Trump in June ordered the creation of Space Force, which would become the sixth service in the US military, arguing the Pentagon needs it to tackle vulnerabilities in space and assert US dominance in orbit.
Mattis last year voiced skepticism on the need to create a separate Space Force.
In a letter to a US congressman, he said he did "not wish to add a separate service that would likely present a narrower and even parochial approach to space operations," adding it would create extra bureaucracy and cost.
On Tuesday, however, the Pentagon chief said he fully supported the idea of creating a new command that would draw in members of existing military branches.
"We need to address space as a developing, war-fighting domain and a combatant command is certainly one thing that we can establish," he said.
The vast US military divides the globe into various commands, such as Central Command in the Middle East or Indo-Pacific Command in Asia, and so a new Space Command would be on an equal footing as these.
Though it would not be a full new branch of the military, it would still require a new headquarters and major organizational changes.
Pence is due to deliver an address at around 11:15 am (local time).
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