Starship is going to be fueled with cryogenic liquid methane. Before launch, the vehicle will spend some time fully loaded on the launchpad. Although methane is lighter than air at normal temperature, at -161.5°C (boiling point of liquid methane) it's 30% denser than air. It has to be heated another 30°C to become lighter again, which is not going to happen fast. Therefore methane can build up around the Starship in significant amounts even if refrigeration is considered, as sealing is never perfect and leakage can happen. Liquid methane expands 590 times. Considering its lower and upper explosion limits with air are 5.3-15.5%, ignition of the rocket could easily trigger a detonation. The danger may seem minuscule, but one cubic meter of gaseous air/methane (90/10) mixture is equivalent in explosion power to 190g of TNT. How does SpaceX plan to deal with this kind of explosion hazard around Starship?
$\begingroup$ slightly related: Rocket explosion compared to kT of TNT; has one ever knocked something over at a distance? $\endgroup$– uhohMay 15, 2020 at 22:45
$\begingroup$ Not sure if build is actually possible or not, but like Space shuttle, used sparks, hydrogen ignitor system. Probably, similar technology can be used ! $\endgroup$– zephyr0110May 16, 2020 at 9:30
$\begingroup$ But hydrogen is lighter than air in any case even at boiling temperature -252.8°C so it cannot buildup substantially around rocket. And you will need 4 times higher concentration of hydrogen per volume than methane to achieve same effect base on stoichiometric combustion. I find methane more dangerous. $\endgroup$– WOW 6EQUJ5May 16, 2020 at 9:51
Boil-off of methane is piped to a Flare Stack and burned off so it won't be vented unburned into the atmosphere. You can see a good picture of the flarestack on the right in the image of Starhopper fueling below:
(Image credit: NasaSpaceflight.com (bocachicagal)
Update Mar 4 2021: SpaceX no longer uses a flare stack. About six months ago they installed a methane recondenser, so the methane is turned back into liquid and reclaimed.
$\begingroup$ Is this true for Starship tank venting and storage tank venting? It seems as if they would need to vent directly from the vehicle close to launch? $\endgroup$ Mar 3, 2021 at 19:37
$\begingroup$ What you see directly venting from the vehicle is liquid oxygen. Starship uses supercooled (densified) LOX, and as it warms it expands and needs to be vented. The header tank contains LOX, and that's what you see venting there as well. The methane is routed to a recondenser and reclaimed. $\endgroup$ Mar 5, 2021 at 2:00
From an addendum to SpaceX environmental impact statement, submitted sometime in 2019-2020:
When the test vehicle is still connected to the ground systems, the methane would be transferred back to the methane tanks. If the vehicle is connected to the ground systems and tests are performed without engine ignition, the methane would be transferred to the flare. When the vehicle has been disconnected from the ground systems, such as during a static fire or test launch, methane would be vented to the atmosphere. Due to risks to personnel, SpaceX is unable to reconnect the vehicle to ground systems when methane remains on the vehicle. SpaceX is currently working to design a safe method to transfer the methane back to the tanks and reduce the amount of methane released.
Mr. Musk more recently tweeted (and as said in the other answer) that the flare stack has been replaced with a solar powered re-condenser.
As for when Starship is disconnected from the ground service equipment, it seems unlikely that there has been a separate re-condenser installed on-board the recent test vehicles.
Given that they are allowed to vent, and knowledgeable persons have tweeted that they indeed are doing so as recently as SN9, it seems some methane is still vented directly from the vehicle.