final 10 days of approach to the comet. When the flyby spacecraft comes within 700 kilometers (420 miles) of the comet's nucleus, this instrument can image the entire comet with a resolution of about 10 meters (about 33 feet) per pixel.

The impactor spacecraft weighs a total of 372 kilograms (820 pounds), with 113 kilograms (249 pounds) of that being "cratering mass" -- dead weight designed to help the impactor make a substantial crater in the cometary nucleus. The cratering mass is made up of copper plates at the impact end of the impactor. The copper plates are machined to form a spherical shape.

The impactor is powered during its brief solo flight by a single 250-amp-hour battery. The computer and avionics interface box are similar to those on the flyby spacecraft; star trackers, inertial reference units and many propellant subsystem components are the same on both spacecraft. Like the flyby spacecraft, the impactor has a group of thrusters to refine its flight path. Because of its brief mission, the impactor does not have redundant backups as does the flyby spacecraft. The impactor's single scientific instrument, called the impactor targeting sensor, is an imaging system identical to the medium-resolution instrument on the flyby spacecraft, but without a filter wheel. A 12-centimeter-diameter (4.7-inch) telescope provides navigation images as well as closeup scientific images of the comet just before impact.

In Depth: Deep Impact (EPOXI)

Unlike previous cometary flyby missions, such as Vega, Giotto, and Stardust, the Deep Impact spacecraft, the eighth mission in NASA’s Discovery program, was intended to study the interior composition of a comet by deploying an impact probe that would collide with its target.

The spacecraft was comprised of two parts: the main flyby spacecraft and an impactor. The flyby spacecraft weighed 1,325 pounds (601 kilograms), was solar powered and carried two primary instruments.

The high-resolution instrument (HRI), the main science camera for Deep Impact, was one of the largest space-based instruments ever built for planetary science. It combined a visible-light multi-spectral CCD camera (with a filter wheel) and an imaging infrared spectrometer called the spectral imaging module (SIM). The medium resolution instrument (MRI) was the functional backup for the HRI, and like the HRI, it served as a navigation aid for Deep Impact.

The impactor weighed 820 pounds (372 kilograms) and carried the impactor targeting sensor (ITS), nearly identical to the MRI, but without the filter wheel, which was designed to measure the impactor’s trajectory and to image the comet from close range before impact.

One of the more unusual payloads on board was a compact disc with the names of 625,000 people collected as part of a campaign to “Send Your Name to a Comet!”

After launch, Deep Impact was put into low Earth orbit, then an elliptical orbit (about 100 x 2,600 miles or 163 × 4,170 kilometers), and after a third stage burn, the spacecraft and its PAM-D upper stage departed on an Earth escape trajectory.

There were some initial moments of anxiety when it was discovered that the spacecraft had automatically entered safe mode shortly after entering heliocentric orbit. By Jan. 13, 2005, Deep Impact had returned to full operational mode following a program to tumble the vehicle using its thrusters.

The spacecraft traveled 267 million miles (429 million kilometers) in six months (including course corrections on Feb. 11 and May 4, 2005) to reach Comet 9P/Tempel.

As the spacecraft approached its target, it spotted two outbursts of activity from the comet June 14 and June 22, 2005.

On July 3, 2005, at 06:00 UT (or 06:07 UT Earth-receive time), Deep Impact released the impactor probe, which, using small thrusters, moved into the path of the comet, where it hit the following day, July 4, at 05:44:58 UT. The probe was traveling at a relative velocity of about 23,000 miles per hour (37,000 kilometers per hour) at the time of impact.

The impact generated an explosion the equivalent of 4.7 tons of TNT and a crater estimated to be about 490 feet (150 meters) in diameter.

Minutes after the impact, the flyby probe passed the nucleus at a range of about 310 miles (500 kilometers) at 05:59 UT July 3 and took images of the crater (although it was obscured by the dust cloud), ejecta plume, and the entire nucleus.

Simultaneous observations of the impact were coordinated with ground-based observatories as well as space-based ones, including the European Rosetta (which was about 50 million miles or 80 million kilometers from the comet), Hubble, Spitzer, the Swift X-ray telescope, and XMM-Newton.

The impactor also took images up to 3 seconds before impact that were transmitted via the flyby vehicle back to Earth.

Controllers registered about 4,500 images from the three cameras over the next few days. Based on the results of Deep Impact’s investigations, scientists concluded that Comet Tempel 1 had probably originated in the Oort Cloud. The data also showed that the comet was about 75% empty space.

Although Deep Impact’s primary mission was over, because the flyby vehicle still had plenty of propellant, on July 3, 2007, NASA approved a new supplemental mission for Deep Impact, known as EPOXI. The name was derived from the combination of the two components of this extended flight: Extrasolar Planet Observations (EPOCh) and Deep Impact Extended Investigation (DIXI).

This so-called “mission of opportunity” was originally focused on Comet 85P/Boethin. On July 21, 2005, Deep Impact was set on a trajectory to conduct a flyby of Earth in anticipation of intercepting Boethin. Unfortunately, scientists lost track of Comet Boethin, possibly because the comet had broken up.

Deep Impact was redirected toward Comet 103P/Hartley (or Hartley 2), starting with an engine burn Nov. 1, 2007. EPOXI’s new plan set Deep Impact on three consecutive Earth flybys, spread over two years (December 2007, December 2008 and June 2010) before the final trek to meet Comet Hartley 2.

These flybys essentially “stole some energy” from the spacecraft, thus dropping Deep Impact into a smaller orbit around the Sun.

Before the second Earth flyby, Deep Impact performed its EPOCh mission using the HRI instrument to perform photometric investigations of extrasolar planets around eight distant stars, returning nearly 200,000 images.

In the fall of 2010, Deep Impact began its investigations of Comet Hartley 2, conducting its flyby of the target at a range of about 430 miles (694 kilometers) at 15:00 UT Nov. 4, 2010. As with the encounter with Comet Tempel 1, Deep Impact used its three instruments to study Hartley 2 for three weeks.

Some of the images were so clear that scientists were able to identify jets of dust with particular features on the comet’s nucleus. The data showed that the two lobes of Hartley 2 were different in composition.

Once past this second cometary encounter, Deep Impact had little propellant for further cometary investigations, but there was a possibility that the spacecraft, if still in working condition, could be used for a flyby of Near Earth Asteroid 2002 GT in 2020.

With that goal in mind, thrusters were fired in December 2011 and October 2012 for targeting purposes. In the meantime, the spacecraft was used for the remote study of faraway comets such as C/200P1 (Garradd) in early 2012 and C/2012 S1 (ISON) in early 2013.

Communication with Deep Impact was lost sometime between Aug. 11 and Aug. 14, 2013, and after considerable effort to contact the spacecraft, NASA announced on Sept. 20, 2013, that it had officially abandoned efforts to contact Deep Impact.

Additional Resources

NASA Deep Impact Mission Profile

NASA Jet Propulsion Laboratory: Deep Impact Page

Key Source

Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.

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