The DIXI component (Deep Impact Extended Investigation) of the EPOXI mission will observe comet 103P/Hartley 2 to compare it with comets observed by other spacecraft missions. Comparisons with data from Tempel 1, taken with the exact same instruments, will be particularly useful for determining which cometary features represent primordial differences and which result from subsequent evolutionary processes. Comets Boethin and Hartley 2 were identified before the launch of the Deep Impact spacecraft as possible targets for an extended mission upon the completion of Deep Impact. Boethin was selected as the primary target since it was a shorter mission and had better encounter conditions. Hartley 2 was designated the backup target in case Boethin was not recovered before the December 2007 Earth flyby.
Boethin was not recovered, which simply means that we were unable to definitively locate the comet's current position according to the best orbital information that we had from the last time it was sighted in 1986. The flyby spacecraft has performed the appropriate Trajectory Correction Maneuver (TCM) to put the spacecraft on course for Hartley 2.
Hartley 2 is smaller and more active than Tempel 1, which makes the comparisons more interesting. This plus more sunlight (because the encounter occurs slightly closer to the sun) means our signal-to-noise ratio (SNR) on all our measurements will be significantly improved.
Currently (Autumn 2008) comet Hartley 2 is just past its aphelion point and is about as far from the sun as Jupiter. It is very faint and will remain faint (dimmer than mag 20) through 2009. However, as it gets closer to the sun and Earth, it will get brighter. We expect that amateurs will start observing it sometime in late Spring 2010 when it becomes brighter than mag 16. We will post observing information on the Amateur Observers' Program website where we will also collect observations. You should also visit the AOP website to see where the Deep Impact flyby spacecraft is in relation to the comet.
The EPOCh component (Extrasolar Planet Observation and Characterization) of the EPOXI mission will carefully study a small number of stars in order to learn more about planets that we know are orbiting those stars, and to search for clues to other planets that might be orbiting the same stars. The number of target stars has to be small, because time is strictly limited for this phase of the mission due to details of the spacecraft orbit, and the stars will each have to be observed nearly continuously for weeks at a time. We have more than enough candidate stars, and we can pick only a few. The experiment team faced complicated and difficult decisions to determine which stars would be observed, and which had to be left for future work - maybe a future space mission.
How do we know that there are planets orbiting stars besides our Sun? As of May 28, 2008, the discovery of 293 planets in orbit around 241 stars had been announced, including 25 stars that have more than one planet. Up-to-date information is available from the interactive catalog of the Extrasolar Planets Encyclopedia. The planets are too small and too dim to see directly, even with modern telescopes and cameras. Wobbles in the motion of the parent star, however, can tell us about the size and orbit of planets that orbit the star and this is how almost all of these planets were discovered.
In a small number of cases, a planet's orbit carries it right between us and the planet's parent star. We say that the planet "transits" the star by traveling across its visible face. If we could clearly see something so distant, we would see the night side of the planet as a black disc against the much larger disc of the star. We can't see anything so clearly at the great distance between stars, not with present technology, but we can see that the star's light is a little bit dimmer every time the planet's orbit carries the planet through a transit. As of May 28, 2008, there were 51 known transiting planets, with new discoveries announced just in early April. Up-to-date information (in technical language) is available at the Summary Table of Parameters for Transiting Planets and also from the Extrasolar Planets Encyclopedia. The EPOXI mission will investigate a few known transiting planets, using the advantages of a telescope in space to learn more than can be learned from a telescope on the ground.
We can learn things from planetary transits like the exact length of the planet's orbital period and the planet's diameter. The diameter of the known transiting planets is equal to or larger than the diameter of our own solar system's planet Neptune. They all orbit very close to their parent star, so they have an orbital period - a year - that can be measured in a few Earth days. These are the kinds of planets that have been found, because a short orbital period is easier to measure with observations that can take place for only a few hours of night-time per day, over a time of several days. Observations of transiting planets are limited by the twinkling (scintillation) of stars seen through the atmosphere, and because the telescope cannot view the star and the planet all the time. This is the advantage of the EPOXI mission: because the on-board telescope is in space, there is no atmosphere and the spacecraft can watch a single star nearly continuously for weeks.
EPOXI target stars are stars with known planets, so we can be certain that at least one planet of each star will produce transits that can be observed. The known transiting planet systems all have only one known planet as of the start of the mission. The EPOXI mission will search for small details in the darkening and brightening of the parent star during a transit, which can provide clues to rings or large satellites (moons) orbiting the planet. EPOXI will search for tiny variations in the orbital period of the known planets, signaling the gravitational influence of other planets orbiting the same star, and EPOXI will search directly for additional transits from smaller planets, too small to observe through the noisiness and uncertainty of the Earth's atmosphere. EPOXI can detect transits of objects down to about half the diameter of the Earth.
This table lists the target stars with known planets that we plan to observe with EPOXI, in the order that we plan to observe them. Our own star and planet are included for comparison, at the bottom of the table. We will observe the Earth several times during the mission, as an analog to the appearance of an Earth-like extrasolar planet.
- Spectral Type1
- App. Mag.2
- Distance (ly)
- Orbital period (days)
- Diameter (x Jupiter)
|Discovery Group||Observation Dates|
|comments||# transits observed|
aka SAO 64638
|Hungarian-made Automated Telescope Network (HATNet)||1/22 -- 2/12
|Low density planet, large radius for its mass||10|
aka TYC 3727-1064-1
|XO Project||2/13 -- 2/19|
|Eccentric orbit, second planet suspected||1|
|Trans-Atlantic Exoplanet Survey||2/20 -- 3/18|
|Short period (31 hours), reflected light target||7|
|XO Project||3/20 -- 4/07|
|Fainter component in wide visual binary, metal rich||3|
|California & Carnegie Planet Search||5/01 -- 5/28|
|Eccentric orbit, unseen planet suspected, star is M-dwarf||8|
|Trans-Atlantic Exoplanet Survey||missed3
|Kepler target, additional planets possible||7|
aka TYC 2636-195-1
|Wide Angle Search for Planets||TBD|
|Strongly heated, reflected light and visible thermal emission possible||8|
aka GSC 03547-01402
|Hungarian-made Automated Telescope Network (HATNet)||TBD|
|Kepler target, even more strongly heated than WASP-3||8|
a really long time ago!
|humans||3/19/08, 5/29/08, 6/5/08, 3/26/09, 9/26/09|
a really long time ago!