# Why not use Halley's Comet as a probe?

Halley's Comet comes around every 75 years. From what I can gather on Wikipedia the comet is not spinning that fast, has a large elliptical orbit, and has been recorded since 240 BC returning on a very regular schedule.

Why would we NOT use Halley's comet as a probe? We already landed Philae on Churyumov–Gerasimenko going 135,000 km/h. Is it that much of a stretch to land it on Halley going 254,016 km/h? (The Sidereal rotation period is ~4.5 times longer for Halley.)

Isn't this rock a super-good candidate for a long-term solar system exploring probe?

• An object travelling at 254 has around 3.5 times the kinetic energy of another travelling at 135. An extra 250% is a lot of a stretch. Further, if you can match that speed or level of energy, there are multiple different paths you might send a long-term probe. The only reason to send it to Halley's comet is if you want to study the comet itself. A lot of study can be done by looking at the spectrums of light its tail absorbs or by passing through the tail (possibly at a much lower velocity than it is travelling). So the benefits of landing on Halleys are not that much more, than a fly-by. Jul 3, 2016 at 12:40
• What do you gain by doing such? Jul 3, 2016 at 22:39
• Related: Harpoon propulsion - what would be the problems? and one very serious problem: Dissipating large amounts of energy fast, in space
– SF.
Jul 4, 2016 at 0:02
• I think a better question would be "why use Halley's Comet as a probe"
– GdD
Jul 4, 2016 at 9:02

There are a number of reasons:

When it's near the Sun (anything less than a few AU) a comet evaporates quickly, so it's not a safe place to be in. If you put the probe in the wrong place, evaporation below it can blast it off the comet.

When the comet is further out that reduces, of course, but it takes a long time for the comet to become completely dormant. Until it's dormant, any scientific instruments measure the comet's atmosphere and not interplanetary space.

Then there's the trajectory. Halley doesn't go anywhere interesting, so there's only a limited amount of science you can do.

You are limited by the comet's rotation, which takes the probe out of the Earth's view at least half of the time.

While normally landing on a comet does not make sense, because it requires the same energy / delta_v as making the spacecraft enter the same trajectory without a comet, NASA is investigating the "Comet Hitchhiker" idea - a new space travel concept, where spacecraft grabs the nearby comet or asteroid using its harpoon, gaining kinetic energy (similar to gravity assist).

• The "Comet Hitchhiker" idea requires a super long tether and small delta_v between comet and spacecraft. A larger delta_v would destroy the tether or the spacecraft. Such a long and strong tether would be pretty heavy. If the harpoon would hit the comet at a suitable point for secure attachment of the harpoon is very difficult to predict. If the harpoon misses the comet, the mission is lost.
– Uwe
Jul 11, 2016 at 13:10